The Design: ...and The Rest

There was an old U.S. sitcom called "Gilligan's Island" that aired in the mid to late 1960's.  It had a sort of ensemble cast and a catchy theme song that introduced the primary characters, but for the first season the song only introduced five of the seven cast-aways, ending the introductions with "...and the rest...".  There were only two actors left to introduce, but they just sort of raced to the finish and did not mention then.

I always think of this when something is nearly completed, but then sort of rushed through right at the end.  There are probably lots of reasons someone would spend a fair amount of time on 5/7ths of a project, only to rush the last 2/7ths at the end.... in my case, I've realized I just won't ever finish documenting my design choices if I write a full description of each.

When I was in the midst of making the decisions, writing the entries was a way for me to explore my own decision making process and was part of the excitement of the project.  Now that it's done, and I've been using the scope, there are other things I'm more excited about, like making the scope perform better, adding an equatorial platform and maybe some DSC's, so the final documentation of the parts that are already done been languishing.

I've already covered the reasons I decided to build rather than buy, what I selected for the size and primary mirror supplier, and my what I chose on how to hold all the bits together.  So, in rapid fire style, here is some information on what I actually ended up buying for the remaining parts, and why.

Secondary Mirror

Selecting a secondary for me was all about the tradeoff between directing as much light as possible from my nice big primary to the eyepiece, and shadowing that exact same nice big primary.  I am building the telescope for visual use, so having the entire field evenly illuminated is not as critical as it would be for imaging.  The link below really helped me think through this tradeoff and ultimate decide on a 3.1" major axis secondary.

http://www.loptics.com/ATM/diagonals.html

This link also contains a super-handy table which has already calculated various primary/secondary combinations.  According to the table, to achieve 0.5" illuminated circle at the eyepiece with my 16" f/4 and secondary placement, I would need a 3.16" mirror.  Since secondary mirrors come in a few standard sizes, I rounded down to a 3.1".  This gives me a central obstruction of 3.7%.  Not too shabby!

Here is a copy/paste from my original worksheet.  The prices are several years out of date, but it shows the suppliers/options I narrowed it down to:

Antares Optical	pyrex	3.1	1/14 p-v	$180
Protostar	pyrex	3.1	1/10 p-v	$195
Astrosystems.biz	pyrex	3.1	1/10 p-v	$175
Astrosystems.biz	pyrex	3.1	1/14 p-v	$195

I ended up going with the Antares Optics 1/14 p-v pyrex secondary, and it's performed wonderfully!

Spider

Daniel Steel at Dobstuff.com, where I purchased the structural kit for my telescope, also sells accessories for the telescope builder.  I decided on a three curved vane spider and he gave me a great price and included it in the shipment with my kit.  A curved spider seemed like a good idea, it spreads the energy from the diffraction of vanes out into a larger area, removing the "spikes" from stars resulting from the diffraction of straight vanes.

For smaller scopes, perhaps a curved vane spider is a workable solution.  In hindsight, for a scope of this size, I would have gone with a standard straight vane spider.  Overall curved vanes have more vane in the light path, so more diffraction overall, even if it's spread out and less noticeable.  Additionally, I feel like, at least some, of the collimation drift I experience as I move the scope in altitude is a result of the curved vanes not holding enough tension to keep the secondary firmly in place.  Early on I adjusted the mounting to put as much tension on the curved vanes as I reasonably could, and this helped quite a bit, but I'm still pretty sure it's angle is changing slightly as the telescope moves.

There are a number of things I want to modify about the telescope now that I've been using it for a while.  Not that it's performing poorly, just that I think it could be better.  Replacing the spider is high on the list.

Focuser

The focuser is really the user facing bit of any telescope.  I knew I wanted something really solid, with a nice feel.  Dobstuff.com sells two really quality brands, Moonlight and Feathertouch, and I think I would have been well served with either.  I went with the Moonlight two speed 2" focuser and it performs fantastically.  It's well made, super-solid, with a great feel as you are using the knobs.  It's got squaring adjustments, is internally baffled, comes with a nice 1.25" adapter and uses compression rings all around.  I can heartily recommend Moonlight focusers!

I did play with the idea of getting a Feathertouch with integrated Teleview Parracorr.  With an f/4 scope I probably could use a coma-corrector, and integrating one into the focuser is a clever idea.  It's also almost $1000 USD..... and coma does not bother me that much :-)

Finders

To round out the package I decided on a two finder system.  I've always liked having a zero magnification finder, like a red-dot or Telrad to get the scope roughly pointed, in combination with a magnified finder for more detailed star hopping or even observing certain extended targets.

For this scope, I decided on, in my opinion, the best zero mag finder there is; the venerable Telrad.  It's a solid design, I like the reticle indicating various FOV's and it's so common many astronomy programs and charts include it's reticle as a default.  It's also not all the expensive and dims down really nicely, something I've found lacking with some cheaper red dot finders that always seem a bit too bright, even at the lowest setting.

I had a RACI 10x50 finder from Orion telescope which performed pretty well which I ended up re-using for this scope initially.  Those of you following closely may remember a sweet 80mm finder I purchased at PATS way back in 2012.  Indeed it has been sitting in my closet all this time, as I left a crucial part back in the U.S. when I moved to Australia.  This simple $15 part cost almost three times this to ship to Australia and I could not find any local suppliers.  I kept holding off purchasing a new one, hoping I could get someone to bring it to me.

Well, finally it's happened and I have the 80mm finder, a telescope in it's own right with swappable eyepieces and a nice set of optics, mounted on my new dob.  Just last month I took it out to the ASV Messier Star Party.... and of course it rained.  I've yet to use it as a finder, but I'm really looking forward to the next new moon.

The Design: Structure

All Systems Go!

