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ATM: Recycling a Telescope


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Recycling a Telescope


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.
  • Portability
  • 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)

Paper Design




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.
I used both Mel Bartel’s program and Newt to determine the optical path performance. I have a 31% obstruction by area with a 1.2” diameter field that has a edge illumination that is 75% of the center. In practice, I have no problem with the performance either from contrast focuser I realize that I depart from conventional wisdom. In practice, the sled holds collimation from one end of travel to the other. The secondary is 1.83” in diameter. This is excessive, but I did not want the lower end of the Paracorr to protrude into the tube. (Why make the central obstruction small only to introduce an additional asymmetrical obstruction?). The placing of the Paracorr and focal plane shift is per a sketch I received from Al Nagler.


Secondary Support



I wanted to have a curved secondary support to eliminate diffraction spikes, but the support has to move with the sled. This design element took some time to resolve. The drawing and photos above show the actual construction. I feel the design is a success because I do not see any diffraction spikes. By making a drawing and calculating the flat length of the support, it becomes easy to bend the stock at the correct locations. The short flats at either end are bent first. The finished support is under quite a bit of stress as the spring stock does not yield easily. This is a benefit, as the spring-like nature of the support causes it to stay in position during the normal kinds of bumps and jolts it gets during handling. The material of the support is .015 thick spring stock. The assembly holds collimation perfectly well during trips to dark sites.


Finder

I stumbled onto an ebay auction for the Melles Griot 82mm achromat lens. Since the price was right, I decided to make the finder based on this lens. The tube and mounting was machined on my grandfather’s 1932 South Bend 9” lathe. It’s kind of primitive, no quick change gears or 3 jaw chuck. It has the original motor which does not have the special starting circuit. I have to start the lathe rotating after I turn the power on. The safety guys would not be amused. The mounting rings are made from scratch as well. I used an “O” ring in the front mounting ring to keep the finder centered. The rear ring uses conventional screws to align the axis of the finder with the main scope. The eyepiece is a Nikon microscope reticle eyepiece that I modified to accept an illuminator (ebay again!). The eyepiece has 20mm of eye relief which makes using it very easy. The diagonal gives an erect image.

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.

Complete OTA

The overall views give a good impression of the final form of the scope. I’m of the school that believes that one should not finish an ATM scope within an “inch of it’s life” because I always want to make changes (improvements). The counterweights are my own from Stainless Stee, replacing the stock weights from Meadel. They were not made on the 9” South Bend; simply not enough horsepower. The weights are threaded onto the shaft. This makes it very easy to change the balance.

Travel

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.


The Mount

The mount is a Meade LXD 500 purchased used. I went thru all the mechanicals, did all the re-greasing and adjustments of the gear drive. I was still not satisfied with the vibration characteristics. Most of the vibration seemed to be coming from the tripod legs. I decided to replace them with something more structurally robust. Photographic style tripods are not suited to astro work. The simple plate that receives the legs (allowing them to pivot) has no way to resist an overturning moment that appears when the scope is set in motion. The mount must not move when that force is applied. To fix this problem, I decided to extend a post downward from the top mounting plate. I could then apply “stringers” from the legs to the lower portion of the post. Now each leg is anchored to the mount via a triangle. Neither the legs or the top plate can move. All the bolts are tightened so that there is no “slop” in any of the joints. This arrangement does take longer to setup than a simple photo tripod. I keep this tripod and mounted fully assembled in my garage. I just carry it out, attach the OTA via the dovetail plate and I’m ready. I can be fully setup in 5 minutes. Transporting this to a dark site requires the stringers be unbolted and the legs swung in. The legs are full height all the time. Making the legs adjustable in length would have seriously compromised stiffness. The improvement of the mount was startling. It’s rock solid. I can say with confidence that the Meade mount is excellent for visual use. The aluminum legs that come with it must be replaced.

As a finishing touch, I made eyepiece holders as part of the tripod down tube. They keep everything right at hand.

Photography

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

To accommodate my Nikon Coolpix 950, I needed to make an attachment to the eyepiece. The Vixen LVW’s have a groove that keeps the rubber eyecup in place. After removing the eyeguard, I put an O-ring in the groove of the eyepiece. Now it was easy to make a threaded adapter that screws onto the Nikon and gets a nice friction fit on the eyepiece .The photo below explains it quicker that trying to describe it verbally.

Conclusions

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.






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