AT111EDT Focuser "Wobble"

[Greg Marshall]

Hi,

I recently acquired an AT111EDT. It is the older style with the R&P focuser. There is a "rotator" on the focuser that looks very good, but I find that when the knob is loosened the rotator assembly is too loose and allows a good deal of wobble. I'm concerned about this because if you loosen to change rotation and then tighten again with the weight of the camera pulling it down, it might end of not straight, putting the image sensor at a tilt. I've talked to Astro-Tech about it and, so far, they claim that this is "normal" behavior for this scope. But I've studied the mechanism and am convinced that it was originally intended to have another part that mine is missing. This part, a ring of low-friction material such as delrin, would fill the gap between the back of the draw tube and the rear surface of the rotator assembly.

In the drawing here you can see the 0.5mm gap between the draw tube in blue (with flange) and the housing in red.

Do any of you have this scope? If so, when you loosen the rotator knob, does it become wobbly, or just allow rotation?

Thanks,
Greg

[Greg Marshall]

UPDATE:

My measurement of 2.5mm is inaccurate - it's more like 2.2mm, making the gap a mere 0.2mm.

I cut some "doughnuts" of plastic sheeting that is about 0.16mm. With 1 layer of this material in there the looseness is greatly reduced, but still noticeable. With 2 layers it is tight and you can adjust how tight it is by turning the setscrews that attach the collar to the rotator housing. I think it's pretty much impossible to get it perfectly smooth in rotation because the metal surfaces will never be perfectly flat. Most people would rather have it a little "wobbly" than have uneven drag in rotation and that's probably why they make it this way. But for astro-photography it's far more important to keep the camera square to the optical axis, so I would recommend the "doughnuts" to anyone using this scope.

I also found that the eyepiece holder will pretty much always tilt any 2" barrel in it when you tighten the thumbscrew. I'm working on a way to directly thread my field flattener onto the draw tube.

[Brian Fredrich]

Hi Greg,

Is your wobble axial or rotation? I just purchased the AT65EDQ with a R&P focuser and it seems to have rotational play if the rotation set screw is not snug. My thought was to do a rough focus and frame then snug down the set screw and do the final focus. At this point, the focuser will rack in and out but there is a bit more resistance to movement.

Let me know if you find a better solution.

Neil mentioned that you may be at ITS at the end of the month. I plan on going the 30th to the 3rd weather permitting.

Hope to see you there....Just noticed your update.

Brian

[Greg Marshall]

Hi Brian,

The part I'm having trouble with is the rotator, so it should be (and is) loose when the locking screw is loose and stops nicely when it is tightened. The problem is (or was) that it would also be loose axially when the screw is loose. As you observed, this requires that you do framing first and then fine tune the focus. But in my case it is worse than that because with a heavy camera hanging 6 inches out from this rotating collar, there is no way that it will end up perfectly straight just by tightening that lock screw.

Ultimately, it doesn't really matter because my reducer/flattener has a rotator also, so I don't need to use the one on the focuser. But I feel better with the fix I described above, as it should ensure good perpendicularity.

I'm looking forward to next weekend - sure hope it works out!

Greg

[Brian Fredrich]

Greg,

I am interested to see your solution / improvement.

Weather looks good for ITS next weekend!