Bias files in DSS cause noise and granulation in stacking RAW files from Canons ? ! ?
I don't know explain, but this is the result of my tests.
Target: NGC 6496
Top photo - with Bias
Bottom photo - without Bias.
First photo: Canon T3
Second photo: Canon 350D - without internal IR filter.
Both photos have HDR convertion, adjustment in curves and level, in Phothop CS3.
Without Bias the image has less noise and the stars have better shape.
With Canon 350D I needed to reduce the intensity of red information. I haven't, yet, a good custom white balance to avoid the general redish color. So I have loose of stars color information, mainly blue. It was necessary to adjust it with reduction of Red in Selective Color-Red.
I was reading again the documentation of DSS. And they tell that dark, flat and bias add noise. They improve some details of the photo, but they really add noise. The more files, more noises.
The documentation has an example of improvement and shows the noise, too: without no files, with two files and with 20 files, over a 32 lights.
Canon RAW has a tool for reducing long exp noise before export as TIFF.
2nd tab. sorry can't remember the name of the tool (Luminance I think). But does a pretty good job reducing noise before processing
I don't use bias frames. Just darks and sometimes flats.
I don't understand why, Kevin? (unless you use Flat Darks). Bias should be subtracted from the darks, flats and lights to remove read noise. Bias frames are easy to take, and easy to apply. Why not do it?
My straightforward procedure:
1. Darks subs - Bias Master (BM), then combine to create DM (or Bad Pixel Map)
2. Flat subs - BM, combine to FM
3. (Light subs - BM - DM)/FM
There seems to be as many opinions as there are imagers. I have several
reputable books on astrophotography, and they all, as I recall, state that
most of the time bias frames are not needed. It seems from what I have
read that just about the only time these authors use bias frames is when scaling darks. I bow to their experience, and don't use them.
raymo
The calibration maths is pretty clear Raymo. You need to remove read noise in some way. If you leave this in your darks, and then take flat darks to remove the dark current + bias from your flats, then I agree you don't need them. If you don't take flat darks, then you need bias. (and if your flats are short duration, then bias == flat dark).
I didn't think they would make much difference. I can see obvious benefits in darks and flats, but if in Jorge's example they actually increase noise.... I'll have to experiment and see what happens.
I have never used bias frames, I thought that bias is in the darks and would be removed along with the dark subtraction.
If you subtract a dark and a bias, wouldn't that subtract the bias twice? (assuming that bias is in the darks) Which would then add in noise?
Jo
That's my understanding. I wouldn't use both unless I'd already subtracted the bias from the dark (like you would if you were scaling your dark frames).
We must analyse the issue in both side of the coin.
1) the main noise in lights is reduced by amount of these lights. The noise isn't created by the camera.
2) there are others electronic noises:
a) in light, dark, flat and bias
b) hot pixels
3) DSS can subtract these electronic noises and clean the image, enabling enhancement of details in photo. But these files will carry electronic noise, too.
And it will be the same for lights. The amount of files can reduce these noises.
I will experiment with much more bias files and stack then again to see the difference in the result. I will post it here. I used only 10 bias files. (How many bias files I will need with my set of equipments and my Canons T3 - 350D ? )
Some very important texts from DSS documentation about the matter:
Quote:
Following the square root rule you will have much cleaner masters if you use a lot of frames to create them. Remember that you are trying to remove the dark and bias signals, not the noise that is coming with it.
For example when you subtract the master dark from each light frame you are adding the noise of the master dark to the noise of the light frame. The smaller the noise of the master dark, the less noise you will add to the light frame. This is also true for the master bias and the master flat.
Quote:
In fact by using only a very small number of frames for the creation of the masters you can easily triple the noise of the calibrated light frame (bias and dark subtracted and flat divided) compared to the noise of the light frame before calibration.
You will then need 9 times (3 squared) more light frames to bring back the noise to the level you could have had by using noise free masters.
Quote:
Hot pixels are pixels that are not behaving normally. They are a very strong signal that is visible in each dark and each light frame.
Of course when you subtract (only) one dark frame to one light frame you will remove the hot pixels which may give the false impression that the dark subtraction did its job.
However, at the same time the subtraction doubled the noise of the calibrated light frame and ruined it thoroughly.
It's all too complicated. I think I'll go back to film
You don't need to be so radical, Kevin. Only professional astrophotographers need to deal with this issue so serious. We, amateurs, can get GOOD images with less care.
As soon they are ready I will post new photos with 100 bias files.
Canon T3 presents more noise. Therefore, here is what happens with 100 bias, no bias and 10 bias.
It seems that 100 bias has less noise than 10 bias and a little more noise than no bias.
All photos are from the same CR2 RAW files, and was used dark and flat files.
No change was done in Autosave.tiff - 32 bits - in DSS enviroment.
All photos were converted from 32 bits to 16 bits with HDR curve in Photoshop and was applied adjustment in curve and level, only.
Conclusion:
1) the improvement of bias files depends from camera noise creation. My Canon T3 can don't like of bias; others Canon T3 can like it.
2) as bias files are used for calibration of the image, some times theirs use can be useful, despite of noise, since they can enhance the details in the photo.
3) the amount of files used to create masters are important to reduce the inherent noise they carry in. A large amount will not enhance its performance.
4) I need to use more than 10 dark, flats and bias. Perhaps 20 or 30 files. But it is impossible work with 100 dark and flats with long time of exposition. Only if it is to sell the photo or to win any competition.
5) the main solution for noise issues is a good camera, with refrigeration. Those with price around dozen thousands dolares.
6) as I haven't that money... God bless the noise ! I love them ! They are beatiful as the spikes distortion are!
It is not only excuse, Kevin. It is the reality. Some cameras produce more noise than others. And the files for calibration of photo don't clean all them.
A hundred bias file is better than ten files, but the noise will be there. Bias are with short time of exposition. But think about 100 dark files with 4 minutes each one. 400 minutes in the same session of lights to have the same temperature. Unreal, right ?
Therefore... the noise will exist and will be cleanned with graphic processing. We can use those calibration files to reduce noise, but only to reduce, and make more easy the task of cleanning.
And another issue, not so evident: the documentation of DSS says that the dark, flat and bias are to calibrate the photo. Subtracting the dark signal, flat signal (vignetting ?) and bias signal. Some electronic noise will be cleanned by colateral action, not as a main goal of these files. Am I right ?
Do you have any description of what is dark signal, flat signal and bias signal ? They aren't the noise, at least only the noise. They carry noise to lights.
I don't know explain, but this is the result of my tests.
a n00b knows not where to turn 
there is no double subtraction
)
God bless the noise ! I love them ! They are beatiful as the spikes distortion are!