A month or so before I left for Australia to work for a while, I placed an order for my primary mirror.  It's a 16" F/4 from Terry Ostahowski which should be ready sometime in March.  With this key decision out of the way, I can pretty much order everything else for the scope as my funds and time allow.

One of the key reasons I decided to build my own telescope was so that I could choose exactly which parts met my particular balance of cost, quality and portability.  I have a good handle on the basic components needed, and some time to figure out exactly which specific part will best serve my needs.  After the mirror, the next thing seemed to be the overall structure... the stuff that would hold all the other stuff together.

The Criteria

Just like the overall criteria for my new scope, the structure itself needs to be portable, and economical, but there are a few more specific requirements:

• I want it to be as small and light as reasonable to firmly and stiffly support the optical system.  
• Ideally it will be quick and easy to take apart and put back together.
• It should have great motion;  Pointing a scope is a big part of using a scope and stiff or irregular movement on either axis can be a frustration.
• Like most things, being aesthetically pleasing is a plus.
• Of course it should be as inexpensive as possible while meeting the above criteria.

Any structure will be some compromise of all these competing criteria.  Some of them, like 'great motion' are tricky to evaluate without seeing one in person.  I've already decided that some variation of the Dobsonian mount.  This is mainly due to my desire for ease of use, but also helps a lot with the budget.  An equatorial mount for a 16" telescope would pretty much consume it entirely.

The Options

The first option is to built it all myself.  This sounds like a fun project, I do have some woodworking experience, and there are good plans of various designs available on the Internet.  In theory, it would be the least expensive monetarily, but would take a large amount of time.  Practically, I don't have a wood shop so I would need to purchase quite a number of fairly expensive tools to really do the job.  There is also a good chance I would not get it anywhere near right on the first try, which would mean putting up with something sort of usable, or spending the time and money to re-do it.... maybe more than once!

Although building from scratch was initially appealing, after a bit of thought, the option of a commercial kit seemed like a much better choice.  I'd love to be able to say with pride that I crafted this great telescope by hand, but even if I produced the structure, I knew I was not going to make the mirrors, focuser, cell, or any of the other pieces that go into a working telescope.   Some sort of kit really fit in with my plan for the scope; To assemble it from the specific pieces I feel meet my criteria best.

Besides,  I already had read about a great kit option, so I knew they were out there.  Even though I was pretty excited about the DobStuff.com kits, I wanted to see what other options were out there.  My search was not exhaustive, but I came up with a few other options:

• Obsession Telescopes:  I heard rumors that one could purchase some or all of a kit from Obsession, but I could not really find any information on this.  If they do sell them, I bet they are pretty nice, and rather expensive.  
• AstroSystems Telekit:  This seems like a nice kit, and it actually comes basically complete with spider for the secondary and a mirror cell.  The cost for a 16" kit is $1670, which after subtracting my budgeted cost for spider ($120) and primary cell ($250) is $1300.
• Hubble Optics: As indicated in this review Hubble Optics will sell you the structure of their ultra-portable dobs.  The price is listed in the review at $1195, which includes the spider and cell, I assume.  This is a bit better than the AstroSystems, and is very lightweight.  I had researched Hubble Optics as a potential supplier of the entire scope, but it seems there is some work to be done to make their scopes really perform, and I'm looking for something solid, even if it weights a bit more.
• DobStuff.com: DobStuff offers kits in a variety of sizes and configurations.  By default it's the structure only, but they also produce mirror cells and sell various lines of parts.  Kits include all the wooden elements, the strut poles, and bearing surfaces (ebony star/teflon) required to build the scope.  All I would have to do is sand, finish, and assemble.  The 13"-16" strut kit is $696.  I saw a few in action at the Golden State Star Party and was impressed.

The Decision

DobStuff kits were high on my list when I started this project.  In a way, it's sort of what inspired it by getting me daydreaming about building a large dob; Knowing I could get the precut parts to assemble made the whole enterprise seem doable.  After exploring the options above, I sent an email to Dennis Steele inquiring about prices and lead time.

He was very helpful and the prices were as quoted on the site.  It also turns out his workshop is near my primary observing locations in Joshua Tree, CA, where the scope would be built and spend most of it's time.  Graciously, he agreed to allow my to pickup the scope from his shop to save some shipping.

I placed an order for a 16" strut kit with the easy transport option for even more portability.  While I was communicating with him I also found that he produced nice aluminum floating mirror cells as well.  I'd already done a fair bit of research on mirror cells, which I'll describe in another post, and I felt his price/quality/design made sense, so I added a mirror cell to the order.

I believe the initial lead time he quoted was something like 4-6 weeks, which seemed fine since my mirror would take four months anyway.  This would still give me plenty of time to work on finishing and assembling the structure before the mirror was ready.  Turns out, he was able to piggy back my order with another 16" kit and cut the parts all at once.  This shortened the lead time to just a week and a half!


On October 20th, Dennis sent me an email with a picture of my completed kit, ready for pickup!

DobStuff 16" kit with mirror cell.  Strut poles are included but not pictured.

It was around this time that I learned I would be going to Australia for several months.  Exciting, but a bit of a bummer at the same time.  I was really looking forward to sanding that bad boy up nicely and figuring out what stain/finish I wanted to use.  When I emailed Dennis to let him know I could not pickup the kit as planned and to arrange for shipping, he kindly offered to hold the kit for me as long as I needed for no additional cost.  That's great customer service!


Current Status

As of now I have almost 2/3rds of a telescope. budget wise at least.  Here is my updated budget with a few the few things I've ordered slotted in with actual pricing.

Item	Cost	Notes
16" Primary Mirror	$2450	Terry Ostahowski 16"
Secondary Mirror	$220
Mirror Cell	$200	DobStuff Flotation Cell
Structure	$700	Dob Stuff 13"-16" Kit
Spider	$100
Focuser	$200
TOTAL	$3870

For anyone keeping track, this is about $1200 more than my original budget.  This is pretty much due to my mirror choice.  I had originally planned on going with a more mass produced JMI 16" f/4.5 mirror with secondary for only $1439, which is a very good price.  However this would have made the scope too long and put the eyepiece out of reach for me when pointing towards the top of the sky.

