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How does fork-mount SCT track its object without a wedge?

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#1 Ballyhoo

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Posted 12 October 2019 - 11:59 AM

This may sound like a very ignorant question, but I have only had gems and I was reading the Imaging Primer book and it was mentioning field rotation inherent w fork mounts. So what then do you see visually following your target  using a forkmount? At some point do you see the mount rotating out of its target (unlike a gem where you would not notice anything?)



#2 sg6

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Posted 12 October 2019 - 12:06 PM

Scope should track the center of the target and the target gently and slowly rotates in the eyepiece.

As the change is 15 degrees an hour you would have to observe it say once an hour over 4, 5, or 6 hours to really notice anything between observations.



#3 FlankerOneTwo

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Posted 12 October 2019 - 12:10 PM

The fork mount drives two motors to track the center of the field, so the image will appear to rotate about its center as the night progresses.

 

From http://kelly.flanaga...ted-telescopes/Orion-Through-the-Night-Alt-Azimuth.png

 


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#4 bbqediguana

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Posted 12 October 2019 - 12:26 PM

And the wedge essentially makes the forks behave like an EQ mount, removing the field rotation. Alternatively, an Alt-Az fork mounted scope can use a field de-rotator like this:

 

https://www.meade.co...de-rotator.html

 

and that will actually rotate the eyepiece (and/or diagonal and/or camera) to compensate.



#5 Eddgie

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Posted 12 October 2019 - 01:42 PM

This may sound like a very ignorant question, but I have only had gems and I was reading the Imaging Primer book and it was mentioning field rotation inherent w fork mounts. So what then do you see visually following your target  using a forkmount? At some point do you see the mount rotating out of its target (unlike a gem where you would not notice anything?)

The answer has already been given, but some terminology is important for future questions.

 

Field rotation is not inherent to "fork mounts."  It is inherent to "alt-azimuth" fork mounts which is what I know you were talking about but best to use the most technical description because there are two modes of operation for a fork, and the mode determines the correct terminology.   

 

When a wedge is used, it is operating as a "polar fork" mount. It is exactly the same fork, but the difference is that the wedge just points the azimuth plane toward the north celestial pole.

 

When in Polar Fork mode, only the RA drive needs to run (assuming perfect alignment).  In alt-azimuth mode, the computer calculates how much the altitude/declination motor has to run to follow the arc in the sky that a star at any given declination would follow, but as the excellent diagram posted by Patrick shows, stars will slowly pivot around the center of the field of view.  Something that crosses the due east horizon at sunset will be almost upside down by sunrise. 


Edited by Eddgie, 12 October 2019 - 01:56 PM.

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#6 twjs

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Posted 12 October 2019 - 09:04 PM

The answer has already been given, but some terminology is important for future questions.

 

Field rotation is not inherent to "fork mounts."  It is inherent to "alt-azimuth" fork mounts which is what I know you were talking about but best to use the most technical description because there are two modes of operation for a fork, and the mode determines the correct terminology.   

 

When a wedge is used, it is operating as a "polar fork" mount. It is exactly the same fork, but the difference is that the wedge just points the azimuth plane toward the north celestial pole.

 

When in Polar Fork mode, only the RA drive needs to run (assuming perfect alignment).  In alt-azimuth mode, the computer calculates how much the altitude/declination motor has to run to follow the arc in the sky that a star at any given declination would follow, but as the excellent diagram posted by Patrick shows, stars will slowly pivot around the center of the field of view.  Something that crosses the due east horizon at sunset will be almost upside down by sunrise. 

and so the target would blur during an AP exposure?



#7 Don Taylor

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Posted 12 October 2019 - 09:27 PM

and so the target would blur during an AP exposure?

Except for extremely short exposures such as for Solar or Lunar AP:

 

Yes, unless a field rotator is used to rotate the camera in combination with the scope moving in altitude and azimuth. All three motions will vary in rate.



#8 Don W

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Posted 12 October 2019 - 09:28 PM

Depends, but generally, yes. The object being imaged would pretty much stay in the center of the image, but the stars, etc. would make a circular trail around it.



#9 carolinaskies

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Posted 13 October 2019 - 03:32 PM

The answer has already been given, but some terminology is important for future questions.

 

Field rotation is not inherent to "fork mounts."  It is inherent to "alt-azimuth" fork mounts which is what I know you were talking about but best to use the most technical description because there are two modes of operation for a fork, and the mode determines the correct terminology.   

 

When a wedge is used, it is operating as a "polar fork" mount. It is exactly the same fork, but the difference is that the wedge just points the azimuth plane toward the north celestial pole.

 

When in Polar Fork mode, only the RA drive needs to run (assuming perfect alignment).  In alt-azimuth mode, the computer calculates how much the altitude/declination motor has to run to follow the arc in the sky that a star at any given declination would follow, but as the excellent diagram posted by Patrick shows, stars will slowly pivot around the center of the field of view.  Something that crosses the due east horizon at sunset will be almost upside down by sunrise. 

Actually the statement should be "It is inherent to motorized Alt-Azimuth mounts".   Whether a single/right/left center support, or a fork arm support.




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