How To Take Bias Frames on a DSLR?

[themos]

If they add a bias level to subtracted frames, how come I see a nice gaussian in the intensity distribution for bias frames? confused...

Subtracting one noisy frame from another noisy frame adds the noise. Ok, it removes any gross, repeatable, constant component but so what? We deal with that with flats anyway...

[brianb]

If they add a bias level to subtracted frames, how come I see a nice gaussian in the intensity distribution for bias frames?

Random readout noise.

[themos]

Ok, I buy that!

Normal Difference Distribution -- from Wolfram MathWorld

[ashworthacca]

Just coming back to this, based upon what has been said Flats must already be bias subtracted? So you only really need lights, darks and flats for calibrating and stacking in dss?

I'm confused even more now, after the other dss calibration thread.

[Psychobilly]

Was the way you were doing things producing images that you were happy with .. if it was stick with it ...

Billy...

[ashworthacca]

I've never used bias frames but was going to when everyone was saying Flat frames should be bias subtracted.

I'm pretty new to all this so to even get a respectable image I was happy.....

[roundycat]

your flats should only be bias subtracted if you do not calibrate them by subtracting a flat dark.

A good master bias does not take long to put together. Shoot 30 bias frames and combine them using a median output - job done. (30 frames will reduce the added noise to 1.7%).

If you use flat darks you probably need to take a set of darks for each flat, one flat for each filter if using a mono camera. That could be thirty x four flat darks (LRGB) and as most of the exposures would likely be fairly short there may not be much of a thermal pixel problem. (unlike in a ten or fifteen minute light frame). So, the best thing to do in theory is to subtract flat darks. In practice it does not make much difference so you see more and more people cutting corners by just subtracting bias from their flats. But you must do one or the other.

Dennis

[ashworthacca]

Thanks Dennis, that is how I previously understood it before Brian posted that all DSLR subs are bias subtracted?

[roundycat]

I thought that reference was to DSLR's using CMOS chips rather than CCD. I don't know one way or the other.

Dennis

[ashworthacca]

I use a canon 1000d which is CMOS isn't it?

I think I'll process with and without Bias frames and see what the results are!

[buzz]

Just found this old thread after doing a search - I understood a master bias file to be the average of many bias frames, to remove the random bit (the read noise).

If a photographic camera is simply subtracting a single bias frame from the image - that surely must be just to get the approximate 'DC' level right.

As someone else said, it would also seem to increase the overall noise as it will inject read noise into the image, that can only be removed by extensive averaging many subs.

If flat frames are used, surely you still need a good clean master bias frame, otherwise the flat frame computation will have gain noise added into it?

[Alien 13]

Ive just come across this thread and theres lots i didnt know i will be making a few changes to how i capture images. I have uses the sensor clean method to remove hot pixels from time to time but was never sure of the value of bias frames and auto long exp noise reduction.

 

Alan

[dph1nm]

Well it is an old thread, but there is a bias signal in CMOS DSLR data. 256 ADU in my Canon 1000D, 512 or 1024 in more modern cameras I believe (and it is not flat). This needs subtracting from both lights and flats before you flatfield.  You can do this equally well using darks (as they have this bias level as well), but you will need separate flat-darks in that case.  Whatever you use,  you should make sure you average enough together to beat down the random noise,  as this gets added in quadrature to the noise in the lights.

NigelM

[IanL]

Well it is an old thread, but there is a bias signal in CMOS DSLR data. 256 ADU in my Canon 1000D, 512 or 1024 in more modern cameras I believe (and it is not flat). This needs subtracting from both lights and flats before you flatfield.  You can do this equally well using darks (as they have this bias level as well), but you will need separate flat-darks in that case.  Whatever you use,  you should make sure you average enough together to beat down the random noise,  as this gets added in quadrature to the noise in the lights.

NigelM

I'm with you on this one. I don't know where the idea that Canon Bias frames are "flatter than a ..." comes from.  They're certainly not and a clear fixed pattern emerges when you stack enough of them. I guess the magnitude of the pattern may not be huge on some models, but given the other limitations of DSLRs every little improvement helps the final image.  (See here: http://www.blackwaterskies.co.uk/2013/09/pixinsight-dslr-workflow-part-1-bias.html).

Where the jury is still out for me is where best to apply the master bias once you have made it.  I certainly subtract it from flats before stacking the master flat.  I am undecided as to whether to use it as part of the dark calibration process or not though.  I've tried a few different approaches so far and the differences are marginal so still on the fence at the moment.

[dph1nm]

Here is a mean bias from my 1000D at ISO800

http://astro.dur.ac.uk/~nm/bias.html

very stripey (and different in the different colours).

NigelM

[lensman57]

Hi,

I want to take bias frames on my Canon 550D.

1. Can I take them once and then never have to do them ever again?

2. Should I take a range of them at different ISO settings?

3. Should I take a number and average them?

4. Can I take them with the stock lens on and the lens cap on, or should I set the camera up in a certain way or perhaps leave it on for 30mins to warm up?

In Short, how do I take them?