I could have gone with a smaller mirror, and therefore a shorter scope, but once I left the JMI mass produced mirror behind, I was in custom territory and the prices went up considerably even for smaller mirrors.  I ultimately decided to spend more now so I would not be as tempted to upgrade again later or regret that I did not get the scope I really wanted after all was said and done.

So that is where the project stands.  I have a mirror on order, perhaps even being ground by now, a finished DobStuff kit waiting to be picked up, and me out of the country.  I knew waiting would be tough, and being on an overseas adventure of sort has made the time go by more quickly, but I am anxious to get started.  I miss observing in Joshua Tree and knowing I could be working on the scope during the day, and observing at night over the holidays, makes it all the tougher to be away.

I hope the holidays find you in good spirits and provides some wonderful winter observing opportunities.  Clear skies and warm fingers!

The Design: Primary Mirror - Part 2

Work has been a bit hectic for me lately, long hours and such, so I've not had a lot of time to post anything here.  Even as busy as it's been, I've found time to daydream about my new telescope.  In my last post I narrowed my list of potential mirrors down to four candidates:

Company	Material	Size	f-ratio	Height	xt8 Mult	Price	$/Inch
Discovery	Pyrex	15	4.2	64.5	3.5	1699	9.60
Pegasus	Borofloat	14.5	4.3	64	3.3	2100	12.72
O.W.L.	Pyrex	15	4	61.5	3.5	2470	13.95
LightHolder	Pyrex	14.5	4.1	61.2	3.3	2440	14.79

It was a nice tidy list, and I contacted each of the manufactures to double check pricing and see about delivery times and such.  The pricing was accurate, and lead times were all around three months, which is pretty reasonable considering the work involved in a mirror like this.  Everybody I've contacted was helpful and responsive, which goes to show the passion they seem to share.

All of the emails did not bring me any closer to a decision, so I decided to try and get some additional input.  There are a lot of helpful and knowledgeable people on Cloudy Nights so it's usually my go-to place for questions I might have.  I started a thread titled 'Mirror Choice for 14-16" Dob Project' and laid out my plans and the current short list.

Quality vs. Price

In no time at all I was getting some insightful comments.  The entire thread is an interesting read, but the most useful posts were not about any specific mirror I had on the list, but were the more philosophical posts and the posts that added perspective.  Two things stood out to me; The first is summed up by these two quotes:

What you are paying extra for is the comfort and re-assurance that your mirror will be excellent. [...] On nights of excellent seeing you can certainly tell an excellent mirror apart from a good one. These nights aren't all that common but it's worth it when the air is good and knowing what you have doesn't get better. - John Bambury 

The thrill and satisfaction of quality is remembered long after the sting of high price. If you intend this to be a "lifetime" telescope take the pain now. [...] Forget the price for now. Aperture, length, and availability is what you need. - Jeff Morgan

A few of my questions boiled down to,  'How good is good enough?' and 'What is the extra money for a premium mirror really buying?'.  These two posts, along with others, sort of put things in perspective for me.

If I purchase a great mirror, from someone with a reputation for quality, there is a good chance that I will be happy for the lifetime of the telescope.  Aperture fever may kick in again, but I will never wonder 'What if I had paid a little extra?'.  Additionally, if I ever do succumb to aperture fever and want to sell my scope, the resale value will probably be higher.


Size Vs. Price

The second thing I realized in reading through the posts was closely related to the first.  Basically, if I was going to spend the money for a 'lifetime' scope, I should be happy with the end product.  Even though a 16" has only 14% more area than a 15" mirror, I know myself, and I'm pretty sure I'd be often wondering what that 14% would be like.

When I had to rule out the 16" mirrors due to price constraints, I was disappointed.  From the start I imagined this as a 16" scope project, even though I was open to a smaller scope if my design constraints (Eyepiece Height, Overall Size, Budget) meant 16" would not be possible.  I was tempted to go beyond my $2500 budget for a mirror and just make it happen....


Wisdom of the Crowd

I spent a fair amount of time trying to find mirror vendors, but I am not at all surprised that I missed a couple.  Two new manufacturers were suggested, Steven Swayze, who I had somehow missed but was very well respected and Terry Ostahowski, who also has a great reputation for quality optics.  I contacted both of them asking for pricing on a 15" or 16" f/4 mirror.

Both of them were responsive and informative.  I even got three different options from Steven Swayze, which was appreciated.  Here is the table with the new possibilities added, again sorted by $/inch.

Company	Material	Size	f-ratio	Height	xt8 Mult	Price	$/Inch
Discovery	Pyrex	15	4.2	64.5	3.5	1699	9.60
Ostahowski	Pyrex	16	4	65	4	2450	12.19
Pegasus	Borofloat	14.5	4.3	64	3.3	2100	12.72
Swayze	Pyrex/Cellular	16.25	4	65.9	3.3	2700	13.03
Swayze	Pyrex	16	4	65	4	2700	13.44
O.W.L.	Pyrex	15	4	61.5	3.5	2470	13.95
Swayze	Pyrex	15	4	61.5	4.13	2500	14.15
LightHolder	Pyrex	14.5	4.1	61.2	3.3	2440	14.79

The Decision

So many great options, and I was really excited to have 16" mirrors back into the mix, even if they blew my budget by 8% or so.  I was leaning heavily towards the 16.25" cellular mirror Steven Swayze suggested.  It's larger, and lighter, than a 16" mirror and would probably have superior cooling and stiffness.  Ultimately, I decided a 16" is more standard, and would make replacing (if I did something foolish to damage it) or selling the mirror (if something else terrible happend) easier in the future.

