HA - longer exposure or more subs?

[MarkF]

Which is better for getting more of the fainter detail out of M27 in HA longer subs or more subs of a shorter duration? I have five 20 min subs so do I continue at that or go for 30mins or longer?

Its on a mono Atik CCD.

[James]

I'm wondering the same (although in relation to a different object). Is it better to have lots of shorter subs and stretch them a wee bit or fewer but longer subs that may not need as much stretching but may have more noise?

James

[daz]

I'm sure there is a thread partly discussing this very thing....

hold on.............

[blinky]

I think it is a balance, longish subs and loads of them to cut down on noise.

[jgs001]

There certainly is a thread on this, but it's quite technical... however, Themos linked to Minimal Exposures which is a very interesting read, and will probably answer your question.

[daz]

I knew it!

http://stargazerslounge.com/imaging-discussion/114320-shot-noise-another-variable.html

Longer subs, more of them!!

[brianb]

Sorry but there's no substitute for collecting photons ... more shorter subs with the same total exposure time will be slightly noisier due to the extra readout noise but OTOH you don't want individual subs to be so long that you get saturation (except in the core of the brightest star images).

[MarkF]

Erm, thanks I think

Well it was interesting reading and it seems a difference of opinions, I guess also the sensitivity of the CCD comes into play somewhat. So in theory six stacked 10 min subs will show as many photons as a 60 min sub. but the longer sub could blow out the detail by collecting too many?

Which doesn't make sense to me but then neither did the links

Can I assume with narrowband you need longer subs than broadband as you are targeting less photons? (ie. being selective in which photons you collect hence less overall)

My 20 min HA subs are very dark until I stretch them and then the very faint HA is noisy and only just visible. Possible clear night tonight and the moon is up so want to grab some more HA if I can.

[MartinB]

Mark, when narrowband imaging, in an ideal world (ha ha) you should expose for as long as you can manage. When doing broadband other factors come into play such as skyglow which severely limits the benefit of longer exposures over more shorter subs.

The problem with hour long exposures are - more susceptible to tracking errors, gusts of wind, cloud, aircraft and satellite trails, neighbours security light going on etc. There is nothing more frustrating than running a 40min sub only for it to be ruined by a small patch of cloud going over the target for 2 mins.

Narrow band subs always need plenty of stretching but if stacking doesn't take out the noise you will be better going longer.

[dph1nm]

Can I assume with narrowband you need longer subs than broadband as you are targeting less photons? (ie. being selective in which photons you collect hence less overall)

Just hopping over from that other thread ...

The problem is that with narrowband you get very little sky (or object) signal, so read noise is an issue for short subs (whereas with broad RGB filters it probably isn't). So, yes, you will need longer subs with Ha in order to get enough photons so that the sky+object noise dominates over the read noise. How long is long? Unfortunately that is something you need to calculate for your particular setup.

This graph was probably the most useful out of all those links

http://www.starrywonders.com/basicgraph.jpg

Basically you do best if your subs are long enough to be on or near the horizontal part of the curve. But you would have to know your readnoise, sky flux etc to reproduce this for your setup.

NigelM

[MarkF]

Thanks all, I think you have answered my question, as long as I can and run the risk of external factors ruining the sub, so more trial and error really. As I am able to sit in the nice warm house and run the setup remotely I won't be cursing too much if I get it wrong. As soon as this moon goes I'll get back to sensible exposure times and lots of them, in broadband

The bulb seems to have blown in the neighbours security light

Thanks Nigel I have no idea how to measure those variables but it makes sense.

MartinB just a quick thank you for your website, I have learnt a lot from it in my processing over the last three weeks.

This Winter really is an experiment for me and everyone's advice and knowledge is really helping me to make good use of the few nights I have had to experiment so far. Roll on the darker nights and narrowband induced images which seem to spit in the face of light pollution.

Thank you from a real beginner.