Are Darks and Bias really necessary?

I am a Sony shooter to and I see no benefit to use darks and bias . Maybe a more technical expert can explain .

Guido

Bias, not really. But darks are helpful because there will always be hot pixels that need to be mapped out.

@turtlecat1000

correct but we need to take the darks under the same circumstances as the exposures :same temperature ...

Guido

Hi all,

without bias, darks, flatdarks, the sensor offset is never subtracted leading to impossible correctl flat-field calibration and colors that are not correct because the offset is there (means the color is there but just not completely right, the colors are a bit more bleak)

So, In my view, if you don't use flats and are happy with less than good color information, you can go without, but I would not know why you want to do that really if darks and bias are easily made? Flats is something many astrophotographers struggle with, but shooting proper bias and darks is not a big problem or is it?

If the sensor has little noise and no amp-glow, i would suggest to use bias for the light and flat calibration where you shoot bias with 0,1 second exposure, then it should work fine also on the newer CMOS  sensors 😉  simply shoot 100-200 bias with 0,1 sec exposure and the noise injection is no issue as well, but you can use flats properly and will have accurate color information 😉

Finally, you really should always use  a Bad Pixel Map to correct hot pixels. Dark subtraction is not the right way to deal with hot pixels since these pixels are not behaving linearly. So make a couple of long darks and make a Bad Pixel Map once, that BPM will work efficiently on all your data for more than a year 😉

Mabula

Posted by: @turtlecat1000

Bias, not really. But darks are helpful because there will always be hot pixels that need to be mapped out.

But BPM does this and dithering as well. Dithering will also cancel out pattern noise.

/stefan

@mabula-admin

Thanks, yes. I tested without darks and the result was not pleasing at all.

/Stefan

With my Stellina setup, all I can really do is darks. I’m not a able to produce bias files because the setup doesn’t allow for it. I’ve wondered if flats are useful for Stellina users because the field rotator would seemingly invalidate the flats as it travels around the circuit. But it is also a small sensor with a 80mm objective so vignettes really aren’t an issue. 

Any thoughts (except those disparaging the Stellina) are welcome. 

@digitaliz-se Yes dithering and a bad pixel map work, if you have no amp-glow at all a good bad pixel map may make darks less important. However, sensors do change and both dithering and darks remove bad stuff to a certain extent, it's always a statistical game basically. Too much and the data gets hurt, too little and you still see the issues. So personally, I always use all calibration files on top of dithering and a map. Best thing to do is to check your data carefully at 100%, checking to see if your bad pixels and other things are properly removed and then you can do away, maybe, with darks. Quickest way is to load in a light, load in your calibration masters and then selecting "l-calibrated" on top of the window. This will calibrate that single sub on the fly for you to see if it works properly.

@turtlecat1000 If you rotate the field all the time then it wouldn't work indeed. I don't know exactly how a Stellina works, but you can't rotate and then keep that rotation fixed? It's Alt-Az?

Posted by: @vincent-mod

@turtlecat1000 If you rotate the field all the time then it wouldn't work indeed. I don't know exactly how a Stellina works, but you can't rotate and then keep that rotation fixed? It's Alt-Az?

Yes, it’s an automated alt-az robotic setup with a rotator to keep the same field of view. It’s an 80mm lens with a Sony 1/1.8” sensor. With a sensor that small, it would likely only capture the best part of the image circle the lens produces. 

Right yes, so that means the sensor is constantly rotating to get the same FOV. Which means dust and vignetting will also differ somewhat each time, that makes taking flats unlikely to work (though I may miss something as I never used alt-az mounts). Darks etc. will be fine of course.

If the vignetting is very consistent as the rotator moves you might still be able to shoot flats that could be pretty effective. Being a more or less sealed unit you probably don't see any dust bunnies as such anyway? Unlike "Traditional" setups where the camera and optics may be disassembled at a frequency up to every night they are used, giving plenty of opportunity for dust to gather where it will be noticeable.

You might not be able to do flats that are as effective as on a more traditional setup, but you might be able to do ones that are pretty good, can you manually control the exposure time on the Stellina? I have never really looked in to them. If you can manually control exposure time and triggering you could use a laptop screen or similar to work out a good fixed exposure for flats, then shoot maybe 25-30 of those and then cover it up and shoot some dark flats of the same length to calibrate with. The worst you might do is invest a couple of hours in to something that does not really help much.

I know someone getting surprisingly good results from a short tube 80mm scope and a mobile phone camera, so no disparaging of the Stellina concept here, it is not my cup of tea but any device that makes astronomy interesting and accessible to people at a price that they are willing to pay is a good thing IMO.

The Stellina really doesn’t have any vignetting that I can see. It’s capturing the sweet spot of the 80mm objective so the corners won’t experience the same degradation as a larger sensor would reveal. There is some ability to set the exposure time on Stellina but it isn’t controlled in the same manner as a typical camera and does not have the same flexibility on timeframes. It is a great instrument but, like every other setup, it has its
flaws. I have seen a few people talk about attempting flats but I haven’t run into anyone doing fits processing who has used them.

If whatever vignetting there is, is really consistent through a range of camera angles (Via the inbuilt rotator) and they don't gather any dust bunnies, maybe with some car you might be able to create an artificial flat in the correct vignetting tool on tab 9, and apply that. I have never really tried to use it as both my systems make taking flats easy. One of them shows very little vignetting to begin with but flats still make a visible difference.