Exposure Duration And Stacking

[pjphilli (Peter)]

Hi - I use a DSI 2 camera and I would like to apply more rationality to the way I select the duration of images and the number that I stack. At present I do this in a rather hit or miss fashion. I would appreciate advice on how to judge selection of these parameters - maybe there is an article that you could point me to about this subject. - Cheers Peter

[Octane (Humayun)]

pjphilli,

How long can you expose without trailing stars?

Take as many images as you can, that show no visible trailing. Also, make sure you are well polar-aligned, otherwise you will suffer from field rotation in your images.

Also, take dark frames and flat frames.

Regards,
Humayun

[Babalyon 5]

Your exposure time will vary from object to object, but generally, the longer the better. With a DSI, you may find that the brighter areas of objects are bloomed out in longer exposures, this is when you will have to do short exposures to get the bright detail and longer ones to get the dark detail and combine them with layering techniques in Photoshop. 10 x5min exposures are more useful than trying to do 1x50min exposure as each frame will fill in the detail and data gaps that are evident in a single exposure. Also, and I think this is the biggest factor, you have to get to know your equipment and how it performs, and this can take a while. DSI's are good entry level units with good software in a single package, learn all you can from these and your scope before you upgrade to better equipment, as it doesnt matter how good the equipment is(within reason, as there is a crap level), if you dont know how to use it, it still wont give you any better results.

[pjphilli (Peter)]

Thanks for these replies. I have no difficulty with guiding or polar aligning so
I suppose the simple answer is to take exposures up to the limit of blooming and do as many of these as time and patience permits for stacking to improve the signal/noise. The layering technique is interesting where an object image had a large contrast ratio. I will try this on the Orion nebula when it gets into a better location. Cheers Peter

[AlexN]

if your tracking/guiding can handle it, go as long as possible..

I finally got all my guiding issues sorted out for me, and was astounded at the difference between 3~4min unguided subs and 20~30 minute subs.. its rather breath taking really..

Go as long as you can before blooming/guiding problems occur..

and, get as many of them as you can... I generally try to get 2hrs data for an image. seems to be a good place to start...

I've currently got 14hrs 30mins data of M42, 9hrs 30mins to go until I stack them.. Just waiting on clear skies!

[pjphilli (Peter)]

Thanks for your encouragement Alex. I too have recently got my guiding issues under control and I should now be thinking of doing more than the three or four minute exposures to which I was previously limited. No doubt about it, the name of the game is good data followed by good processing.
Cheers Peter

[TrevorW]

I've found that on some nebulous stars longer than shorter is the go depending on overall brightness if you want to see more of the nebulosity

Example I've stacked 40 by 30 second exposures of M45 and even after processing I found it difficult to get the filament to show around the stars without washing out the rest of the image.

All the best