white balance using a gray-card

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psy1280
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white balance using a gray-card

Post by psy1280 »

I recently used a gray-card to get a reading for my red and blue white balance while using a L-pro LP filter. The readings I got from the gray-card, using the L-pro, were: Red 78; Blue 60. I read on one of the threads discussing the ASI294 where Robin suggested using 50-50 for the white balance in order to take darks. I want to make a dark library, should I use my gray-card readings or 50-50? I know the ASI294 is quite finicky (to say the least) when it comes to vignetting.

Joe
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Menno555
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Re: white balance using a gray-card

Post by Menno555 »

Hi Joe

I have no experience with a gray-card but when I had the ASI294MC Pro, the R50 B50 white balance setting gave good darks.
It did also work fine with taking lights, flats and darkflats, but I used that because I do stack and process my captures afterwards.

Menno
psy1280
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Re: white balance using a gray-card

Post by psy1280 »

Much thanks Menno. Since you've worked with the 294, did you have a solution for the vignetting that I, and others, have with flats? If so, would love to hear about your flat protocol.

Joe
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Re: white balance using a gray-card

Post by admin »

Hi,

you can use whatever settings you like - some prefer to work with 50/50 neutral settings, others with balanced settings.

What you *must* do is use the same settings for lights and darks. If you don't do this your darks will not subtract correctly.

thanks,

Robin
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Menno555
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Re: white balance using a gray-card

Post by Menno555 »

Ah yes, that.
My experience was that the general "rule" of making flats for the 294 with around 25000 ADU works fine as long as you do not have your light source too bright. Also, in contradiction to what in general is said, only changing the exposure time. So making flats and darkflats with the same gain, white balance and brightness/offset settings as your lights. With all that, the vignetting was really reduced.

I have a flatpanel and did read somewhere that the vignetting was less with lower light but longer exposures. I was used to do flats with around 0.2 seconds. But with less light and exposures of 2 seconds, the vignetting was greatly reduced. And than around 50 of them.
Especially with the Optolong filters. Those are very sensitive with this and making the light source as dim as possible, really works. I even had flats with 4 seconds exposure time and worked great :)

Menno
psy1280
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Re: white balance using a gray-card

Post by psy1280 »

Thank you Menno,

I wasn't clear on what you said

You said you "used to do flats with around 0.2 seconds." And then you said with less light exposures of 2 seconds. Which was more productive, the 0.2 or the 2 second exposures? And 50 flats...didn't realize I should take that many. Much thanks for clarifying this...it really helps.

Joe
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Menno555
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Re: white balance using a gray-card

Post by Menno555 »

Joe

The 2 second exposures were better for less vignetting.
And yes, since the flats exposures are so short it's just handy to make a lot of flats. I did experiment with that and found that around 40 or 50 flats give good results. 30 flats were also okay but 50 was just that bit more better. Also tried 100 but that is just overkill, there is almost no difference.
So in general I do 50 flats, 50 darkflats and, depending on the exposure settings, 15 to 30 darks.

Menno
psy1280
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Re: white balance using a gray-card

Post by psy1280 »

Fantastic! Thank you so much Menno! I hope to try that tonight....I'll let you know.

Joe
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Re: white balance using a gray-card

Post by oopfan »

I too capture 50 flats. The exposure depends on a number of factors: your sensor's QE and the brightness of the light source. I use an electroluminescent (EL) panel. It has a noticeable blue tint, so exposures are shorter using the blue filter versus using the red filter. The worst case happens when using the Ha or SII filter (deep red). Also my sensor is least sensitive to red. All combined, a single Flat requires 40 seconds exposure with the EL panel. I just started capturing Dark Flats with Ha, and I think it makes a difference. Joe, you probably already know this, but you know you have the proper exposure when the peak of the histogram is around 32,000 ADU. Increase the exposure if less than that, and decrease if greater. Take a few test exposures before committing to a 50-frame run.

Brian
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Re: white balance using a gray-card

Post by Menno555 »

oopfan wrote: Thu Jun 24, 2021 11:02 pm .... but you know you have the proper exposure when the peak of the histogram is around 32,000 ADU. Increase the exposure if less than that, and decrease if greater. Take a few test exposures before committing to a 50-frame run.

Brian
Brian
For fighting the vignetting (but also to fight the "color blotching" if it's there) of the ASI294, higher ADU's are not recommended. In my mails with Zwo, they told me that and with experimenting, that was true. They suggested 18000 to 25000 ADU and the latter worked best.

Menno
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