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Flats, dark frames, light frames, bias frames...some assistance needed please!


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Hi All,

I understand the principles of light and dark frames, basically subtracting the noise from your camera depending on the temperature that the frames were taken but can somebody explain the best methods of taking them? I've a small Startravel 102 scope and ASI183MC non-cooled camera. I'd prefer not to have to spend loads of money on some light panel, etc.

Cheers
Daz

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I think you've been looking at NINA so that app can manage them all for you. Darks and Flat Darks just need dark and then the flats can be done using a uniform sky and white t-shirts etc to get the correct exposure. There is a "Sky Flats" option in the Flat section of NINA.

If you keep a consistent gain and offset that will make things easier then the flats/flat darks need to match exposure as do the lights/darks.

You'll also need to maintain the layout of the optical train, ie scope, camera, focus, filters etc for the flats and lights.

Hope that makes sense and I didn't miss anything obvious

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The Dark frames should be at or close to the temperature of the light frames. As you have the uncooled version of the 183 this can be tricky. Do you know what the sensor temp was for the lights? Some people pop the camera in the fridge (usually about 4 degrees C?) to get close. Full darkness required so put a lens cap on the camera. The exposure time, gain and offset should be the same as the lights.

Bias frames are not usually used for CMOS astro cameras.

Calibrated Flat frames remove vignetting and dust marks from lights. They're important. Flats don't need the same temp, exposure time, gain or offset as your lights, and the white t-shirt method is the easiest. Ideally point to the sky in early morning, but pointing at a well-lit blank wall will also work. The optical path should be identical to that used for the lights - so don't refocus or rotate the camera. If you're using filters you should take flats for each filter. 

You should also take Dark Flats to match your Flats - these use the same exposure time, temp, gain and offset as the flats, and they need full darkness so again pop on a lens cap, and/or stick the camera in a box.

Dark frames last for months, so once you have a set matching your common exposure times, gain and offset values, you can keep using them. Flats and Dark Flats though will have to be retaken reasonably regularly, especially if you dismantle your setup after every session. It's a bit of a pain, but you get used to it.

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I won't restart the Dark Flats vs. Bias wars here, but will note that for many camera and processing software combinations, one can indeed use a set of "bias" or no-signal frames instead of dark flats. They are shot as darks with the shortest exposure your camera can do consistently.

There are indeed some CMOS cameras for which bias frames do not work well, but "most"? Citation needed! The only one I'm aware of is the Panasonic sensor in the 1600.

Really it's a horse apiece for one-shot color cameras, so long as your flats are always done at the same exposure there's little need to reshoot dark flats. Although you could make a case that the thermal signal could be a significant part of the value for dark flats, but not for bias. In that case one set of bias frames and one of darks for each temperature suffices, whereas you'd have to shoot and manage a per-temperature set of dark flats too.

But flats are also pretty short exposures so that can be safely neglected.

Mono imagers tend to rant more about bias since we often have a different flats exposure for each filter. In my case for my mono (CMOS!) camera that's one set of bias or seven sets of dark flats. Even I can do that math.

Edited by rickwayne
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  • 2 weeks later...
10 hours ago, Ohgodwherediditgo said:

There's no need to spend big on a flats panel. I use a Tracing Panel. I bought it from Amazon for £12 if i recall correct and it's dimmable.

Does it flicker when dimmed? Some do, giving inconsistent exposures.

Olly

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I mostly use the sky to do flats and strap a white pillow case over the aperture usually folded to dim the light and ensure evenness and no clouds etc in the flats.  
 

l did have one if those tracing boards and it worked well until l dropped it !!!!   Now it flickers so is useless.  so yave gone back to sky flats. 
 

carole
 

 

Edited by carastro
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I don't notice any flicker at all. I need to turn it nearly all the way down to take flats, but for Sharpcaps' sensor analysis i had to lay a sheet of paper over the top, which was effective. It is USB powered so can be plugged into my camera which is nice and easy.

Edited by Ohgodwherediditgo
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  • 4 weeks later...
On 28/02/2022 at 07:40, ollypenrice said:

Does it flicker when dimmed? Some do, giving inconsistent exposures.

Olly

I was just doing sensor analysis in Sharpcap and for illumination i had the tracing panel powered via the usb hub on my camera, this did produce a flicker. I then plugged the tracing pad in to my power bank and the illumination was flicker free. It was like the transfer of data was having an impact on the voltage to the panel. HTH.

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I found that my flats panel created 'strobing' effects regardless of exposure time unless it was on the brightest setting. I then needed to use a number of Neutral Density sheets to reduce the exposure time to a couple of seconds.

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I use the following morning’s sky and the t-shirt method for flats. I normally go for flat exposure times between 3 and 10 seconds since shorter exposures have been alleged to be problematic with the ZWO ASI2600.  I match the exposure of my dark flats to my flat lights. I use a master dark flat in place of master bias. 

Since the 2600 is cooled I have a set of preprepared master darks obtained at different set temperatures and duration. 

I gave up taking darks when I used my canon 450D and instead used dithering.  Taking darks under clear skies seemed such a waste of time and dithering between frames seemed to work pretty much as well. I don’t know whether that approach might work with the ASI183 uncooled  camera.  Might be worth a try. 

Edited by Ouroboros
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On 27/03/2022 at 08:11, prusling said:

I found that my flats panel created 'strobing' effects regardless of exposure time unless it was on the brightest setting. I then needed to use a number of Neutral Density sheets to reduce the exposure time to a couple of seconds.

I remember watching a YouTube video about reusing dead led screens. There is a defusion layer over the leds which could be used over the tracing panel or as part of a DIY project. I have a Samsung TV in my shed that suffered from a big purple spot that has plagued many Samsung tvs, I intend to strip it and give that a try at some point in the future. 

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