I have just acquired an Astro-Telescopes 102mm achromatic refractor that I intend to mount directly on the CF tube of the 12.5" astrograph to use as a guidescope.
The main resons for choosing this particular refractor are as follows:
1. Economical ($USD 499) but still having good (achromatic) optics - more than adequate to achieve good guiding.
2. Relatively long focal ratio (f/11 - remember the days when these f/ratios were common?). Not only does this give a good focal length for guiding relative to the main scope (1122mm vs 1600mm), but the longer tube permits wider spacing of the tube rings for greater stability.
3. Reasonable aperture (102mm) without being excessively heavy (just over 6kg with rings).
However, although there were good reviews for the optics and it sports a nice tube with a generous dew cap, I expected that it's Achilles heel would be the generic (made in China) crayford focuser (I had seen one review on the web where the owner had problems). Nevertheless, I thought that even without the focuser it was a good deal so I purchased it. Since it will be used for guiding, I intend to add an extension before the focuser such that the camera will be in focus near minimum drawtube extension (for stability). I took some measurements of the focus position to facilitate this (see picture showing the AT102 on a fully adjustable AltAz-Beanbag mount
).
However, I wasn't wrong about the quality of the focuser
Crayford Focuser Repairs- The first thing I found was an annoying ding in the end of the 1.25" adapter. On inspecting the box again, I found a slit right through it that had escaped my notice which lined up suspiciously well with the focuser. Oh well, not the fault of the focuser and just cosmetic ...
- When I turned the focuser knobs, I found that it sounded very scratchy and the fine focus knob turned in unison when I turned the coarse focus knob (not correct - the fine focus should turn 10 times faster when you turn the coars knob). I found that some of the grub screws were not tightened, but when I tighted one I found it was cross threaded and turned freely without tightening. On investigation, I found that the base of the threaded hole was not well formed and the grub screw was too short (so did not engage the upper hole/threads). I replaced this with a longer (M4) grub screw that I had and this solved the problem.
- The next problem that I found was that the fine focus seemed to "grab" every third of a turn, as if there were 3 detents per rotation. Of course, this should not be, so I began investigating; Gary Hand (HandsOnOptics) said that the most likely cause was a bent pin. This is used as the centre "gear" for the planetary gear system that enables the slow motion (central pin rotates and contacts three large ball bearings which are set in a brass fitting that is attached to the main drive shaft. I found a good article on how to disassemble a similar GSO crayford focuser (see http://backyardvoyager.com/astronomy...ur-gso-focuser), but it didn't go far enough. I needed to completely disassemble the fine focus ball bearing mechanism to get at the centre pin. Also, instaed of a hex nut holding it in, my focuser had a round nut with a slit (similar to a straight screw head), so I had to make my own tool to undo it (see attached pictures). Once I managed to disassemble it completely, I found that the central pin was perfectly straight
. It had to be something to do with the pin I reasoned, as the false "detent" matched the rotation cycle perfectly. The ball bearings are held against a groove around the circumference of the pin near one end, so I inspected the groove with an eyepiece, and found a very slight mark/depression within the groove (see the picture to get an idea of how small I am talking about - probably of the order of 10-20 microns deep!!!). This had to be it, as combined with the high mechanical pressure applied, this would mimick a detent I reasoned. I mounted the pin in my lathe (a recent acquisition - I'm no expert by any means!), and smoothed the groove using #320 wetanddry, followed by some metal polish / cloth giving a nice shiny surface (even using an eyepiece). I then re-assembled the focuser and .... voila !!! The false detent problem was gone 
- Just as I was feeling really pleased with myself I noticed another problem: the focuser drawtube was not square with the focuser body !!??!! I soon found that one of the 4 roller bearings was set crooked and had even badly scored the drawtube where the edge of the roller had dug in. There is an M3 set screw that hlods the roller bearing against an aluminium pad that was not screwed in square to the hole (cross threaded). After a few attempts, I managed to screw it in square; but the screw only engages the hole by about 3mm, hardly enough to ensure a good structural fitting (a screw should engage by 2 x D in soft metal like aluminium - so 6mm would be ideal in this case). I will replace all 4 screws with longer ones later.
The end result is that it seems to be performing well, and the fine focus now has a smooth feel to it
