Back in April 2009
I wanted to have a more portable system using a more lightweight mount. thus reducing the weight of the imaging gear is a good thing.
I’d seen in the forums on Cloudynights.com that some folks were using 50mm finder scopes as guiders. I decided I had to try modifying one myself. SO…..
I already had a Meade DSI Pro, and PHD, and the tools I thought I’d need. So I bought an inexpensive GSO 50mm finder and a scope ring to mount it to. Luckily I had a couple chunks of aluminum to machine the other parts I needed, that would have cost me another 10 bucks or so.
The GSO finder’s FL is about 180mm. The objective is in a threaded lens cell for focusing and has generous travel… I didn’t measure it, but it’s close to 1 cm. The stock eye piece was just threaded on, so removal was a snap.
I did not have to cut the tube to find focus with the DSI. You can see in the pictures below how the DSI is positioned. Note my DSI has a low profile adapter (from ScopeStuff) on the front for accepting T-threads or a 1-1/4″ nose piece. I’m using the nose piece.
I made a 1-1/4″ nose piece adapter for the finder from some 2″ aluminum round stock I happened to have. It’s sized to have a precise (but not interference) fit with the tube 50mm bore . There is a single layer of 5 mil Mylar tape around the adapter that provides a press fit with the tube. The adapter is rock solid, but could be removed if necessary without damage to the tube. I drilled and threaded the tube/adapter for two 8-32 nylon tipped clamp screws.
The tube mounting block is 1″ thk aluminum bored .020″ bigger than the tube OD of 53mm. A layer of thin Teflon sheet protects the tube OD. The block is secured to the main scope mounting ring with 3 screws so it’s really solid.
I’ve used the guider now several times. I often use it with a 2x barlow to get the extra FL with hopes that it might make the guiding more accurate, but I don’t know if it does or not. Focus is easy to achieve either with the DSI alone or with a 2x barlow, however the barlow had to be positioned 0.1″ further back than the DSI without the barlow. One issue with the barlow is that I can’t focus to nice tight star images. I think it has to do with precise alignment of the objective lens with the barlow, but since PHD calculates the centroid and guides just fine so I haven’t investigated further.
The total weight of the assy complete with camera is 2 lbs. That knocks about 12 lbs off compared to my usual side-by-side setup. Even though it feels very rigid, I’m still a little worried about flexure using only the one ring for mounting the guider. However, I bought an extra ring so I can make a simple front tube support if needed.
Here’s some pics of the final assembly:
Hope these details are helpful
al