Lovejoy 12/30

[dvj]

Finally a clear night. Lovejoy this morning from Coonabarabran. FSQ, Proline 16803, 180s x 7, average combine, stacked on the comet.

BTW, so far I am unable to remove stars from the background with any sigma or median combine routines when stacking on the comet. Following numerous web site instructions to no avail. Not sure what I am doing wrong.

j

[rat156]

Hi John,

Great image.

You can't remove the background stars as you don't have enough delay between exposures. When I do this I usually take a series of LRGB frames, each with a 30 s delay. So that means for 60 s exposures it's at least 6 minutes (closer to 7 in reality with CCD download times) between each L (or R or G or B) exposure. That way when you align on the comet the separate images of the stars don't overlap, hence they can be sigma rejected and median combined out (mostly) of the image.

Attached is a bit of a non-rejected sum of the same comet, just showing the stars, you can see the pattern (also that one of my Luminance frames is out of alignment!).

I really like your image, the tail is so pronounced that the star trails don't matter.

Cheers
Stuart

[Rigel003]

Lots of detail there, John. Thanks for that advice too, Stuart. I've been grappling with the same problem.

[dvj]

Hi Stuart,

I tried an assembly of exposures 6 minutes apart (every other sub), with the same result. Then tried exposures 12 minutes apart to insure there was a significant gap between the stars. No rejection. Hmmm. OK, I'll do more reading on this. It would seem simple enough as I see many of these done on the web and posted here, and there is a lot of ion tail streamer detail that should pop in a starless image.

j

[gregbradley]

Use Minimum combine not median therein lies the problem! Minimum may leave some artifacts but they can be cloned out.

The alternative may be to make starless images and stack them then add to the aligned/stacked star image.

Greg.

[dvj]

Hi Greg, I am using MaximDL v.5. No Minimum combine there that I can find in the tools. Maybe I should try the Deep Sky stacker routines.


j

[gregbradley]

Minimum combine is an option in CCDstack. There's a free 30 day trial.

Greg.

[cometcatcher]

Lovely tail detail there John!

You could simply halve your exposure and do a median stack on the stars. I don't think you would lose much detail that way. It's pretty much all I do. Can't expose the tail for this comet too long anyway it changes every 15 minutes.

[dvj]

Thanks Kevin, I'm going to give that a crack tonight. Plus try some LRGB.

I've actually noticed ion tail changes between my 3 minute subs. Quite interesting to watch the data come in for each image.

jg

[rat156]

Anyone tried a red continuum filter to remove the stars from each sub? Bert has one and uses it to remove the stars from his narrowband images. Something to try if you have one of those filters. I might try it with so combined L images with the comet removed...

Cheers
Stuart

[Peter.M]

I don't think that will work, a continuum filter is a narrowband filter with the bandpass set to somewhere that hydrogen and sulfur emission don't occur. Because the image of the comet is taken with luminance filters, the stars will be smaller than the ones recorded in the comet image, and worse the comet might show up in the continuum image and be subtracted away!