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Just an observation on field rotation

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#1 GSBass

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Posted 19 July 2021 - 11:44 AM

So after doing several shots and not really understanding the math I’ve had very good luck keeping stars round with my 102 f7 with 2.9 pixel cameras using sharpcap recommended values… example is when I imaged the Eagle nebula the other night I was using 11.5 second exposures and was able to go three hours….. in comparison if I went 15.1 seconds on my 180mak I could see trailing almost immediately. So anyway just taking notes of when I see stars out of round, how close to zenith or horizon I am and I think I can become comfortable knowing whether the sharpcap recommendations will work. When I first started I think I was attributing any issues with just tracking errors which can occur if the Az pro is not leveled properly but am starting to understand the effect of field rotation. Sharpcap does well doing the actual turning of the image overtime but that is an independent thing from the actual effect you get from actual sub exposure time. Please chime in if you have thoughts on this…. Right now I don’t know exactly how long the subs can go with the f7 or which parts of the sky are most forgiving but it’s obvious if I can get my subs right then sharpcap will do the rest for as long as I want to go


Edited by GSBass, 19 July 2021 - 11:47 AM.


#2 Alien Observatory

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Posted 19 July 2021 - 12:15 PM

This link will take you to an in-depth discussion on Field Rotation...  Pat Utah :)

 

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


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#3 GSBass

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Posted 19 July 2021 - 12:22 PM

Thankyou, reading it now….. I am finding it interesting because I don’t think I understood how software field rotation worked…. It just derotates the subs but the subs themselves still rotate…. Kinda makes me wonder why camera manufactures have not tackled this also…surley software field derotation could be done as the light is recorded?

This link will take you to an in-depth discussion on Field Rotation...  Pat Utah smile.gif

 

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


Edited by GSBass, 19 July 2021 - 12:41 PM.


#4 Alien Observatory

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Posted 19 July 2021 - 12:33 PM

A Field De-Rotator was made by Meade Telescopes many years ago.  A few folks are working on making de-rotator's using 3d printing, stepper motor's and a Raspberry Pi interface.  Pat Utah :)



#5 GSBass

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Posted 19 July 2021 - 01:05 PM

That will be cool… just thinking you would not need one at all if cameras could do this real time, seems like a arm chip and a memory module could handle anything a minute or two long and that’s about the most you need anyway for eaa

A Field De-Rotator was made by Meade Telescopes many years ago.  A few folks are working on making de-rotator's using 3d printing, stepper motor's and a Raspberry Pi interface.  Pat Utah smile.gif


Edited by GSBass, 19 July 2021 - 01:05 PM.


#6 photoracer18

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Posted 19 July 2021 - 01:05 PM

Camera can only do de-rotation if the sensor has IBIS, so that deletes astro cameras and leaves only those DSLR/MILC bodies that have IBIS and 5-axis shift sensors. That pretty much eliminates everything except the Pentax cameras with either the built in O-GPS or that can take the external O-GPS-1 module. Otherwise its easier just to let the stacking software align the subs. The amount of de-rotation needed changes depending on where you are pointing relative to the pole and the celestial equator. Only reason the Pentax cameras can do longer exposure star-scapes is because it takes a GPS reading before you stick it on a fixed tripod and it can tell where its pointed so it knows how much field de-rotation to dial in using sensor shift. However  doesn't work when you point it thru a telescope because the small amount of actual sensor movement limits the length of the subs based on magnification. You can make it work for camera lenses up to about 300mm but at that amount of magnification the subs have to be pretty short. Too bad we can't program it ourselves.


