Field Rotation and Fork Mounts

Discussion of using SharpCap for Deep Sky Imaging
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Calamity_Jane
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Field Rotation and Fork Mounts

#1

Post by Calamity_Jane »

Hi:

I have taken on the job of observatory manager for Cardiff Astro Soc. They have quite a nice Celestron 11" Edge HD, but it is mounted on a fork mount in an observatory building with a rather low roll-off roof. There may not be room to re-mount it onto an equatorial mount.

My question is: if we were to attempt deep sky photography, could SharpCap deal with the considerable field rotation?

Thank you very much.

Regards,

Jane.
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admin
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Re: Field Rotation and Fork Mounts

#2

Post by admin »

Hi Jane,

yes, live stacking will cope with rotation no problem - the limit will probably come when the loss of image data to the corners of the image (due to the rotation meaning they are no longer receiving new data from each frame) becomes unacceptable. The center of the image will be fine, so you can always crop to that afterwards.

Steer clear of observing near the zenith (where the rotation rates from an alt/az mount are the worst) and you should be able to stack for a reasonable amount of time (at least an hour or two). You might also consider that a rotator device (ie https://pegasusastro.com/products/falcon-rotator/) will help - while this will not do a 'counterrotation to keep the image steady', it will allow you to easily wind back to the correct orientation every so often. Currently this would involve manually pausing stacking, rotating back then resuming stacking.

cheers,

Robin
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Calamity_Jane
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Re: Field Rotation and Fork Mounts

#3

Post by Calamity_Jane »

Thanks, Robin, very useful. :)
MunichAtNight
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Re: Field Rotation and Fork Mounts

#4

Post by MunichAtNight »

Hi Robin
admin wrote: Tue May 10, 2022 1:51 pm.... You might also consider that a rotator device (ie https://pegasusastro.com/products/falcon-rotator/) will help - while this will not do a 'counterrotation to keep the image steady', it will allow you to easily wind back to the correct orientation every so often. Currently this would involve manually pausing stacking, rotating back then resuming stacking ...
I was looking for help on something different ( Masterflat ) and found your hint for falcon-rotator. I thought to buy this, but finally I skipped the idea because I was afraid with the rotator I won't be able to use any longer any flats. I thought with flats I am doing correction for fixed to the camera effects as garbage on camera window and correction which are influenced by rotation of the camera in the OAZ, too? Therefore when I was rotating the camera in the OAZ to get a better and more convenient FOV I did a new flat. Am I wrong? :? I spoiled my time? :roll:

I am doing EAA with a SW AZ-EQ5 in AZ-mode so my target isn't to aim for stacking time in many hours, but as you mentioned depending as closer as the observing position is to the zenith as bigger is the effect with dark corners.

My idea of using the falcon-rotator would be that it will "derotate" automatically during the complete session. It is mentioned on the pegasusastro page ...
falcon-01.jpg
falcon-01.jpg (45.02 KiB) Viewed 2383 times
I hoped but there wasn't a clear approval for this, and it might be somehow naive thinking that the falcon-software will be able to get by ASCOM the position and will be able to calculate and control rotation derotation speed according to the position at the sky. :P

I am willing to buy and test this piece of hardware and it should be not a problem to get necessary help and support from pegasus-astro for there unit. ASCOM driver / Software is available with ASCOM Support 6.5

But I would be very interested what you think and know? I guess out from your answer ...
admin wrote: Tue May 10, 2022 1:51 pm.... it will allow you to easily wind back to the correct orientation every so often. Currently this would involve manually pausing stacking, rotating back then resuming stacking ...
... you understand and perhaps based on your personally knowing this unit that it will be necessary to de-rotate it manually. In case it won't be in the end automatically corrected by "Falcon" software during the stacking/observing process, a manually job would help any way to reduce the "dark-edge-effect".

Nevertheless when using such a manual procedure:
  • stop-stacking
  • rotate back
  • start-stacking
finding the parameters, which are on top changing during the stacking process for
  • how long to wait until stopping stacking
  • how far to rotate back each time
I assume it will be a real challenge to find the somehow right values.

Perhaps there is already help for this, finding right values with sharpcap, I don't know in the moment? I learn every night something spectacular new with sharpcap! :)

Will it be now already ( or in future with a new sharpcap version ) when the rotator unit will be installed by hardware (USB?) and by software (ASCOM driver?) that a control section / window for the falcon rotator will show up inside Sharpcap Camera Control Panel ? As it is now for example with a ZWO EAF focuser?

Kind regards - Ewald
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Re: Field Rotation and Fork Mounts

#5

Post by admin »

Hi,

I was actually unaware that the Falcon had the ability to do live counterrotation - I guess it must connect to the mount ASCOM driver to get the current pointing co-ordinates, from which it can then calculate the rate at which to rotate. In that mode SharpCap doesn't even need to know that the falcon is actively de-rotating, it should just work.

