An Interesting Astrophoto Question (Maybe)

[wallis23]

I was just wondering, what would capture the best image - a 10 minute single exposure photo, say, or 10 1 minute exposures stacked together? Or would they both give the same results?

Also, is there any point in doing a 1 hour single exposure photo? (if it's possible.) Is there a usable limit of exposure time?

Questions apply to planetary and DSOs so if one is different than the other then please comment... Thank you.

[Ant]

I think that this depends on the camera / scope combinations (or rather the arc second/pixel value), something to do with well depth as well seemed to crop up...

Also I think it depends on the background sky brightness.

Someone who actually knows what they are talking about will be along soon

Ant

[Kaptain Klevtsov]

One way of thinking about this is, what if a plane flies through the shot? If you're doing a single 30 minute shot, like with old fashioned film, then the shots ruined. If you do 10 X 3 minutes stacked, you might end up with 9 useable shots to stack. Not good, but at least all is not lost. Same issue with clouds turning up, you can pack in part way through if you're using stacking, but not for a single shot.

Kaptain Klevtsov

[Macavity]

I think it's an interesting question - AND also for "short" exposures too. I imagine stacking these LATTER is more to do with removing atmospheric "seeing" variations. However, on short webcam AVI's, and exposures (whatever these actually are!), I felt "image post-processing" effected a more dramatic improvement than stacking? But who (really) knows...

[rikyuu]

I'm not an expert, and haven't tried this to any extent, but my understanding is that a long exposure gathers more light and many short exposures stacked, improve the detail and noise removal, for a DSO anyway. Please correct me if I am wrong as I'm not 100% sure!

eg for a faint object, 1 minutes worth of exposure may reveal nothing, as not enough light photons have hit the CCD to register on the sensor. Another minutes worth still reveals nothing, so stacking them is like stacking 2 blank images. I guess it's simlar to taking a photo in a dark room, no matter how many photos you take and stack, if there's no image then there will still be none when stacked.

However for a brighter object a 1 minute exposure may produce a reasonable image, but it may be noisy and rough around the edges, lacking detail and the back ground may be noisy too. The second 1 minute exposure will capture different details and different random noise. When 100's of images are layered, it brings the details out and averages out the noise speckles.

On the other hand, a long 60 min exposure will allow enough light to hit the CCD to register an image of a very faint object. The disadvantage is the CCD and/or electronics warm up over a longer exposure causing a glow across the image which requires cooling. It also requires very accurate tracking and no shaking else the image will blur.

[dph1nm]

If you had a perfect photon counting camera, then there is really no difference between stacking lots of shots and taking one single shot the same length - you end up with the same number of photons from your object. However, in real life all cameras have read-out noise which is added into your signal every time you read the CCD. So if you stack N shots you get sqrt(N) *readnoise, whereas with one long shot you only get 1*readnoise. This is the usual problem people worry about when deciding stack lengths. Whether this is significant depends on the ratio of readnoise to other forms of noise - often the noise from the sky glow will dominate and you can ignore read-noise.

There is a separate issue of not getting enough photons to register on the CCD, This only applies if you have a CCD which requires several photon to trigger 1 ADU, and you can often set the gain or ISO to remove this problem. Also, as the arrival time of photons is essentially a random process, you may occasionally get enough arriving to trigger an ADU, so it is not just a straight case of either you get a signal or you don't.

Of course, stacking short exposures does have the advantage for bright images, where a long expsosure would saturate the CCD - and tracking issues etc are minimised.

NigelM