Andromeda Galxy - M31

Thanks Jed,

Keith..

Keith - lovely wide-field shot. Noise is your biggest enemy.

Your image reduction should include dark frames at the least. Its not strictly necessary to take them at the same time as your exposures. But you do need to make sure your dark frame temperature and duration is the same as your exposures.

What I do is create a 'library' of dark frames - different exposure times ranging from 1 sec up to 5 mins. I take the required darks at whatever temperature it is outside (unless I already have a set at the same temp). This can be a handy job for a cold night (or day) even if its cloudy. Just cover the lens/scope and off you go.

Take a number of darks at each exposure duration and average them to get a 'master' dark for that temp and duration. Just wheel it out when you need it.

Bias frames are a bit trickier. They are the shortest duration your imager can take and record the 'inherent' noise in the imager, irrespective of exposure duration. They can be used to 'scale' a dark frame of a different duration as your light frame. e.g you can use a 1 minute dark on a 2 minute exposure using a bias frame. I've not used this technique so I'm not sure how it works. One thing I have seen mentioned though is that in taking a series of bias frames, you must median-combine them - do not average them.

A final image reduction technique is to use a flat field fram. This is an image taken through the imaging system whilst it is fully illuminated by a diffuse, uniform light. Some folks use light-boxes. Others use a white t-shirt over the scope. A flat-field frame records optical defects (gradients, dust motes etc) and are subtracted from your light frame. You should should take darks for your flat fields...

As Jed mentions, a good imaging programme will use whatever reduction images you provide. Some of the higher end packages have step-by-step routines you can use to generate the reduction sequences.

Anyhow, love the image and keep them coming!

Cheers

Dave

Dave, Thanks for outlining those processes in detail, there's a lot to it!, but i'm sure you would get used to it as time goes on.

Do you subtract the dark frames in ps7? Getting them done as you describe is not a problem, it's actually the technique or steps that I don't understand.

Do I just load up my shot - then load up the dark frame, and click apply image to the original shot? Maybe i'm on the wrong track here... :oops:

Keith..

What I do is create a 'library' of dark frames - different exposure times ranging from 1 sec up to 5 mins. I take the required darks at whatever temperature it is outside (unless I already have a set at the same temp). This can be a handy job for a cold night (or day) even if its cloudy. Just cover the lens/scope and off you go.

Dave,
I'm not sure this works with the 300D/DSLRs, I took an exposure of 4 minutes of an object. The battery died soon afterwards and I charged it back up for 30 minutes or so, it didnt get warm as it was around 0 degrees at the time. I then took a dark frame and the noise was considerably less then in the image even though the air/camera temperature had not changed. What had happened was that when I took the image, the camera had been on for the previous hour or 2 and had heated up "internally" due to the heafty amount of internal electronics etc....
When it turned off, rechagred the battery and turned it back on, its bits'n pieces had cooled down which gave me a useless dark frame even though the air/casing temperature had not changed.
Moral of the story, if you're using the 300D, take the dark frame immediately after the main image.

With the relatively small amount of electronics in the DSI, I'd imagine that this would not be anywhere as serious a problem.

Aaaahhhhh. The process....

I've always used the imager software to do the dark subtract for me. But I think ( : ) if you use PS7, you should do the apply image (with both light and dark frames open) and subtract the dark from the light.

I think Registax may also have the abilty to do dark subtract too.

On reading Davel L's post, there may be a major rethink here anyway... I guess I didn't know that the DSLR's generated so much internal heat. So, take the dark immediately after your exposure and forget about putting together a library.

HTH

Dave

Thanks for your help guys! So dark frame after the main shot(s), then put ps7/regisatx to work - i'll start playing around :arrow:

Keith..