Lots of short subs versus less but longer subs

[SteveL]

With the distinct lack of long batches clear skies, you tend to get set in your ways and have less nights "experimenting". What I`d like to see isa discussion of sub lengths.

Say you had a two hour window of clear skies, and you were all set up. Which would give you a better end product:

120 x 1 min

or

24 x 5 mins

or

8 x 15 mins

(Ignore any tracking issues, satellites, aircraft, burned out cores, etc)

Also, is it in any way dependant on what the object is that you are imaging? I see Celescope does lots of short subs and gets some stunning images, and then I see other images elsewhere (here and on other forums) who are taking 10-15-20 min subs. I personally use 10-15 min subs, but when I do get aircraft/satellite/wind related artifacts in my subs, I lost a much longer length of imaging time than if I had taken lots of shorter subs.

So, discuss....

[Kaptain Klevtsov]

ISTR Martin did a good article on this, but as I'm up for work tomorrow I'll leave the die-hards to dig it up. e mentioned shot noise, readout noise and (I think) sky glow. Interesting stuff.

Apologies in advance to whoever wrote it if it wasn't Martin.

Kaptain Klevtsov

[anthony]

10min x 12 or 15min x 8. I always try and take them unbinned. Remember, if you bin 2x2 you are increasing sensitivity and so can get away with shorter exposures.

You wont get a scientifically precise answer from me but apparently the longer the sub the less noise you have. In general you get readout noise which by and large stays the same whether your sub is 5 seconds or 15 minutes. All other noise associated with the exposure will increase over time. However, your signal, (the photons you are collecting), increases at a greater rate. In theory, the longer the exposure the better a ratio between noise and signal and so a smoother image. Also one that you can spend more time processing the "dimmer" areas.

Typically the dim areas being the noisiest are the hardest to process without making it look carp.

So I go for longer subs. But consequently, I am fussier about not bothering on those patchy nights. When really knackered little excuses are handy!

Cheers

Anthony

[Milamber]

Longer subs for 16 bit, shorter for 8 bit. Since a 16 bit image *can* hold a lot more info there's little point in not letting it do so where an 8 bit image would overflow as it were.

Arthur

[Roy Batty]

Although i'm no expert on this, (yeah i know it's a bit of a cliched opener) (i'm sure others are far more qualified here to comment) it seems to me that because there are lots of variables involved (spec of CCD, scope, mount, seeing, time etc...) you are not going to get any definitive conclusions, although some general rules of thumb may come out?

Am i right in thinking that depending on the sensetivity of the CCD chip used you will reach a maximum practical exposure for a given time and thereafter the benefit of prolonging exposure will become nugatory? Does this also depend on the objects brightness, i.e. sum of the light falling on the chip in a given time?

Well, nuf said from me....coat check

RB

[Milamber]

That's what I said

[Cloudwatcher]

What's a sub?

CW

[Roy Batty]

What's a sub?

CW

an americanism for a baguette

RB

[CELESCOPE]

I think a lot depends on the equipment we use really Steve, my C11 has so much light grasp most of the images i,m taking at the moment i can see very plainly on the sub, , for myself i,m trying to achieve at least 7 or 8 mins with the C11, i,m nearly there , just need some more fine tuning with the settings in PHD, i can run 5 mins at the moment that shows a pretty good image as far as the stars go, but maybe just a tweak for 100%.

To bin or not to Bin, well depends on what you want i guess , if you want a huge picture to print then no binning is best i guess, i,m doing both at the moment, and can get far more detail binning,

Then of course the processing , i have spent so much time learning all this in the past, from early days with carp images and turning them into something presentable.

So in answer Steve ,i would go for ,and this is what i would do 24x5 mins.

Rog

heck that was longwinded .

lol

[Cloudwatcher]

What's a sub?

CW

an americanism for a baguette

RB

:laughing3:

Ah! That'll be it then! Had a feeling our American cousins were involved somewhere along the line.

CW

[CELESCOPE]

Sorry CW,

a sub is is a download of a given exposure ,IE, 1 min exposure download or 10 min exposure down load, we can do as many downloads of those times and then stack for a final image , so each one is called a Sub, to finally make up one framed final image.

Rog

[Milamber]

... as in subframe - a single frame in a range of subframes that will make up the final image.

Arthur

[Roy Batty]

:laughing3:

Ah! That'll be it then! Had a feeling our American cousins were involved somewhere along the line.

CW

well they wouldn't want to borrow even a word en francais.. that mite turn them into surrender monkeys

RB

[coatesg]

Think the answer depends on what you are imaging, what the full well depth is, if you have ABG on your CCD or not, the level of read noise in your camera and the background level of your sky. The basic idea is that you want long enough subexposures to ensure that the background overwhelms the read noise, but ensuring you don't saturate the chip in doing so (particularly for bright objects - though we can work round later this by combining exposures of different length). Going longer gives limited returns; going shorter quickly results in a decreasing signal to noise ratio.

Some interesting stuff (for the mathematically minded!):

http://hiddenloft.com/notes/SubExposures.pdf

Useful page also at:

http://www.ccdware.com/resources/subexposure.cfm

Cheers,

GC

[Cloudwatcher]

Thinking about it the Yanks are probably right. I mean,120 - 3min baguettes sounds a bit underexposed underdone. They are bound to turn out grainy!

CW (Who not only has his coat but has left the building.)

[centroid]

Having a 16 bit 'single shot' colour camera, I will always opt for the longest practical exposure.

However, being practical, this has to be balanced against loosing a 15 min sub, for the reasons you said to ingnore.

I generally, limit my maximum exposure to 10 minutes, as being a good compromise.

As Graeme has already said, at one end of the scale, you get 'diminshing returns', while at the other, SNR., starts to become an issue.

Dave

[cygnusx1]

Interesting topic, thanks for the input.

Of course unguided exposures are limiting so until i get a dual mount im stuck to 90s on my c9.25 or 3-4 mins on my 66mm apo.