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ATM: Recycling a Telescope
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The photo to the left shows the original configuration of my uncle’s 6” F13.5 scope which he built when living in San Diego in the 1950’s. He gave me the scope during a family visit in 1963. The photo is from approx 1967 when I was a senior in High School. Even by then I had already modified it to have an equatorial mount with a coffee can filled with concrete as a counterweight. Originally it had an alt-azimuth mount. I convinced another uncle to make the part that tilted the axis to be at the correct angle. The counterweight is concrete in a coffee can with a 2” pipe inserted so I could connect it to the mount. The only eyepieces I had were a war surplus Erfle, and a ?” FL Ramsden from Edmund Scientific. The original scope is quite remarkable in the fact that every part was hand made except for the mirror cell and the main bearing (the azimuth - a 1950something Studebaker front wheel bearing).
I used the scope in this configuration for the better part of 6 years, until
marriage, college and a real job intervened. It sat in my garage until 1993,
when I got the itch to “make a scope” that was easier to transport, store
and use. The 1993 redesign was filled with problems, mainly because I still
had too many other demands on my time. Cloudy Nights hadn’t been invented
yet, and as I was only familiar with an F13 scope, I was blissfully unaware
of the collimation requirements of a short FL scope. The F4 mirror I purchased
wasn’t too good and bad collimation made it worse. That version just sat
in my basement until last year. I stumbled onto Cloudy Nights in ‘04, which
really renewed my interest in the sky. I decided to try again, this time
taking as long as necessary to build a scope that met my requirements.
OTA Design Criteria
My intent was to build a scope that gets me back into the astronomy environment, is portable, high quality, and is a design that can be scaled up to a larger size when I am ready to do so. Currently I live in the Chicago metro area and dark skies are only seen on visits to my in-laws in rural Iowa. Since the old six inch reflector of my uncle’s wasn’t being used, I had no qualms about making use of whatever components I could find a use for. ATM scopes really have no sale value, they’re built for the owner by the owner, and he is usually willing to make compromises on design/construction that he is willing to live with, but another owner might find the compromises a bit hard to take. So for this iteration, these are the criteria:
- 6” mirror, short focus. I settled on F4.
- As high a quality image as I could get.
- Really quick set-up and take down.
- Designed/engineered on paper before constructed
- Accommodate occasional film & digital photography
In addition, I wanted a really good finder with lots of eye relief, and a really rigid mount (but I still wanted portability)
The design is centered on the recycled aluminum tube section from the old scope. The original scope was made from (2) tubes joined by a flange. The flange is riveted to the tube and keeps it circular. The tube flange also provides mounting points for the (6) rods that support the mirror cell. After reading countless threads on Cloudy Nights, I opted to have an open mirror cell area to allow free air motion. I also wanted to include an active cooling component. To do that I discarded the rear element of the University Optics mirror cell and made a new one that is machined to accept (3) 27mm dia. 9 volt fans. AA batteries are velcroed to the 7” tube. The mirror assembly is covered when not in use by a QuickTube cover closed with a plywood end. The mirror is a refigured version of the mirror I purchased in 1993. Cary Woden really did a nice job at refiguring the mirror. My personal breakthrough was understanding the collimation requirements at F4 thanks to Vic Menard and Nils Olof Carlin. It was tough to accept the fact that I needed an $85 autocollimator to finish the assembly, but I believe this single item is the key to getting optimum performance from F4 scopes.
Also shown in the above is the focus mechanism. A half-nut engages a 3/8 –28 UNF threaded rod. Turning the knob moves the sled left or right. The photo shows the scope after I added an Orion electric focus motor. Because the motor turns slowly, I’ve made parfocal rings for my eyepieces. The depth of focus of an F4 scope is very small. I can notice the change in focus from a 1/8 turn of my focus knob. The electric focus combined with the parfocalized eyepieces make changing eyepieces a quick effort. The electric focus is also a help at star parties as people seem to get the idea of the electric focus much quicker than understanding that one has to be gentle with a manual focus knob. Also visible in the photo is my Tasco Red Dot sight, no longer available, but perfect for my use.
One excuse for this scope is that it is a travel scope. To that end, I built
a case from a molded plastic storage trunk. The scope fits on (2) pins built
into the bottom of the trunk. There are two corresponding holes in the dovetail
plate on the scope. The fit is precise so that the scope cannot tip over,
it’s held pretty well. A bungy cord wrapped around the scope and fastened
to the handles on the trunk and the OTA really fix it in place.
As a finishing touch, I made eyepiece holders as part of the tripod down tube. They keep everything right at hand.
This setup is not for serious photography. But from time to time I wanted to be able to do snapshots with either film or digital. I’ve used Hasselblad’s for years, so it seemed natural to make an interface to be able to use a film back in the 2” focuser. The photo below shows the setup. The cool thing about the Hasselblad system is that they make a groundglass that attaches just like the film back. So focusing is pretty easy
This whole exercise worked out quite well for me. I got my scope just the way I want it, and it pretty much meets all my objectives. If anyone is interested, I can share my CAD drawings in almost any format (DWG, DXF, DRW, WMF). Just send me a PM with your email addy & I’ll send it out. Sometimes it’s easier to start your design by carving up someone else’s design. And to finish, one moon shot with the Nikon 950. One exposure, nothing stacked, no mosaic. When it comes to photography, I guess I’m a die-hard. Thanks to everyone on CN for advice (even when they didn’t know they were giving it). You guys(and gals) certainly got me back up to speed quickly.