Order of events

[Moonshed]

I keep hearing about darks and and subs. Could someone please explain what order to do things in, Such as take a number of exposures and then take a dark, which I believe is just an exposure with the lens cap on? After that I believe next comes stacking, I have Registax for that, then image processing, in Photoshop in my case. Is that about right? What about the darks? Is this something I have to do during image processing?

Appreciate any advice you can give.

[daz]

1 - Light frames. If your camera does not support cooling, note the approx. temperature you captured the light frames

2 - Flats. Evenly illuminated image, at the same focus position as the lights. Temperature does not matter. Focus position does! Aim for around 21 frames to get a good average

3 - Darks. Same temp (or as close as possible) as the lights. Cover the scope/camera as much as poss. Aim for 21 frames to get a good average - these can be done on cloudy nights but again, at a similar temp as the lights.

4 - Bias. Zero length (or as short as possible) exposures. Temperature not important. Aim for 51 (or more) for a good average.

 

Get Deep Sky Stacker - it's free. You tell it which frames are which and it does the magic for you! Plenty on here/Net about DSS and how to use, before passing to PS for processing.

Basically, Flats will help to take out uneven illumination and dust from the image train. Darks will counteract the thermal noise from the sensor, and Bias is the read-out noise from the sensor. All of these will introduce artefacts into the signal so have to be (as much as possible) removed.

If you have a CCD, you can create libraries of the bias and darks - not sure you can with DSLRs, but someone else may be able to offer more here...

 

Hope this helps, initially!

 

[Moonshed]

Hi Daz. Many thanks for that explanation, it's a big help. I have started reading a book on digital Astrophotography and hopefully all will become clear. Thanks again. 

Keith

[daz]

Welcome!

You will find huge discussions/discourse on the number of flats/darks/bias to take - the numbers I have quoted are my standard ones, but you may need to experiment. Depends on your kit...

 

[Alien 13]

If you look at the Deep Sky Stacker manual and help files they do a good job at explaining what the different types of files do and how to shoot them.

On a personal note I have found the most important ones are the Lights or image files and the Flats which contain the lens/sensor defects, you can in a lot of situations get by with bias frames only to control the noise, dark frames can often make things worse if not correctly matched.

If I am shooting wide field with a background which requires a single shot rather than stacked or just a few exposures then I let the inbuilt dark subtraction routine to run, it does provide a closely matched dark frame in terms of exp and temp. 

Another option if you are using a Canon lens is to use the lens data in the camera to correct for vignetting and other defects but it will not fix dust motes on the sensor (there is a menu option for this though).

Alan