Constructing a guide scope from a finder scope

UPDATE I have modified my finder guider to use a Baader T2 Quick Change system, which is available from Alpine Astro. The part number is "#T2-6,7". Here are some pictures of the new configuration. The advantage is that the camera is much more secure this way than using the PVC pipe. The construction is basically to file the inside of the finder tube until you can press fit the T2 quick change female part in it. I cut some slots around the edge of the tube to allow it to expand as the quick change adapter is driven in.

The following instructions involve using a PVC adapter and works well and costs much less than buying the T2 quick change adapter. I would recommend adding 3 screws to retain the guide scope versus the 1 screw I use in these instructions.

I have been using an off-axis guiding setup for several years now. I started with a piggyback refractor on my imaging telescope, then I moved to a tandem arrangement where I mounted the scopes side by side. My goal has been to reduce flexure as well as minimize overall weight. On day a fellow astroimager showed me his setup using a SBIG STV autoguider and the eFinder 100mm lens. It was the type of lightweight guiding solution that I'd been looking for all this time. I decided to take on the challenge of coming up with an DIY version.

I had an extra 9x50 finder scope laying around and proceeded to take it apart. The finder scope I had was from an Orion telescope, but I believe most inexpensive 9x50 finders these days have a similar design. On my finder you can unscrew the eyepiece as well as the objective lens. I took the eyepiece off and held my Meade DSI up to the end of the finder and found the focus position. I worked out the proper position for focus and found it produced a fairly good image! Next I needed to work out the focal length of the finder objective, which turned out to be around 200mm. This should be adequate for imaging up to at least 1000mm using modern sub-pixel guiding techniques.

What follows is a step-by-step guide for building a guide scope from an inexpensive finder scope. I've been using one for several weeks and I've successfully guided my Atlas EQ-G and Sirius EQ-G mounts using telescopes from 500 to 1250 mm, using Maxim DL as the guiding program.

This guide scope should work with any CCD camera, as well as webcams with a 1.25 inch nosepiece adapter.

Here are the parts you'll need:

I was able to find the spacer at Lowes and the 6-32 parts at a local hardware store with a well stocked hardware section (with all the slideout drawers for bolts/nuts/etc).

First you will need to remove the eyepiece:

and then the objective:

Both just unthread from the finder scope.

Next we need to remove the spacer on the objective end of the finder scope. If you do not remove this spacer you will probably not be able to reach focus with your guide camera.

There is some nasty gunk on the finder scope to hold the spacer in place. I used mineral spirits to remove this gunk before proceeding:

I left a little gunk on the threads so that when we put the objective back on it will have some holding power. Otherwise the objective might move around while imaging. Alternately you can just use some handy blue tape to keep the objective from rotating once you get the focus position set.

The next step is to prepare the spacer block. It needs to have some spots sanded down. I used a Dremel tool with a grinding attachment. The first step is to remove a bump from the outside of the spacer:

You can see the bump on the outer left side of the spacer. File or sand it down flush with the outer part of the spacer:

Test fit the spacer into the eyepiece end of the finder scope, with the ridged end of the spacer pointing outwards. File or sand this ridge until the spacer block will fit snuggly. If you need to remove the spacer and its stuck in the finder scope, use a screwdriver and a hammer and gently tap it out from the other end.

Once the spacer fits nicely we have one more adjustment to make. If you look inside the spacer you see there is a ridge on this inside. This ridge is not quite large enough to let a 1.25 inch nosepiece from a guide camera fit through. Sand this ridge down until your nosepiece or guide scope just fits through. Having a good fit here helps make sure the camera will fit snugly and minimize flexure.

Here is an image of the spacer block being test fit over the nosepiece on my Meade DSI.

Next you will want to drill and tap 3 holes for the retaining bolts for the spacer block. I used a tap kit from a local hardware store:

I recommend using a 6-32 bolt size.

After this step the eyepiece end of the finder scope will look like this:

Now insert the spacer block and then some small bolts to hold it in place.

Next we are going to add the thumbscrew which will hold the guide camera in place. Drill and tap another 6-32 hole through the outside of the finder scope as well as the spacer block. Insert a thumbscrew and you should have something like:

We're almost done now. The inner diameter of the spacer block is slightly larger than 1.25 inches. I used some Velcro inserted to get the size approximately correct. Insert the Velcro strip so that one is exactly opposite the thumbscrew, and the other two are off to the side to provide additional support:

Now insert your guide camera. You will need to work it into place so the end of the nosepiece of the camera fits inside to small ridge on the inside of the spacer block. This inner ridge will support the end of the guide camera and keep it from moving around. Then tighten the thumbscrew to hold the guide camera into place. As you tighten the thumbscrew watch the guide camera and make sure it doesn't end up tilted relative to the optical axis of the finder. This may require some additional Velcro shimming.

Finally you need to thread the objective back onto the finder. You will want to thread it all the way on.

Now hook the camera up to your computer and point the finder at a distant object. Slowly unthread the objective outwards until you reach focus. At night when you reach focus you will probably want to use some masking tape to hold the objective in place so it doesn't move from the focus position.

This is my first light exposure from my garage looking out into the yard:

I use a standard Synta-style finder scope holder to hold my guide scope. I put some shims into the holder as I tightened down the screws on the finder scope to make sure the guide scope cannot move around inside the finder holder. I purchased some additional bases for the finder holder so I could one on each scope I use. Then I just insert the finder holder dovetail in the base and tighten it down. So far this arrangement has worked well. I was able to take some 3 minute exposures on my C8 reduced to 1280mm with the finder guide scope attached this way.

For reference, here is a 1 second exposure with my Meade DSI of 17P/Holmes on 2007-11-06:

Here is an image taken with the C8 and a focal reducer (1280mm effective focal length) composed of 20 frames, each 3 minutes long. The stars look good - the odd stars away from the center of the image are due to the focal reducer (note the radial pattern of the elongation). I did notice some flexure, however, as the image did drift over time. The drift per frame was not enough to elongate the stars, but over time the object moves away from the center of the frame. This can be good as it provides some offset between frames which can be desirable when combining frames. However I am going to move to a more secure mechanism to mount the finder. The vendor ScopeStuff has some nice finder holders, for example.

I have never needed to use more than 1 second for my exposure time to find a guide star.

Please let me know if you have any questions or comments.

Example Uses

Mounted on a Celestron C8:

Custom Mounted With a Canon 200mm f/2.8 EF lenses (More info)

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