The other option that you have is to use the SharpCap sequencer (full sequence editor). A few weeks ago I added sequencer steps (found in the rotator section) that allow the current orientation to be saved (using plate solving), then will allow SharpCap to return to that orientation later by a second plate solve operation followed by commanding the rotator ASCOM driver to adjust for any changes. There's a thread on here somewhere about it, but I cannot find it at the moment :(

cheers,

Robin
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Re: Field Rotation and Fork Mounts

#6

Post by MunichAtNight »

Hi Robin!
admin wrote: Sun May 22, 2022 1:40 pm I was actually unaware that the Falcon had the ability to do live counterrotation -.....
The other option that you have is to use the SharpCap sequencer (full sequence editor). ...
Both options sounds very interesting, I think I will get this device and will find it out what may working. Thanks for your opinion. I will report.

What do you think about my question. May I use or each "new" derotated situation all the time the same flat? So flats are not influneced by rotating the camera?

Thanks for your support - MunichAtNight
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Re: Field Rotation and Fork Mounts

#7

Post by admin »

Hi,

I think that strictly speaking you should take new flats each time the rotator moves - this is because one of the things you are trying to correct for with flats is the vignetting towards the corners of the sensor. If the sensor is possibly slightly off-center then the rotation can change the vignetting. Of course, if you are simply using flats to correct for dust shadows on the filter/sensor window then those should stay in a fixed orientation when the rotator moves (providing the filter is on the camera side of the rotator), in which case you might get away with using the same flats.

cheers,

Robin
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Re: Field Rotation and Fork Mounts

#8

Post by MunichAtNight »

Hi Robin!
I jump in at the deep end and got the falcon rotator. In the moment I wait for thread adapters to be 3D printed to screw all together. But I did a indoor testing and it is true, that the falcon software gets coordinates by the ascom driver. It shows in the windows software the actual rate it is doing. And when changing the mount position this value changes accordingly. The values I have read looks sense-full to the coordinates of the mount.

But it has to be proofed with the mount under the sky during night.

I guess there is no need to pause stacking during live derotation. But it has to be tested. I agree your thought for the flats. I will see! I am very excited! I will report!

Kind regards Ewald
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Re: Field Rotation and Fork Mounts

#9

Post by MunichAtNight »

Hi Robin!

I got the falcon rotator to work! ;)
falcon-skywatcher-01.jpg
falcon-skywatcher-01.jpg (952.1 KiB) Viewed 2065 times
I 3D-printed the necessary adapter and included a Baader coma corrector filter drawer as well.

ASCOM based it was easy to integrate it to Sharpcap! Thanks to your engineering genius! And now when activating the camera inside sharpcap the falcon software starts as well autoamtically.

sharcap-rotator-05.jpg
sharcap-rotator-05.jpg (200.23 KiB) Viewed 2065 times


Inside the Falcon software I had to activated the De-rotate Field. By the way I couldn't find an option to activate this mode inside Sharpcap!
sharcap-rotator-06.jpg
sharcap-rotator-06.jpg (191.23 KiB) Viewed 2065 times
My first light I did with M13 -
  • Skywatcher 200PDS
  • ASI294MC (Uncooled)
  • 724 frames, 4 s: 2912 s
  • Azimut/Height ~230° | ~ 74°
  • Mount SW AZ-EQ5 at AltAZ-mode
M13_ALL-1500_Stack_724frames_2896s_WithDisplayStretch.jpg
M13_ALL-1500_Stack_724frames_2896s_WithDisplayStretch.jpg (644.6 KiB) Viewed 2065 times
Interesting facts, it seems to de-rotate very well but it looks like that the mount is to fast or to slow :? when tracking. During stacking there was already visible a growing, black border at the below and right edge.

I will check next CS night! I guess it may end that I need guiding! :shock: As first idea I will try to manually modify the tracking rate value in the hand controller of the mount.

Do you have any idea what might cause this borders? Do you think I am right with to slow/fast tracking?

Servus! MunichAtNight - Ewald
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Re: Field Rotation and Fork Mounts

#10

Post by admin »

Hi,

thanks for the report - glad to hear that things are now working for you :)

You are completely correct that the darkened borders of the stacked image are down to image drift - SharpCap is correcting for this in software, but the areas to the bottom and right have drifted out of view, so they are not receiving any new data and gradually become darker than the rest of the image. I can see from your screenshots that you are using a Celestron mount, presumably in AltAz mode to require the derotation. Errors in the initial alignment of the mount could lead to the tracking problem (for instance if the base is not level).

Unfortunately, ASCOM does not provide a way for SharpCap to send the message to turn on/off the live derotation function to the Falcon driver, so that must be done in the Falcon window.

cheers,

Robin
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