Flat Field Calibration pops up warning when Bias not used  

Hi,

yes I am getting same error message as I dont use Bias frames...so wil ne interested in any replies...

Olly

I realised my problem. I get this error when I'm not using darks. The exposure times where short on the cooled cam that noise wasn't a problem, however APP didn't like this.

I get it when I use darks and flats alone , but not when I use either bias or dark flats with them...!

Olly

Posted by: AstroCookbook Kev

I read somewhere that calibration without BIAS on the ASI1600MM yeilds better results. So I don't use them.

Instead I take FLATS, and immediately cover the scope and take DARKFLATS - so all settings are the same.

APP pops up this warning though:

CRITICAL WARNING: flat-field calibration can not be performed correctly!

To be able to perform accurate flat-field calibration,
the provided flats need to be calibrated by either

- a MasterBias, of the same ISO/gain value as the flat frame, or
- a MasterDarkFlat of the same ISO/gain value and the same exposure time as the flat frame, or
- both

I'm using the second approach - DarkFlats with my Flats, so what is it complaining about? Do I need to create Master DarkFlats first, then remove the subs and then re-calibrate?

Kev

 

Hi @itarchitectkev & @astroolly,

The warning

CRITICAL WARNING: flat-field calibration can not be performed correctly!

To be able to perform accurate flat-field calibration,
the provided flats need to be calibrated by either

- a MasterBias, of the same ISO/gain value as the flat frame, or
- a MasterDarkFlat of the same ISO/gain value and the same exposure time as the flat frame, or
- both

will and should appear if you have loaded flats without bias and/or dark flats or masters of these.

If you have loaded dark flats and/or bias, together with your flats, the warning should not appear. Does it in your case ? If so, I need to investigate and solve this a.s.a.p. 😉

The warning is there to ensure to the user that flat-field calibration is working and will be applied correctly. To be able to get correct flat-field calibration, bias and/or dark flats are needed.

If you load the dark flats as darks, then they will not be used for the flats. To use them for the flats, you need to assing them explicitely as dark flats 😉

Please let me know if this clarifies the appearance of the warning and/or if there might be a bug here.

Kind regards,

Mabula

Mabula, I have loaded dark flats to match the flats, but I don’t load biases and I too get this warning. 

Are you sure that the dark flats are loaded correctly, .i.e. for the same channel ?

If so, can you show a screenshot of the warning and the loaded frames in APP ?

The warning should not appear if dark flats are loaded with lights and assigned to the same channel/session.

Mabula

I only bet the warning when I have not loaded either Dark flats or bias, ie, a lot of my old data has just darks and flats, so I load

1. All lights

2. All flats

3. All darks

i then get the warning, if I load either bias or dark flats too, then I DO NOT get the warning, hope that clarify my issue...

Mabula, I *think* I know what may be happening. I have a new type of camera, the FLI Kepler KL 4040. It uses what’s called a Scientific CMOS (sCMOS) chip. This camera reads the chip twice for each exposure, once to generate a high gain  sub and once to generate a low gain sub. You process each set of subs separately with low and high gain darks, then use special software to merge the low and high masters into what amounts to a High Dynamic Range final master, which you then do your postprocessing on. However you don’t need to have low and high gain flats. I created my flats using just high gain. I was just doing Ha and created Hi and Low gain channels. But I assigned the flats to All Channels. Would that cause the issue? 

I don’t see why we need to use bias frames, as the bias signal is contained in the darks, so I only use darks and flats with my KAF8300 CCD camera, so as we dont need to use bias, this warining should not appear and tell us that we do...??

could someone explain why we should or need to use both darks and bias....??

Olly

In order to not use biases, you need darks exactly matching lights in terms of temperature and exposure duration. If for example you have a library of 2-minute darks and you want to use them on shorter exposure lights, you need the bias to accurately scale the darks. I frequently did this for ccd images but doesn’t work so well for CMOS. 

wayne