The longest HA exposure

[pyrasanth]

I'm trying to capture the faintest HA details on the outer edges of the jets in M82 using an Atik 460 mono camera & Baader HA 7 nm filter. I have some earlier frames that were exposed for 30 minutes last year on a 2x2 bin & they seem okay but still don't reach into the very faint territory that I'm looking for despite having 20 of them.

So the question is- am I trying to capture something that is potentially limited by seeing conditions or should I increase the 2x2 bin exposure to far longer. The question is how much longer before I get hit by the law of diminishing returns. I can pretty much now guide for any duration and it does not matter about star bloat as my stars are processed separately from an alternate layer.

Should I go for 60 minute subs or longer but understanding that astro dark from my location is 6 hours & I need at least 3 subs for dithering hot pixel reduction.

Let me know your thoughts on this as I don't want to waste imaging time if the potential return has already reached its limits in the 30 minute subs.

[ollypenrice]

Why are you worried about hot pixels and noise? Have a think about what the role of this capture is going to be in your final image. You are not (I guess?) trying to make a standalone image of Ha in the galaxy. What you will be applying to your LRGB image is only the brightest part of the Ha capture where it outshines the red layer. Noise below that is of no importance because it won't find its way into the final result. The Ha will appear in so little of the final image (but be important none the less) that you can clean it by hand if you have to, but I doubt you will. Ha for galaxies can be stretched to the point of screaming blue murder because only the top end of the signal will be used. Don't saddle yourself with the standards of an L layer when shooting galaxy Ha. Stretch it like a madman, noise reduce it, wring its neck: it will work fine in its intended role. I wouldn't post my Ha galaxy layers on here. I'd be tried for cruelty to animals or something.

Olly

[MartinB]

The problem with Ha is that it it takes a long long exposure before you are over the read noise bump.  Making the read noise insubstantial will help the very dim Ha stand out from the background.  I know I'm not over the read noise issue with my super ha sensitive QSI 532 after 30mins even from a relatively light polluted location.

There's an excellent tool to work out your optimum exposure time http://www.ccdware.com/resources/subexposure.cfm

[pyrasanth]

Tonight is clear so I've started some 30 minute subs on a 2x2 Bin- I will see what that gives me & if Okay I will push it to 1 hour on a 2x2 bin

[pyrasanth]

I'm now pushing the subs to 1 hour on a 2x2 bin- the outer HA wisps are incredibly faint- I just don't have the quality of sky darkness- so I'm going for more data. A 2x2 bin is 4 times the sensitivity so I'm hoping this will be enough. If the weather holds I should be able to get another 8-10 hours of HA data to put into the M82 image. Then next session I can capture some stars to finish the background.

[sharkmelley]

5 hours ago, pyrasanth said:

Tonight is clear so I've started some 30 minute subs on a 2x2 Bin- I will see what that gives me & if Okay I will push it to 1 hour on a 2x2 bin

Won't moonlight be your main limiting factor tonight?

Mark

[pyrasanth]

8 hours ago, sharkmelley said:

Won't moonlight be your main limiting factor tonight?

Mark

The moon does have an effect but not so bad with HA. I captured a nice extra 6 hours of data this morning and it was better than what I've ever previously caught. I think its down to better technique rather than the weather. I would like another six hours tonight as well to finish off.