I have read many places that for the best calibrated astrophotographs, you should be taking bias frames, dark frames, and flat frames.

But on a forum I read the following statement

You usually don't need to. DSLRs with CMOS sensors (as opposed to CCD) automatically take and subtract a bias frame every time an exposure is made. If they didn't, the noise would be horrendous.

Almost all current models of DSLR use CMOS sensors; some older Nikons used CCD.

Is this true? Does this mean that all the astrophotography literature I have been reading is either outdated, or just wrong?

Is there no point taking bias shots to subtract from my flat frames and light shots?

EDIT: I should clarify that a bias frame is not a dark frame. The quoted person went on to indicate that all CMOS sensors take a bias frame afterwards, and SOME have the option for take a dark frame, which is taken with the same shutterspeed as the light frame (bias frame is always taken with fastest shutter).

  • \$\begingroup\$ I think the forum poster should have said, "SOME cameras have the OPTION of taking a dark frame automatically." This isn't a default option, and where it is, it isn't on be default either. So, your literature is correct. \$\endgroup\$
    – OnBreak.
    Commented Nov 29, 2017 at 23:00
  • \$\begingroup\$ Actually further discussions went on to make the distinction that bias frame is NOT a dark frame. And this optional dark frame is in addition to the light and the bias. I will update main question. \$\endgroup\$
    – Scorb
    Commented Nov 29, 2017 at 23:01
  • \$\begingroup\$ There are probably some cameras somewhere that do not measure bias from the sensor with each exposed frame. So until you specify a specific camera the question probably can not be definitively answered. \$\endgroup\$
    – Michael C
    Commented Nov 30, 2017 at 0:24
  • \$\begingroup\$ The question does not pertain to a specific model. The question is reduced to "based on the statement that all CMOS sensors take a bias frame, do I still need to take bias frames for astrophotography on CMOS sensors"? The question is totally valid and independant from a specific model. \$\endgroup\$
    – Scorb
    Commented Nov 30, 2017 at 0:58
  • 1
    \$\begingroup\$ It'd be good form to include a link to the forum where the quoted text comes from. \$\endgroup\$
    – Caleb
    Commented Dec 3, 2017 at 7:23

1 Answer 1


I've had a couple of tries with Siril and found it very complex and probably designed for the astronomy enthusiast who takes photograph's rather than the photographer who takes the occasional astro picture. That said, results are possible.

https://free-astro.org/index.php/Siril:Tutorial_preprocessing states as follows:

WARNING: Remember that dark frames are always composed from real dark signal and bias signal. If you don't apply dark optimization, you can leave the bias signal and your masterDark will be in fact . In consequence subtracting this master to the light frames will remove both signals. However, applying dark optimization makes things different by multiplying masterDark by a coefficient factor not equal to 1. In this case, you must subtract masterBias from each dark frame.

http://www.rawastrodata.com/pages/typesofimages.html states it a bit more clearly (it also describes the different types of images used quite nicely):

Note: Dark frames also contain the bias signal which is why I can get away without subtracting bias frames. I have read that subtracting a bias from the darks can help improve DeepSkyStacker’s dark optimization feature.

It seems that if you are using the Dark Frames made necessary by long exposures, you don't need Bias Frames as the information is already in the Dark Frame.


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