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


Self Guiding with SBIG Camera

Nov 14 2009 – My first real try at using the internal guider in my SBIG ST-2000xcm camera.

For some reason it’s been hard for me to get up the nerve to set up and use the internal guide chip of my SBIG camera. Up to this point I’ve always used Craig Stark’s wonderful PHD program along with a separate guide scope and camera. PHD is a pleasure to use and takes a lot of the guesswork out of the guiding process. However, using the internal chip in the camera has substantial benefit in simplifying the equipment setup and eliminating flexure as a potential problem.

Recently, I’ve been working on improving my ability to work with my more portable mounts, a GM-8 and a  CG-5. These mounts can’t handle much weight, and I figure the only way I’d be able to image at longer focal lengths would be to keep the weight down (to keep friction in the mount at a minimum) and minimize flexure. Using the internal chip helps meet both those goals.

First off, there is a great page on the Starizona website ( HERE ) which helps get you started on the process and settings. I read it twice and still needed to refer back to it.

I first tried guiding with CCDOps, and got it working fairly well…. but I hit some snags alnog the way (I’ll describe in a minute) and I decided to try CCDSoft v5. I found that CCDSoft picked up the settings I had input in CCDOps so I hadn’t lost any ground, and with a few more tweaks I had it guiding pretty well. I was trucking along guiding at about +/- 1 px (which equated to about 1.5 arcsec at the 1200mm FL). Not too shabby.

So here are the snags, gotchas, and ah-ha’s:

I found that the Starizona instructions were really good. Using them as a starting point gets you 90% there. A few tweaks after you should be getting real close to good guiding. One problem I had was that one direction wouldn’t calibrate. I checked all four axis before the calibration run, but after calibration (which reported itself successful) one of the Y directions would have become “unchecked”. I found the problem was that I selected a star that was too close to the edge of the frame and the calibration steps took it out of bounds. Chosing a star near the center of the frame fixed it.

The other issue I had was that CCDOps doesn’t seem to have an easy way to get guiding going separately, and then letting you start taking a series of images (like doing “autograb”). If there is a way, it’s well hidden in the docs. I didnt’ find it. This is why I jusmped over to CCDSoft after I got guiding working in CCDOps.

I got things going pretty quickly in CCDSoft as it’s routines are similar to CCDOps.  One small thing I don’t like is there isn’t a nice graph of the tracking like in CCDOps.  However, it does have a numerical readout, so you can still get a good feel for how you’re doing.


I found that I had to set the aggressivness down to about 5 to keep the guiding to smooth out, higher values would make it overshoot.

These were the settings I used in CCDSoft for calibration. While guiding I had the aggressiveness around 5 and the guide exposures at 3 secs.


Here’s a graph of the guiding log.  I’d say not too bad for the first time with it, and  I’m pretty sure it can be tweaked some more.


My first Ever Blog Post!!!!


This blog is going to tell the tales of my triumphs and failures in pursuit of collecting photons from deep space.  I plan to share info on equipment, locations, methods and processes. I’ll tell what has worked for me  and what hasn’t.  Projects that were worthwhile, and those that weren’t.

Hopefully this info (my hard luck lessons) will be of value to others.