Comet stacking in DSS

[Dawson]

Deep Sky Stacker is not able to stack my comet data. It keeps smearing the comet, whatever option I select. I’ve checked carefully that the comet is selected in each light frame.

Has anyone else encountered this and found a solution? I do wonder if a small star very close to the comet is causing an issue.

James

[Peter Carson]

Hi James,
I’ve sometimes had issues with DSS that I’ve never understood…it’s not just you!
It could well be that the comet centroid is being biased by the nearby field star. DSS is not very clever and needs a well-defined comet movement between each frame to distinguish it from the background stars. Try stacking the first, last and middle frames of the series and see if that makes any difference.
Good Luck
Peter

[Dawson]

Thanks Peter. I think I may have discovered the issue. I hadn’t appreciated it was possible to use roller ball on my mouse to zoom right into the comet to get a better placement of the purple ring, and also I hadn’t been pressing the save icon on the right, under the comet icon. I’m still not sure if I am meant to press the save icon on EVERY sub, or just at the end of marking the comet on all the subs. Anyway, the first stack I’ve done since implementing these changes seems to be OK, now I’m trying to keep the stars and comet static so it taking my poor laptop even longer!

I will report back with results.

Thanks for replying.

James

[Alex Pratt]

Hi James,

As Peter commented, your blurry stacking might be due to a nearby star. I had no problems using the comet stacking workflow in the DSS User Guide for my C/2020 F3 (NEOWISE) series of exposures

https://britastro.org/observations/observation.php?id=20200716_224500_72d8be81be01185c

I zoomed in to each image and click / registered the bright centre of the coma.

The sky motion of the comet will also be a factor, which is more evident in narrow field images rather than wide-angle views. The exposure duration and number of images in the stack can exaggerate the trailing. For my recent images of C/2022 E3 (ZTF) I took 20 x 4s exposures through my C11 and just used the default ‘star field stacking’ in DSS.

Alex.

[Eric Watkins]

Would not Astrometrica be ideal for stacking for asteroids or comets. When the MPCOrb file updated one can select the objects motion. The software is relatively affordable and there is a 100 day free trial. It’s a piece of software that should be in ones tool box in any event. The only reason that springs to mind that may be the reason for non use is the input file extensions (.fit) if DSLR type cameras are being used.

[Dr Paul Leyland]

In my experience, it is much easier to convert a DSLR image to FITS than it is to persuade Astrometrica to pay any attention to an embedded WCS.

For JPG and PNG files, nova.astrometry.net will convert to FITS and plate-solve for free. There are any number of free apps to convert from RAW format to something more amenable, including FITS. A very quick search on “RAW to FITS converter” dug up several on the first page.

The major problem with Astrometrica, in my experience, is that it runs natively only under Windoze and barely runs under WINE. I don’t trust the photometry very much either. If the source code were available we could check and perhaps enhance, but it isn’t and we can’t.

Other than that, parts of it are excellent and it is likely what I would use, had I not written a script to track on an object moving from frame to frame.

[Eric Watkins]

Paul, thanks for your experiences with Astrometrica. I wasn’t aware that it did not run under WINE and the info re converting raw to fits is useful for me.
Thanks,

Eric

[Dr Paul Leyland]

My phrase was “barely runs”. I did manage to get it to run eventually, but it seems to be much less stable and reliable for me than is reported by those who run it natively. You may have more success.

Paul

[Dawson]

I’ve resolved the streaking comet. Now I have streaking stars when they are not meant to be. An overly stretched image shows the subtle streaking. I’ve posted this question on SGL too, but I do wonder if this is somehow related to my flats or darks, or maybe because I had the ISO too high (3200 for Canon 6D). More work in progress. It is just a case of finding the time to sit down and change the myriad of variables, stack and see if things have improved. I am SLOWLY getting there.

https://stargazerslounge.com/topic/407703-streaky-stars-in-dss-comet-stack/

James

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