Comet 46P/Wirtanen

[Davide Simonetti]

Comet 46P/Wirtanen...I've been looking forward to imaging this comet since I heard that it was on its way to our part of the Solar System. The last time I imaged a comet was 18 months ago and I had fotgotten how tricky they can be to image and process. The only software I have that 'freezes' comets to avoid streaking stars is Deep Sky Stacker (DSS) which I'm not a fan of but it's very useful for this sort of image without performing contortions in Photoshop. The resulting image still had some faint streaks but they were easy enough to remove but the image also needed a big boost in saturation to bring out the colour. All in all I'm pleased with the result (though I would have liked to have resolved the tail). At the moment the 46P/Wirtanen is still low in in the sky close to the southern horizon so the scope was pointed at the murky London light pollution. It would have been better to wait until it's higher in the sky (it reaches perihelion on December 16th and should be significantly brighter) but with the weather here being what it is you never know if there'll be another opportunity...I hope there will be. 

062 x 60 second exposures at Gain 180 cooled to -20°C
065 x dark frames
060 x flat frames
100 x bias frames
Binning 1x1
Total integration time = 1 hours and 2 minutes

Captured with APT
Guided with PHD2
Processed in Nebulosity, Fitsworks, Deep Sky Stacker and Photoshop 

Equipment
Telescope: Sky-Watcher Explorer-150PDS
Mount: Skywatcher EQ5
Guide Scope: Orion 50mm Mini
Guiding Camera: ZWO ASI120MC
Imaging Camera:  ZWO ASI1600MC Pro
Baader Mark-III MPCC Coma Corrector
Light pollution filter

[PhotoGav]

That’s a good looking first image of this currently rather murky visitor! Hopefully it will only improve as it increases in altitude from here in the UK. 

Did you guide on the comet or a star? I’m presuming a star?

Can you please give a quick overview of your data processing workflow? Does DSS create one image that deals with both the movement of the comet and of the stars or does it create two images that you then merge? I am hoping to have a go at 46P soon too and have never tried stacking multiple subs of a comet... thanks in advance!

[Davey-T]

Well done Davide, bit better than my effort with the same problem of it lurking at minus 20 degrees or so hopefully it's racing northwards now.

Dave

[Davide Simonetti]

1 minute ago, PhotoGav said:

That’s a good looking first image of this currently rather murky visitor! Hopefully it will only improve as it increases in altitude from here in the UK. 

Did you guide on the comet or a star? I’m presuming a star?

Can you please give a quick overview of your data processing workflow? Does DSS create one image that deals with both the movement of the comet and of the stars or does it create two images that you then merge? I am hoping to have a go at 46P soon too and have never tried stacking multiple subs of a comet... thanks in advance!

Thanks Gav,

Guiding was done on a star...it was too faint to show up on my guide cam and guiding on a star gives me the option to do a time-lapse of the comet's movement later on. The processing was done as follows: Calibrations were done in Nebulosity 3 because I like the format in that program and I was hoping to avoid using Deep Sky Stacker (DSS). I used Fitswork 4 to debayer the subs because I think it does a better job of it and it's easy to use. I put the calibrated subs into DSS, registered the frames and used the comet stacking option (which means clicking on the comet's nucleus for each frame). I then chose the option to align on the comet and the stars so processing is done twice...it takes much longer but it's the only way to have the stars and the comet looking sharp. The stacking mode was standard and the alignment method bilinear. The stacking method was Median. The result was a single image of the comet on a starfield but there were faint residual streaks of the stars and the image was very grey with almost no colour. I put the combined image back into Nebulosity and used the Digital Development tool to brighten it up. I also adjusted the colour scaling and the colour background (offset). Finally I put the image into Photoshop where I stretched the histogram with levels, tweaked the colour balance and increased the saturation. I used several Photoshop plugins as well: Gradient Exterminator to remove a gradient, and Astroflat Pro to smooth it some more. I removed the streaks with the Clone Stamp tool. I also used some of the Astronomy Tools v 1.6 actions - Increase Star Colour, Make Stars Smaller, and Reduce Blue/Violet Halos. I created a layer mask to desaturate the residual green on the background without effecting the comet. Lastly I increased the vibrance. I think that's about everything I did...hope it helps ?

[Space Oddities]

Thanks Davide for the explanations! I'd like to photograph 46P as well, but that's my first comet, so your explanations will be helpful  Well, if the clouds that are here since weeks finally decide to annoy someone else...

[PhotoGav]

Davide, that's brilliant, thank you for your detailed explanation. Fingers crossed that we all get some good clear skies to have a go at photographing this beauty!