After owning my MI250 and AP130EDT for a couple of months, I started thinking of ways to add some sort of piggy-back mount. I wanted to be able to shoot the same area of the sky, at three different magnifications, at the same time. While I put great thought into how to create such a mount, I am still working out some details. I have completed the preliminary construction of the piggy-back mount and will probably continue to enhance it as I need. Below are some shots of the piggy-back mount with a bit of information on its construction.
Here is a full view shot of my MI250/ piggy-back setup. I wanted to do some "through the lens" wide field astrophotography using my Nikon F2 and 180mm f2.8 ED Nikkor. My thought was to come up with a system that would allow me to shoot through the scope and piggy-back camera at the same time. The challenge was to come up with a design that wouldn't interfere with the scope and not add too much weight.
I started with an aluminum plate that would hold up to two cameras on it (13.5" X 19" X 1/4"). I drilled holes (not shown) to match the hole pattern on my MI250 that allow the Losmandy DA Dovetail plates to be secured. I also drilled some extra holes and threaded them (10-32) so that I could directly attach the Losmandy Dovetail plate to the piggy-back plate. This makes it much easier to get the plate mounted on the MI250.
The camera mounting adapters were the most critical part of the setup. I wanted to make the adapters as stiff as possible to avoid any flexure. With this in mind, if I was off at all on my measurements, the lens and camera would be very stressed when locked down. The bottom of the lens was about 1/4" lower than the bottom of the camera body so I had to mount the body up on a 1/2" mounting block. The lens mounting adapter was the most difficult piece to construct. I am not a machinist and don't have access to a milling machine. I basically drilled a bunch of holes, cut out the inner circle part with a saber saw, and filed out the rest. I also added some foam rubber to the lens mounting adapter to save the finish and allow for some miss-alignment.
This is my high tech counterweight system ;-) I have been working on a better (and hopefully lighter) counterweight system but this works for now. It is basically two dive weights (4 and 2 LB) that I drilled holes in. I have had thoughts of a system that would hang off the edge of the plate, could slide forward and back, and could be screwed in and out. This would provide adjustment in the two axis needed and could be done with a lot less weight. It would also make room for the second camera to be mounted.
One last thing I needed to do. The Losmandy DA dovetail plates needed a bit of clearance so the tightening screw could be turned. I placed 1/2" spacers in between the DA plate and piggy-back plate. I suppose I could have just cut out part of the plate and saved the half inch but that would have made it harder to tighten down. Since the two dovetail plates are all holding up about 5K worth of equipment, I decided that it was more important to make it easy to tighten down.
The project took me about 20 hours of work and probably twice that in design. I got the aluminum from an aluminum wholesaler that I talked into selling me some of his small 'scraps'. All in all I spent $75 on aluminum (with enough to make a second set of camera adapters) and another $25 on fasteners and drill/tap sets. Tools used: Drill Press, Saber Saw, various taps, and hand files.