Convert a Cheap Meade 60mm Spotter into a CCD Guide Scope
After trying my hand at autoguiding through an "off-axis" guider (boy what a pain!) I desperately wanted a guide scope. Since my wife would string me up if she even knew half of what I've spent on this stuff so far I had to find an inexpensive way out. I was at Service Merchandise one day and looked up among the Jasons and Bushnells and there sat a cute little 60mm Meade "Zoom" Spotter that was just begging for a ride on my 8 incher. So I bought it and took it home to the operating table. I ended up with a nice little guide scope and ring setup that cost less then the Celestron Radial Guider it was replacing. I liked the fact that the scope was stubby and not hanging all off the tube, but was concerned that the focal length would be too short for accurate guiding. I spoke to a few knowledgable people on the subject and they determined that a focal length of 1/3 (some even said 1/4) the main tube would be adequate for CCD GUIDING (not eyeballs). Since my main OTA focal length is 2000mm that meant I would need 666mm. I figured the 60mm optics to be approximately 375mm focal length. I shoved a 2X barlow in it to bring this effectively to 750mm. The scope guides very nicely like this with a Pictor 208XT Autoguider. If you have a longer FL scope you may need to use a 3X barlow or stack 2 barlows to assure a long enough focal length for accurate guiding. What little it may lack in focal length I think it gains in three other areas, it will not suffer from mirror flop (like an ETX type guide scope can), since it is short and lightweight flexure has not been a problem, and it can remain on the scope all the times since it really isn't in the way. So anyway on to the modifications.....scalpel!
I made these mods a few months ago and don't remember the exact specifics but generally it was easy and straightforward. If anyone performs these mods please refresh my memory and I'll post the exact specifics here. Basically you need remove the "zoom" portion of the scope but keep the helical focuser intact. If I remember correctly you remove the 3 screws holding the rear focus/zoom assy to the tube. Remove a threaded ring on the very end and the zoom portion of the scope can be extracted out the back of the focuser. You will need to cut the tube length down in order to reach a focus with 1.25" EP's and the CCD. I cut mine down to a length of 8" (this measurement is from the rear of black ring on the front to the end of the tube at the rear, the white visible part is 8"). Cutting the tube neatly is the trickiest part. You could use a hacksaw then clean it up, if you have a metal chopsaw you are homefree. I used a large pipecutter. If you use this method you will have to find something to place in the tube to keep it from crushing as you tighten the cutter down on it. Otherwise the end will "flare" in and you will have one hell of a time getting the focuser back in, like I did ; )
NOTE: This measurement works fine with all my 1.25" EP's, my Pictor 208XT, a Celestron Ultima Barlow, and an Orion Shorty Barlow. If you will be using a full length barlow or 3X barlow you better double check to make sure you will have enough back focus and adjust the length accordingly. I highly recommend a short barlow for maximum stability and minimum lever arm action.
Reassemble the helical focuser to the tube. You will notice that the helical focuser is actually very tight and stiff for a cheap scope which is excellent for our purposes. The only problem now is the hole in the focuser is too big for a 1.25 barrel. After searching high and low I found that the inner ring from a cap of black Krylon spray paint is the perfect size to use as a bushing. You will need to use a razor knife to carefully cut the ring out of the cap, then cut around the ring to separate it from the end. The inner ring has 4 flanges that will need to be cut off also. Take a few minutes and clean it up smooth as possible with the knife. Use some sandpaper to bevel one edge slightly. Now insert it halfway into the focuser, insert the barlow into the ring and push (it is a VERY snug fit and will require considerable force) till the barlow is fully seated into the focuser. I suggest you use an old or spare barlow so you can permanently leave it in the focuser, since it will also act as an eyepiece holder. It needn't be a great barlow, the CCD is not real picky. That's it, you're done. If you look through it at night you will notice that the images look terrible (a zillion rings around the star and purple flares shooting out), but it doesn't seem to bother the autoguider one bit. Images during the day are actually quite good!