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#7 GSBass

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Posted 19 July 2021 - 01:26 PM

I think I was thinking the camera would do a reference image and then just compare and align subsequent pixels…. I mean the iPhone records photos before you even touch the shutter so the data would be there to figure out any movement…… I’m not saying this would be easy to implement…. Just saying i don’t see any technical hurdles because once you have those first milliseconds of data then it’s just a mapping thing to know where those brightest pixels should be falling

Camera can only do de-rotation if the sensor has IBIS, so that deletes astro cameras and leaves only those DSLR/MILC bodies that have IBIS and 5-axis shift sensors. That pretty much eliminates everything except the Pentax cameras with either the built in O-GPS or that can take the external O-GPS-1 module. Otherwise its easier just to let the stacking software align the subs. The amount of de-rotation needed changes depending on where you are pointing relative to the pole and the celestial equator. Only reason the Pentax cameras can do longer exposure star-scapes is because it takes a GPS reading before you stick it on a fixed tripod and it can tell where its pointed so it knows how much field de-rotation to dial in using sensor shift. However  doesn't work when you point it thru a telescope because the small amount of actual sensor movement limits the length of the subs based on magnification. You can make it work for camera lenses up to about 300mm but at that amount of magnification the subs have to be pretty short. Too bad we can't program it ourselves.



#8 photoracer18

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Posted 19 July 2021 - 01:56 PM

Well a camera with Pixel-Shift imaging maybe. That right now only includes Pentax and Olympus in regular cameras. Some medium format maybe. What we would need is an AP hacking program. Speculation at this stage.


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#9 GSBass

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Posted 19 July 2021 - 03:32 PM

Might be developed by some of the robotic peeps since they have all embraced Az so far… they might be doing it already except I think they are limiting exposures to 10 second subs which seems to work fine as long as they stick to fast scopes. Actually it’s long enough for most stuff as long as your overall exposure is long enough

Well a camera with Pixel-Shift imaging maybe. That right now only includes Pentax and Olympus in regular cameras. Some medium format maybe. What we would need is an AP hacking program. Speculation at this stage.



#10 Clouzot

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Posted 20 July 2021 - 01:59 AM

A Field De-Rotator was made by Meade Telescopes many years ago.  A few folks are working on making de-rotator's using 3d printing, stepper motor's and a Raspberry Pi interface.  Pat Utah [smile.gif]

 

There's also this: https://www.primaluc...de-rotator.html

I don't know if and how it can manage the derotation in alt-az, but it says it uses 1-arcsec steps



#11 alphatripleplus

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Posted 20 July 2021 - 08:01 AM

Field rotators can cost almost as much or more than an Alt-Az mount, so they are not inexpensive items. Here's one for around $600.



#12 Alien Observatory

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Posted 20 July 2021 - 11:13 AM

Field rotators can cost almost as much or more than an Alt-Az mount, so they are not inexpensive items. Here's one for around $600.

I do not believe this would be of any use as a Field De-Rotator.   The minimum Motor Step Angle is 0.0625 degrees per step (about 225 arc seconds).  The typical de-rotation speed at the Celestial Equator is about 5 arc seconds per second.    Pat Utah smile.gif


Edited by Alien Observatory, 20 July 2021 - 11:14 AM.

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#13 GSBass

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Posted 20 July 2021 - 11:13 AM

True dat, if it’s going to be a hardware solution then the price needs to drop…. I really do believe though camera manufactures doing a pixel alignment routine during capture along with the field rotation already being done by sharpcap ,you could make Az capture indistinguishable from a polar aligned scope. Some may ask why not just buy a gem …… the honest answer is just because Az mounts are more pleasurable to use

Field rotators can cost almost as much or more than an Alt-Az mount, so they are not inexpensive items. Here's one for around $600.


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#14 alphatripleplus

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Posted 20 July 2021 - 12:50 PM

I do not believe this would be of any use as a Field De-Rotator.   The minimum Motor Step Angle is 0.0625 degrees per step (about 225 arc seconds).  The typical de-rotation speed at the Celestial Equator is about 5 arc seconds per second.    Pat Utah smile.gif

I didn't check the specs on the step size, but you are right that a field rotator needs to be able to operate at arcsecs/ second during tracking. The Pegasus model may just be for  electronic orientation of a camera or filter rather than continuous field rotation during tracking.



