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Old 03-08-2021, 06:39 AM
gregmc (Greg)
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gregmc is offline
 
Join Date: Sep 2018
Location: Sydney, Australia
Posts: 35
Quote:
Originally Posted by g__day View Post
Let me restate - I am still trying to improve both my polar alignment and guiding. When one is within 2 arc minutes of the celestial pole do you really think frame rotation is a big thing - I am not doing 30 minute subs anymore since I switched to ZWO camera - I rarely to 6 minute subs even! Stacking takes out any small field rotation easily.

I do not have an unobstructed view of the celestial pole - and after long discussions between both the Bisque brothers and the author of PEMPro - the Bisques kind of won saying a large Tpoint model will always be superior for all points in the sky over prolonged drift alignment over only any two points in the sky... as it adjusts for all telescope and mount behaviours over all sky.

Now I ponder that the best polar alignment I might ever achieve is something like is between 30 arc second to 2 arc minutes from the refracted celestial pole with my gear and current skill level.

But that was never the point of my post. It was I have potentially competing tracking algorithms going that will compete - so how do I analyse them to achieve the best outcome?

The SS2K may also believe I am say 2 arc minutes off the SCP - so depending where in the sky I am pointing the SS2K controller can either be told to adjust RA speed and DEC or not. Given I normally have it on it could and probably would be creating periodic slight adjustments to my mounts position (I don't know the correction frequency or amount). The amount the SS2K may be changing the RA and correcting DEC is likely to be very small. It might be saying run the RA at 0.999998 sidereal and send a +0.0001 to the DEC motor ever 26.7 seconds for instance - picking numbers out of my head for an example when a star is on the equinox at say 30 degrees elevation above the horizon. Now that might perfectly adjust my guide star for polar misalignment - or it may not - but one thing for sure my guide program would see it and either say well that wasn't me - but it helped - thanks - or whoops - was that just seeing - ignore it for a few seconds and see if it goes away or opps I am not tracking correctly - issue a guide command to correct for drift. Either way I now have an extra feedback system trying to help - and it may or may not be succeeding - I simply don't know.

The easiest thing I could do would be to guide for a few hours and say every twenty minutes switch on and off the mounts polar misalignment correction and see if the guiding improves or worsens according to the guide camera.


My gut says that for visual or on a rig that isn't within say 30 arc seconds of the pole it will help - outside that it would only help if the adjustments were really, really fine tuned and the gear was perfectly balanced with no backlash - otherwise you have in effect two different people trying to steer the car so to speak and they are probably interfering with each other!


Does my thinking make sense to folk - or would you suggest a different approach?


Thanks, Matt
Stacking can not remove field rotation. It can align images that have been rotated which is different.

Field rotation is where during the exposure, the camera does not rotate exactly with the earth during the actual exposure.
The stars in one part of the image will move at the guided rate but the stars at the other end of the sensor may not in a noticeable manner.

If a short focal length, a smaller sensor or lower resolution, shorter exposures (etc?), it may not be noticeable so may not be a concern.
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