Darks?

[Celeste]

I finally managed to get the scope out for a bit the other night, I tried imaging M42 again, but this time using it in 2" size (so I could get a little bit more image on the small CCD) I also took darks this time too....now my question is, do darks make the image better? Or at least, are they supposed to? My image is pants to say the least with darks added. I can get more data out without the darks, than with. I don't seem to have captured much data at all this time, I took 60secs subs like last time, but I did add the coma corrector (I coudn't get focus without it) Would that cause me issues? I took my darks at the same time as my subs, nothing was changed or moved....only adding the cap to the end of the scope, as the manual tells me to. Is there a 'knack' on adding darks? I use Astro art 3 or DSS.

[Guest bunnygod1]

Dark frames are made at the same exposure time and ISO as the subject light frames, but with no light reaching the sensor. Their purpose is to remove the thermal noise/hot pixels from the light frames. Normally you want between 10 and 20 dark frames during each imaging session.
 

[Celeste]

The manual suggests 1 dark frame per sub....same settings etc. I did just that, 20 60sec subs and 20 60sec darks, add the darks and my image has low data, minus the darks it's a little better. Although the subs I have now aren't very good, I'm wondering if it's because I used a coma corrector? GAH this imaging malarky is a right PITA

[+Ibbo]

don't worry about the calibration frames just yet just get used to using the ccd and software to start with

and of course setting up the mount

 

 

 

 

Steve

[+RonC]

I thought the better rate was 1 dark to 5 lights, that's what I use?

[+Graham]

I was told by a very clever guy who designs TV cameras this.

The max number of dark's you need is 21.

Because of the multiplication factors involved to get the same signal to noise ratio of 21 Dark's the next number will be something like 85 Dark's.

He also told me that if you have too many Dark's the stacking program will convert consistent hot pixels into stars.

It is better to use an odd number of Dark's and a ratio of 1 dark per 5 lights up to 21.

Do not ask me to explain any of technical details of this as it is way above my head.

I have taken his advice on board and stuck to it ever since and it does work.

Edited February 6, 2013 by Graham

[Guest Kheldar]

As per Graham, anything over 20 is going to be a lack of dimishing returns - same applies to flat and bias

[Celeste]

I've come to the conclusion this set of subs are pretty naff, to be polite (to myself lol) My last lot of 60sec subs, on 1.25" size and IR filter....were MUCH better than 2" size, coma corrector and no filters (I dont have any 2" filters) I can't get much from this 60sec lot

 

I'm saving for an Atik 314l+, ST80 and hopefully an ED80 soon anyway (well, soon as in after summer hols!)