lock042

@Spaced Out: a positive answer could help all the community :).

Hello @Spaced Out. As indicated in the topic, you need to check the dither option. If you made a stack with Siril, in 32bits mode, you wouldn't have needed it, though. In order to improve the documentation, could you send me your stack output image (WeTransfer), and give me the rights to use it for the Siril documentation?

Yes this is allowed. Siril is free, open source, under the GPL3 license terms.

Please update. This behavior will disappear.

Ok. Sorry but I stop here. This assertion is so wrong that I couldn't go further. Have a nice day.

We are talking about removing dark flat to flats. We don't care about light pollution. This is my whole point. We are just talking about flats here. Of course darks are very important to remove all unwanted signal to light frames.

Be careful. Your ampglow can be negligeable at 1.8s, like with 294MM. If you look your image very stretched, of course you will see it, but statistics don't lie.

Everything is explained here: https://siril.org/tutorials/tuto-manual/#background-extraction

Did you tried to remove background on each sub? It works pretty well. Siril works only with monochrome or CFA flats.

To change working directory you can use the "home" button at the top.

Try relative path. I know it works in script mode.

Could you send a screenshot please?

Symlink do work with windows development mode on. But conversion does not change pixel value.

Uncheck the "normalize output" button.

OK, what you were asking for before is not the test you have been doing, as nowhere in your procedure you were asking to subtract a master. What I gather from your experiment is that: First line is the stats of a single bias Second line is the stats of same bias with a constant value subtracted Last line is the stats of same bias – a masterbias. Fair enough, it makes “more sense”. At least if the discussion from the start had been about calibrating biases… which is not at all the point. So here is the test that we have been talking about. Made 200 darkflats of 0.9s, matching the flats done for a session from last summer. ZWO ASI294MC, gain 120, offset 30, T -10C for reference. Stacked 1, 10, 25, 50, 100, 150 and 200. Yes, of course, the std decrease with increasing subs… Then checked the stats of a flat (1st CFA channel only, it would not make sense on a color sensor to measure all channels altogether) from which I have substracted a synthetic offset of 1920 and masterdarkflats made with 50, 100, 200 stacked frames respectively. And here-below the results: I’m not even going to claim that there is significant reduction in noise in the calibrated flat, because the stds are probably within the uncertainties. But to be honest, this kind of measure is the basis for questioning the usefulness of using darkflats (or bias) to calibrate the flats. I hope this will encourage others to test with their setup. So now I think we've done enough testing (here and with the post). We're not forcing anyone to adopt synthetic bias, but we've shown in different ways that it's worth a try.

Of course not and this is why subtracting master bias or master dark flat adds noise, because they have noise. And this is why subtracting synthetic bias (i.e a constant) don't. I think you don't get the point at all.

I'm sorry but this is signal processing basics. With image operations, even with subtraction, the noise is added in the result, even with a master dark flat of 100 images. In the case of the synthetic bias you only use a constant. So noise stays the same. So ok, difference is not huge with a master containing a lot of frame. But why would you add noise, even if it is not too much.

Sure. And as DSLR use power of 2 you can take 2048 as constant (you even can enter this value in the preferences : =2048). This is the value I use with my DSLR.

With DSLR it is very simple. Take your master offset you did used. Measure the median value and it will be your level.

By not adding noise at all. Subtracting image adds noise. Subtracting level don't. Another test from Christian Buil (famous French astronomer which as created IRIS): http://www.astrosurf.com/buil/asi294mm.html Comment. The offset signal is so uniform that I recommend for processing to equate it to an image of constant level: a synthetic bias, so the constant level is equal to the average (or better, median) intensity of a typical offset taken with the settings selected (gain, offset, temperature). This initiative is one way to reduce processing noise.

Thanks a lot @clouzot for your tests that perfectly match ours. That shows that we are trying to say. Make your own tests with your setup, and I bet that in most cases you will find no differences. (Oh wait, maybe less noise )

You say than ampglow has different ADU values. But take a look at the figure we did: the plot is inside the AG area on purpose!!. These differences are negligible in comparison to the flat. They mean nothing.

In this image, prove me you cannot replace the darkflat by a constant (right figure) (of course darkflat plot has been y shifted to be visible on the flat plot): It is not because you see ampglow it will be a problem. Especially when you scale the darkflat to the flat.... We agree it is not really a problem of noise.

In the flat???? The bias only applies on flats. And if you take a look at our plots. That is really not a big deal. Of course master dark is very importants for lights. Because lights have a very low level in comparison of flats.

Not if you use constant....