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2022-05-29: APP 2.0.0-beta2 has been released !

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Appropriate exposure time for Flat, Bias in DSLR


 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

Hello everyone

How do I decide the proper exposure time when shooting FLAT with DSLR?
When I shoot FLAT using fuji X-A2, there is a peak level around the center of the camera's histogram.

When I look at this FLAT on the APP's histogram (upper right of the screen), it is located on the left end of the histogram.

Is this too dark?

Also, BIAS is taken with 1/4000 exposure time.

Is this appropriate?

Thank you!


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4830
 

Yes, you want to aim for the peaks of the channels to be at least at 50% of the range, preferably even at 60-70%. Just make sure they don't clip on the right side of the histogram. Shooting bias at he fastest setting is technically correct, but these days we advice to do this at 0.5 seconds per sub, this will still mainly have bias signal in it and prevents issues that some sensors have at faster settings.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

@vincent-mod 

Thank you to your answer.

"at least at 50% of the range, preferably even at 60-70%"

Is this a DSLR camera histogram, or an APP histogram?

Thank you.

 


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4830
 

The linear histogram, APP shows this when you select "no stretch" for the preview. This is not specific to a DSLR or APP, but a general way of showing the histogram. Many DSLR's show a logarithmic histogram which makes it harder to see how long you should expose for (it's mostly used for regular photography), the linear histogram shows the full range properly.

image

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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

@vincent-mod 

I think I understand well.
DSLR displays histograms in logarithm, which is different from App. I didn't know, so thank you for telling me.

May it be a starry sky.


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1723
 

@mo-mo Would it be possible to set the histogram of the DSLR to linear mode? That would help you get your flats correctly illuminated.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

@wvreeven 

Thank you for advice.

Unfortunately, my DSLR could not set a linear histogram.

So, I tried to make it closer to the right end with DSLR's histogram.
Last night's test was the sensitivity of ISO2500, and it was a histogram like an image in 50 seconds to 60 seconds exposure time.

This is the result of loading from Flat in tab 1) and double -clicking one from the file list at the bottom of the screen.

I do not calibrate FLAT.
I do not use CFA of tab 0).

The balance of the RGB is noticeable, but I guess it is the influence of a nearby streetlight. Is this a good result for a calibration for Light?

1
2

 

This post was modified 2 months ago 2 times by momo

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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4830
 

I don't think so as now the green channel seems to be clipping on the right hand side. So I'd advice to expose a bit shorter to get that in the range. It's tricky though as your blue channel seems to be way lower.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

@vincent-mod 

OK

I took a picture of the FLAT in another place (the light pollution was slightly reduced).
The image is 40 seconds exposure. ISO2500, (F4.5)
I think I can extend the exposure time a little more when I look at the histogram, but what do you think?

Thank you for your kindness.

3
4

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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4830
 

That looks like it should be, it's not clipping anywhere. 🙂


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

@vincent-mod 

Thank you! 👍 😊 😊 


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4830
 

ps. I noticed you're using 40 second exposures, these are sky-flats? Even then, usually the aim is about 1-2 seconds. So your light source is very dim?


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

@vincent-mod 

Yes, I photographed the night sky and got Flat.
A translucent resin plate was attached to the tip of the camera lens and shot.


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4830
 

Ah, so taking a proper flat is usually done the following morning (when doing skyflats), so just before the sun really gets up, but it does create a nice and even sky illumination. Clouds etc will work less well, you really need a flat background. This is why I switched to using a flatfield generator as I can use that whenever I want.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

@vincent-mod 

What is a Flatfield Generator?
I have never used it.
I use a translucent resin plate to intend to make the brightness uniform.

I hope that the resin plate will spread the light without directly shooting the cloudy night sky.

I would like to use FLATFIELD GENERATOR if it is more appropriate.


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4830
 

I like using it as you can control everything very nicely. I don't want to advertise any particular brand, but I use something like this on my Newton (there are more options!):

image

The key is to have a nice flat field (so illuminating the same across the entire panel) which sends out broadband light and to be able to control the brightness.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

@vincent-mod 

Thank you.
I use a refractive telescope, so I'll check it on the net.


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(@carlesa25)
White Dwarf Customer
Joined: 4 years ago
Posts: 27
 

Hello: I don't know what software you use to acquire your photos but many (NINA, APT etc...) have very good help to automate the taking of Flats, specifically NINA makes the job much easier since it perfectly calculates the best exposure for achieve the most balanced flats possible, naturally starting from a good, natural light or a good flats generator as suggested. Health


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 8 months ago
Posts: 92
Topic starter  

@carlesa25 

I have never used those software, but I am trial and error to get the right FLAT.
Thank you for your information.


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(@wheeljack)
White Dwarf Customer
Joined: 2 years ago
Posts: 21
 

I use a DSLR. For Flats, I use a tablet and a flat-frame app (basically just a white screen) and place over my telescope. I put the camera in AV-mode, which seem to do the trick nicely. I also control the camera via APT (Astro Photography Tool) which does the rest for me.


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