Comparing Darks

[Carole]

Following on from my Markarian Chain thread.

DSLR450D
6 minute dark done at 2 degrees 800 ISO
1/4 of the frame stretched in levels (grey slider) to 2



Same again but done at 17 degrees


Also, what is this?  Amp glow but also a light leak?
I thought I might take a few more darks the next morning as it was still 12 degrees.  Taken with cover over flattener (still attached) (but Camera not in telescope) in daylight.  Camera in shade. 



Carole

[Mac]

Yup looks like a light leak.

you dont need the camera attached to the scope for darks,
just fit a lens and a lens cap, and do your darks that way.

Mac.

[Carole]

Yup looks like a light leak.

that's what I thought. I'll try it again without the flattener with either a lens cap or the cover for the body only. 

Carole

[MarkS]

Carole,

It certainly looks as if you have light leakage somewhere.

The general rule affecting darks is that the dark current (and hence pixel brightness) will double for every 6C rise in CCD temperature. Therefore, for a 12C rise in temperature it will double and then double again i.e. quadruple.

A rise from 2C to 17C is a difference of 15C - it will results in darks around 5.5x brigher.

I'm still using the same set of darks I took over a year ago because I can scale them up or down as necessary to produce a "synthetic dark" for my processing sequence.

Mark

[Carole]

Yes Mark, I am collecting darks and when I have same length/same temperature I re-use them.  I don't have a way of measuring the sensor temperature so have to measure air temperature.  If any-one can tell me how to measure the sensor temperature I would like to be able to do this (provided it's not too technical).

Bit worrying about that light leak.  Going to try some experiments.

Carole

[mickw]

The light leak - is it just on one dark ?

It looks like a light shining on a square thing with a bit of glow at the edge (not the amp glow).  It looks as though the lens or whatever was not covered properly at that time.

[Fay]

If daylight is around......it will get in!

[Carole]

The light leak - is it just on one dark ?

No it was a set of them.

I've just tried again with the same cover (in the daylight) and got no leak, so I don't know what happened there, maybe something was slightly loose or the cap not on properly.  

Carole                                                                                                                                                                                                                                                                        

[Mac]

Bit worrying about that light leak.  Going to try some experiments.

one of the most common place for light to leak in is through the view finder, (where you put your eye for the non technical)
so it might be wise to just cover that as well when you are taking darks.

Mac