Thanks

Daniel

Hi,

You take them at the fastest avilable shutter speed of your camera with the lens cap on, you need about 50+ of these . You have to take them at the of the imaging session as despite rumors the sensor temp is important and you want them at the same temp as you captured the lights. You can not use a master Bias or a Master Dark from a library for a DSLR as these  are highly TEMP dependant and as you have no control over the sensor TEMP as you could with a cooled CCD. As the sensor gets older the Bias and Dark noise patterns change so you need fresh ones from time to time even if you were using a cooled CCD with set point temp cooling.

A.G

[dph1nm]

Can't say I have ever noticed any change in bias on my 1000D - I happily use biases from months or even years ago.

NigelM

[IanL]

Can't say I have ever noticed any change in bias on my 1000D - I happily use biases from months or even years ago.

NigelM

Ditto.  Darks are definitely more problematic but it is possible to get satisfactory results with a bit of effort.

[jrista]

I just thought I would chime in here. I found this topic while searching on a similar topic. 

When it comes to Canon DSLRs, they do not really subtract a bias frame from the image. There isn't a separate exposure or anything like that. Canon CMOS image sensors simply use CDS, or Correlated Double Sampling. This is an active process that occurs during readout. CDS units on each column sample the dark current noise flowing through the sensor at "reset" time. When the sensor is read for the full image, each pixel is sampled, and the dark current sample from the reset is subtracted. That's it. CDS greatly reduces the noise contribution from dark current (it doesn't eliminate it, but it does reduce it considerably.) Canon sensors do use a bias offset (calibrated via a border of masked pixels on the sensor that are also saved to the RAW file), so negative signal is preserved. Canon does have some noisy downstream electronics, and a lot of the banding that shows up in Canon RAW images is thanks to those downstream electronics, not the sensor itself (their sensors are actually quite good, and their CDS is top notch, so if Canon can ever figure out how to reduce downstream noise contribution, they would make even better astro cams than they do now.)

I've always wondered about the value of a Bias Frame myself. I've only more recently gotten deeper into astrophotography, but I've experimented it for some time. In my experience, every time I have involved bias frames, my noise seems to get worse (at least with Canon sensors). Not exactly more, however it does take on a less desirable characteristic...usually noise ends up becoming patterned, forming barely visible angled bands across the background of the image. When I exclude bias frames from my calibrations, and only using dark frames, noise maintains the nicer, entirely random aesthetic.

If you use a Canon DSLR for your astrophotography, you might want to forgo bias frames. Or at the very least, try processing a few of your images with and without them, and compare the quality of the noise. Specifically the noise quality, not the amount of noise, as overall the amount doesn't seem to change...it just seems to get more unsightly with bias frames. I would also experiment with CCDs. To my knowledge, CCD sensors were using CDS long before CMOS sensors ever came along. I don't know as much about the actual construction of astro imaging CCDs on the market today (they mostly seem to be Kodak, but beyond that I don't know the specifics of their construction). I would expect CCDs to use CDS...its about the most fundamental and ubiquitous form of hardware level noise reduction there is for imaging sensors.

Where you should really be concerned is if you use a Nikon camera. Nikon has long been known to clip negative pixel values, rather than use a bias offset. They have also been known to use a variety of sensors from a variety of sources. That makes it difficult to fully know the characteristics of Nikon cameras in general. You have to investigate the specifications and behavior of each Nikon camera to know how it will affect your images. Nikon also has indeed employed additional forms of hardware noise reduction beyond just CDS in their RAW images. Again, this differs from model to model, but it can impact your results.

If I were to decide whether to use bias frames or not, it would be as follows:

Canon: Probably not

Astro CCD: Maybe

Nikon: Who knows!

Well, hope this helps. 

[dph1nm]

Well you have to use either a bias or a dark if you want to flat field, and the dark contains the bias, so either way you are subtracting the bias!

NigelM

[lensman57]

I just thought I would chime in here. I found this topic while searching on a similar topic. 

When it comes to Canon DSLRs, they do not really subtract a bias frame from the image. There isn't a separate exposure or anything like that. Canon CMOS image sensors simply use CDS, or Correlated Double Sampling. This is an active process that occurs during readout. CDS units on each column sample the dark current noise flowing through the sensor at "reset" time. When the sensor is read for the full image, each pixel is sampled, and the dark current sample from the reset is subtracted. That's it. CDS greatly reduces the noise contribution from dark current (it doesn't eliminate it, but it does reduce it considerably.) Canon sensors do use a bias offset (calibrated via a border of masked pixels on the sensor that are also saved to the RAW file), so negative signal is preserved. Canon does have some noisy downstream electronics, and a lot of the banding that shows up in Canon RAW images is thanks to those downstream electronics, not the sensor itself (their sensors are actually quite good, and their CDS is top notch, so if Canon can ever figure out how to reduce downstream noise contribution, they would make even better astro cams than they do now.)

