Flats in Sharpcap

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DuaneS
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Joined: Thu Dec 06, 2018 4:08 am

Flats in Sharpcap

#1

Post by DuaneS »

I have a general question on setting up flats in Sharpcap. I have a LED flat panel and 3 sheets of opaque paper. I am using a native 16 bit camera (ZWO ASI6200). The issue I am having is if I put any gain on the camera it pegs the histogram to the right even with the flat panel set to the lowest setting. If I set the gain to 0 I can get the true ADU to 32k. I would like to have the gain set to 121 or higher when doing realtime viewing. For an effective flat I always thought the flats should be the same gain as used with lights?? Should I get a few more pieces of opaque paper? Or can I use a flat with gain 0 and lights with some gain applied?

Thanks
Duane
nexusjeep
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Location: Gloucestershire

Re: Flats in Sharpcap

#2

Post by nexusjeep »

I believe the gain should be the same which is the way I shoot them, could you not add some additional sheets of paper to make the output dimmer. I have to do this with my flat generator it is the Geoptik one as it is too bright with out I used the plastic sheets they use for photography for soft light filters and think it took about four of these too tame it as I do not like using it on minimum settings in case there is slight under illumination of an area

Not so bad with narrow band though so you can always remove a few if necessary.

cheers
Nick
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admin
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Re: Flats in Sharpcap

#3

Post by admin »

Hi,

there's no real need for the gain to be the same when shooting flat frames, but if you are going to use dark flats and/or bias frames to subtract the bias level from the flat frame then the settings should be the same between the two.

I'm guessing that you already tried turning down the exposure to reduce the brightness – have you perhaps hit the minimum exposure for this camera?

Cheers, Robin
DuaneS
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Joined: Thu Dec 06, 2018 4:08 am

Re: Flats in Sharpcap

#4

Post by DuaneS »

Thanks Nick - i will get more paper

Thanks Robin - Yes I have it all the way down. Although I am looking at the specs and it works from 6v to 12v. I am using a controller to turn this on and off and it is variable. The issue is it goes from 8v down to 5v and nothing in between. I does not turn on at 5v so I am guessing if I can get it down to 6v it "might work".

Duane
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Re: Flats in Sharpcap

#5

Post by admin »

Hi,

Not sure if we are talking at cross purposes... Usually you can deal with flats that are too bright simply by reducing the length of exposure time that the camera is taking (nothing to stop you taking flats at 1ms exposure or even less)

Robin
nexusjeep
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Re: Flats in Sharpcap

#6

Post by nexusjeep »

Hi Robin,
I usually aim for 200 - 300ms as I did try very short exposure flats can't remember whether it was with the old 183C ASI1600MM or the new ASI2600MC and at really short exposure I had some weird patterning but not sure if this was the sensor or to do with the frequency that the flat panel was running at when dimmed as I am not sure if the dimming is achieved by chopping the output so PWM hence the reason I added some plastic sheeting to increase the time + intensity. I have not tried it with the Geoptik flat panel though so might have an experiment not sure what kind of flat panel Duane has and must confess never thought to ask how long a flat was being taken.

Cheers
Nick
PetePeterson
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Re: Flats in Sharpcap

#7

Post by PetePeterson »

When running an SBIG CCD the software accommodated separate sets of darks, flats, darks for the flats and bias frames. The terminology and the process made sense and the CCDSoft imaging software integrated these with lights. There was no gain to contend with. And I understood what I was doing.

* Is the terminology "dark flats" the same thing that I know is taking a separate set of darks at the same temperature and exposure as the darks themselves? So that they may be subtracted from the flats? Or are "dark flats" flats that have already had darks applied? I've not found anything in SharpCap that applies separate darks for flats to the actual flats themselves.

SharpCap has a checkbox that integrates flats and bias frames. Cool. And since both are done at the same time the bias temperature is the same as the flat and there's no problem there. Again, cool. I've been checking that bias box every time flats are run.

BUT I run twilight flats or cloudy day tee shirt flat. Short exposures produced strange flat results. I learned on this forum that for an ASI2600MC-P flats should be 2 seconds or longer. This exposure forces me to set gain to 0 if ADU is to be kept midrange. While I'm still struggling with "unity gain" I think it's set to 100 on this camera. And not being sure if there's variation on that setting I've added a safety margin and adapted a gain of 120 for all imaging.

With only minimal control of brightness of the light source and constraints prohibiting running flats at sub 1-second exposures, it's probably not possible to run my flats with bias box checked with gain set at 120.

* In this situation should the bias box be checked when running flats or should bias be ignored?

Geeze do I feel ignorant.

Pete
nexusjeep
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Re: Flats in Sharpcap

#8

Post by nexusjeep »

Hi Pete,
Dark Flat or Flat Dark are just an exposure the same as your flat with the scope covered, not sure why you have to shoot 2-3s flats with the ASI2600MC as mine works fine between 200-300ms which is were I am normally running them with my flat panel with the same gain as my other calibration frames. Is the funny pattern potential the weave in the t shirt at shorter exposures?

Cheer
Nick
PetePeterson
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Re: Flats in Sharpcap

#9

Post by PetePeterson »

First of all a correction. The flats exposure info I'm basing my questions on WAS NOT FROM THIS FORUM as I'd indicated. It was from the ZWO AIS camera Facebook site. Sorry 'bout that.

I'd been led to understand that longer exposures for flats are necessary for CMOS cameras because many of these cameras used different read configurations for long and short exposures. Remember that CMOS sensors aren’t specifically designed for astro imaging. The sensor is designed to do high resolution video as well as long exposures.

Supposedly when the camera is set to short exposures it uses read circuitry to optimize data transfer rates that aren’t used for longer exposures. Therefore longer flat exposures will be a better match to the long light exposures.

Since initially experiencing problems with taking decent flats and having recent success with 2 second and 4 sec flat exposures on an ASI2600MC-P I've bought into this. However, part of my bias toward longer flat exposure is a leftover from CCD experience with a mechanical shutter. And the same ASI group had many contributors who successfully use very short exposure flats.

Based upon your feedback Nick, I'll experiment with the shorter exposures once again.

OK. So "dark flats" are the same as my understanding of darks taken at the same exposure and temperature as the flats, and they are subsequently subtracted from the flats before the flats are applied to the lights. Thank you for that clarification as well. BUT with SharpCap's ASI2600MC-P configuration I have not encountered a way to take "dark flats" and then to subtract them from flats.

What am I missing?

Pete
nexusjeep
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Location: Gloucestershire

Re: Flats in Sharpcap

#10

Post by nexusjeep »

Hi Pete,
I only take dark flats as I use them in Astro Pixel Processor not SharpCap from what Robin was saying in my post about Darks with the ASI2600mc Flats + Bias is sufficient.

Cheers
Nick
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