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Old 10-12-2019, 09:13 AM
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Merlin66 (Ken)
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Why use bias exposures?

Other than compensating for different dark temperatures why do take bias exposures???
Doesnít seem to be any point.
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Old 10-12-2019, 09:17 AM
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Quote:
Originally Posted by Merlin66 View Post
Other than compensating for different dark temperatures why do take bias exposures???
Doesnít seem to be any point.
Neither do I
Biasí are very useful if you want to do Dark Scaling and you need an accurate measurement of JUST the dark current. Other than that, darks are all you need.
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Old 10-12-2019, 10:13 AM
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You really donít need bias (nor darks) for calibrating dithered frames captured with ICX 674/694/814/834, even when chasing the faintest narrowband bits.
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Old 10-12-2019, 11:06 AM
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I use them all the time to subtract from flats prior to divide into the light frames.
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Old 10-12-2019, 11:07 AM
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Marc,
Why?
The flats and the lights both contain bias....
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Old 10-12-2019, 11:11 AM
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Quote:
Originally Posted by Merlin66 View Post
Marc,
Why?
The flats and the lights both contain bias....
If I don't use bias frames when flat fielding my master flat always over or under correct. That's my experience with calibration in CCD Stack. So I got into the habit of doing it.
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Old 10-12-2019, 11:38 AM
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Quote:
Originally Posted by Merlin66 View Post
Marc,
Why?
The flats and the lights both contain bias....
Itís the difference between a division and subtraction. If you donít use a bias or dark frame to calibrate your flats to make a Master Flat you end up dividing noise instead of subtracting it from your lights. As Marc says, it ends up under/over correcting your lights and doesnít remove the bias.
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Old 10-12-2019, 11:58 AM
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Ahh, I take dark flats as per AstroArt recommendations.
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Old 10-12-2019, 01:28 PM
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Originally Posted by Merlin66 View Post
Ahh, I take dark flats as per AstroArt recommendations.
So do I. Granted my dark flats are 0.5s to match my flats (no shutter). Iíd say that as a general rule of thumb with CMOS sensors, match everything as closely as possible because of the non-linear nature of dark current progression.
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Old 10-12-2019, 05:50 PM
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This is the method that I use, as recommended by Deep Sky Stacker.
Attached Thumbnails
Click for full-size image (DSS stacking with flat darks.JPG)
70.7 KB50 views
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Old 10-12-2019, 05:56 PM
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Ok, we agree....
So why do some still take bias images???
Just take a dark frame with both the light and the flat?
Ken
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Old 10-12-2019, 06:42 PM
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When using a preprocessing script in PixInsight, it requires bias for calibration with flats. Other than that, I find no use for bias, while darks are not needed at all, even when stacking hundreds of dithered 20-minute subs.
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Old 10-12-2019, 06:51 PM
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Taking bias frames may be a hangover from when DSLR's first came out, getting darks to match the lights is an issue temperature wise.

Also some folks do not even bother with darks at all and just substitute a Bad Pixel Map instead. Suavi doesn't use darks and I think Strongman Mike just uses a BPM.
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Old 11-12-2019, 07:16 AM
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I only use darks and dark flats (flat darks?) on my cmos camera, as recommended on a PI forum I saw somewhere. I found that I was getting clipping of my darks if I also calibrated with bias frames.

That took a while to figure out ...
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Old 11-12-2019, 10:24 AM
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For my ASI294, ZWO recommend not using bias frames. By my understanding it is related to the issues getting really effective flats for that camera. Any exposure below about 4 seconds is timed on chip by the SoC (System on a Chip) componentry of the IMX294 sensor itself, beyond that the ZWO circuitry takes over. That means the noise profile is different either side of the threshold so flats need to be 4 seconds + and bias frames would have a different profile.

That 4 seconds is an estimate I have seen, I have not found the exact figure unfortunately or I would use that. It means that flats need a very dim light source.
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Old 12-12-2019, 02:47 PM
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Quote:
Originally Posted by Merlin66 View Post
Ok, we agree....
So why do some still take bias images???
Just take a dark frame with both the light and the flat?
Ken
I routinely scale darks when using my CCD cameras so need bias images. For my ASI183 I have found that the darks don't scale so always use the correct exposure. Bias frames aren't necessary then but I tend to use the same processing pathway and take them anyway.
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Old 14-12-2019, 11:41 AM
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I also routinely scale my darks even though they are done at exact temperatures.

I subtract a bias from the master flat when applied not when making the master flat.

I also check the callibrations are working as I am imaging rather than assume they will. I tend to trial and error it a bit as often the calibration does not work that well until I get it looking right.

To add to the complication my FLI cameras have a slow download and a fast download speed with different read noises. So its usually not a problem but I have to make sure if I was using the faster download speed to speed up focusing I remember to switch it back so my calibration files match.

Callibrating astro files is a bit touchy especially now almost every scope has a reducer or flattener.

Greg.
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