A Tracking Mount for Astrophotography
By Clayton Kessler
As I was planning my recent vacation in Tucson, I realized that the ability to do some astrophotography while I was there was very important to me. Unfortunately I was flying, and luggage restraints precluded carrying my Meade 8" SCT. I spent a fair amount of time researching the portable alternatives.
At first, I thought I would build a classic "Barn Door" mount. I have seen these work very well with normal and wide angle lenses. Upon reflection I thought that a more sophisticated system would be more useful in the long run. There are several commercial camera tracking mounts available, and two had been reviewed in Sky and Telescope in the last year. More research!
The most highly regarded camera mount is the venerable Byers "Cam Track". This was apparently a very robust and accurate mount and is no longer made. Judging by the cost of the "Cam track" on the used market, some of the components must be machined from solid gold! Lack of availability was the downfall for this mount. A search of Astromart showed many more requests to buy than offers to sell, and this is probably why the cost of the few available is so high!
Pocono Mountain Optics sells a camera mount that they call the "Series II German Equatorial Mount". This mount was reviewed in the March 1998 issue of Sky and Telescope. I read the review several times and came to the conclusion that the reviewer thought it was "OK" at best. The mount sells for $309.00 and does not include any kind of tripod or declination controls. The mount could not be guided and would only support 1 camera with a limitation on the lens focal length. There were no polar alignment aids built into the mount so a good polar alignment was difficult to achieve. I see, in the current adds that a quartz controlled drive is available which brings the cost to $435.00. And a second camera adapter is available for an additional $24.95. The biggest problem with this is the lack of declination control and difficulty with polar alignment.
The next thing that came to mind is the Apogee Multi Purpose Fork Mount. This was reviewed in the January 1999 Sky and Telescope. This is a small equatorial fork mount with an RA drive and declination slow motion controls. This mount is large enough to accept a Celestron C90 or a Teleview Pronto. The mount comes with a light wooden tripod and a hole bored through the polar axis. This hole can be used to get a rough polar alignment and a drift alignment can finish the process. The RA drive includes a hand-box with variable drive rate controls and fast and slow buttons. This allows guiding in RA for astrophotos. Guide with what? The reviewer noticed that the polar bore hole was large enough for a 3/8" diameter screw. This allowed a camera tripod ball adapter to be bolted to the bottom of the RA shaft and a second camera to be mounted there. The reviewer mounted a C-90 to the fork mount and used it to guide photos of up to 25 minutes and up to 185mm focal length with a camera mounted to the ball head. The cost for this mount is $399.00 and I started to get very interested in it. The downfall on this mount is the tripod. A much heavier tripod would be needed to take accurate photos reliably.
As a result of my research, I felt that none of these mounts would satisfy my camera platform desires. I started a search for a used "Super Polaris" mount with RA and Dec motors. Unfortunately I was not able to find one in a timely manner that had a working drive system.
Time was beginning to get short and I had to make a decision. I had seen, in my cybertravels, numerous places that were selling a GEM mount made in Taiwan. This mount seemed to be everywhere from Orion Telescopes and Binoculars (the Skyview Deluxe mount) to the Europa Mount in Great Britain. Many small manufacturers were using this mount for 4", 6" and even 8" newtonians. I found, on the web, that Internet Telescope Exchange was offering their version of this GEM, the First Magnitude Mount, with dual axis drives. I called Bill Burnett at ITE and discussed this mount with him. Unfortunately, the dual axis drives were not available, the drive manufacturer was located in the Florida Keys and heavily damaged by hurricane Georges. Bill did have a single axis drive available with a hand paddle for guiding. This I promptly ordered at a cost of $400.00.
A week later the drive hit my floor and I was very impressed when I unpacked it. The construction was robust and both RA and Dec seemed tight and precise. This mount is WAY beyond the normal, inexpensive, equatorial mount that comes with most mid priced scopes from Meade and Celestron. In size, the mount is slightly smaller than the venerable "Super Polaris" mount maybe the size of the older Polaris. It includes worm gears on both axis and slow motion controls. A very nice feature is the polar alignment scope. The reticule shows Polaris with the proper offset and several of the stars surrounding the pole. The mount has fine adjustments in altitude and azimuth to ease the chore of polar alignment.
I quickly built a dual camera platform out of some ½" Lexan that I had laying around. I designed this in the shape of an elongated diamond. I included a machined dovetail bar in the center to attach a guide scope. This gave me the ability to attach my Orion "Short Tube 80" or my C-90 as a guide scope. Two cameras can be attached, one on each end. Balance is achieved in the normal way with a GEM and the counter-weight supplied was equal to the task. I really wanted to try the system out before the Tucson trip, to have a chance to work out any bugs. Unfortunately, Michigan weather being what it is there were no clear nights available for this.
This system is fairly compact but I did not want to carry this for the entire trip so I shipped the mount UPS to my folks place in Tucson a week prior to my leaving. This worked very well as the mount arrived 5 minutes after I did and I saved my back to haul around golf clubs!
A couple of days after my arrival I gave the system "first light" at the TAAA Empire Ranch dark site 30 miles from Tucson. The results were pretty good. The polar alignment scope allowed me to get a decent polar alignment quickly but I had trouble finding the additional stars shown in the reticule. This resulted in some declination drift visible in the guide scope. If I were not quite so lazy, I would have done a drift alignment but I am much too lazy for that. The Declination drift showed up as guiding error in the 200mm shots and to a lesser extent in the 135mm shots. A good drift alignment would have minimized this. Even better would have been a declination motor. A simple bump to the dec. axis once in a while would have made the use of 300mm and 400mm lenses possible. RA guiding was great. The motor adjusted well and held speed nicely. The Orion Short Tube is a 400mm focal length scope and my 9mm guiding eyepiece gives a 44x magnification. I did not notice any great amount of periodic error in the drive gears with this magnification.
All in all I am very pleased with this setup. I do plan to add a declination motor, which will allow more accurate guiding. I used the mount on three different nights and took about 70 astrophotos with it using two cameras simultaneously. All but a few (kick the tripod dummy!) came out very well and this gives me a wealth of negatives to scan and print. This will make a nice addition to my astrophotography arsenal and I will get a lot of use from it!