Dark, Flat and Bias Frames on Sigma 100-400 DG DN OS for DSO - Forums

Topic starter

Jun 7, 2022 · Edited: Jun 7, 2022

Hello People,

I know this have been asked before here and is widely talked about over the web however I am little confused with the information I have come across so posting it here for more clarity.

I have a Sony A7III w/ Sigma 100-400 DG DN OS lens which I use for DSO imaging (only had a couple of chances to try this out on my Star Adventurer though).

The problem I ran into with this lens was that there is no lock for the focal ring and not a very good marking showing which focal length I am at so while taking the Calibration frames (darks, flats, biases) I noticed that the focal ring is so sensitive that it moved when I tried to put the lens cap on.

Now my confusion is with respect to the calibration frames where I need to put the lens cap on after my imaging session to take the calibration shots.

From what I read, for Darks and Biases I don't have to worry about the focal length getting changed while taking the shots however for Flats I need to make sure that everything in the setup should remain the same (focal length, filter, etc.) however as I cannot lock the focal ring it is going to retract if I put anything on top of it at the slightest touch.

This is what I understand from reading/watching over the web:

Dark: Need to match the shutter speed, ISO and the temperature (of the sensor) of the light frames by covering the lens cap. Basically, take these post imaging session.

Bias: Need to shoot at the highest shutter speed available on my Sony A7III, keeping the ISO same as Light frames in a dark environment. Temperature is not important so can be taken at any point.

Flat: Need to put the Camera in Av mode, keep the ISO and shutter speed same as Light frames and covering the lens with a paper/cloth with an even light source. Is Focal length important here or it can be anything (i.e. different than the light frames??)

Please modify/correct and advise the correct way of taking these calibration shots and let me know if I have these understood correctly or missing something there.

I hope I am able to explain it clearly.

Thank you in advance.

Regards,
Nick

Earle Waghorne

Jun 7, 2022 · Edited: Jun 7, 2022

Hi Nick,

You have this right, alghough I would add one proviso.

Let me bore for a second. There are three things on your image (any image) 1- the thing you are photographing, 2 - an effects from dust on lenses, camera sensors etc and 3 - any artefacts from the camera electronics.

So flats remove the effects of dust, etc. and darks remove the effects of artefacts from the camera sensor. Of course flats are images and we need to remove the artefacts from the camera electronics from them.

So your calibrated image is essentially = Sub - Dark - (Flat - Dark Flat)

For CCD cameras the sensor artefacts are 1-read noise, that is always the same and is corrected with biases, and 2-thermal noise which at a constant temperature is essentially linear in time. So knowing the exposure time of any dark it is possible to estiimate the dark for any other exposure time and this is fine for flats, which are generally short exposures. In essence the Dark Flat is calculated by subtracting the bias from the Dark and then multiplying the Dark by the ratio of the exposure times.

For CMOS cameras, and I don't profess to understand why, the thermal dark current isn't necessarily linear in time (there is a post on the Pixinsight forum about this for the ZWO 1600 and cameras with the same Panasonic sensor). In this case the calculation of the Dark Flat from the exposure time (and a reference dark of known exposure time) is less accurate and probably depends on things like gain.

So for CMOS cameras it is common to measure Dark Flats directly (darks with the same exposure time as the flat) rather than estimating them. This adds a few minutes but guarantees proper calibration.

Hope that this is of some use.

CS

Helpful Insightful Respectful

andrea tasselli

Frequent contributor

Jun 7, 2022

Dark: Need to match the shutter speed, ISO and the temperature (of the sensor) of the light frames by covering the lens cap. Basically, take these post imaging session.

Bias: Need to shoot at the highest shutter speed available on my Sony A7III, keeping the ISO same as Light frames in a dark environment. Temperature is not important so can be taken at any point.

Flat: Need to put the Camera in Av mode, keep the ISO and shutter speed same as Light frames and covering the lens with a paper/cloth with an even light source. Is Focal length important here or it can be anything (i.e. different than the light frames??)

Please modify/correct and advise the correct way of taking these calibration shots and let me know if I have these understood correctly or missing something there.

Dark: Correct but do not fret about the temperature.

Bias: Correct.

Flat: Keep the ISO the same and shutter speed whatever suits the situation. If shutter speed slower than 1/30s take also as many darks with the same exact shutter speed and use those to calibrate the master flat frame.

As for zoom moving, a well known problem, my solution is to mark the focus point on strip of electric tape and block movement of the zoom ring with a substantial elastic band.

