M27

[Jim]

Managed to get the guiding sorted last night and squeezed in M27 before the clouds rolled in.
Meade 127 APO, Atik 16IC, Guiding with QHY5 on a ZS66.
4 x 300 sec subs, Darks & Bias. Processed in Maxim.
Figured out a better way to do GOTO. Do a good Polar alignment, then a 1 star alignment on the nearest star to the target that's in the Starbooks database. Saves a lot of time, although it's still difficult to find the object on the Atik's small chip.
 

[Rocket Pooch]

Well the guiding is certainly bang on, this is a nice image.

Chris

[RobertM]

I expect it is a really good image but there's one problem ... it's not visible

Where has it gone since Chris looked at it ...

[Jim]

Thanks Chris. I guess there must be a law of diminishing returns regarding image quality (S-N) and exposure lenght. Is there a way of calculating that that you guys use? I notice that a lot of imagers tend to standardise on 300 sec subs, guess that's a good compromise bearing in mind our busy skies. I'd like to experiment and compare say a 1 hour exposure against 12 x 5 mins stacked. Anyone done that?

[Carole]

Well done Jim.  I'll get the filter wheel and filters to you on Thursday and then you can add some colour to this.  Should look great.  But just think you'll have to get it on the chip all over again!!!!

Carole

[Rocket Pooch]

Hi Jim,

There are a couple of hints.

1 - your image subject should not be more than 40,000 ADU, this is to ensure you do not saturate the image in any way.  But the reality is unless you either have a) very dark skies or b) your using narrowband filters this is never going to happen because the sky background will also be high.

2 - A lot of people use 5 minute subs because the guiding is not spot on, or there are loads of planes and satillites.  There is nothing worse than doing a 20 minute sub with three planes and a satillite in the field of view.

3 - More subs = less adv noise when stacked, so again you can get 12x5 mins in an hour or 3x20 subs, the 3x20 might have more signal to noise, but the adv of the 12x5 will have 16x less ranom CCD noise.  You have a very low noise camera, which is also highly sensitive so take your pick.  Have a look half way down the page here, its a cool example of no sub's v's noise http://www.qsimaging.com/why_qsi.html

4 - Evaluate your 1st image, if you use software with a profile tool, AA or Maxim for example its quite easy to evaluate your 1st exposure, for example does it look ok?  Is the subject I'm imaging greater in ADU value than the background noise.  I do this and leave the imaging going, sometime I change my imaging strategy based on what I see, for example if the guiding is all over the place due to the weather I might take shorter subs on a brighter target.

5 - If you can take longer subs and the S/N is ok you will not need flats unless the scope really have issues, your combo won't have optical issues.

Chirs

[Jim]

Thanks Chris, lots to learn.

[Fay]

You are suddenly getting in the swing, Jim, Nice M27

[RobertM]

Can see it from home so must have been filtered out at work  

Very nice image Jim, well, nothing that can't be fixed with more photons but that's always the case.

Chris has made some good points there.  The sensor on your camera is very sensitive and has a deep well depth so by all means try longer subs to get a better dynamic range but be mindful that loosing a couple can equate to a lot of lost imaging time.

You might find this a short interesting read http://www.cloudnet.com/~dsastro/ST-7eAutoguide/ST-7eAuto.htm

Robert

[Jim]

Thanks Robert, interesting article. There was less difference than I thought there would be. It also makes a good point about seeing conditions over long exposures. So maybe keeping subs to 10-20 mins and doing lots of them is the reasonable compromise.

[MarkS]

Can see it from home so must have been filtered out at work  

I have the same problem at work. 

Jim - that's a great image for such a short exposure - well done.  Those round stars show your guiding was spot on.

As for optimal exposure length - a good rule of thumb is that the sky background should swap the CCD read noise - beyond that point longer subs don't help at all.  But how to calculate that - well that would take a whole article!

Generally speaking, the darker the sky, the more worthwhile are longer exposures.  Using a low noise Atik around here then 5 minutes is almost certainly adequate.

Mark

[RobertM]

Here's an interesting article about sub exposure length but might be of more interest to Mark...

http://www.samirkharusi.net/sub-exposures.html

[Jim]

Ok, had another go at this last night. Guiding was awfull though, think my polar alignment was well out.
60 x 60 secs and 12 x 300 secs. Meade 127 + Atik 16IC. QHY5 + ZS66 for guiding. Processed in Maxim.
I don't have Photoshop but I tried doing some processing in a photographic package to bring some of the additional nebulosity out in the second picture. I've no idea what I'm doing but the structure looks interesting. I really need to learn more about post processing. Will try in colour next.