Something else to worry about?

[Peter Drew]

This afternoon I was observing the Sun with my PST modded Helios refractor. It's the popular standard 6" F8, similar to the Skywatcher series in recent years with the 2" type rack and pinion focuser. The objective had misted up a bit and I removed the eyepiece and looked op towards the objective via the blocking filter. I was surprised to see that the image of the objective was significantly misalligned. I checked the drawtube for slop and that seemed ok so I removed the PST body and put a well adjusted laser collimator into the eyepiece holder. The resulting spot on the objective was way off centre, rotating the laser made little difference. I next undid the screws that hold the focuser housing and push-pulled it untill the laser spot was central resulting in a considerable gap where the main tube met the focuser housing. I had shortened the main tube as part of the mod but I've done this sort of thing many times and am confident about getting things square. Puzzled, I put the focuser in the lathe, the focusing tube held between centres. On running, the outer housing ran out wildly, altering the setup showed that when the housing ran true, the focusing tube ran out. This was on a unit that focused smoothly with no slop. By using a combination of pinion adjustment and re-setting the small tension adjusting screws I was able to get the complete assembly running true again. If you have one of these popular telescopes it might be worthwhile popping in a laser collimator to check this for accuracy.

[David Galvin]

Good point Peter, I have seen focus slop on Newtonians but always took it for granted that refractors would be okay. It will be interesting to see if you have an improvement in image quality when re-assembled.

[John]

I've had to address this when using Chromacor's in Synta 6" F/8's and, more recently my current Meade AR6. The Chromacor is very fussy about being right on axis which means a properly squared focuser and getting the objective collimation correct. The AR6 is more challenging because it does not, as far as I can see, have those small hex screws on the top of the focuser with which, combined with the 4 screws holding the pinion onto the rack, you can get the focuser squared. At the moment I have my AR6 "there or therabouts" but I feel that a properly collimatable focuser such as a Moonlight might be the ultimate solution.

[Merlin66]

Use the laser to centre the focuser/ objective then a Cheshire to align the objective to the optical axis....it's the only way.