Bias frames have a too long exposure

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Menno555
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Bias frames have a too long exposure

#1

Post by Menno555 »

SharpCap Pro 3.2.6338.0
Zwo ASI385MC
Windows 10 Pro 64bit

When taking Flats with the Subtract Bias selected, the Bias frames are taken with 1ms. But in the SharpCap manual it states that it is taken with the minimum of the camera. This is also mentioned online: use the lowest exposure possible. In my case that is 0.032ms.
I was getting weird results when using Flats with Bias and think this is the cause.

Menno
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admin
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Re: Bias frames have a too long exposure

#2

Post by admin »

Hi,


The documentation here is out of date – we found that some cameras behave oddly when at or very close to their minimum exposure values. A 1 ms exposure is a safe compromise between being short enough to have minimal effect in terms of thermal noise and being far enough away from minimal exposure values to avoid the problems. From the point of view of creating a working flat frame, the length of the bias exposure is largely irrelevant as long as it's less than the actual flat exposure by a good amount. The point of the flat frames to work out the relative sensitivity to light of the different parts of the sensor and this can be done quite nicely by taking (for instance) a one second exposure and then subtracting a 0.25 second exposure from it.

Cheers, Robin
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Menno555
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Re: Bias frames have a too long exposure

#3

Post by Menno555 »

Ah okay. This is de-bugged then :)
Thanks
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turfpit
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Re: Bias frames have a too long exposure

#4

Post by turfpit »

Have a read of this by Gary Palmer:

https://www.skyatnightmagazine.com/astr ... otography/

In the Calibration Frames section Gary says:
Use darks, flats and bias frames – despite there being some debate on whether the latter upsets the stacking of images, they do work for me.

I capture bias frames at around two seconds and that prevents any problems upsetting the subtraction in processing.
I have seen this type of statement before relating to bias frame exposure length on modern CMOS cameras, with exposures around 1s being offered.

On an Altair 183C I have been using minimum exposure EXPTIME = 0.000264 for bias frames which seems to work
https://www.astrobin.com/full/p63izu/0/. I leave the black_level (offset) and gain the same as for the dark frames.

Dave
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