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ASI533/2600 darks, bias, do I need to do?


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As titled really. I'm guessing I don't need to do darks at all. Do I still need to do a master bias or flat dark? Was a necessity with my 1600MM a few years ago. Would I get away with flats and lights? If I can't use a bias how do I calibrate the lights? What do 2600 users do?

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You will definitely need to calibrate your flats with flat darks or bias and your lights with darks or you will get overcorrection of flats.

I use bias for flats since my flats exposure time is only 0,13s.

Darks are of little use with how little dark signal the cameras produce but its still needed to weed out hot pixels and calibrate lights (removing offset mostly).

I have a dark master made of 50x 30s exposures at -10 and use that for everything, even though often i take longer exposures or the camera is cooled lower than -10. But at -10 or lower it really doesnt matter anymore as the total amount of dark signal in a stack could still be less than 1 electron over many hours.

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I have IMX 571 OSC cameras and calibrate the lights with master darks, flats and dark flats. I have no idea if the darks are needed but they don’t seem to do any harm and it’s no hardship to use them.

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We spend thousands on kit and countless hours trying to capture precious photons, why wouldn't we aim for the best calibration we can possibly get?  You may get away without using Darks, but why do this when building a Dark library is no hardship and they can be reused over a season?

Darks - same exposure, temperature, offset and gain as lights

Flats - aim for a couple of seconds if possible (use t-shirt, sheets of white paper etc to reduce brightness of the light source), same temperature, offset and gain as lights

Flat Darks - match your Flat exposure time, same temperature, offset and gain as lights

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6 minutes ago, Starflyer said:

Flats - aim for a couple of seconds if possible

This might be important for some models of camera, but for IMX571 based cameras there is no reason to take long flats as the sensor has a linear response all the way to saturated (99% + linear).

A few milliseconds is enough if the panel is bright enough and there are no issues with calibration.

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Thanks. So same as the 1600. I might crank up the flat panel brightness and do short flats and use a master bias. Can't be dealing with different flat exposures and shooting flat darks for each filter again!

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Posted (edited)

When I imaged with an ASI1600 I also had to take long  exposure flats (2 sec.)  and calibrate them with dark flats, as this particular sensor has some issue with very short exposures (I don't remember which). Dialing the panel brightness to match the exposure time of the master dark flat was really a pain.

Now I image with an ASI183, and I  take short exposure flats (typically 0.1sec luminance and 0.3 sec chrominance). For calibration, instead of a master bias I use a synthetic bias, and it works very well. Essentially just a value  that you need to measure once for all.  Some details here :

https://siril.org/2021/12/enough-with-dark-flats/

https://siril.org/tutorials/synthetic-biases/

I also used this procedure  successfully with an ASI533MC.

Dan

Edited by Dan_Paris
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On 03/06/2022 at 08:45, Dan_Paris said:

When I imaged with an ASI1600 I also had to take long  exposure flats (2 sec.)  and calibrate them with dark flats, as this particular sensor has some issue with very short exposures (I don't remember which). Dialing the panel brightness to match the exposure time of the master dark flat was really a pain.

Now I image with an ASI183, and I  take short exposure flats (typically 0.1sec luminance and 0.3 sec chrominance). For calibration, instead of a master bias I use a synthetic bias, and it works very well. Essentially just a value  that you need to measure once for all.  Some details here :

https://siril.org/2021/12/enough-with-dark-flats/

https://siril.org/tutorials/synthetic-biases/

I also used this procedure  successfully with an ASI533MC.

Dan

The issue with both 183 and 1600 is they produce a lot of amp glow. The darks serve to remove that. With the newer IMX sensors there's no amp glow so it's questionable if darks are needed at all. After shooting a darks library anyway I can confirm the bias signal is around 2000ADU at 100 gain and 50 offset so this needs to be removed. I used a master bias for the short flats I took the other night and master dark for the lights. No issues with calibration.

12 x 300s each SHO gain 100 offset 50

SHO.jpg.d23cbe311af3b52e340619ba0fc3cc79.jpg

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