Looking at the price per square inch, and doing some research about the acknowledged quality, finally made my choice clear.  Today I put down a deposit on a 16" f/4 mirror from Terry Ostahowski.  Delivery is estimated sometime in March of next year... that's going to seem like long time, I'm sure!

Unlike the larger 80mm finder I bought a while back at P.A.T.S. this is the first, scarily big, step towards my new telescope.  Since the dimensions and focal length of the mirror sort of anchor the design, I can pretty much finalize all the other options I have to consider.  I can figure out what size secondary to use, find a spider to hold it, double check my decision on a structure, and think about focusers and other accessories.

It's exciting to think that I could have a working telescope as soon as April or May, 2013.  Seems like a long time from now, but I have a lot of things to consider, and observing to do with my current XT8 in the meantime!

Here's hoping for clear skies to keep me occupied...

The Design: Primary Mirror - Part 1

Quick Recap

In my last new telescope related post, I finalized my decision to build a telescope rather than buy.  Towards the end of that post, I did some thinking about my budget.  After some quick calculations, it became pretty clear that the mirror has a huge impact on the cost. 

It seems that no matter what size mirror I buy, the rest of the scope is going to be pretty much the same price.  A couple of parts, namely the primary cell and secondary, vary a bit with the size of the mirror, but everything else remains constant.  I'm estimating this 'base price' for the telescope, sans mirror, at about $1500.

The Long List

After looking around for a bit online, here is my long list of 17 potential telescope mirrors.  They are ordered by name of the company that produces them:

Company	Material	Size	f-ratio	Height	XT8 Mult	Price	Link
Discovery	Pyrex	15	4.2	64.5	3.5	1699	link
Discovery	Pyrex	16	4.5	73	4	1699	link
Hubble	Pyrex / Sandwich	14	5	72	3	1775	link
JMI/GSO	BK/7	16	4.5	73	4	1439	link
LightHolder	Pyrex	14.5	4.1	61.2	3.3	2440	link
LightHolder	Pyrex	16	4	65	4	2870	link
Lockwood	Pyrex	16	4	65	4	3870	link
O.W.L.	Pyrex	15	4.5	61.5	3.5	2470	link
O.W.L.	Pyrex	16	4	65	4	2900	link
Pegasus	Borofloat	14.5	4.3	64	3.3	2100	link
Pegasus	Borofloat	16	3.5	57	4	3100	link
Waite Research	SUPREMAX 33	14.5	4	60	3.3	2650	link
Waite Research	SUPREMAX 33	15	4	61.5	3.5	2800	link
Waite Research	SUPREMAX 33	16	4	65	4	3000	link
Zambuto	SUPREMAX 33	14.5	4.5	67	3.3	2650	link
Zambuto	SUPREMAX 33	15	4	61.5	3.5	3360	link
Zambuto	SUPREMAX 33	16	4	65	4	3924	link

*XT8 Mult is how many times more area this new mirror would be than my current 8" mirror

Two obvious points, there are a lot of options (this may not even be a complete list), and they vary quite a bit in price.  If I choose the least expensive option, the mirror will make up 50% of the $3000 cost of the telescope.  If I choose the most expensive, the mirror will make up 73% of the $5500 cost.

So this is the decision that seems to have the most impact on the cost, and probably quality, of my new telescope.  A telescope I hope will last a long time.  I'm beginning to dread this decision.

A Disappointing Realization

When I was thinking about the size of my new telescope a while back, I felt that something in the 14-16" f4.5 range met my criteria for portability, and for flat footed observing.  Along with price and overall aperture these are two of my most important considerations.  After a recent experience at the Pacific Astronomy and Telescope Show, I think my allowable size range might be a little off.

Me and a LightBridge 16" f4.5 Telescope

You can pretty much see in the photo above, that my eye is NOWHERE NEAR the eyepiece...

In order to observe with my feet on the ground, without a ladder, step-stool, box, or the like, I have to be able to reach the eyepiece through most of the range of motion of my new telescope.  Observing with the telescope straight up is somewhat awkward, so I'm willing to forego a bit of sky close to 90 degrees, but I am really committed to a flat foot telescope.  This means that the eyepiece height has to be at or below 62", which is the distance my eyes are from the ground, for the vast majority of angles.

The main factor in how high the eyepiece of a dobsonian telescope is from the ground is the focal length of the telescope, which is determined by the size of the mirror and it's Focal Ratio.  The focal ratio (or f-ratio) of a telescope is the relationship between the size of the mirror and the distance away from that mirror the converging light beam reaches focus.  In short, the higher the f-ratio the longer a telescope will be for a given size mirror.  There are some other design issues that can raise or lower the eyepiece a few inches, but really it's the focal length of the scope.

In retrospect, it should have been obvious to me that a 16" f4.5 scope, with an eyepiece height at zenith (pointed straight up) of somewhere around 73" would be no good.  I was hopeful that my rough calculations of eyepiece height might be off.  After all, eyepiece height is not exactly the focal length of the scope since the light path is turned at a right angle and the mirror is not sitting on the ground.  I also figured that even if the height at zenith was too much, that pointing the scope down just a bit would get the eyepiece and my eye better aligned.  

Seeing a 16" f/4.5 scope in person, standing next to it, moving it to various altitudes, really showed me just how wrong I was.  It was not until lower than 70 degrees that the eyepiece got to a height I could actually view through.  This means I would give up the upper 40-50 degree patch  of the sky, which is pretty much the darkest and most awesome part.  

So I am ruling out any mirror that will produce an eyepiece height greater than 65".  Even at 65" I could not use the scope straight up, but it would let me get close.  I think this height would be comfortable for me, and the area of the sky that would be inaccessible would be pretty small.  

