Bias or flat darks - Forums

Topic starter

Oct 28, 2017

I have been having some issues with bias taken at 0.00032s so reading different
forums to get an idea of what is happening I found where Jon recommended that no bias or
flat dart sub should be taken at less than 0.2s with 0.5 as maybe best exposure.
I am trying to get dark flats/flat darks using my Lacerta LED Flatfield Box and
I have been able to reduce light by 60% but still there is no chance of getting
up to 0.2s with RGB filters. Do I have to reduce light even further or is there
another way of getting up to 0.2s?

Bernard DELATTRE

Oct 28, 2017

Hello Kenneth,
I also stopped doing bias, to take flats field with my lacerta, I decreased the light (beyond the drop in voltage) with filter sheets of neutral gray density, between the led and the screen.

Tim

Oct 28, 2017

I've just added sheets of white paper between my flatbed and scope. A stretched white t-shirt over the scope works as well.

Kenneth Hoynes

Topic starter

Oct 28, 2017

Hi Bernard
I place a sheet of paper between led and scope, works great. Since thefilters are no longer evolved do you take a dark flat for each exposure length
or do you just take one at average exposure length?

bobzeq25

Oct 28, 2017

People should note this is a camera specific issue. The 1600, in particular, seems to have a problem with short exposure biases. My ATIK 460 takes them just fine at .001 sec, flats just fine at .1 (don't need to go lower).

Helpful

Kenneth Hoynes

Topic starter

Oct 28, 2017

Camera specific yes, thats why its posted in the ZWO ASI1600 forum.

fielderda

Oct 28, 2017

I have a ASI1600MM - Cooled and use a Spike_a _Fielder (EL) for my flats. Similar to some of the comments above, I too put 1-2 sheets of white paper between the EL and the telescope.
For Bias, I use Sequence Generator Pro and have set the exposure to 0.00032 sec and haven't had any problems.

Bobzeq- what were the types of problems encountered with the 1600 short exposures?

Bernard DELATTRE

Oct 29, 2017

Kenneth Hoynes:
Hi BernardI place a sheet of paper between led and scope, works great. Since thefilters are no longer evolved do you take a dark flat for each exposure length
or do you just take one at average exposure length?

Hi Kenneth,
I no length take dark flat, with a gain greater than 139, I consider that the noise read is negligible compared to the signal of the flat frame

Kenneth Hoynes

Topic starter

Oct 29, 2017

Hi Bernard
Do I understand correctly that if you use gain over 139 you do not use dark flats or bias frames??
Kenneth

Bernard DELATTRE

Oct 29, 2017 · Edited: Oct 29, 2017

Kenneth Hoynes:
Do I understand correctly that if you use gain over 139 you do not use dark flats or bias frames??Kenneth

yes, that's it (we are several to do so). This is the big difference between Cmos and CCD.
I do not remove the bias of dark frames either (now I do not use bias)

Kenneth Hoynes

Topic starter

Oct 29, 2017

Thanks all, think i'll try Bernards method of no bias or dark flats.

Tim

Oct 29, 2017

I use flat frames to get rid of vignetting as well as dust and other markings on my imaging train. I get this regardless of gain. I used to use bias frames but testing has shown that the ASI1600mm-c does not produce consistent results under .2 seconds. So, thats why so many of us have moved to dark flats instead of bias frames. After shooting my flats I just remove the panel, cover the scope with its cap and shoot the dark flats.

Helpful Concise

fielderda

Oct 29, 2017

Interesting Tim - I'm interested in reprocessing a couple of my images in PI using dark flats only instead of bars frames.
When you remove the panel and cover the scope with the cap, you're now shooting darks and not dark flats? Are dark flats processed using flats and darks?

Tim

Oct 29, 2017

Dark Flats are to Flats the same as Darks are to Lights. So, when shooting Darks they are the same temperature, gain and exposure time as the lights. Dark Flats have the same temperature, exposure time and gain as the flats. So, for calibration, dark flats are processed with flats to get a master flat. That master flat is processed with a master dark and the relative lights.

This is fairly easy to do in Astro Pixel Processor - the program I use for calibration and stacking. I know DSS does this automatically when you load the relative frames. I've only used PI a couple times for this and am probably not the best person to say how to do it in PI. However, what I described above is what is required regardless of the program you use. Hope this helps.

Helpful Concise

fielderda

Oct 29, 2017

Thanks for the clarification Tim!
With my Spike-a-Fielder EL, it will be easy to do the Flat darks right after taking the Flats as you indicated.
Clear Skies!

Well Written Respectful

Christophe Brun-Franc

Nov 2, 2017 · Edited: Nov 2, 2017

Bernard DELATTRE:
Kenneth Hoynes:
Do I understand correctly that if you use gain over 139 you do not use dark flats or bias frames??Kenneth

yes, that's it (we are several to do so). This is the big difference between Cmos and CCD.
I do not remove the bias of dark frames either (now I do not use bias)

That means you are not calibrating at all the flat ? - you are only integrating flat to create a master flat ?
Thanks

Christophe