Hi Peter,
In addition to what Eric said...
What is the issue with having a 5arc min eccentricity? I have ecentricity when I rotate my focuser rotator due the the camera chip not being centered on axis, its minor though.
Quote:
Originally Posted by PRejto
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.
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I can not imagine there will be eccentricity in the rotator alone, you might see it if you mount the rotator slightly off the optical axis, but not in the rotator. so, my guess is tilt, and im guessing the laser is not parallel with the rotators rotating axis or the laser is not perfectly collimated.
I can not see miscollimation (if you have any), having an effect on the rotation test either.
Peter, if you can get your hands on one, get a micrometer dial guage with a needle and magnetic base. mount this on your scope and place the needle on the end of the rotating tube... then rotate the rotator 360 deg and see the runout, if any. this test will tell you if you have tilt right there. This is what i did. However, any tilt between the camera and focuser should be measured with focus runs on different parts of the chip and shimmed accordingly.
To deal with tilt in the adapter between the OTA and rotator, use your laser in the focuser to see where it is pointing with respect to the objective. even if there is tilt at the mounting face of the rotator (or your laser is not collimated) a rotation of the rotator should have the laser scribe a concentric circle with respect to the objective (assuming the laser is not parallel with the rotator) if not, add shims between the OTA and rotator until it does. Now I understand that there is complexity in this step because you say there is lateral movement in how you can mount the rotator to the OTA. my suggestion would be to use a combination of shimming and lateral movement until the laser scribes a concentric circle with the objective.
Quote:
Originally Posted by PRejto
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.
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I would agree, as long as you do not change collimation while rotating, which im sure you wont.
Quote:
Originally Posted by PRejto
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?
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If it is tilt you have, I would agree, but it may not be, it could be eccentricity and a laser that is placed not parallel to the rotating axis. You might even find tilt in the ccd, that is why you need to address this with the camera on with several focus runs.
Quote:
Originally Posted by PRejto
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.
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From our phone conversation, try moving the rotator lateraly on the OTA to reduce the eccentricity. I appreciate it is hard to differentiate between eccentricity or if the laser is just not rotating true. so i would try it with the laser, then move the rotator laterally, and look what happens. if you see a big difference in the size of the circle the laser makes, then you know a portion of the circle is due to eccentricity.
regards
Josh