Cut #1 - Eyepiece Height

All of that long winded explanation means I have to go with something more like a 16" f/4, or a smaller f/4.5 mirror.  This limits my options a bit, which is good, because too many options are proven to make people anxious and less satisfied with their decisions.  Sadly, 16" f/4.5 is a very common size, and common tends to equal less expensive.  Here is the slightly shorter list, now with 13 options:

Company	Material	Size	f-ratio	Height	xt8 Mult	Price	Link
Discovery	Pyrex	15	4.2	64.5	3.5	1699	link
LightHolder	Pyrex	14.5	4.1	61.2	3.3	2440	link
LightHolder	Pyrex	16	4	65	4	2870	link
Lockwood	Pyrex	16	4	65	4	3870	link
O.W.L.	Pyrex	15	4	61.5	3.5	2470	link
O.W.L.	Pyrex	16	4	65	4	2900	link
Pegasus	Borofloat	14.5	4.3	64	3.3	2100	link
Pegasus	Borofloat	16	3.5	57	4	3100	link
Waite Research	SUPREMAX 33	14.5	4	60	3.3	2650	link
Waite Research	SUPREMAX 33	15	4	61.5	3.5	2800	link
Waite Research	SUPREMAX 33	16	4	65	4	3000	link
Zambuto	SUPREMAX 33	15	4	61.5	3.5	3360	link
Zambuto	SUPREMAX 33	16	4	65	4	3924	link

Cut #2 - Price

I could theoretically save up money for as long as I need to get the most expensive mirror on the list above, but every month I save is another month I don't get to use my new telescope.  That's why I set a goal of one year; I'd love to have the telescope ready to go for the summer star party season 2013.  Spending more than $4000 on this project is probably unrealistic if I want to meet my goal.  So I'm going to have to spend less than $2500 on the mirror to make it work.  That brings us down to these 4 options:

Company	Material	Size	f-ratio	Height	xt8 Mult	Price	Link
Discovery	Pyrex	15	4.2	64.5	3.5	1699	link
LightHolder	Pyrex	14.5	4.1	61.2	3.3	2440	link
O.W.L.	Pyrex	15	4	61.5	3.5	2470	link
Pegasus	Borofloat	14.5	4.3	64	3.3	2100	link

I still think those are a lot of good options, based on some of the tests and opinions I have found so far.  Ultimately, there is only room for one primary mirror in my new telescope, so I'm going to have to figure out some way to evaluate these mirrors.

The Short List

Now that I have a manageable list, I need to figure out what I need to do to convince myself to buy one.  All things being equal, I want the most bang for my buck.  A good first way to compare these options is dollars per inch of area.  Here is the same list, with dollars per inch replacing links, and sorted by the same:

<

Company	Material	Size	f-ratio	Height	xt8 Mult	Price	$/Inch
Discovery	Pyrex	15	4.2	64.5	3.5	1699	9.60
Pegasus	Borofloat	14.5	4.3	64	3.3	2100	12.72
O.W.L.	Pyrex	15	4	61.5	3.5	2470	13.95
LightHolder	Pyrex	14.5	4.1	61.2	3.3	2440	14.79
LightHolder	Pyrex	14.5	4.1	61.2	3.3	2440	14.79

Now this is a list I can do some thinking about!  It's reasonably short and ordered by value.

I'm hoping to decide in the next week or two and see if I can place an order.  Here are my thoughts on each of these options, and what I want to look into:

Discovery 15" f/4.2: There is a virtue in being the least expensive.  The money saved could potentially go for a nicer focuser, or perhaps a nice fan system.  I have not read any information about their quality, so I'll definitely need to do a bit more digging

Pegasus 14.5" f/4.3: About 7% smaller than the 15", but this does not sound like much.  I've heard generally good things about Pegasus mirrors, but is the extra quality worth less aperture and more cost?

O.W.L 15" f/4: Almost $800 more than the Discovery, but slightly lower f-ratio for a shorter scope.  In fact, I'd be able to use this one straight up!  Not sure how the quality compares to the Discovery mirror. This is another company I've not heard much about, so I'll have to try to find some reviews.

LightHolder 14.5" f/4.1: The most expensive inches of aperture of the bunch, but LightHolder is supposed to be very good.  Again, will I notice?  Is it $340 better than the Pegasus?

So my next step is to try to learn about mirror quality, and how much difference it makes at the eyepiece.  I'm sure that there are differences in quality that can be measured with various machinery, but are these differences perceptible?

Another consideration is lead time.  Since most of these companies make mirrors to order, I'm going to email each one to see what their current wait/lead time is.  I'll let you know in a future post what sort of responses I get.


A Eulogy for the 16"

I'd like to pause here for a second to mourn the passing of the 16" options.  There is just something nice, round and magical, with the idea of upgrading to a telescope that offers 4x the light gathering of my existing telescope.  16" offers 21% more aperture than the 14.5", and 13% more than the 15" mirrors.  If I was able to go with an f/4.5 and still reach the eyepiece, 16" would be within my grasp!  Curse my shortness!

I'm very much tempted to open up my budget a little bit to get to the 16" mark.  I'd have to go at least $2900 to get there.  That's $1100 more than the Discovery 15" above!  I'm just not sure it's worth it for 13% more light gathering.

One very interesting option that I dropped due to price was the Pegasus 16" f/3.5.  For $3100 it would have given my the LOWEST eyepiece height out of any of the options, a mere 57" off the ground!  That complete telescope would cost roughly $4700, about 46% more than a 15" based on the Discovery mirror.


What do you think?

I'm going to do some research on manufacturing methods, reputation, and test reports, but ultimately, How much difference does the quality of a mirror make to the visual observer?  Have you had any relevant experience?  Do you know, or have you heard, anything about the mirrors on my short list?  Are there any I missed?

