Choosing Sub Times

[Roy]

At the risk of stirring up a hornets nest, can I ask how do you choose the length of your imaging subs for optimum quality data that can then be stacked? Is it based on trial and error, i.e. experience, or are there formulae one can use?

Roy

[JohnP]

Depends a lot on your polar alignment & how good your guiding is & sensor size of camera/ focal length of scope. I'm sure Mark or Robert will give you all the formula's.. typically I take either 5 or 10 mins subs if setup/ conditions allow but that's with a Narrow Band Ha filter in place from Light polluted Bromley. I know Mark gets unbelievable results with 30 sec exposures but then his ccd/ scope combo allows that. I've never seen any performance gains in going past 10mins subs..

John

[Carole]

With DSLR I always used to use 5mins 800 iso, although I did experiment once with 600secs 400 iso which also worked. 

With Cooled CCD really depends on your Filter/ target.

Luminance normally maximum 600secs
RGB you can bin x 2 and do 150secs, or not bin and do 300secs

With narrowband, you can do up to 1800secs (1/2 hour), but then there is more of a risk of getting a plane fly through to FOV.  I generally do 900secs - 1200secs Ha and the same for Oiii and Sii or bin both Oiii and Sii.

Target depends on how bright it is, as with brighter targets you can risk blowing them out if using broadband and the subs are too long.

It's all a balancing act really, because the more subs you stack the better for noise, so if you are not going to get a lot of time on a target, I sometimes choose slightly shorter subs to have more to stack. 

Well that's the way I decide anyway.

Carole

[MarkS]

Are you using a DSLR?  If so, which one? 

You want the combined noise from the skyfog and the thermal signal to swamp the read noise.  A good rule of thumb for a DSLR is to get the peak of the histogram (on the back of the camera) approx. 1/4 or 1/3 from the left hand edge.  Also use an ISO high enough so that there is no obvious horizontal or vertical banding.  ISO 800 or 1600 is usually OK.

Mark