Image Integration When Field Rotation Is Present?

[jv]

Hi,

I wish to align 36 sub-frames where field rotation is present over the imaging interval.  How do I proceed using PixInsight 1.8?

Thanks,
--jv

[jv]

Does the lack of response indicate this is not possible in PixInsight 1.8 ?

[oldwexi]

No!

[mmirot]

You should be able to fix rotation with Star Registration before integration.

[jv]

Could you elaborate on how to use Star Registration in this case?  It produced a very bad end result in Image Integration; off-axis stars were smudged arcs.

I had no such issues using Nebulosity 3 to align and stack theses images.

I have previously used PI successfully to process images, but this is the first time I have tried to use it on sub-frames exhibiting obvious field rotation.

[Geoff]

If I understand you correctly your subs have rotation arcs, so the stars are not round.  In that case there is no way that stacking them will give round stars.  The only remedy left is to correct your polar alignment problem and retake the pics.

If, on the other hand, the stars are round in the subs, then stacking should not be a problem.

An example file or two would be helpful in diagnosing the problem.

[jv]

Thanks for your response Geoff.

Sorry for the confusion.  The imaging session comprised 36, 3-minute sub-frames over approximately two hours.  In each 3-minute sub-frame, the stars are relatively round.  However, during the 90-minute session, field rotation became apparent.  Post capture, the processing workflow was:

• Script | Batch Processing | BatchDebayer
  
• Process | ImageRegistration | StarAlignment
  
• Process | ImageIntegration | ImageIntegration
  

At the completion of step (3), the stars were smudgy arcs despite being relatively round in individual sub-frames.  Arc lengths increased with distance from the center.

I did not see a PI repository to receive uploaded images; could you tell me where to look?  Each raw sub-frame is ~12mb.

[Alejandro Tombolini]

Hi jv, welcome to the forum.

For your description I assume that you did not calibrate the images (using bias, darks and flat). If that is the case, it is possible that your images have hot pixels and StarAlignment detects those pixels as stars and your images are aligned with the hot pixels.
If your stars are arcs after image integration seams to indicate that the images were not previously aligned. You can use Blink tool to inspect the aligned images.

See this article about hoy to share image in PixInsight Forum.

Hope it helps.

Saludos, Alejandro.

[jv]

Thanks for your reply, Alejandro.

Hi jv, welcome to the forum.

Thank-you.

For your description I assume that you did not calibrate the images (using bias, darks and flat).

That is correct and intentional.

If that is the case, it is possible that your images have hot pixels and StarAlignment detects those pixels as stars and your images are aligned with the hot pixels.

Hot pixels are present in the images, but that was not an issue in previous image processing sessions.  I typically enable dithering between successive image captures, and PI has done a great job of detecting/ignoring the defective pixels.

If your stars are arcs after image integration seams to indicate that the images were not previously aligned. You can use Blink tool to inspect the aligned images.

Good suggestion.  This should reveal whether dithering was being applied between successive image captures.

See this article about hoy to share image in PixInsight Forum.

Great!

Hope it helps.

Very helpful. 

At this point, from the responses, it is clear PI does account for field rotation in the registration process.  Therefore, I will reexamine my images.  If I can not find the mistake I'm making, I will followup and provide some sample images.

[pfile]

Hot pixels are present in the images, but that was not an issue in previous image processing sessions.  I typically enable dithering between successive image captures, and PI has done a great job of detecting/ignoring the defective pixels.

outlier rejection is something that happens during ImageIntegration... if StarAlignment was thrown off by hot pixels then the results of II will probably be garbage (as you have seen).

StarAlignment has a mode called "Detected Stars" that will produce an image showing where it thinks all the stars in your image are. you can run this on one of your subs (open the sub as an image on your workspace, then switch the mode and apply SA to the image). if you then zoom into the output image it should be clear if it's detected a bunch of hot pixels instead of stars. look under the "star detection" tab in StarAlignment and you can fiddle with the hot pixel settings and see if the Detected Stars map improves. if so then you should be able to register all your images with those settings.

rob

[jv]

 

Hot pixels are present in the images, but that was not an issue in previous image processing sessions.  I typically enable dithering between successive image captures, and PI has done a great job of detecting/ignoring the defective pixels.

outlier rejection is something that happens during ImageIntegration... if StarAlignment was thrown off by hot pixels then the results of II will probably be garbage (as you have seen).

StarAlignment has a mode called "Detected Stars" that will produce an image showing where it thinks all the stars in your image are. you can run this on one of your subs (open the sub as an image on your workspace, then switch the mode and apply SA to the image). if you then zoom into the output image it should be clear if it's detected a bunch of hot pixels instead of stars. look under the "star detection" tab in StarAlignment and you can fiddle with the hot pixel settings and see if the Detected Stars map improves. if so then you should be able to register all your images with those settings.

rob

Thanks for your response Rob.

The information on the Detected Stars and Star Detection features, of the StarAlignment tool, were very helpful.  I need to spend more time learning PI capabilities!

I followed your suggestions and, after some experimentation, got a great integrated image!  StarAlignment had, indeed, locked onto the defective pixels.  I'm using a new imaging train (except for the detector) and defective pixels are now apparently more of an issue.

 
--jim

[pfile]

that's good news. i guess if you calibrate the subs things will get a little easier. and if there are still hot pixels hanging around after that (which either mess up SA or are difficult to reject in II) you can also try out CosmeticCorrection to clean up the hot pixels in your calibrated subs.

rob