question about lights/darks

[Mark-mck]

my question is if i take a series of different lenght exposures do i need to take a series of darks at the same exposure length?

as in if i take 20x 15, 20,25 and 30s subs do i need to take the same ammount of darks at the same ex[posure lengths or would i get away with only 1 set at a certain exposure length?

sorry to be a pain and ask the same question you've all been asked a hundred times before,

Thanks

Mark

[themos]

The more the better, but there is a law of diminishing returns. Take at least 5 if you're going to bother.

[Mark-mck]

would that be at each exposure length?

[themos]

Yes, at each exposure length.

[Mark-mck]

cheers for that themos, i thought as much but thought i'd check.

didnt want to go wasting any clear night time if i dont have to.

[themos]

You can take darks on cloudy nights. Just make sure the ambient temperature is the same. You also need to worry about the camera reaching thermal equilibrium. Mine (Canon 450D) got warmer and warmer over an hour and the darks were getting noisier and noisier. That's why some people take darks between the lights.

[AstroManDan]

Is there a way to tell what the temp. of a Canon EOS is at?

Or were you just guessing the camera was getting warmer due to the noisier darks?

[themos]

There is a temperature field in the EXIF data ( ExifTool GUI )

I think it measures the temperature in the DIGIC processor inside the camera. But yes, I was just looking at the increase in noise. See

http://stargazerslounge.com/imaging-discussion/88921-using-iris-measure-performance-canon-450d.html

and the file "dark stability.pdf"

[red dwalf]

is`nt there some sort of dark`s libary where someone has taken the time to get loads of darks over loads of temperatures that we lazy folk could just download ?

if not why not, or am i onto something here

[themos]

Darks are camera-specific, my darks would be useless to you. That's because no two sensors are the same and the defect-set is unique to each camera.

[ollypenrice]

I wonder why you would want to take so many different exposure lengths? Sometimes short exposures are needed to cover the dynamic range, the classic being the Trapezium in M42. But 95 percent of my images are taken at a single sub length, usually 5, 10, 15 or 20 minutes with CCD. I would just settle on a time that works for your mount's tracking. As long as possible would be best.

Olly

[Mark-mck]

Thanks for the advice folk's.

Olly - i was planning on taking the different exposure lengths on my next outing as i'm trying to figure out what works with my camera and scope. ATM everything will be unguided so 30s will be about the limit i'm guessing.

i'm only "playing" with AP at the min, just trying to get to grips fith some of the finer details. looking at geting an ED80 pro after the summers out of the way, should give me time to learn decent PA and get my head aroud some of the processing too. and also to learn the night sky a bit better too.

Mark.