Tuesday, August 28, 2012

A "Found" Telescope

I have several nice telescopes.  One of the first questions that people ask when looking through them is "How much did it cost?"  Often I quote a replacement cost - what it would cost me to purchase one like it.  That is the answer they usually want to hear.  When at the Grand Canyon Star Party, they don't want to hear that my Celestron 14" on the G-11 mount cost me $75 out-of-pocket, which is the truth.  When I do quote that number, I usually explain that I work for a university - I couldn't afford to pay retail for a telescope like that - also the truth!  I generally either make my telescopes from scratch, or need to wheel-and-deal to get something serviceable.

The Setup shown here at left, for example...  The Losmandy G-11 mounting is normally about $2,500.  I got it as a gift - sort of...  Peter Ceravolo, when in town for several weeks to nightly observe Comet Hyakutake back in 1996 needed a van to haul gear and people.  He used the van, and left the mount as a barter exchange. 

The 14" Celestron is a more-involved story.  Normally about $4,000 new, my good friend Bob Goff who had a garage optics shop in town, originally bought the telescope on the used market, in order to replace the corrector plate that a customer had broken on his telescope.  This one, without corrector, was of little use to anyone.  Bob was in the process of making a new corrector plate from scratch when he died.  His widow Valerie ended up giving me the tube assembly.  At a recent Riverside Telescope Makers Conference (well, 8 years ago...), I had befriended an optician who worked for Celestron.  I had happened to call him on his last day at the company before he started his own business making telescope mirrors.  Steve was able to send me a reject corrector plate for nothing (had a small scratch near the central perforation).  Mixing random primary mirrors and correctors normally result in mediocre performance, so I ended up figuring the little secondary mirror to fix the half-wave of spherical aberration the combo had...  It is now a superb performer, though it carries a paper mask reducing the aperture down to 13.5" - a turned down edge that I may fix someday!  The $75 investment I actually made in the combo was for the dovetail adaptor plate to connect telescope to Losmandy mount.  The setup has served me extremely well the last 8 years - at about the size limit I can set up by myself for public events like the Grand Canyon, or for personal imaging from a dark site.

Fast forward to the present...  This summer I found the holy grail of telescope mounts!  The Gold Standard in sturdy and beefy, yet portable mounts is an AP-1200, made by Astro-Physics in Rockford, Illinois.  An acquaintance of mine not far from us in St Charles had upgraded, and informed me he would make an "offer I couldn't refuse!"  So for the first time in memory, I actually spent money for an item - a decade or so old, but still the ultimate in mechanical quality!

At the same time, there has been a telescope in my "storeroom" for some number of years.  It is a Schmidt camera - a 10" aperture of very short focus, this one about F/2 or 20" focal length.  Designed as a film camera, these Schmidts cover large patches of sky for the aperture, the drawback being that the film needs to be warped to a curved surface to remain in focus.  This telescope was made by a buddy of Bob Goff (first name Tom - his last name escapes me).  He died before Bob, his estate scattered.  Well, no one uses film any more - digital is the way to go. One day at Starizona I spotted his Schmidt back in the store room. Dean offered it to me, not knowing himself what to do with it.  Can't say no to a telescope like this, particularly after knowing the maker!   I had been thinking of repurposing the telescope for some time, since the Schmidt is limited as-is. 

With a 10" aperture, I assumed it had a 12.5" diameter mirror, and with the 48" long fiberglass tube, I was thinking that I could regrind and figure the mirror to a nice F/4 parabola for imaging with a substantially longer focal length that the Hyperstar plus C-14.  So finally, the other night, I lugged out the scope (seemed heavier than I remembered), and disassembled the thing.  I was surprised to find that instead of a 12.5" mirror, it had a full-thickness (something like 2.5" thick) 14.25" mirror!  And that stuffed inside a 15" ID tube!  Conventional wisdom says you should have at least an inch or more space between the mirror and tube, so something like 16.5" ID tube...  So I've been thinking hard what to do - the 2 choices are to lengthen the focal length to something like F/3.5 to squeeze into the present tube, or get a proper 16"-17" tube a little longer for an F/4 to F/5 Newtonian.  Either way the new AP1200 mounting should handle it just fine.  I'll post my decision and progress on this project - stay tuned!

2 comments:

David Allen Harvey said...

damn dude you "find some of thee best stuff can't wait for first light!

Anonymous said...

Dean, you look so happy when you're holding your telescopes! ~Ewica