#15 roelb

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Posted 20 July 2021 - 06:00 PM

SharpCap do it much cheaper...


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#16 GSBass

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Posted 20 July 2021 - 06:25 PM

Sharpcap does fine as long as your subs are short enough

SharpCap do it much cheaper...


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#17 steveincolo

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Posted 03 September 2021 - 06:33 PM

Based on the formulas in that RASC article (thanks Alien Observatory for the link!), it seems like you could do longer exposures with an AltAz mount in a couple situations.  First, when the target is due (or near due) east or west.  Second, if you didn't care about the stars at the perimeter trailing and just wanted to view a small object in the center, such as most of the planetary nebulas in last month's challenge.  According to the formulas, the effects of field rotation grow linearly from the center of rotation, so they will be small near the center.  But the article by Dr. Glover about exposure makes me question the value of longer exposures anyway, especially with a low noise camera in light-polluted skies.  


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#18 Alien Observatory

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Posted 03 September 2021 - 08:41 PM

Much to my surprise I discovered my Stellina (80 mm F5) has a built in Field De-Rotator.  This is the first time (ever) I can gather +30 minutes of frames and show no Field Rotation in the stacked image.  If I knew this many years ago I could have saved myself from Wedges, EQ mounts and a lot of trouble with no view of Polaris.  I truly like this option.  Pat Utah :)


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#19 steveincolo

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Posted 04 September 2021 - 09:48 AM

Much to my surprise I discovered my Stellina (80 mm F5) has a built in Field De-Rotator.  This is the first time (ever) I can gather +30 minutes of frames and show no Field Rotation in the stacked image.  If I knew this many years ago I could have saved myself from Wedges, EQ mounts and a lot of trouble with no view of Polaris.  I truly like this option.  Pat Utah smile.gif

Such a nice feature!  Much easier and cheaper, I guess, to include in a purpose-built device like the Stellina as compared to retrofitted onto typical astro equipment.


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#20 GSBass

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Posted 05 September 2021 - 12:14 PM

Sadly my Vespera will not have a field derotator , it will be software like sharpcap does, but on the bright side as long as the object is framed correctly then you don’t lose too much after it crops out, it’s all about how much space you have around your target, I have not suggested it yet but I think if enough people were trying to image with Az mounts, Robin might incorporate the brain function to adjust exposures based on where in the sky the object is…. Simply doing what we are guessing at… ie shorten exposure with larger gain…. All you would need to do is tell sharpcap what kind of mount you were using and it would calculate max exposure and adjust gain to compensate. I’m not sure how Vaonis does this since  they use 10 seconds for just about everything and that can be too long in some parts of the sky… they must be compensating with some algorithm 

Such a nice feature!  Much easier and cheaper, I guess, to include in a purpose-built device like the Stellina as compared to retrofitted onto typical astro equipment.


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#21 steveincolo

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Posted 05 September 2021 - 12:58 PM

Sadly my Vespera will not have a field derotator , it will be software like sharpcap does, but on the bright side as long as the object is framed correctly then you don’t lose too much after it crops out, it’s all about how much space you have around your target, I have not suggested it yet but I think if enough people were trying to image with Az mounts, Robin might incorporate the brain function to adjust exposures based on where in the sky the object is…. Simply doing what we are guessing at… ie shorten exposure with larger gain…. All you would need to do is tell sharpcap what kind of mount you were using and it would calculate max exposure and adjust gain to compensate. I’m not sure how Vaonis does this since  they use 10 seconds for just about everything and that can be too long in some parts of the sky… they must be compensating with some algorithm 

The Vespera does have a larger FOV than the Stellina, so cropping isn't such a big deal.  And it's more compact (and quite a bit easier on the wallet).  The Vaonis folks seem to have their heads on straight, design-wise, so I'm sure they had good reasons for leaving the derotator out.  I wonder if the target is poorly placed, near the zenith, maybe the Vespera app will warn you.  