Also, if I'm going to have this telescope for a long time, will I end up disappointed if I don't spend 50% more and get a 16" scope?  That fast 16" Pegasus is tempting!  I'd love to hear what you think.

Build Vs. Buy: Build for the Win!

My work has been a bit crazy lately, but I was fortunate enough to get some needed time off and spend several nights observing in Joshua Tree last weekend around the new moon.  The observing at night was great, once I get my photo's together and have a few free moments, I'll post a report.  During the day I had some time to compare some options and think a lot about my next telescope, and I've decided...


I'm going to build

Ever since the idea for this project really solidified in my head, I always imagined building the telescope I want;  Not from scratch, but by assembling the parts I hand picked.  However, I wanted to take a look at what's out there and make sure I could not get a better deal with a commercially available telescope.  Something already built that would fit my requirements.

To help make my decision, I took a look at all the parts that make up a telescope, so I could look at the commercial options out there compared with what I could buy.  It helped a bit to solidify what I was looking for in a telescope, and ruled out several commercial options (for more detailed look at each of these commercial options, check out this post from 10MinuteAstronomy):

• Orion XTxx Scopes: There are several goto, and push to, scopes in 14 and 16" size from Orion.  Across the board they are just too heavy, and I don't really need the extra cost and complexity of the Goto/Push to.
   
• Hubble Optics 16" ultralight dob: Price seems good, very portable, but the reviews indicate mechanical issues.  If I have to make a lot of modifications to make it smooth and stable, I might as well get something more solid right off the bat.
   
• Meade LightBridge 16": Price is very good, but they are pretty heavy.  There have also been mixed reports regarding the motion and overall build/design.  It seems like with a bit of work these can turn out to be nice scopes, but they are still larger and heavier than I think are needed.
   
• T-Scope, Obsession, StarStructure, et. al.: All very nice options, and they all seem like they could be configured to meet my needs, but I'm pretty sure I can assemble a scope just about a nice, and make some more specific decisions along the way, for less money.  

Additionally, I do like to work with my hands, and I'm sure I'd enjoy at least some parts of assembling my own telescope.

Assembly Budget

Part of evaluating commercial scopes, particularly the high end ones that seem to meet my requirements, was figuring out if they were more or less I would spend to build.  Armed with my broad list of parts, I fleshed out a rough budget.

None of these numbers are very specific, they represent average costs for the components I would need.  As I have time, I'll research each of these components and figure out which specific ones will work best for me and my budget.

Item	Cost	Notes
16" Primary Mirror	$1400	Based on JMI Primary/Secondary Kit
Secondary Mirror	$0	Included Above
Mirror Cell	$250	Multi Point Flotation Type Cell
Structure	$700	Based on Dob Stuff 13"-16" Kit
Spider	$100
Focuser	$200
TOTAL	$2650

This assumes a 16" scope, which may not be the size I choose based on a recent experience at the Pacific Astronomy and Telescope Show (more on that later), but the pricing for every component except the mirror and mirror cell is valid for anything roughly 13-16".  In figuring out an ballpark price for each part, it was pretty clear that the mirror is both the most expensive, and the most variable component.

You can buy $500+ dollar focusers, and that would be two or three times the cost I budgeted, but the change to the overall budget would be only 11% or so.  Conversely, I might be able to save a bit of money if I build my structure from scratch, rather than buying the DobStuff kit, but it would not be much compared to the overall budget and I'm not sure it would be a good investment of my time.

The Mirror, however, is the big variable.  In looking around, the JMI option listed above was the least expensive option I found; The costs can easily go to double that price.  If the mirror were $2800, that would increase the overall budget by a whopping 52%!


Next Step: Mirror

If I'm going to try to meet my one year timeline, I'll need to eventually start purchasing stuff, and the mirror seems like the first decision to be made.  It's the largest component, cost wise, and it's size, weight and mounting will influence other decisions.  So I figured it's the component I should research and decide on first.

I've put together a preliminary list of manufacturers, but they are a bit hard to find and I expect my list is incomplete.  Please leave a comment if you are aware of, or have seen reviews for, any mirror manufacturers or suppliers in the 14-16" range.  I'd love to hear what you think.

Build Vs. Buy: The sum of the parts

To those of you in the U.S. Happy Labor Day.  For me it's a great day to spend some time researching my new telescope.

I've now done some thinking about the size of my better scope, and I've decided on something in the 14-16" range.   Large enough to be a big step up from my current scope that can fill my observing needs for long time to come, but small enough to be manageable and affordable.  As I continue to try to scrape together some pennies, I'm left wondering what my options are.

Since I started this project,  I've been leaning heavily towards building my own scope.  Not from scratch, but more by assembling the parts I wanted.  In my mind this seems like a good way for me to control all the various trade offs between price/performance and get just the scope I want, but it also seems that it could end up being a boondoggle of sorts.  I could end up spending more than a commercial scope and getting less.  The tagline on my blog is a joke of sorts, Saving one penny at the cost of two, but I don't want it to be an omen...

To help put my mind at ease, and to get some ideas for the design of my telescope, I've been surveying what is available commercially over the last few weeks.  As luck would have it Matt Wedel, of 10 Minute Astronomy fame, just posted a great overview of telescopes available in the size range I was looking at.  He shares some of my concerns about price and portability, so his pros and cons are instructive for me as well.

With such a wide range of options, and a similarly wide range of prices, how can I go about really knowing which decision, Build Vs. Buy is right?  If I decide to buy, how do I go about choosing which one?  If I decide to build, how can I figure out how much I need to spend?

I think the answer to all of these questions is to look at which basic parts make up a telescope, try to judge their impact on the overall telescope experience and figure out which features are important to me.

Sum of the Parts

I've been around telescopes for a while, and if you are reading this, I bet you have too.  In my observing experiences with my own telescopes and others, each telescope has a different feel... some are easy to use and seem to disappear into the observing experience.  Others assert themselves with distracting issues that interrupt the zen-like flow of observing.

