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My first ever comet! Captured 12th Feb 2023 around 22:00 before the moon had risen.


Equipment: Celestron AVX mount, Vixen 81SD, 0.67x reducer/flattener, Altair 183C.
Capture: SharpCap 4.0, 60x60s lights, 25 each bias/flats/darks. Camera gain=200, black_level=50.
Processing: PixInsight using Comet Alignment module.

The combined optics have a focal length of 418mm, which gave me a good field of view.

The comet's co-ordinates were obtained from https://theskylive.com/comets after configuring my location. The comet's RA and DEC were input into the mount handset and I had a lucky first time hit (using the Display Histogram Stretch).

The processing workflow was:

• calibrate, debayer & register the lights

• create a comet master (no stars)

• create a star master (no comet)

• merge the 2 masters using comet_master+star_master in PixelMath

• dynamic background extraction

• minor histogram adjustments

There are many possible adjustments to be made in the Comet Alignment. I can probably improve on this result if I spent time trying various settings (although it may be a case of diminishing returns).



Dave

Hi Dave,

well done on getting the image to come out so well! One of these days I must really put some time into learning how to use PI properly, rather than just dabbling, since I'd have no idea how to do comet alignment in it

cheers,

Robin

Thanks Robin.

The book Inside PixInsight by Warren A Keller page 319 'Comet Techniques' has 6 pages describing how to use the Comet Alignment module. This is what I used to process my data. Basically it is about clipping high pixel values. Of course there is a slider to make this easy to trial. The comet is protected (or removed) during the 2 runs of the module. The book represents the best 20 quid I ever spent. Of course no-one sees the many hours spent getting to grips with the processing software.

Dave

Hi Dave,

thanks for the recommendation - just ordered a copy. In this hobby, £20 is about as cheap as it gets... I have 'Lessons from the Masters' already (same series), which is heavy going, but has good stuff in it.

cheers,

Robin

Robin

I have Lessons from the Masters. I found it a good read with maximum benefit obtained after a few years of hands-on experience (mistakes!).

Dave

A couple of screen shots from the capture session. Focus with a Bahtinov Mask and Capture in Progress.




Dave

Hi Dave,

Nice comet capture and thanks for the details.

I also got things to work in PI processing - like you by combining a comet master with a stars master - but it really was a bit clunky and am not at all sure whether or not it was the best way.

The problems were these ...

) The comet master had all the lines of multiple star streaks on (exactly as in Sharpcap comet stacking - I could about equally have used the SC autostack). I first made these (lines of multiple discrete spots) a bit smaller using Blur Xt and was then able to completely remove them using StarXterminator. So that left a reasonably clean starless comet image. However the background still showed a streaky pattern corresponding to the movement of the comet.. not sure how that could be avoided though?

2) The star master had the converse problem -- a bright elongated patch of comet that had to be removed. Again I used StarXterminator to create a starmask. This worked but the extracted stars were not clean. An elongated green patch was removed along with the stars. I could have removed this manually using clone stamp but that was messy and long winded. In the end I used SharpCap to image exactly the same coordinates on the folowing night and then aligned this new star image with the original using dynamic alignment. In the end it was this set of clean stars that were then dropped back into the comet image.

Interested to know whether you faced similar issues?

Tim

Thanks Tim.

As always when trying new techniques, issues do arise. I think I had similar experiences to you.

With the Altair 183C, the comet_master seemed clean (some very faint stars) and the star_master had a very faint comet residue. I considered these 'good enough' to produce the image in my original post.

With the Altair 183M (next night 60x60s), the comet_master was ok (some very faint stars) but the star_master had a more pronounced comet residue. The final mono image had a slight 'grainy' look with close inspection revealing some 'streaking'.

In ImageIntegration in PixInsight, for Pixel Rejection, I used Windsorised Sigma Clipping with Sigma Low = 4 and Sigma High = 0.

I have probably done as much as I can with my current knowledge. With some experience of comet processing behind me, I might gain more now from reading around the web. There is also Deep Sky Stacker to investigate. I will have a look at creating an animated GIF.

Using https://theskylive.com/comets and entering my location gave me first time hits on both nights I imaged the comet.

The best data capture would have been late January but the weather went against me.

Dave



Dave