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How to image a comet?

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#1 vpcirc

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Posted 28 January 2013 - 09:49 PM

I'd like to image a comet at some point. Is there any special things I need to know, for example does tracking rate need to be increased? Where can I find resources from experienced comet imagers?

#2 Rick J

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Posted 28 January 2013 - 11:41 PM

This is the one place I sometimes wish I had OSC. With filters I find it best to take a lot of images with the exposure short enough the motion isn't very great. I cycle through the LRGB filters so the order might be LRGBLRGBLRGB... This helps later

Now you have a ton of images through all filters. I make two versions, one aligned on the stars with a messed up comet and one aligned on the comet but thanks to rejection stacking and gaps created by not taking all with one filter at the same time, most stars are gone. Clone out the few that remain. Then remove the comet from the star field and combine the two resulting images.

Also a movie can be quite amazing, this is where OSC would help, I'm stuck in mono but it is more sensitive. For instance you can see gasses flowing down the tail in this movie.
http://picasaweb.goo...718307535701698

I hope this works, I'm just learning Picasa. I just found out while our club used to host the animation the new webmaster removed most members photos for reasons unknown.

Rick

#3 pfile

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Posted 29 January 2013 - 12:05 AM

i agree that the mono+filters thing is kind of tedious. i failed to get a decent result against LINEAR with that technique. i have been idly thinking about getting an OSC for the upcoming comets. of course that's a recipe for causing those comets to fizzle.

i think the Astro-Physics ascom driver can handle comet rates directly from the ephemeris. not sure what the bisque options are.

my G2 can handle different RA and Dec divisors, but i did not figure out how to compute those from the ephemeris. so one thing i tried was to guide with an external guidescope, and just pick the head of LINEAR as a guidestar. it's not optimal - the mount is fighting the autoguiding continuously. regardless, this worked for a while, until the comet moved so close to another star that PhD got confused and jumped onto the star. the result was not really that satisfying:

Posted Image
C/2012 K5 (LINEAR) by pfile, on Flickr


LINEAR was moving pretty fast, so i took a bunch of 60s L exposures which i stacked into an image. i also made a movie from the images, but since the exposures are short it looks kind of *BLEEP*.

Posted Image
Comet C/2012 K5 (LINEAR) at 08:08:00 01/03/2013 UTC by pfile, on Flickr

movie:

http://flic.kr/p/dJJuyP


i think with the upcoming 'great' comets, widefield may be more appropriate. in that case you probably don't need to do too much special stuff, as long as you can mount a camera lens to your CCD, that is. with really wide FOVs you probably don't have to guide on the comet itself, and it won't appear to move that much during a normal-length exposure.

#4 Rick J

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Posted 29 January 2013 - 02:04 AM

Swan had mostly a gas tail. Gas tails are far more active than dust ones like in your LINEAR movie. I tried a few mostly dust tailed comets but the results were far from what I got with Swan. I didn't post them. There was a predicted tail disconnect from Swan I wanted to catch but it didn't happen until a couple hours after it set behind my neighbors trees. You can see the tail starting to pinch off at the head in my 30 minute animation but not the disconnect unfortunately. Movie ended because my wife lit newspaper to light the logs in the fireplace and it was right over the chimney. Suddenly I had more IR sources (streaks) than stars. I wasn't using a blocking filter to get the most light possible. Seeing was ruined as well. But it hit the trees only 10 minutes later so hardly mattered.

I did have one comet, can't recall which right now, that the sigma rejection stack didn't work for very well because the star field was so rich too many stars replaced others as it moved they didn't vanish like usual. Took lots of cloning but most work well if there's enough gap between frames in the stack. Takes some playing with the parameters some times.

Rick

#5 pfile

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Posted 29 January 2013 - 02:08 AM

yeah - i shot hyakutake on film and it had a really awesome gas tail. hale-bopp too for that matter. but hyakutake had a very detailed and twisty gas tail like your Swan movie.

that's a funny story about the fireplace. i'm glad the session was almost over, i think i would be raging if my wife did that :)

#6 astrosharp

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Posted 29 January 2013 - 03:36 AM

I can recommend Martin Mobberley's excellent book:

http://www.amazon.co...my/dp/144196...

Cheers
Ian

#7 vpcirc

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Posted 29 January 2013 - 04:22 AM

Thanks!

#8 John Wunderlin

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Posted 29 January 2013 - 04:45 PM

I'd like to image a comet at some point. Is there any special things I need to know, for example does tracking rate need to be increased? Where can I find resources from experienced comet imagers?