These experiences often center around one or other piece of the scope which is not working as well as I'd like.  I've thought about them in the moment, but never taken an inventory of each part and what I would like to see from them.  This will come in handy as I evaluate commercial scopes and put together a budget and parts list for building a better scope.

Most of these will warrant a post to themselves as I try to figure out the key measures of quality for each and evaluate different options and suppliers.  Here are my main areas of consideration in no particular order:

Mirrors
Size is king, but there are also considerations such as focal length, thermal properties of the material, weight vs. deformation and shape (conical vs. flat).   Each manufacturer seems to have different standards of, and reputations for, quality.  In my experience the mirror is probably the largest factor in the overall experience of a telescope and I've already spent some time trying to figure out where the value/dollar sweet spot is.

Mass produced, or machine produced, mirrors are less expensive but quality can be hit and miss.  Most commercial scopes will have some sort of mass produced mirror.  Hand figured mirrors are usually higher quality, since some person spent time testing and fine tuning them, but can be pretty expensive.

Focuser
Unless I am lucky enough to have a full set of parfocal eyepieces some day, I'm going to need to refocus my telescope.  A cheap, sticky, sloppy or overly fast focuser can be a big annoyance over a night of observing.  With faster focal ratios, the critical focus plane gets thinner and thinner, so a good quality focuser is a must.  A very nice focuser can run upwards of $250 and this is an area I've seen some commercial scopes cut corners.

Motion
This is not really a part, per se, but it's directly dependent on the design and quality of the moving parts of the mount.  It's the feeling one has when pushing the telescope around the sky.  Is it easy to move?  Does it require the same amount of force to move in altitude as it does in azimuth?  Can it be knocked off target by a stray bump of the eyepiece?

I've already decided on a dobsonian style scope, so the basic design will be the same no matter if I build or buy, but there are a lot of different ways to implement the same design.  This is an area where the details matter.  The sizes of the various surfaces that rub against each other, the materials they are made of, and the weight and balance of the overall telescope are all critical to the motion of the scope.

Aside from a few, very high end, commercial scopes, it seems like this is always the first area people try to modify when they purchase one.  I've done a fair amount of tweaking on my XT8 to improve the motion, with some success, but I want my next telescope to be a real joy to use, without a lot of modifications.

Finders
Since I'm not a fan of goto or push-to systems, I spend a lot of time with my finders.  Locating deep sky objects can be a fun challenge, but working with a bad finder is a major frustration.  Aside from the mirror, the is probably the next most important aspect for me.  I like having a unit (non-magnified red dot or reticle) finder for getting the scope in the right part of the sky, and a magnified correct image finder for fainter star hopping.

From what I have seen so far, commercial scopes either come with a unit finder (if you are lucky) or some sort of terribly small straight through finder.  I'm figuring that regardless of my build/buy decision, I'll be purchasing a new finder, so it's sort of a wash.

Spider
The spider is the structure that holds the secondary mirror near the top of the telescope.  It is not a component I really think about, until it comes time to collimate my telescopes.  Having to re-align the secondary is infrequent, so this is not a huge concern, but there are some options that impact the appearance of bright objects.  Since the arms of the  spider are in the light path, they will diffract light and cause scattering and other noticeable optical effects.

I don't think a particular spider design is going to make or break my build vs. buy decision, but some people are passionate about it, so I don't want to ignore this subtlety in a scope I hope to have for a long time.

Mirror Cell
With my largest telescope being 8" I've never had to think about the mirror cell.  It's the thing that holds the mirror and joins it with the rest of the telescope.  At a minimum, it has to keep the mirror from falling out and have some way to adjust it's tilt to get the telescope properly collimated.

An 8" mirror does not require anything that special for support.  The mirror is going to be pretty light, and since it's somewhat small it's not going to flex much under it's own weight.  As mirror size and weight increase, supporting it without changing it's shape becomes a trickier task.  14-16" is where this component starts to matter.  What good is a wonderfully figured mirror if it is deformed under it's own weight as the telescope it moved around the sky?

The mirror cell is not going to be like a bad focuser, reminding you of it's presence all the time, but stability, adjustment, protection, and air flow are all important issues to think about.  They will all affect the overall performance of the scope, and in ways that might not be immediately obvious like a bad focuser.  I've not checked into the commercial scopes I might consider, so I'm not sure how they handle these issues.  If I choose to build, I think this is going to be one of the more difficult parts to research and source.

Structure
The Structure is the skeleton of the scope... the stuff that holds all the other stuff together and keeps it aligned correctly.  It holds everything off the ground and lets the mirrors be pointed at various parts of the sky.  Ideally, it keeps everything aligned perfectly as the direction of gravity changes.  The larger and heavier the mirrors and other components, the more difficult building a good structure becomes.

With smaller scopes, like my XT8, the structure is a solid tube with the mirrors, spider, focuser, finders, etc, all bolted on.  A solid tube is strong, and stiff, but with larger telescopes they can be heavy and overly long.  Larger scopes need something a bit more clever.  There are a lot of different solutions to this, each with their own weight/size/stiffness tradeoffs.

I've seen commercial scopes with very different designs, and even more variety of home built telescopes.  Since size and weight are important considerations for me, and I want a telescope that will last a long time and move well, the structure of the scope will be nearly as important as what it's holding together.



Those are some of my thoughts on the criteria I can use to evaluate different commercial options, and to think about the parts I might need to build one of my own.  I'd love to hear any thoughts you might have on how important you've found each of these, or if I've overlooked some pieces of the puzzle.

