Saturday, March 28, 2015

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.

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 Opticalpyrex3.11/14 p-v$180
Protostarpyrex3.11/10 p-v$195
Astrosystems.bizpyrex3.11/10 p-v$175
Astrosystems.bizpyrex3.11/14 p-v$195

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


Daniel Steel at, 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.


The focuser is really the user facing bit of any telescope.  I knew I wanted something really solid, with a nice feel. 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 :-)


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.