dark frames and flat frames

[Logan Run]

Hi everyone many thanks for the warm welcome

I was wondering,  do I need to do dark frames and flat frames with the atik 314L ccd camera , how do I go about doing flats  , do I  lay white t shirt over the scope first then the flat panel on top / or do I lay the flat panel on top of the scope without the t-shirt and how long of exposure time do I need for each filter.

can I do Dark frames / flats frames at anytime or as it to be on the same night as imaging.

 

[wongataa]

Dark frames need to be done at the same time as light frames so everything is at the same temperature as when you took the light frames.  Flat frames can be taken at any time provided the orientation of the camera in regards to the telescope is the same as when you took the light frames (theoretically it shouldn't matter but it could do in the real world) so it makes sense to also take them at the same time before packing up.

[scotty38]

If it's a cooled camera (and I think it is) there's nothing stopping you setting up a library of dark frames at varying temperatures/settings/whatever. Then when you come to process your images just pick the darks that match the settings you used for the lights. Once every blue moon you can update the library with new files.

[Logan Run]

yes it is a cooled camera , oh i never thought of doing that,  thanks 

[daz]

Don't forget bias frames as well - 0 length exposures.

Flat frame exposure will vary according to filter - but RGB ones will be very short. You'll need to experiment but you are aiming for the histogram to be between 1/2 and 2/3 of the way across - somewhere between 28000-35000 ADU counts is typical.

The flat captures irregularities in illumination from vigneting and dust bunnies. While the temperature is not super critical for flats - they are way too short for dark current build up - focus position and camera orientation are. Once you take the camera off the scope, you'll never get it back perfectly aligned so the flats won't work.

As for the t-shirt, I think you'll find it's not needed. The 314L will go down 1/1000th of a second exposure, so unless your flats panel is mega-bright, you should be ok

 

[daz]

What capture software are you using?

[Logan Run]

hiya there am using APT software

[Tim]

3 hours ago, wongataa said:

Dark frames need to be done at the same time as light frames so everything is at the same temperature as when you took the light frames.  Flat frames can be taken at any time provided the orientation of the camera in regards to the telescope is the same as when you took the light frames (theoretically it shouldn't matter but it could do in the real world) so it makes sense to also take them at the same time before packing up.

I hate to disagree a bit, but am going to 

Dark frames need to be at the same temperature and duration of the lights. Using cameras with set cooling you can do them at any time, and even have a library of them ready to go as suggested above.

It is my experience however that flat frames are best taken on the same night as the lights, at the end of the session. because they not only remove vignetting, but also dust bunny artifacts, and these might move, present differently if focus point changes due to varying ambient temperatures (affects the scope not the camera) and sometimes the dust motes actually turn out to be ice crystals on some cameras, and they won't be the same on a different evening. All things considered, the flat frames are going to be most likely to coincide with the issues for any particular light frame if you can take them with conditions as absolutely similar as possible. So as Wongataa suggests, take them as your session finishes.

20 to 30 flat frames should do, with 50 to 100 bias frames to calibrate them.

Have fun

Tim

[Logan Run]

when you say 20 to 30 flats does that mean for each filter

[scotty38]

Yes flats per filter as it's a change in the optical system and different bits of dust etc etc.

Edited December 10, 2021 by scotty38