The Design: Size

As wonderfully pointed out by Matt Wedel over at 10 Minute Astronomy in a recent post, Apeture Matters. It allows you to see more detail, to see fainter objects and was my Requirement #1 when thinking about A Better Telescope. So in a very real way, I want to get the largest telescope possible. Sadly, for me that has nothing to do with the largest telescope that can be manufactured, I'm well under that threshold. Possible for me is the confluence of two factors... Size and Cost. Keeping in mind that I have pretty much decided on a Dobsonian Design, I should be able to figure out how large a scope I am aiming for.


Aperture vs. Size

The first difficulty with very large scopes is portability. Since I have to travel to do my best observing, my telescope has to be reasonably portable. It can't be too big to fit into a wide range of cars, as I don't own a car and am never sure what will be available when I rent. Ideally, the largest component, probably the mirror or rocker box, would fit in a trunk. Failing that, it has to be able to fit in a back seat.

The telescope also can't be too heavy for me to handle easily myself, as I sometimes observe alone. I figure the heaviest component I'd really want to carry would be somewhere around 50 lbs. This is a bit arbitrary, but moving something too heavy around has risks, especially in the darkness after a long night of observing!

The final consideration in size is the eyepiece height. I want to be standing flat footed for all of my observing.  I'm about 5'6", so I'm hoping for an eyepiece height, near the zenith, of around 5'. In general, the larger the aperture, the longer the focal length and the higher the eyepiece when you point the scope up.

The exact length depends on the focal ratio (focal length/aperture) of the mirror I choose.  You can get very fast (low f-ratio) mirrors that will reduce the length of a given aperture, but they tend to be more expensive and have other undesirable optical characteristics; namely coma. Realistically, for reasons I'll cover in a specific post about my mirror options, I'm probably going to settle on an f-ration between 4 and 5... They are generally available, don't necessarily require a coma corrector, and won't be too long.

A spreadsheet seemed like the best way to juggle all of these variables and come up with some concrete numbers. Luckily, before I actually finished one, I ran across a nice Telescope height brainstorming spreadsheet a user on Cloudy Nights worked up!  It will also do rough calculations of mirror weight.  Awesome.

Playing with the numbers for a while, it seems that my most stringent requirement is probably eyepiece height.  Any scope with a mirror or rocker box large enough, or heavy enough that it violates my restrictions has already exceeded the eyepiece height.  So simplifying a bit I came up with this table which gives an overall sort of range:

Diameter	f-ratio	8" mult	Weight	Eyepiece Height
14"	f5	3.1	23	6' 1"
15"	f4.5	3.5	26	5' 10"
16"	f4.5	4	30	6' 2"
16"	f4	4	30	5' 6"
17"	f4	4.5	34	5' 10"
18"	f4	5	38	6' 1"

From left to right I've listed the mirror size, f-ratio, how much more light than my current 8" scope it will gather, the weight of the mirror only, and the eyepiece height pointed straight up.  

I'm a bit suspect of the eyepiece height calculation.  The math seems sound, but in spot checking some listed eyepiece heights for various scopes on the web it seems high.  I'll have to do some more specific calculations when I'm closer to choosing a design to build.  In any case, I'm probably not going to use the scope straight up because it's somewhat uncomfortable with a dobsonian, or any alt-az mount for that matter.  

So I think any of these will meet my three requirements, although the 18" would probably be pushing it on the mirror box size, and I've never really seen a 17" mirror around.  However I want to keep an open mind, and 5 times the light gathering of my current 8" scope would be wonderful.  


Aperture Vs. Cost

This is a trickier trade off to optimize.  The cost of the mirror is the largest, but not only, cost in a telescope and not all the components scale in price the same way.  Some things, like a good focuser, are going to be the same price regardless of aperture.  Others, like the secondary mirror, or support structure, do get more expensive with increasing aperture, but at rates different from each other, and different than the mirror.

I'm going to crib off of the work of Matt Wedel again and copy a section of a chart he posted about What Aperture Costs, and expanded it a bit to include a couple of the more common larger scopes:

Meade 10″ Lightbridge – 250mm – 78.5 in^2 – $700 – $8.92/in^2
Meade 12″ Lightbridge – 300mm – 113 in^2 – $1000 – $8.85/in^2
Orion XT12i – 300mm – 113 in^2 – $1100 – $9.73/in^2
Orion XX14i – 350mm – 154 in^2 – $1700 – $11.04/in^2
Meade 16″ Lightbridge – 400mm – 200 in^2 – $2000 – $10.00/in^2
Hubble Optics 16" - 400mm - 200 in^2 - $2300 - $11.50/in^2
Obsession 15" classic - 381mm - 177 in^2 - $4500 - $25.42/in^2
Obsession 18" classic - 457mm - 254 in^2 - $7000 - $27.55/in^2

From left to right he's listed the brand/size, aperture (in MM), square inches of mirror real-estate, scope cost, and most interestingly, price per square inch.

Not all these scopes are equally equipped; some have goto, some do not, and they vary in the quality and types of accessories.  However, one thing is pretty clear, larger aperture costs more money!

I've not fully worked out my budget yet, but I think I'm aiming to come in under the $4000 mark. Based on what I have saved so far, this seems like the absolute max I could really do in a year.  True, the year is somewhat arbitrary, but that is my goal and I'd like to stick to it.


The Verdict

Based on all the factors so far, I'm going to build my design around a 15, or 16" f4 or f4.5 mirror.   I've not done all the budgeting yet on every scope, but it seems like I really can't afford anything larger than 16".  Depending on the design, 16" might even stretch my budget and portability requirements, so I'm going to design and budget a couple of options right around that size.  

The difference between 15" and 16" may seem small, but the aperture increases with the square of the diameter, so the 16" has 15% more aperture.  However, it's eyepiece height is also 4" higher, and I bet that extra inch of diameter means a noticeably larger mirror box as well.  

I'm going to have to see what is available in commercial scopes or parts, figure out the specific dimensions for a given implementation and decide exactly what size a bit later.  Until then, Clear Skies!