Got Field Rotation? No Problem! De-Rotation in Planetary Stacker and see how much to expect!

Added: 2026-05-13

[00:00:00]Got field rotation?

[00:00:02]You got no problem.

[00:00:04]Welcome to SharpCap Astro.

[00:00:09]>> [music] >> So, as always, make sure you have the latest version of SharpCap Suite Pro.

[00:00:17]Head over to sharpcapastro.com under Astro Programs SharpCap Suite Pro. We're on version 1.15. You'd use Python 3 12, 13, or 14. There's the download here in the mirror site for the normal installers. There's pip install SharpCap Suite Pro.

[00:00:31]Use Homebrew on the Mac machine, or you can run it directly from the GitHub. For those of you guys running Alt-Az mounts up there where you're doing maybe a shoot, move, shoot with a dob, or just taking longer exposure with an Alt-Az mount, let's just briefly go over what causes field rotation and then look at the new stuff.

[00:00:48]Okay, I simply have Stellarium Web opened up here, and I'm centered on M5.

[00:00:55]And we're in Alt-Az mode here, and if I just move the time forward, we'll see how the sky rotates around M5 if we keep looking at it until it finally starts setting in the west here.

[00:01:10]And we'll roll it back, so you see how it's rotating. So, any Alt-Az mount is going to suffer from this.

[00:01:16]And if we zoom way up in here like you're actually imaging it, as the night progresses, it really just looks like it's rotating around a central point there.

[00:01:26]And that rate of rotation is really what causes all the issues in uh long Alt-Az mounts, as well as if you're doing like a shoot, move, shoot situation with uh like a like a dob.

[00:01:41]So, all these things are going to introduce uh field rotation. So, let's look at Planetary Stacker in SharpCap Suite Pro that can deal with field rotation.

[00:01:51]And then we can also look at a new tool as well to show you how much how much rotation you're going to be getting uh if you want to try doing some longer exposure photography. The first thing I want to do is just run a stack of some lunar shots here.

[00:02:02]Now, these were done on a dob where it was a point, shoot, move, shoot, point, shoot, move, shoot kind of a situation where uh over the course of, you know, a few minutes we're actually getting a decent amount of ro- rotation here. And what I want to do first is just stack it uh the normal way prior to me making any of these de-rotation updates. So, I'm simply going to analyze it, give it a good amount to stack, and click stack now.

[00:02:27]And it's done, and let's check out the result.

[00:02:30]Ooh. Look at that big swirly mess. And that's absolutely because this particular set of exposures was over enough time to cause some significant field rotation.

[00:02:41]We can go ahead and look back at the keepers in here, and I can just show you as we move forward through them. You can see how the moon kind of changes its orientation. Even though it's centered, you know, it's kind of rocking back and forth as we're going through the better quality frames here.

[00:02:55]And that's what's causing that uh motion blur. And we can throw the same set of images even in AutoStakkert, and if we move this uh around, you can see how the moon is kind of rocking back and forth.

[00:03:05]Let's go ahead and just analyze these.

[00:03:07]I'm going to go ahead and stack them.

[00:03:13]Let's go ahead and open up the result.

[00:03:15]And it is different than the initial one, but I hope you guys could see it is quite a bit of tiley breaks, it looks like, all over the place, and large-scale smearing around the edges because of all the rotation. I mean, you can see these large-scale smears.

[00:03:34]And if you give it just even the slightest bit of sharpening here, now you can absolutely see what's going on. It just cannot handle the de-rotation uh very well at all, and we get all sorts of multiple exposures along the limbs and all sorts of tiling breaks and giant smears and and what you would expect.

[00:03:56]So, let's go over the the new addition here and Planetary Stacker. I'm going to go ahead and open those uh lunar frames back up. Going to go ahead and open the stacker. And now I have a spot down here that says field rotation. Let's go ahead and turn that on. You can adjust the max rotation angle, how much it's going to actually look at rotation to see how far out your rotation is. Obviously, if you don't even have any idea what the rotation is, you might want to have a bigger number in here like 20°. I'm just going to click analyze now, and it does the same quality measurements, but now during the SSD refinement, you're going to see something come up here where it has a bunch of angles. Now, it's actually calculating the angle offset of all these frames for us. And I'm going to go ahead and just stack a bunch of these since there weren't many. I'm going to click stack now. And there we go. Let me close this.

