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A Bit Late To The Party...


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But finally managed to capture some semi-decent subs of Comet Catalina C/2013 US10, I have still got some more processing to do as it passed close to M101, but I find processing comets to be extremely slow going and painful (in a nice way). So here's the result so far...tip: don't run HLVG on a comet...it takes away all it's glory!

Baader modified Canon 6D @ ISO1600
Canon 500mm f/4 L IS Lens
120x 60 second subs.

Captured 15th January 2015

Thanks for looking!

(Larger version by clicking on the image)

24213255710_8c14c33561_b.jpg

Comet Catalina C/2013 US10 by Stuart, on Flickr

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Very nice. Still trying to get my head round comet photography. So did you track Cat manually taking the images for stacking and then layer the stacked comet on a single background image? If so must involve some manual editing of anything in the faint whisps.

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That is a cracker, well done. How do you go about capturing that kind of image? I haven't done much research yet, so apologies for such a wide open question. More specific questions are:

What mount did you use for this capture?

What tracking mode do you use - is there a 'comet' mode?!?

What do you guide on - the comet itself I guess? Which means an OAG is totally useless!

How do you align and stack the images? I use Nebulosity on the Mac, not DSS on a PC.

I won't even touch on the processing side of it quite yet!!

Thanks,

Gav.

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Thanks everyone! I really need to process this data again as I feel there is more lurking in those murky subs :) I also have some M101 data, but the overlap is minimal as Saturday would have been much better than Friday.

Very nice. Still trying to get my head round comet photography. So did you track Cat manually taking the images for stacking and then layer the stacked comet on a single background image? If so must involve some manual editing of anything in the faint whisps.

See below...

That is a cracker, well done. How do you go about capturing that kind of image? I haven't done much research yet, so apologies for such a wide open question. More specific questions are:

What mount did you use for this capture?

Avalon M-Zero - though being only 60 second subs almost anything will do...
What tracking mode do you use - is there a 'comet' mode?!?

Tracking mode was standard sidereal.
What do you guide on - the comet itself I guess? Which means an OAG is totally useless!

I guided on stars as normal, but kept the subs short. Surely using an OAG will still allow you to guide on the comet, or am I missing something? Can't imagine the difference between OAG guiding and side-by-side will make?
How do you align and stack the images? I use Nebulosity on the Mac, not DSS on a PC.

I used a combination of PixInsight and PS for this, see below...I don't think it matters too much as any decent tool should suffice but the process is slightly different to DSO imaging.
I won't even touch on the processing side of it quite yet!!

No time like the present...the more you play around with the data, hopefully the better you get at it, it is a constant learning exercise...

Thanks,

Gav.

I used a combination of PI and PS for this. Obviously there are two stacks, one for the comet and one for the stars and then blended in the final steps. To me, PI is not very good for stacking comets as it assumes that each sub in the stack is spaced exactly the same distance apart, this isn't always the case if there are dropped subs so I ended up with 4 separate comet stacks that were then stacked together. A much better method would be to select the comet on each and every sub, it may be time consuming but would be better in my opinion.

A good resource for the processing method is Tony Cook's (Tonk) astrobin image of Comet Jaques:

http://www.astrobin.com/151422/

You will probably have to adapt steps for your data and processing/stacking software as it will almost certainly be different, but the gist of it is all there...

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Thank you for all your answers Stuart. That's interesting that you guide on a star, I'll give that a go. As for the OAG issue - it wouldn't be possible to guide on the comet as the oag picks off a section of sky outside of the camera's view, so definitely no comet if it's in the main image... Sounds like that's not going to be an issue though.

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On 22/01/2016 at 21:51, PhotoGav said:

Thank you for all your answers Stuart. That's interesting that you guide on a star, I'll give that a go. As for the OAG issue - it wouldn't be possible to guide on the comet as the oag picks off a section of sky outside of the camera's view, so definitely no comet if it's in the main image... Sounds like that's not going to be an issue though.

Thanks for that clarification...makes sense now...

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Hi Stuart - that is  a cracker. I forgot all about Stargazers Lounge for quite a few months so I'm a bit late coming along to see the action here - and WOW!

 

To answer others questions. By far the best tool for comet stacking is Deep Sky Stacker mainly because its got a built in time stamp interpolation algorithm that yields perfect alignments (disclosure - I had a hand in getting Luc to add this feature to make my life easier - this was back in 2008). To use this feature your  RAW comet images need a time stamp when they were exposed (all RAW formats have this embedded in the data - so that bit is easy. Anyone using tiff/jpegs etc. will suffer here - as time stamps gone!). After you have loaded your light images you just need to use the comet selection tool in DSS to mark the comet in the FIRST, LAST and REFERENCE image. You don't have to do any of the others - yay!.

 

To get the comet isolated from the stars - use Kappa-Sigma stacking with iterations around 10 and Kappa set to somewhere between 1.5 and 1.2 (experiment). Its also best that you shoot as long as possible as the star rejection improves with more sample images. Stuart points at one of my tutorials - again you can improve star rejection by sorting the collection of lights into "sieved" stacks to avoid the same star overlapping with a bit of itself in the adjacent image in the time sequence - these overlaps leave artefacts.

Star isolation can be done differently - median stacking is best followed by a classic background extraction technique to remove the blurred comet (better still - reshoot the background when the comet has gone).

I've vastly improved the "stars only" and "comet only" blending method using "screen" blending but I've still to do a tutorial for that. The classic way is "lighten" blending - but that does have artefact quirks with stars embedded in the coma.

 

Tracking the comet sounds a nice idea* - but the stars in each image become short trails that this makes automated image alignment and star rejection just a bit harder to do - sometime its not even possible without a lot of manual effort. Just keep your images short and take lots. Then you can get great images like Stuart's.

 

* - guiding isn't the best method here - if you do want to try this and you have a mount that can do custom tracking then feed in two ephemeris positions for the comet and the time interval between these positions. The positions should be close to the time you start imaging and when you estimate you will stop (comet set or dawn)

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