Cable wrapping on Rotator (was: Help understanding rotator behavior)

[Wesley D. Grimes]

I am using a takometer on a TOA 150, with SkyX on a Paramount MX.

I'm just trying to get things up and going with a new set up. Mount, scope, rotator are all new.

In watching where the rotator is positioned during image capture and then later during autoflats....using a flip-flat (new too!!)....there seems to be a difference. It is not always the same difference, but seems to be off by 10-20 degrees sometimes. I know the rotator is doing an east and west version of flats....I understand that....I think. I'm seeing something different I think....not a 180 degree off rotation. When I look at camera orientation relative to focus knobs during image capture, it is not the same as when doing flats sometimes.....often it is, but sometimes it is clearly different.

How do I 'calibrate' the rotator??

Appreciate all suggestions and knowledge passed my way!!!

Wes

[Bob Denny]

Without knowing more specifics, I can't say what's happening precisely. However, one key bit of information is that the rotator is "self-calibrating" via ACP and its plate solutions. Every time ACP solves a plate (pointing update and final/science/art images) it looks at the true sky PA versus the indicated rotator PA angle, and stashes away the difference. For the next rotation, ACP takes the requested sky PA, applies the stashed difference, and then sends the rotator to the resulting mechanical rotator PA.

A side benefit of this is that rotation errors are self-correcting. Let's say the rotation, which happens prior to the ACP pointing update, slips a little. Now the pointing update re-measures the sky PA and finds it is off. So in addition to jogging the scope to remove any pointing error, a pointing update may jog the rotator to remove any angle bias.

Now let's say your rotator is slipping. You finish up observing on one night and the last PA bias has been recorded. Then the next night, you want to do dusk flats. Well ACP has no choice but to use the previous night's PA bias to calculate the mechanical angle needed to achieve the desired sky PA. Normally this is no problem as things remain stable. But if the rotator slips while doing dusk flats then things will be off.

Post a couple of log files from ACP and I will tell you if your rotator is slipping. Or you can look carefully for messages about PA differences.

The above is guesswork....

[Wesley D. Grimes]

Thank you Bob. I'll take a look.....I'm sure it is just my understanding that is lacking here. I appreciate the explanation.

W

[Bob Denny]

Post a log from one of your recent runs, preferably one with multiple targets or at least multiple PAs.

[Wesley D. Grimes]

I see what has been happening. When the camera rotates around too much I'm getting a power cutoff because power cord gets stretched too tightly.

How do I get ACP to rotate camera the opposite way?? Or how do I figure out which direction it is going to rotate all the time so I can route my power cord correctly??

Thx,
W

[Bob Denny]

Oh, excellent, you found the problem. Good work!!

The rotator, depending on the brand/model, has some sort of wrap-control feature. I have no control over that low-level of operations and control. All I can do is tell the rotator to move to a given angle, period. How it gets there, which direction it goes to get there, how it keeps cord wrapping under control, etc., is the responsibility of the particular rotator and its controller. Contact the rotator manufacturer for info on wrapping control. Or maybe it is covered in the docs that came with your rotator.

[Wesley D. Grimes]

Okay. Thanks.

Wes