M1 difficulty

[tonybarry]

Hello good folk of the forum,

I have been experimenting with the Gstar-EX monochrome camera for some time, and have tried my hand at an LRGB image of M1 (the Crab Nebula).

The Luminance part of the image seems OK to me; but when I come to combine the R, G, and B parts, I am faced with an apparently insurmountable difficulty.

The problem is that I have no reference for relative colour, or if you prefer, the relative brightness of one channel with respect to the others. I can do anything I want in Photoshop, and intuition tells me that it's most likely to be wrong (as there are many paths to mistakes, but only one path to the truth).

How do I combine RGB channels and ensure their relative brightness is "correct"? Stars come out white, as one would expect; if I normalise the image parts so a given (non-saturated) star is equivalent in brightness across all three RGB parts, and an area devoid of stars or nebula is also given a different but equivalent (dark) brightness, then the nebula appears a bit blue.

I am attaching the L channel image to this post; and would be pleased to send the original FITS files (LRGB parts) made under Nebulosity 2 to those persons interested to enlighten me with regard to this problem. However these images are around 7MB each, so they are not small files.

The L image consists of a Drizzle stacking of four hundred GStar-EX images of M1 taken on the 18th of January this year. The scope is an LX90-8" with no guiding; about one hundred images were discarded during the initial scrutiny due to scope relative motion against the image field. The images were captured using a Firewire Canopus ADVC55 to iMovie on an Intel Mac using the time lapse facility of iMovie.

The GStar-EX has been a very worthwhile addition to my meagre store of astronomical devices, and its low-light collecting abilities from a very light polluted area of Western Sydney has me quite delighted. I was unable to see M1 through the same scope using an 18mm Series 5000 UWA Meade eyepiece and averted vision (or even the eyes of faith). But the GStar was quite able to display it on screen in real time.

Thank you for your time.

Regards,
Tony Barry

[TrevorW]

Someone more knowledgeable probable can comment here but as i understand it it's all about the SNR (signal too noise ratio) and the longer the expsoure the higher SNR.

There is really no point in stacking multiple short exposures except where the object is fairly bright

[RobF]

I can relate totally to your delight (and frustrations!) Barry. The thing that really got me excited about imaging through my scope was the amount of faint detail that could suddenly be discerned. Its like suddenly having your own huge dob on hand, but as a bonus you get to keep a souvenier of your nights work too. If its a pretty pic you end up with that's nice too, but even faint glimses of the horsehead and galaxies from surburbia can be amazing.

M1 is one of those objects I always wanted to see with my old scope, but I now realise I didn't have a hope. You certainly seem to have captured the essence of the object and plenty of tantalising detail. 3min shots through a pollution filter with my Canon 450D struggled to show as much info as you have there.

If you want to be purist about the RGB you really need to identify a standard star in your image with known colour characteristics (with something like Maxim/The Sky). Might I humbly suggest you just get in there and have a go though? Although its tempting to try and get everything as perfect as possible, the reality of imaging is you're constantly learning new processing tricks so that you'll want to revisit old data and objects with what you've learned along the way. That's half the fun really. Plus the fact that one person's interpretation of an object is rarely the same as anothers.

Sorry if my post is a bit short on helpful tech detail. Hopefully the Gstar or RGB experts here will have help to give.