[00:04:43]And here's our beautifully, perfectly aligned moon now. All the frames were de-rotated uh just perfectly. And you can go ahead and open up like your multi-scale decomposition.

[00:04:55]I'm just going to crank that first one all the way up.

[00:04:58]Now, you can really see all the details just jump out of the moon. Here's the before and the after.

[00:05:06]No weird edge seams or anything because the whole frame was de-rotated properly.

[00:05:11]The other thing we can look at is the blink the keepers now that we've done the de-rotation, and now when we go through uh these here, you're going to see the the moon doesn't even look like it's moving at all. So, for those of you out there with like a dobsonian or something else where you're like taking an exposure, having to move it again to get another exposure, moving it again, uh now you can fully de-rotate your frames in Planetary Stacker. It also works for surface tracking. It's going to create the center of rotation around whatever surface anchor you put in there. Uh so, just an excellent tool now to use for de-rotation of the field itself in Planetary Stacker. The next tool I want to show you is actually an addition to What's in My Sky. So, click What's in My Sky, and uh I'm just going to go ahead and calculate for all this stuff here.

[00:05:58]So, we have all our targets here. I have a whole 'nother video for What's in My Sky. But now, if you choose any of these targets and right click it, you'll see an option for field rotation, and there's also a button up here for Alt-Az field rotation. So, this is going to be if you're shooting on an Alt-Az mount, again, like a dob, or even something like a Seestar or something that's in Alt-Az mode, other other telescopes like the Origin if there's no wedge.

[00:06:26]There There's a bunch of telescopes that have Alt-Az, right? So, if you click field rotation now, it's going to ask you some things. Uh put in your focal length, uh your your pixel pitch, your sensor width, your sensor height, and how much movement in the corners you think is acceptable. If you want it super tight, you could leave it on one pixel. That's one pixel of motion in the corners of your sensor. A more moderate amount is just five pixels of motion in the very corners.

[00:06:56]And uh click calculate. So, what this is going to show you is the longest exposure length you can get prior to getting those distortions. In this case, we said five pixels of distortion in the corners. Remember, as the object moves across the sky, it's rotating.

[00:07:13]So, how long can you expose before that amount of rotation blurs the corners, in this case by five pixels? So, for NGC 2227, it's saying it's actually pretty good for this night. The median longest exposure is 22 seconds.

[00:07:27]And for an Alt-Az mount, uh it says everything's good between 20 and 60-second exposures.

[00:07:33]The best exposure length you can get out of it is 48.8 seconds.

[00:07:38]The worst exposure where it's going to be highest in the sky is only 16.6 seconds. And here's some medians that are other tolerances. Like if you want to say 10 sec or 10 pixels of blurring is okay for you, you may be able to go all the way up to 44 seconds. The other really cool thing about this is if you click exposure over the night, you can see what the max exposure is going to be throughout the night. So, uh you can see it starts out with max exposure of about 30 seconds. It dips down just below 20, right? And then it comes back up. Obviously, once it goes below the horizon, you can't help that anymore. Let's go ahead and just try another object here. This one's saying it's really good. Lot of exposure to be had here.

[00:08:23]If you look at the exposure over the night, so this is one that's starting out essentially due east, where you almost get no field rotation, and it just caps off at 120 seconds of exposure.

[00:08:35]And then it dips as it approaches the zenith. Remember, right at the zenith, you're actually getting uh infinite rotation rate. So, as you're progressing through the night, that's going to quickly drop off.

[00:08:48]Once it's overhead, you almost can't even image it, and then it's going to come back. But for a big portion of this night, you're going to get well over, you know, 20, 30, 40, 50 seconds of exposure that you can do on this target prior to getting uh the rotation of five pixels in the corner.

[00:09:07]So, this is just going to be an excellent tool for any of you guys out there with an Alt-Az mount to show you exactly how long the exposures can be uh prior to getting rotation distortion. I hope you guys get a lot of uh use out of these tools, both the Planetary Stacker and uh the new Alt-Az field rotation calculator in What's in My Sky.

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