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Field Rotation in smaller FOV

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#1 Agreegator

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Posted 26 February 2021 - 08:18 PM

I started DSO imaging about a month ago with a 70mm APO. But now I have been trying to image small DSOs with my 5" MAK Scope. It needs a lot of hard work and FOV is only 0.5 Deg. wide at best .. but it gives very sharp results and I love imaging DSOs in more depth with it. 

 

Last night was partially cloudy so it wasn't possible to polar align the scope. So I decided to do autoguiding in Alt-Az mode just to see what Alt Az limits of MAK are. I went for Leo Triplet which was about 30 - 40 Deg Altitude, in the West.  

 

- 30 Sec Guided Exposure - Nice and round, pinpoint stars.

- 120 Sec Guided Exposure - Overexposed stars - still rounded, with no field rotation. I was imaging in west so I thought this would be achievable.

- 300 Sec Guided Exposure - I reduced ISO to the minimum possible, to avoid overexposed stars. To my great surprise, there was still no field rotation anywhere in the image ! I investigated every corner of the image, but nothing seemed elongated at all. Every star was very well rounded. 

 

Now, I'd give benefit of doubt to 2 mins exposures - But I cannot make sense of why 5 mins exposures would have no field rotation. Is it because of the small FOV that I'm working with ? Every thing I have read places Alt-Az limit at 20 sec - 120 sec max (with no mention of FOVs)

 

Does a small FOV make that much of an impact that I can more than double the exposure times, without field rotation ?  I have checked my image several times since last night and I'm completely confused. I'd love to hear an explanation of this.

 

Thanks in advance.


Edited by Agreegator, 26 February 2021 - 08:20 PM.


#2 MikeK314

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Posted 26 February 2021 - 08:30 PM

Ah, but you were pointed at precisely the place (due West [or East] at a relatively low elevation) where field rotation is minimized.  Had you been pointed at, say, M81, the results would have been quite different.

 

Here's one writeup that explains some of the details.  There are others out there.

 

https://calgary.rasc...ld_rotation.htm


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#3 kathyastro

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Posted 26 February 2021 - 08:39 PM

Field of view does not have much impact on field rotation if the guide star is in the FOV.  "X" degrees of rotation is still "X" degrees, no matter what the image scale or FOV is.

 

Yes, east or west gives you the minimum field rotation.  Crossing the meridian gives you more, and the zenith is the worst.


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#4 TOMDEY

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Posted 26 February 2021 - 08:44 PM

Field rotation is of course centered on the guide star and is simple harmonic sinusoidal with a period of a day. It can be counterintuitive that the rotational pendulum is Pi/4 (90o) out of phase with the instantaneous correction vector. That is to say, when the corrections executed by the drives are most aggressive, the rotation is actually least, and when the corrections are at a min, the rotation is at its worst. The amplitude of the rotational sinusoid is given by my generalized handout below. I derived that back in the days when we were doing 2-5 hour single exposures onto film for Deep Sky work, typically at 2000mm focal length. Under those conditions, accurate polar alignment and guide correction were a real nightmare.   Tom

Attached Thumbnails

  • 124 field rotation Tom's Dec Drift handout p1.jpg
  • 123 field rotation Tom's Dec Drift handout p2.jpg

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#5 Iamhondo

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Posted 27 February 2021 - 06:15 PM

Field of view does not have much impact on field rotation if the guide star is in the FOV.  "X" degrees of rotation is still "X" degrees, no matter what the image scale or FOV is.

 

Yes, east or west gives you the minimum field rotation.  Crossing the meridian gives you more, and the zenith is the worst.

FOV does influence the perception of field rotation. Yes, the field rotation is a fixed angle for any specific set of image duration, alt, az and observation site. However, the telltale arcs of alt/az field rotation (and any serious polar alignment error) won't be detected by visual examination if the field of view is too small to reveal an arc.

The stars farthest from the center of the image will show the greatest arc. If you shrink the FOV, the arcs are linearly shorter even if they reflect the same field rotation angle. At some point of shrinkage of FOV, the arcs won't be perceptible (without software).

Other factors can disguise field rotation (and polar misalignment and PE). Bad seeing makes for chubby stars and less obvious arcs. Ditto poor focus. Short enough exposures can also stifle the perception of arcs because it reduces the angle of the arc.

The acid test is simple: Mount a camera with a wide lens on fixed tripod aimed at Polaris and take a long exposure. The stars near the corners will exhibit long arcs. The arcs get shorter (linearly) near Polaris. If you crop the image to only Polaris, you might not notice its short arc if the focus poor, seeing is bad, and/or the duration is short.

 

Joe


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#6 Iamhondo

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Posted 01 March 2021 - 06:12 PM

Field rotation is of course centered on the guide star ...

Field rotation due to polar misalignment of a tracking mount is centered on the axis of the optical train -- not a star. That presumes the center of the sensor is the point where the optical axis intersects the plain of the sensor.

 

Field rotation shows up whether or not the mount is being guided.

 

Joe



#7 kathyastro

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Posted 01 March 2021 - 06:16 PM

Field rotation due to polar misalignment of a tracking mount is centered on the axis of the optical train -- not a star. That presumes the center of the sensor is the point where the optical axis intersects the plain of the sensor.

 

Field rotation shows up whether or not the mount is being guided.

 

Joe

Yes and no.  The centre is as you say if unguided.  If guided, the centre becomes the guide star.



#8 TOMDEY

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Posted 01 March 2021 - 06:59 PM

Field rotation due to polar misalignment of a tracking mount is centered on the axis of the optical train -- not a star. That presumes the center of the sensor is the point where the optical axis intersects the plain of the sensor.

 

Field rotation shows up whether or not the mount is being guided.

 

Joe

Wrong, Joe. For the generalized case, both rotation and displacements are involved, which are transformable to pure sinusoidal rotation about a unique point, which is generally outside (but may be inside) the instrumental "field or regard". That's readily solvable, but of no practical concern. The conditionally-specific case of auto guide correction on either a star or equivalent AGC algorithm operating on the field of a ~Guide Scope~ correspond to my equations and are generalized for all such cases, even (especially) when the guide camera and imaging camera are inadvertently or intentionally misaligned to each other. Note that OAGs most always and intentionally use a guide star significantly displaced from the center of the imaged field. Here's an example of mine. Supernova 1991T imaged onto 24x36mm ("35mm") film using the ESOAG and original SBIG ST4 single-star-based AutoGuldeCorrector. Imaging EFL 1905mm, AGC EFL 2000mm, single two hour exposure. At that long EFL and (especially) long single exposure... all polar alignment, bull-dog AGC, agile, backlash-free corrector drives... were critical back then.   Tom

 

~click on~ >>>

Attached Thumbnails

  • 15 Supernova 1991t 35-244_SN Tom Dey jpg.jpg

Edited by TOMDEY, 01 March 2021 - 07:01 PM.


#9 Iamhondo

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Posted 02 March 2021 - 10:51 AM

<insert "white flag waving" graphic>

 

There's already too much thread drift from the OP's question about FOV and polar misalignment. I'm terminating my contribution to more drift.

 

Also, I need to go back to my 100's of severely field-rotated images and explain that they're wrong because they didn't obey the guide-related side thread.

 

Joe




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