Comet alignment module

[Jfakatselis]

Would someone kindly explain the function of the " Subtract image" functionality of the Comet Alignment module?

When should the subtract function be used? Before or after comet alignment and on what image set, comet aligned or star aligned?

I am confused when I read Mr Tombolini's Comet Processing page on the PixInsight Resources site.
Harry's Astro-Shed tutorial does not cover the Subtract function either, hence my question.

Thank you in advance.

Jim Fakatselis

[Alejandro Tombolini]

Hi Jim,
What I did in this processing example was to generate two images, one with the comet and other with the stars. The subtract function was used to generate the stars_only image (useful if you did not take it in a different night).

The steps could be:
First integrate the comet image
Second subtract the comet image from each individual images
Third integrate the stars only image

You can also see this post about Comet Alignment Module

Saludos, Alejandro.

[Jfakatselis]

Hola Alejandro,

Thank you for your response. I am still confused however because I have done the steps as I interpreted them below and don't see much difference when the integrated comet image is subtracted. This is what I interpreted as the steps.

(My clarifying comments are in [] brackets.)

" The steps could be:
   First integrate the comet image
   Second subtract the [integrated] comet image from each individual [comet aligned?] images
   Third integrate the stars only image "

In addition when I read your tutorial on the Lovejoy processing you did, when you do the subtraction, the screen capture for that step shows that you used what I believe are the star registered images (they have the _c_d_r extension). This is why I am confused.

Attached is the screen capture from the tutorial.

Sorry, but it is not clear to me.

Best,
Jim

[Jfakatselis]

Sorry for the strikethrough text marks, it was unintentional. Fat fingers! Please include that text.

Jim

[Jfakatselis]

Alejandro,

I think have been making an error, duh! I believe I had to first register the original individual calibrated images to the stars and THEN perform the comet head registration. I would be then registering a stars centered set of images.

Instead I was using the calibrated and debayered files in the comet module directly without registration, assuming (falsely!) that the comet module would align them properly. That's why I was confused, I think. Sorry about that.

So are these the proper preliminary steps? :

     1.- Calibrate and register the original (comet+background stars) set after calibration, (therefore now have stars registered but comet trailed)
     2.- process this set with the comet module to derive a set of registered images with the comet nucleus aligned
     3.- Integrate the comet aligned images into a single integrated image  (and hopefully stars removed as much as possible with proper rejection)
     4.- Subtract the single integrated comet head aligned image from the individual comet aligned images
     5.- Then I would get to the more difficult part of removing any residual (subtracted) comet pixels from the set

Thanks for your trouble,
Jim

     
]

[Warhen]

Jim, Craig hits it well here in reply #64 at http://pixinsight.com/forum/index.php?topic=3980.60

Jim states-

* Align light frames on the stars
* Use CometAlign on these star aligned light frames
* Integrate the comet aligned frames
* Use this "comet" Integration image and run CometAlign again, but this time using the subtract option which will remove the comet from each star aligned light frame
* Now integrate these comet subtracted frames to get a star only Integration
* Finally recombine the comet only and star only integrations