Auxiliary Equipment

Celestron 5 Guide Scope

I successfully used a 6" f15 Cassegrain on my 8" Cave Astrola. My original thought for the 22" guide scope was to use the 6" Cassegrain from the 8" Cave Astrola. I decided to try the C5 Because I had some Aluminum tube that I could make some rings from, it is lighter, and quite a bit smaller.

I personally believe that the guide scope should have about three times the focal length of the photographic instrument. The 22" has a focal length of approximately 95". This means that the guide scope should have an effective focal length of about 284". The C5 is a f10 so the focal length is 50". I need a 4mm eyepiece or an 8mm with a 2X barlow to achieve a 3X guide ratio. I am not sure how suitable the C5 will be given the difficulty in achieving a 3X magnification over the 22" at prime focus. I may find that guiding is difficult with the 4mm eyepiece due to the short eye relief. I may need to return to the 6" Cassegrain used on the 22". Then I can use an 8mm eyepiece or a 16mm with a barlow.

I made the guide scope mounting from a piece of 2" channel and 6" diameter tubing. I made two rings by parting the tubing at 1.500" on the lathe to make two rings. The two rings are bolted on 2" channel and the telescope is held in the rings with 2 springs and 2 micrometers in the back and 4 pivot head set screws in the front,  Figure 1.

Figure 1

 The two micrometers push against two strong spring for acquiring a guide star. The channel has two 1/4" 20 bolts for attaching the guide scope to the 22" tube box.

A Vixen guiding reticule is used to provide an illuminated reticule. I also mounted a small finder on the C5 to aid in finding guide stars, Figure 2. I found a finder useful in finding suitable guide stars.

Figure 2

 The tension springs that the micrometer push against and are opposite each micrometer, Figure 3. The tension springs need to be strong and long enough to provide full movement within in the mounting ring. Stiff tension springs are necessary to keep the guide scope from moving.

Figure 3
 
 

 The front of the tube is supported in the mount ring by 4 set screws with a swivel built on the end, Figure 4. These items can be found at specialty fastener stores. I had them from some scrap parts. A ball end set screw would also work.

Figure 4

 The telescope is mounted on the telescope tube box. Figure 5.

Figure 5

 The preliminary experiments with the guide scope have shown that finding a guide star was difficult. That is why I added the finder. I hope to make some images in a couple of months when my mirror returns from being coated. I am not totally sure how suitable the C5 will be as a guide scope but time will tell.

I hope that the simple guide scope mount will encourage others to make such items. The Rings could have been made from PVC, or any suitable  pipe. The micrometers can be replaced with any fine thread screw. Even though I made the rings in a lathe they can be made with a hand saw. I have done this many times. I think that the entire mount could be made with hand tools.

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