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Alt/Az tracking near zenith

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#1 Rac19

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Posted 30 May 2020 - 06:18 AM

I was attempting 20 second subs on M83 tonight which happened to be very close to the zenith (Alt 86 degrees). The first problem I noticed was that some images were OK and some had elongated stars, with clear signs of frame rotation. I also noticed that tracking was not very accurate and that attempts to correct with "east" and "west" slews were not successful.

 

Having read up on the subject, it seems that there is a no go area around the zenith for Alt/Az mounts, which makes sense when you think about it. I guess this is one situation where, despite the limitations, a wedge would make a difference.



#2 descott12

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Posted 30 May 2020 - 07:27 AM

yep. my evo 8 couldn’t track anything above about 80 degrees

#3 junomike

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Posted 02 June 2020 - 08:50 PM

Strange, I'll have to test my Evo on this error.

My synscan Dob's have no problem tracking at Zenith



#4 Rac19

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Posted 03 June 2020 - 03:24 AM

Strange, I'll have to test my Evo on this error.

My synscan Dob's have no problem tracking at Zenith

I found this quote somewhere (I can't find it again). While the technical "zenith blind spot" may be > 89.25 degrees, I had problems with longer (20 second) exposures at 84 degrees. Some frames seemed OK and some were distinctly streaky.
 

 

When tracking at elevations close to 90°, the azimuth axis must rotate very quickly; if the altitude is exactly 90°, the speed is infinite. Thus, altazimuth telescopes, although they can point in any direction, cannot track smoothly within a "zenith blind spot", commonly 0.5 or 0.75 degrees from the zenith. (I.e. at elevations greater than 89.25° or 89.5°.)"



#5 SkipW

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Posted 03 June 2020 - 05:45 AM

Even if you might be able to more or less accurately track close to the zenith, field rotation is going to kill you (unless you have a field derotator, and even a good one is probably not going to give great results very close to zenith). Without a derotator, there's nothing that you can do about this.

 

As far as I know, the Celestron alt-az mounts adjust the tracking rates in altitude and azimuth every 30 seconds to approximate sidereal tracking. This is probably adequate for a lot of purposes, but is unlikely to be sufficient near "gimbal lock" position (i.e. when the optical axis is parallel to the azimuth axis) because the azimuth speed increases rapidly as zenith angle approaches zero. I suspect that even small errors in leveling will cause large errors the calculated azimuth speed near zenith because the mount's idea of the zenith angle will be slightly different than the true zenith angle, and you're calculating the azimuth speed by dividing by the sine of the zenith angle, which is very small, so even small errors become significant.



#6 Rac19

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Posted 03 June 2020 - 05:57 AM

Even if you might be able to more or less accurately track close to the zenith, field rotation is going to kill you (unless you have a field derotator, and even a good one is probably not going to give great results very close to zenith). Without a derotator, there's nothing that you can do about this.

 

As far as I know, the Celestron alt-az mounts adjust the tracking rates in altitude and azimuth every 30 seconds to approximate sidereal tracking. This is probably adequate for a lot of purposes, but is unlikely to be sufficient near "gimbal lock" position (i.e. when the optical axis is parallel to the azimuth axis) because the azimuth speed increases rapidly as zenith angle approaches zero. I suspect that even small errors in leveling will cause large errors the calculated azimuth speed near zenith because the mount's idea of the zenith angle will be slightly different than the true zenith angle, and you're calculating the azimuth speed by dividing by the sine of the zenith angle, which is very small, so even small errors become significant.

Possibly the subs that were "streaky" were a result of the 30 adjustments occurring during those exposures.

 

It's kind of what ai suspected, I was just not really sure how Celestron Alt/Az mounts operate. I was already aware that Alt/Az speeds were constantly recalculated to avoid stepwise Alt/Az movements during tracking.

 

It sounds to me that, as the zenith is approached, the speed changes are larger and required more frequently than 30 second intervals.

 

EDIT: I guess also that within the "zenith blind spot" the situation becomes totally impossible, for even the most sophisticated Alt/Az mount.


Edited by Rac19, 03 June 2020 - 06:05 AM.



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