I've imaged a fair number of comets now. I also use a mono camera at the moment. A few things I've learned:

- Experiment with various exposure lengths to determine how long you can go without elongating the nucleus. Typical times are 15 seconds to a minute or so depending on their relative motion against the background stars.

- DSS's comet stacking works ok, but the 'comet freeze' function causes the background stars to be kind of streaked. I prefer to stack the exposures twice- once on the comet and once on the background stars and then digitally combine the shots in Photoshop.

- Nebulosity works well for a quick stacks and allows you to stack either way.

- If you use color filters, Nebulosity has a terrific option to alternate RGB filters using an automated script during acquisition.

Here is a link to a thread I started on comet ISON which includes a couple of shots I did recently.

#9 John Wunderlin

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Posted 29 January 2013 - 04:46 PM

One more thing- creating a video is fairly involved... I would hold off on that to start with to avoid a lot of frustration :)

#10 schmeah

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Posted 30 January 2013 - 09:05 PM

One more thing- creating a video is fairly involved... I would hold off on that to start with to avoid a lot of frustration :)


Actually a simple animation like the one I did here was pretty easy and was quite a lot of fun! Give it a go.
http://www.pbase.com...image/122734666

Derek

#11 rigel123

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Posted 30 January 2013 - 09:26 PM

That's a great animation Derek!

#12 neptun2

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Posted 30 January 2013 - 09:31 PM

For those like me that did not try the comet stacking mode in DSS here is a link with instructions:

http://deepskystacke...m#cometstacking

#13 vpcirc

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Posted 30 January 2013 - 09:54 PM

Thanks again for the resources

#14 John Wunderlin

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Posted 31 January 2013 - 11:02 AM

Actually a simple animation like the one I did here was pretty easy and was quite a lot of fun! Give it a go.
http://www.pbase.com...image/122734666Derek


Derek- how did you stretch the individual frames? I ended up using ImageMagick, a command line tool that is not for the faint of heart! My raw data would not display comets unless they are quite bright.

#15 vpcirc

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Posted 31 January 2013 - 11:07 AM

That's amazing how the stars kept in place

#16 schmeah

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Posted 01 February 2013 - 02:57 PM

Actually a simple animation like the one I did here was pretty easy and was quite a lot of fun! Give it a go.
http://www.pbase.com...image/122734666Derek


Derek- how did you stretch the individual frames? I ended up using ImageMagick, a command line tool that is not for the faint of heart! My raw data would not display comets unless they are quite bright.


I think Lulin was pretty bright by that point because I don't recall having had to any stretching beyond usual. It has been such a long time since I've tried to image a comet, that I completely forgot how to do the animation. I remember layering the individual images in PS, then making the movie by transporting it to Image Ready. Tom King posted a nice tutorial which I followed in the Yahoo DSI group at the time which is here:
http://tinyurl.com/apmw936

Derek

#17 John Wunderlin

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Posted 01 February 2013 - 03:05 PM

Thanks! For comets that need stretching, I haven't found a good way to do that in Photoshop- maybe someone else can jump in if they know how to apply the same stretch over multiple images- seems like there must be a way to do that.

#18 WillCarney

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Posted 01 February 2013 - 03:48 PM

I've done a few and I found the best shots really come from using a seperate guide scope guiding on the comet. Just ignore the stars. It will give you streaks but you get more detail in the comet tail. Lumicon also has a narrow band filter that is made for comets. For most comets it really does help.

I had a few problems stacking some images from my old ST7 so I just made a video of the comet moving against the stars. I also did this for other images. The first if from the ST7 and the second with a Canon DSLR. Both 103P Hartley.

http://s658.photobuc...graphs/?acti...
http://s658.photobuc...graphs/?acti...
William

#19 pfile

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Posted 01 February 2013 - 07:39 PM

well, at least in pixinsight there are a few ways to do it.

if the statistics of your frames are all very similar, you can come up with a single histogram transformation that you like and then use ImageContainer to apply it to all of your images.

if your frames have different illumination/mean values (like due to clouds or a moonlight gradient or something like that) then you need a different histogram transformation for each image. in this case the Animation script can be used. Animation can apply an AutoSTF to each image, convert that to a histogram transformation and write out the files with the HT applied. the autoSTF tries to stretch to the same background values each time.

in PI1.8 the animation script has been removed. there's a new process called Blink that should be able to do the same thing as Animation. Blink is in the latest 1.7 as well but as far as i can tell it can not save the files with the HT applied.






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