Helpful Concise

kuechlew

Jun 7, 2022

Hi,

Darks and Bias Frames depend on the characteristics of the sensor only. So you can even take them without a lens attached.
Flats capture the specifics of the image train, mostly dust and vignetting. Taking them with the same focal length as your lights is crucial. Once you decided on your focal length for imaging, fix your zoom ring with tape as good as possible.

It has become common practice to ditch the Bias Frames and take Dark Flats instead. These are essentially Darks taken at the temperature, iso and exposure time of your Flats.

Correction to Earles post: Calibrated Light = (Light - MasterDark) / MasterFlat

For Details see here: Guide To Calibration Frames - Lights, Darks, Flats, Dark_Flats, and Bias (nightskypix.com)

Clear skies
Wolfgang

Helpful

nickastrobin

Topic starter

Jun 7, 2022 · Edited: Jun 7, 2022

Earle Waghorne:
Hi Nick,

You have this right, alghough I would add one proviso.

Let me bore for a second. There are three things on your image (any image) 1- the thing you are photographing, 2 - an effects from dust on lenses, camera sensors etc and 3 - any artefacts from the camera electronics.

So flats remove the effects of dust, etc. and darks remove the effects of artefacts from the camera sensor. Of course flats are images and we need to remove the artefacts from the camera electronics from them.

So your calibrated image is essentially = Sub - Dark - (Flat - Dark Flat)

For CCD cameras the sensor artefacts are 1-read noise, that is always the same and is corrected with biases, and 2-thermal noise which at a constant temperature is essentially linear in time. So knowing the exposure time of any dark it is possible to estiimate the dark for any other exposure time and this is fine for flats, which are generally short exposures. In essence the Dark Flat is calculated by subtracting the bias from the Dark and then multiplying the Dark by the ratio of the exposure times.

For CMOS cameras, and I don't profess to understand why, the thermal dark current isn't necessarily linear in time (there is a post on the Pixinsight forum about this for the ZWO 1600 and cameras with the same Panasonic sensor). In this case the calculation of the Dark Flat from the exposure time (and a reference dark of known exposure time) is less accurate and probably depends on things like gain.

So for CMOS cameras it is common to measure Dark Flats directly (darks with the same exposure time as the flat) rather than estimating them. This adds a few minutes but guarantees proper calibration.

Hope that this is of some use.

CS

Thanks a lot for the explanation and quick response @Earle Waghorne ! Much appreciated

nickastrobin

Topic starter

Jun 7, 2022 · Edited: Jun 7, 2022

andrea tasselli:
Dark: Need to match the shutter speed, ISO and the temperature (of the sensor) of the light frames by covering the lens cap. Basically, take these post imaging session.

Bias: Need to shoot at the highest shutter speed available on my Sony A7III, keeping the ISO same as Light frames in a dark environment. Temperature is not important so can be taken at any point.

Flat: Need to put the Camera in Av mode, keep the ISO and shutter speed same as Light frames and covering the lens with a paper/cloth with an even light source. Is Focal length important here or it can be anything (i.e. different than the light frames??)

Please modify/correct and advise the correct way of taking these calibration shots and let me know if I have these understood correctly or missing something there.

Dark: Correct but do not fret about the temperature.

Bias: Correct.

Flat: Keep the ISO the same and shutter speed whatever suits the situation. If shutter speed slower than 1/30s take also as many darks with the same exact shutter speed and use those to calibrate the master flat frame.

As for zoom moving, a well known problem, my solution is to mark the focus point on strip of electric tape and block movement of the zoom ring with a substantial elastic band.

@andrea tasselli , you are always one of the few to reply to these posts and I appreciate it very much! Thank you for confirming and clarifying my doubts. Much appreciated! BTW, this docis a pretty good one and I have it bookmarked.

Well Written Respectful Supportive

nickastrobin

Topic starter

Jun 7, 2022

Hi,

Darks and Bias Frames depend on the characteristics of the sensor only. So you can even take them without a lens attached.
Flats capture the specifics of the image train, mostly dust and vignetting. Taking them with the same focal length as your lights is crucial. Once you decided on your focal length for imaging, fix your zoom ring with tape as good as possible.

It has become common practice to ditch the Bias Frames and take Dark Flats instead. These are essentially Darks taken at the temperature, iso and exposure time of your Flats.

Correction to Earles post: Calibrated Light = (Light - MasterDark) / MasterFlat

For Details see here: Guide To Calibration Frames - Lights, Darks, Flats, Dark_Flats, and Bias (nightskypix.com)

Clear skies
Wolfgang

@kuechlew , good to hear from you again! Your advice and information is much appreciated. Thanks a lot!