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hi all,

this is 232 lights of 15 & 20s exposures 60 darks 60 bias with a total exposure of 1h 8mins stacked with DSS. Is there any way to improve on this? or is this a good as it will get?

thx for any input and help

post-31022-0-84007400-1375212181_thumb.j

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flats will help, as will longer subs.

Also looks a little washed out, improved post processing can sort that.

Looks relatively sharp and smooth though, so certainly not bad.

As Freddie said.. kit needs listing to give a decent opinion

Derek

Edited by rfdesigner

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Apologies for that, taken with the nexstar 6, eos1100d unmod@ prime focus in alt az mode. took 500 lights over 3 nights which was about 180 lights 20 darks n bias each night then stacked in DSS which took 232 lights with a total exposure of 1h 8mns, just post pro in DSS had a quick go in CS3 but just seemed to get worse. i was planning on doing another session on it with 20s exposures, would this help improve it? as well as maybe doing a few sessions at 25s even if it throws 10-20 of the subs in?

thx for the help

adz

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As above, flats are the number on essential. I agree with RFdesigner that longer subs would be better, too. I'm arguing with another member on this subject at the moment in a different thread but I can promise you that my half hour subs are here to stay!

There are ways of boosting colour without resorting to saturation, which is noisy, but they would take a while to explain.

Olly

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The reason longer subs help is simple.

You want to swamp the camera readout noise (an avoidable source of noise) with skyglow and dark-current (unavoidable sources)

If you shoot with a VERY low current noise camera (the ATIK460 for instance) under extremely dark skys (as Olly does) then your unavoidable noise will be so low you will need very long subs to swamp the readout noise in each frame.

If you use a camera with higher dark current (like a DSLR) under most UK skys (with some degree of skyglow) then you don't need extreme sub lengths, 5 minutes will probably be fine. (my 383L+ under mag 20 skys at f5 really need 10 minutes for zenith shots, and an hour for narrowband.. but I'm making to with 5mins for the moment)

The thing is to know how skyglow+dark current swamp readout noise.

The maths is like this:

total noise = sqrt( readout_noise^2 + skyglow + darkcurrent) + a few other bits that we'll ignore.

(remember this is done in units of 'electrons'.. don't worry about it too much though)

Most cameras have readout noise between 5e and 10e.. interestingly DSLRs have good readout noise, lets assume you have a 7e DSLR camera.. so that means you need 7e ^2 or 49e skyglow+darkcurrent just to equal the readout noise. Assuming you have a camera gain of 2 (depends on ISO on DSLR) then that means the 'glow' in each sub (after removing a bias frame) should be >>98. I generally like to see a value over 1000 (edit: on 16bit CCDs.. on 14bit/12bit DSLR, you'll comparatively need less).. then readout noise is thoroughly swamped.

You however have another problem.

I noticed you are using Alt-Az.. this as I'm sure you realise will give you field rotation over time. My first upgrade to your imaging setup would be a wedge of some kind.. as that will open the door to much longer subs and all the benefits that brings.

Derek

Edited by rfdesigner

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I think you could afford to stretch the image a bit more in CS3........all depends on what kind of result you are looking for.

I have done a quick tweak using the Dodge and Burn tools on the galaxy,darkened the background and used the sponge tool to bring out a bit of colour in the stars.

post-849-0-46391700-1375274610_thumb.jpg

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thankyou for the replys and advice peeps, ive never used flats yet, they seemed a bit too complicated but i guess i should really give it a go.

@skywatcher thankyou for your time at having a go at it,the detail u got in the middle of the galaxy is kind of detail im after so ill have another go at cs3

@rfdesigner thats some very detailed info and u clearly know what ur talking about but most of that went waaaaaay over my head lol sorry. ive been reading that 15 20s subs is the same as 1 5 min sub but then some people are saying it isnt? which is why im wondering if doubling my total exposure with more sessions of 15,20,25s would bring out more detail?

i suppose the question im asking is,is building the total exposure time up the same as taking 1 long exposure of the same time? but then again it seems ppl have diff opinions on it so maybe i should just do it and see what happens?

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Ok, sorry I lost you.. lets keep it simple.

Total exposure time helps... more time is always better.

But each exposure adds one 'lot' of camera readout noise.

Ideally if you only cared about noise you would use just one exposure for the whole shot. (I used to do this with film.. 1hour shots)

You do however get all sorts of benefits from taking multiple short shots such as compensating for imperfect alignment and removing transient events (aircraft satellites)

As each 'light' frame has one dose of 'readout noise'.. the question is how many doses of readout noise can you have before you start losing out?

It's a balance between the 'glow' and the readout noise...

If you have grossly short shots (as you do).. then doubling the length of the subs within the same overall timeframe will half the total number of shots, and so half the readout noise.

i.e.

lets say you have 120 x 30s shots.

You can double performance by:

A: doubling the imaging time... so 240 x 30s shots (2 hours)

or

B: doubling the sub length, so 60 x 60s shots (just 1 hour.. good)

if you do B, you can repeat and get nearly another doubling in performance by going to 30 x 120s.

If you keep doing this eventually you don't get much benefit... getting close to that point is what it is worth doing.

There is no perfect sub length, but 5 minutes should be enough that you shouldn't see much benefit going to 10m subs.. and that's the point, it's best to row back just a bit from perfection if you experience the odd dud sub... it's better to have a dud 5m sub than a dud 10m sub.

Summary..

Subs of a few minutes are roughly where you should be... but you need the tracking good enough to make this work.. no point in having long smeared shots.

Derek

Edited by rfdesigner

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just had a go at a process... just for fun

post-8988-0-09433100-1375441411_thumb.jp

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