I've always wondered about the value of a Bias Frame myself. I've only more recently gotten deeper into astrophotography, but I've experimented it for some time. In my experience, every time I have involved bias frames, my noise seems to get worse (at least with Canon sensors). Not exactly more, however it does take on a less desirable characteristic...usually noise ends up becoming patterned, forming barely visible angled bands across the background of the image. When I exclude bias frames from my calibrations, and only using dark frames, noise maintains the nicer, entirely random aesthetic.

If you use a Canon DSLR for your astrophotography, you might want to forgo bias frames. Or at the very least, try processing a few of your images with and without them, and compare the quality of the noise. Specifically the noise quality, not the amount of noise, as overall the amount doesn't seem to change...it just seems to get more unsightly with bias frames. I would also experiment with CCDs. To my knowledge, CCD sensors were using CDS long before CMOS sensors ever came along. I don't know as much about the actual construction of astro imaging CCDs on the market today (they mostly seem to be Kodak, but beyond that I don't know the specifics of their construction). I would expect CCDs to use CDS...its about the most fundamental and ubiquitous form of hardware level noise reduction there is for imaging sensors.

Where you should really be concerned is if you use a Nikon camera. Nikon has long been known to clip negative pixel values, rather than use a bias offset. They have also been known to use a variety of sensors from a variety of sources. That makes it difficult to fully know the characteristics of Nikon cameras in general. You have to investigate the specifications and behavior of each Nikon camera to know how it will affect your images. Nikon also has indeed employed additional forms of hardware noise reduction beyond just CDS in their RAW images. Again, this differs from model to model, but it can impact your results.

If I were to decide whether to use bias frames or not, it would be as follows:

Canon: Probably not

Astro CCD: Maybe

Nikon: Who knows!

Well, hope this helps. 

The practice of massaging the raw data is not restricted to Nikon, I read an article by Craig Stark, the author of PHD guiding and Nebulasity, while testing a Canon DSLR and he

conclude that something strange was going on behind the scenes with the RAW data. I am not clear however , if all stacking software follow the same routine for calibration or not. In Nebulasity for example,  first a master Bias is made then subtracted from individual Flat frames before these are stacked to create a master Flat, from here on Bias plays no role in the proceedings as the Bias in the Dark frames will take care of the lights. The problem with excessive noise in the DSLR captures has more to do with mismatched or inadequate Dark frames.

Regards,

A.G

[oldpink]

excuse the newbie dumb questions here but I am reading up on dark's, flat's and Bios shots and how and when to take them

in preparation of the arrival of my Baader modified Canon 600D that will be used with DDS and either BYEOS or ATP (maybe both)

having read several threads tonight on here discussing this I'm getting a bit lost

even though all the hype about the 600D and 1100D having very low noise ratio's I assumed I'd still need to to do Darks, Flat's and BIOS images to achieve the best results
and liked something I read about building a library of blacks etc at different temps to save time and shutter use

and how its not good to use the auto that takes its own darks to work with as it uses up imaging time and shutter count

now I'm reading that this isn't necessary as the camera will sort it out its self with another exposure it takes on its own (short) to correct the exposure

or have I got this completely  wrong (thats a high possibility) 

 

[wxsatuser]

Don't worry about that incamera short exposure, it does'nt really matter to us.
The latest dslrs are low noise but the lower the temperature the better.
Technically you don't need bias if you use darks.
You need bias to calibrate your flats.
 
Yes you can use a library of darks but make sure your software can scale the darks.

[jrista]

excuse the newbie dumb questions here but I am reading up on dark's, flat's and Bios shots and how and when to take them

in preparation of the arrival of my Baader modified Canon 600D that will be used with DDS and either BYEOS or ATP (maybe both)

having read several threads tonight on here discussing this I'm getting a bit lost

even though all the hype about the 600D and 1100D having very low noise ratio's I assumed I'd still need to to do Darks, Flat's and BIOS images to achieve the best results

and liked something I read about building a library of blacks etc at different temps to save time and shutter use

and how its not good to use the auto that takes its own darks to work with as it uses up imaging time and shutter count

now I'm reading that this isn't necessary as the camera will sort it out its self with another exposure it takes on its own (short) to correct the exposure

or have I got this completely  wrong (thats a high possibility) 

The only thing that is really in question is bias (not bios, that's computer firmware...bias) frames. When it comes to bias frames, you should take some and compare calibration and stacking results with and without, and use whichever approach produces the best results.

As for dark frames, flat frames, and dark flat frames, you should still take those. When it comes to dark frames, they will include the bias offset information as well, hence the reason why you probably don't need separate bias offset frames for calibrating standard light frames. When it comes to flat frames, if you do not take dark flats, then the recommendation is that you should at least calibrate them with bias frames. Personally, I take dark flats to calibrate my flats with, as that handles all the various forms of noise that you get from the camera (where as the bias frames alone won't necessarily take care of it...kind of depends on exactly how long your flat frame exposures are.) 

So, if your doing astrophotography...take lights, darks, flats, and dark flats. That should give you the best results. If you want to take bias frames as well, just compare and contrast with and without, to make sure they aren't actually reintroducing noise.

[Russe]

Dumb question: how do you make dark flats in DSS?