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Hello guys (and girls),
i need some help with making correct flat frames. First of all, what is the big difference between a linear and nonlinear histogram and how do i see if i'm using a linear or nonlinear histogram?
Second, when i used my dslr i never had issues maken flat frames aiming at the sky with a folded white thsirt on my telescope. however since i am using a cooled ASi294mc i can't get good flats anymore using the same technique…. i end up with gain 0 and -10C with a exposure of 0.5 sec max.... What is the best way (or a way) with this camera to get about 2-3sec flat frames. I do not own a LED screen or photobox to light my telescope….. Now i can't get rid of my dustbunnies…. 🙁
Thanks in advance!
Cheers,
Michel
Hi Michel,
Ahhh, flats, no worries they are the most challenging regarding taking calibration frames. But when they are ok, they help a lot. Stick with it, it might be frustrating but when you get it... sooo worth it. 😉
1. A linear histogram is what a camera sensor actually records in the amount of bits it has (e.g. 12-bit, 14-bit). Your monitor only has 255 values, so for you to be able to see the image at the expected brightness, you need to transform that linear signal into a non-linear one, usually some kind of logarithmic curve is fitted to the linear data. A DSLR on which you can show the histogram, usually shows you a non-linear histogram. This is a bit unfortunate for taking flats by looking at that, as the ideal situation for flats is to take them at about 50% of its linear histogram (this will make sure the sensor is well lit and not clipped either side). I always use Sequence Generator Pro also for my flats as it always works in 16-bits, no matter what camera I use. This is great because now I can always use its flat-wizard to just take the frames at about 30.000 ADU (~50% of 16-bit).
2. So it appears you camera is much more sensitive (good!). But you would need to cover up the telescope with more t-shirts or something like that. This is exactly the reason I like a flat-panel so much on which I can adjust the brightness.
Thanks for the clear explanation! Feels like i’m starting all over with the new camera type and options....
Guess i’ll be looking for a flatscreen then!!!
