A 50mm Finder as a Guidescope

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


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