Is a field rotator good enough for planetary?

[Stonius]

Hey all,


I was thinking of getting a biggish dob (18"), and am hoping to use it for visual deep sky as well as some planetary photography.


Is a field rotator going to be enough to keep the 3 minute or so videos in alignment? I figure it's better than trying to derotate the images in post (in addition to derotating the planet itself in Winjupos).


Cheers


Markus

[bojan]

It's not necessary for this kind of work.
De-rotation is automatically performed by Autostakkert.

[Zuts]

Hi,

Bojan is correct. If you search the web you will see that most people use an alt-az mount for planetary, one of the best examples is Damien Peach, unless of course you have an eq mount then that is fine as well.

[Tulloch]

Well - kind of. For short videos the amount of field rotation you get can be handled well by AutoStakkert, but it doesn't automatically perform field derotation per se. There is an option to remove field rotation in AS!3, but it's only enabled for monochrome videos, trying to perform the technique on raw (undebayered) videos destroys the bayer matrix and screws up the colours, and trying it on RGB (debayered) videos will not work at all.
https://www.cloudynights.com/topic/771817-imaging-with-large-dobsonian/?p=11109977

WinJupos can remove both field and disc rotation on longer videos, and since 11.1.6 it also preserves the Bayer matrix (so debayering can be performed in AS!3).

Short answer - if you are imaging just the planetary disc for short times (<3 min for Jupiter, <5 min for Saturn), and are not trying to image the moons of these planets, then field rotation is not an issue. If you try imaging for longer than these times or you want to image the moons, then you need to look into using WinJupos to remove the rotations.

Of course, this is only applicable for planetary work, for DSOs that's a whole other story :).

Andrew

[Stonius]

Thanks for the replies.


I wonder how far you can push things here?


For example, could you capture DSOs using an alt az mount, large aperature, field rotator, wide field of view, and many shorter exposures stacked. Downside is read noise obviously.


I guess it all comes down to the granularity of the drive impulses. It won't be a Single gear turning at sidereal, but two gears turning at variable speeds to try to achieve the same thing.




Markus

[Tulloch]

This is what I did on my altaz scope without a rotator, taking lots of 5 sec frames and stacking them in DSS.
https://www.cloudynights.com/gallery/image/90616-ngc-1566-imaged-1732020/

SharpCap's Live stacking removes shifts and rotation on the fly, it's pretty impressive what it can do.

However, an eq mount is preferred ...

[bojan]

Well, one can learns something new each and every day :-).. yes, you are correct.


As for Markus' question about pushing things to the limits, there are plenty of solutions for tracking (and guiding) with AltAz mount and compensating for field rotation at the same time (which is not constant BTW, maximum is around celestial equator and minimal towards east or west directions, see here (https://www.californiaskys.com/field-rotation.html)).
I am still using Mel Bartels solution (https://www.bbastrodesigns.com/StepperSystem.html) for my dob (http://www.iceinspace.com.au/forum/showthread.php?t=12171&highlight=bartels), but there are other options:
OnStep (https://onstep.groups.io/g/main/wiki/3860), SoundStepper. (http://soundstepper.sourceforge.net/).. etc. I tried those two and they were working OK.


But, yes, EQ mount is preferred. Or EQ platform for dobs.