M16 from my backyard

[multiweb]

Wednesday night I managed 40 minutes on the eagle nebula.
I think this time I got this offset right on my QHY8 [gain 0 offset 50] . I got the sky glow down to under 800ADU. It made a huge difference to the final. When the clouds eventually rolled in I left the scope and took about 200 min worth of darks (5x light exposure time) + 100 bias and around the same number of flats.
I'm using CCD Stack to calibrate the pictures. I managed to dark substract the subs but applying the flats afterward screwed things up real quick. Doing a pixel math and dividing by the file though held a much better result. I thought that's what it did automatically. :confuse3: Need to work on this still. Anyway here's the shot with no flats applied:

http://www.multiweb.com.au/astro/M16_f.jpg

My previous attempt was at [gain 1 offset 110] and required a massive amount of processing to get rid of the noise - so the change in offset made a massive difference with the raw data:

http://www.multiweb.com.au/astro/M16.jpg

M16 is on my list of target from Wiruna along with M42 this week-end. So I should have some real crisp data to play with next week.

[peeb61]

Marc,
Great images mate, this is one I always have trouble with, I think its the cluster of stars toward the centre. I always seem to bloat them out somewhat but you have nailed it.

Paul

[AlexN]

Geez... The full res one looks VERY smooth.

Not only smooth, but you definitely centered it in the FOV! :D did your mount do that or did it take some tweaking??

A very nice image indeed.. I'm hoping to capture some data on M16 this weekend from dark skies...

A question, your gain and offset settings, How come so low? I was under the impression that gain and offset settings were generally to be set around 50 and 110~125 respectively??? I know you can set them where ever you like, but I had thought that for general use, those settings would almost always be ok....

I understand that Gain being 0 is a lot less likely to generate noise, however, does it not also force you to run longer subs to pull similar detail?

Alex.

[Geoff45]

Nice sharp image, but a few dust donuts. Did you use flats?
Geoff
Added: Should have read on before replying after just looking at the pic. What was the problem with the flats?

[multiweb]

No tweaking. I just pointed and shot. Got lucky.



Hi Alex, Gain & offset are still a bit of a mystery hey? ;)
I never use any gain. I just leave it to zero now. There's no point bumping it up. But reducing the offset allows me to control the background sky glow. I target 800-1000ADU. So that will depend where you're imaging from. My backyard: offset 50. I used to have it at 110 but even under dark skies my background ADU would still be around the 6000ADU mark. I only tried 50 last night. Was a first and I got better results. I'll try again this week-end from Wiruna see how it goes.

[multiweb]

Yes the calibration [lack of] really shows. I got my flats peak at 16000ADU. 16bit camera / 30% of the full well. I thought I was within range. Got inverted vignetting. Very dark in the center and bright on the outside when flats were applied so I clearly got the ranges wrong. Gotta talk to Monte this week-end and pick his brain at Ilford. I think I'm doing something wrong with the software as well.

[multiweb]

Thanks Paul. The stars have a bit of halos and are slightly bloated. My guiding was spot on but I guess the seeing is never that great from the burbs. An the clouds rolled in with a bit of haze. I have a Halpha 7nm baader filter arriving next week. I'm looking forward to use it and see if I can get some luminance data and start playing with combining channels as in Ha RBG. I wonder if narrowband will make the star tighters even when it's slightly hazy?

[Gama]

You should be able to get much more detail if you stretch the image more.
Try spending a little extra time on this, as you have a real gem here.

Also, why waste time on darks ?, you dont have ccd noise to worry about.

Theo.

[multiweb]

Thanks Theo. Hardly a gem though. I agree with the darks. I applied the master dark to the subs and I see absolutely no difference. nada. So that was a waste of time. As for the flats I just figured out what I'm doing wrong. It's the normalisation between the lighst and the flats. I've shot my flats on very short exposures (0.05s) with an A4 piece of paper in front of the scope illuminated by a beam of light from a source about 10m away. I think I'm going to invest in an EL sheet and make a lightbox.