 

I was thinking along the same lines as you, after reading the materials about the Brain and Smart Histogram.  The view I had of M33 last night was close to due east, so there was little field rotation and cropping even after 25 ten-second frames.  GazingOli did 16-second exposures of M33 with very nice results, so it may have been placed similarly.  So it would be nice for the Brain to figure out maximum exposure based on location.

 

On the other hand, Dr. Glover's analysis shows that in moderately light-polluted skies, with a low-read noise sensor, eight seconds is an optimal sub exposure, which will work for most of the sky.  


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#22 GSBass

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Posted 05 September 2021 - 01:03 PM

I just put the idea in his feature suggestion forum, honestly don’t expect it though because I imagine it might be a good bit of work to incorporate it. But I am happy with the results I am getting with my Az mount overall

The Vespera does have a larger FOV than the Stellina, so cropping isn't such a big deal.  And it's more compact (and quite a bit easier on the wallet).  The Vaonis folks seem to have their heads on straight, design-wise, so I'm sure they had good reasons for leaving the derotator out.  I wonder if the target is poorly placed, near the zenith, maybe the Vespera app will warn you.  

 

I was thinking along the same lines as you, after reading the materials about the Brain and Smart Histogram.  The view I had of M33 last night was close to due east, so there was little field rotation and cropping even after 25 ten-second frames.  GazingOli did 16-second exposures of M33 with very nice results, so it may have been placed similarly.  So it would be nice for the Brain to figure out maximum exposure based on location.

 

On the other hand, Dr. Glover's analysis shows that in moderately light-polluted skies, with a low-read noise sensor, eight seconds is an optimal sub exposure, which will work for most of the sky.  


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#23 GSBass

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Posted 05 September 2021 - 01:17 PM

Steve, in general I think people who disparage using a Az mount for dso simply have not tried it lately…. Technology marches on and sharpcap has made that door wide open. I never claim it’s as good as being polar aligned but I’m not trying to win any contest either…. For my level of enjoyment and expectations I am taking some very pretty shots, 

 

The Vespera does have a larger FOV than the Stellina, so cropping isn't such a big deal.  And it's more compact (and quite a bit easier on the wallet).  The Vaonis folks seem to have their heads on straight, design-wise, so I'm sure they had good reasons for leaving the derotator out.  I wonder if the target is poorly placed, near the zenith, maybe the Vespera app will warn you.  

 

I was thinking along the same lines as you, after reading the materials about the Brain and Smart Histogram.  The view I had of M33 last night was close to due east, so there was little field rotation and cropping even after 25 ten-second frames.  GazingOli did 16-second exposures of M33 with very nice results, so it may have been placed similarly.  So it would be nice for the Brain to figure out maximum exposure based on location.

 

On the other hand, Dr. Glover's analysis shows that in moderately light-polluted skies, with a low-read noise sensor, eight seconds is an optimal sub exposure, which will work for most of the sky.  

 


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#24 steveincolo

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Posted 05 September 2021 - 01:38 PM

Steve, in general I think people who disparage using a Az mount for dso simply have not tried it lately…. Technology marches on and sharpcap has made that door wide open. I never claim it’s as good as being polar aligned but I’m not trying to win any contest either…. For my level of enjoyment and expectations I am taking some very pretty shots.

Very, very nice!  54 minute exposure, how much of the frame did you lose to field rotation?  I'm going to have to try this on our deck, can just leave the setup running for an hour.  



#25 Clouzot

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Posted 05 September 2021 - 01:50 PM

Nice image GSBass! @steveincolo, you can even guide and dither in alt-az, and that helps a lot getting less of the "windshield wiper" effect one gets with noise trailing in circles. I did some tests with my faithful Celestron Evo or my AZ-GTi in alt-az mode, and the EAA results were quite enjoyable


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