When to redo calibration?

[Ar Bit]

Hi,

I am aware that calibration can be reused if the rotational alignment of the guiding system is maintained.

When redoing the polar alignment (say for a portable system) or taking the OTA on and off, I'd think it would introduce minor differences in the absolute orientation of the guiding system wrt to the celestial co-ordinates.

Does that call for a recalibration? Or does only the relative orientation of the guiding system (scope + camera) matter, and as long as guidescope / camera are locked together the above are irrelevant?

Thanks

[Bruce Waddington]

You’re correct, what matters is the rotational position of the guide camera relative to the sky.  If you want to avoid doing one calibration at the start of the night, you would need to be sure that rotational orientation doesn’t change by more than a few degrees.  If in doubt, re-calibrate.  Polar alignment doesn’t affect this because you don’t change the orientation of the guide camera with respect to the sky.

 

Regards

Bruce

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[Ar Bit]

Thanks. 

In some mounts without an inbuilt zero, you have to reset zero, and that can cause a few degrees change in orientation.

So calibration every night it is!

[Bruce Waddington]

Hi Ar.  If you’re talking about registration marks in the drive systems or some kind of “home” position on the mount, that really has nothing to do with the PHD2 calibration.  What we’re interested in for calibration is the rotational orientation of the guide camera with respect to the sky – that isn’t going to be affected by anything you do with the mount, it’s a matter of how the guiding assembly is attached.

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/6c03f9da-d419-4524-9801-fa3b44d59731n%40googlegroups.com.

[Ar Bit]

Hi Bruce,

To clarify - lets take CEM26 (my mount).

It does not have an internal zero position. It has to be set and marked manually. When the mount is dismantled, the home position is "lost".

When setting it up, the marks are lined up by eye and that is then set as zero position via HC / Commander software. Plus, each night the az-alt knobs are adjusted manually, and the reported PA is always different each night. Due to the manual operation, a small rotational error is inevitably introduced compared to the previous setup (Guide camera fixed to OTA fixed to mount which has changed its absolute orientation).

So for example, I set up one night, set to zero, and do a platesolve and sync. The error is say 0.5 degrees. PA is off by 15 sec in RA and 10 sec in Dec.

When I set it up again the next night , there is a slight change in the physical orientation, so the platesolve error could be say 2 degrees, with PA off by 10 sec in RA and 30 sec in Dec. (Actual numbers)

So from Day 1 to Day 2, the physical orientation of the OTA/guide assembly relative to the celestial coordinates has changed.

I suppose this would not be an issue if its permanently mounted, or the zero position is built into the mount, or the mount can be setup within a degree or so each night.

Is that reasoning wrong?

[Bruce Waddington]

I don’t really follow all this but if you’re literally dismantling the mount, I suppose something could affect the axial positioning.  Varying PA errors don’t do that.  I don’t think it’s worth flogging this though, I probably don’t grasp the full picture.  Since you’re willing and able to start the evening session with a fresh calibration, that eliminates the issue entirely and is the most reliable solution.

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/f74ce280-4a4b-46a6-8ac3-b7851cdc0956n%40googlegroups.com.

[Ar Bit]

Agree, not worth flogging this when a fresh calibration is possible :-)

Thanks