What is the difference between Darks and dark flats?

[bluesilver]

Hi,  I am guessing it is a basic question, but after doing some research i can't find an answer on what is different between these two,  Dark frames and dark flat frames.

I am using DeepsSkyStacker and my camera is a ASI2600mc

So i know the Darks are taken with the cap on the end of the scope and the camera temperature, exposure, and all settings are the same as what you used for the lights

The Flats are taken with a illuminated white light at the end of the scope and  the camera temperature, exposure, and all settings are the same as what you used for the lights

The Bias is taken with the cap on the end of the scope and the camera temperature, and all settings are the same as what you used for the lights except the exposure is as fast as the camera will take them.

 

But this Dark Flats has got me stumped,  from what i have researched,  it is exactly the same as the Darks,  but this can't be right otherwise why would it be called Dark Flats.

So how do i go about taking these dark flats?

So say you don't have to take bias frames with this camera or it might of been dark frames,  not sure,  i was just going to take all of them and see how things turn out.

 

But mainly interested in finding out how to take these dark flat frames.

Any help would be appreciated.

Thanks.

 

[vlaiv]

Dark flats are to flats the same as darks are to lights.

Flat exposures although short tend to also accumulate some of dark current and bias signal (especially narrow band flats which tend to be longer than broadband and give more time for dark current to accumulate). You want to remove these and for that reason you want to take dark flats.

They are taken at exact same settings as flats - but with scope cover on.

For proper calibration of your subs - you need just darks, flats and flat darks. You don't need bias, and actually in this simple sort of calibration - bias is redundant. Bias is useful when you want to do dark scaling (ok for CCD - not as much for CMOS) or you want to do approximate calibration and you substitute say flat darks with bias because flats have short exposure.

[bluesilver]

Thanks for the reply,

I think i might see my error now,

 Darks are taken with the exact same settings as your lights,  but just with the cover on the scope,  I think that is correct?

Flats are taken with target ADU  ( this is what i completely forgot about )  only just got the ASI2600mc,  use to use a DSLR

So for this camera,  i think the ideal ADU is around 30000  ( i am using APT to do all this by the way )

Is there a target ADU or a target histogram that i should be looking for when doing flat darks?

I take it that also still should be running the camera at the same temperature as the lights and darks?

 

Edited October 21, 2021 by bluesilver

[vlaiv]

11 minutes ago, bluesilver said:

Is there a target ADU or a target histogram that i should be looking for when doing flat darks?

You don't look at histogram nor ADU value for that.

You simply repeat whatever settings you used for Flats - just with your scope covered.

Say you needed 1.2s exposure for your flats. You cover the scope and using same settings - gain, offset, temperature and exposure length of 1.2s - you take your flat darks.

13 minutes ago, bluesilver said:

So for this camera,  i think the ideal ADU is around 30000  ( i am using APT to do all this by the way )

When doing flats - use assistant to tell you exposure length needed (although you really don't need assistant to do that - take flat exposure - if too bright, dial it down, if to faint - prolong it until you get it right) and remember that exposure. Take all your flats at the same exposure time and take your flat darks at the same exposure length.

Btw, APT flat assistant has been known to mess up flats, so I recommend taking them manually.

ADU of 30000 and histogram are just guidelines. You want your histogram not to be clipped - no over exposed bits and you want your flats to be strong in signal not weak.

You want histogram peaks to be to the right - but not as far right so that it clips in highlights - there should be room to the right. This is if you set histogram to whole range 0-65535.

 

[daz]

11 minutes ago, bluesilver said:

 Darks are taken with the exact same settings as your lights,  but just with the cover on the scope,  I think that is correct?

 

Yep

11 minutes ago, bluesilver said:

Is there a target ADU or a target histogram that i should be looking for when doing flat darks?

 

You're not looking at the histogram for flat-darks. For flats, yes, but not for flat-darks. You're just doing the same as the darks for the light frames - same temperature, cover on.

For flats, you're aiming for somewhere between 1/3 to 1/2 way on the histogram, and somewhere between 50-75% saturation. Focus position and camera orientation need to be the same as when you took the light frames. If you move the camera, the dist donuts won;t match the flats. You'll need to experiment for your camera... 

Doesn't matter for the darks, there is nothing to see - as long as the temperature is the same.

 

[daz]

#typefaster

 

[bluesilver]

Thanks heaps,

That has explained it extremely well,  

Learned a fair bit also on just the flats,  i had it a tad wrong.

Very much appreciated and great explaining also.

Wish  i went here and asked first up,  my researching hasn't done me well.

Looking forward to putting it all into practice now.

Cheers.