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Rotator Problem Is Making Me Insane
Apologies, this will be a long post.
Problem: Adding an Optec Gemini to a TEC180 (which has not been done before, or, I was the first to try this.
Solution: Precise Parts adapter based around the design of the TEC Large Focuser that came with the TEC180FL. This is a conical shaped fitting held in place with the original focuser's locking collar.
Result: Imprecise and non-repeatable fitting of Geminir to OTA. Taking a 4 min time exposure of a centered star while making a complete rotation shows about a 5 arc-min off center circle.
Experiment: Removed rotator and ordered another PrecisePart fitting so that a Glatter laser can be fitted directly on rotator. Collimated laser and projected beam 12 feet to wall. Rotation is not perfect but according to Optec is about as good as it gets and seems to be in spec for the Gemini.
Question: Is the rotational error due to eccentricity (i.e., lateral side to side motion, or tilt changes as the rotator turns? I cannot tell and I don't know how to determine this. It could be a bit of both.
Experiment 2: Placed rotator back on TEC180FL this time with a paper mask over the objective and a centered dot. Collimated laser and aimed dot to center on mark. Performed star test/rotation and still get a 5 arc-min error in rotation.
What to do? First I assume the TEC180 is collimated though it has no adjustment on the lens cell. I did check it with a Cheshire and it appears to be collimated however I found it difficult to use and I'm not certain. In any case, collimated or not, I cannot reason out that this would have any effect on the rotation test. Also, I don't feel that my ability to truly mount the rotator without any tilt is good enough using the paper mask method...but, I'm having a hard time thinking up a new method. Appeals to Precise Parts for help have fallen on deaf ears.....I'm basically stuck with this adapter and I don't know where to turn for another in terms of design. Optec initially offered to try to make one but in the end said they didn't have the correct specs on the TEC180 to proceed. Any ideas?
More questions.
1. Am I correct that the rotator test I'm doing is going to be unaffected by collimation? I reason that a star placed on the center of the CCD will be centered and remain centered during rotation regardless of collimation.
2. Tilt is problematic. Once I put a camera on the rotator there are two different sources of tilt. 1. The rotator attachment to the OTA. 2. The assembly behind the rotator. As far as #2, the fittings are a massive dovetail from the Gemini and then all screw fittings to the TEC flattener, FW, Camera. I think I can probably assume (?) that the fittings behind the Gemini are pretty close to concentric and tilt free and that the problem is probably mostly the adapter from the Gemini to the OTA. Is this a reasonable assumption?
Since there are two independent issues at play (the inherent rotator eccentricity or tilt, and the tilt of the adapter) I'm very unsure how to attack this problem. I've considered just trying to shim the adapter until I can reduce the rotational error to min. But, obviously no amount of shimming will reduce the eccentricity error (or inherent tilt error) of the rotator. I don't know how to separate these issues. And if there is tilt behind the rotator I can't see how to chase the adapter tilt issue without possibly being influenced by potential tilt behind the rotator.
Any and all advice will be greatly appreciated!
Peter
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