Sunday, September 30, 2012

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:

CompanyMaterialSizef-ratioHeightXT8 MultPriceLink
DiscoveryPyrex154.264.53.51699link
DiscoveryPyrex164.57341699link
HubblePyrex / Sandwich1457231775link
JMI/GSOBK/7164.57341439link
LightHolderPyrex14.54.161.23.32440link
LightHolderPyrex1646542870link
LockwoodPyrex1646543870link
O.W.L.Pyrex154.561.53.52470link
O.W.L.Pyrex1646542900link
PegasusBorofloat14.54.3643.32100link
PegasusBorofloat163.55743100link
Waite ResearchSUPREMAX 3314.54603.32650link
Waite ResearchSUPREMAX 3315461.53.52800link
Waite ResearchSUPREMAX 331646543000link
ZambutoSUPREMAX 3314.54.5673.32650link
ZambutoSUPREMAX 3315461.53.53360link
ZambutoSUPREMAX 331646543924link

*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:

CompanyMaterialSizef-ratioHeightxt8 MultPriceLink
DiscoveryPyrex154.264.53.51699link
LightHolderPyrex14.54.161.23.32440link
LightHolderPyrex1646542870link
LockwoodPyrex1646543870link
O.W.L.Pyrex15461.53.52470link
O.W.L.Pyrex1646542900link
PegasusBorofloat14.54.3643.32100link
PegasusBorofloat163.55743100link
Waite ResearchSUPREMAX 3314.54603.32650link
Waite ResearchSUPREMAX 3315461.53.52800link
Waite ResearchSUPREMAX 331646543000link
ZambutoSUPREMAX 3315461.53.53360link
ZambutoSUPREMAX 331646543924link


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:

CompanyMaterialSizef-ratioHeightxt8 MultPriceLink
DiscoveryPyrex154.264.53.51699link
LightHolderPyrex14.54.161.23.32440link
O.W.L.Pyrex15461.53.52470link
PegasusBorofloat14.54.3643.32100link

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:

<
CompanyMaterialSizef-ratioHeightxt8 MultPrice$/Inch
DiscoveryPyrex154.264.53.516999.60
PegasusBorofloat14.54.3643.3210012.72
O.W.L.Pyrex15461.53.5247013.95
LightHolderPyrex14.54.161.23.3244014.79
LightHolderPyrex14.54.161.23.3244014.79

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.



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