Exposure time for flats

[carl20171]

I was wondering if anyone knows if .1 seconds is ok. I see a lot of info that shows about 1 second.

I have some nice flats that have a good saturation of about 25000 and seem to work well to calibrate with,

But due to the lighting they were only taken at a tenth of a second. Would I get better data at one second exposure time or is it just the saturation being correct that matters?

[Phillyo]

I think the only difference comes from what type of shutter you have. If it's mechanical then you want it to be longer, if it's an electric then I don't think it matters as long as you're getting the correct adu value.

Phil

[swag72]

I think that as long as the flats are doing their job does it really matter what exposure they are at? I would say not, mine can be pretty short at times with no adverse affects.

[Ant]

I normally go for about a 1/3 saturation. About 20,000 in Maxim.

But it will depend on the chip, the light source and also the filter used. RGB tends to be fairly short somewhere less than a second. But when I am using the Ha filter it can be up to around 5 or 6 seconds.

When I take flats for the sun, it's 1/1000s or faster - but that's a completely different area

As Sarah says, if your flats are working then that's great! Stick with it.

I went though a period of my flats being really difficult to use, so I took three sets at the end of every session. 15,000 20,000 and 25,000 ADU. I then processed the data with each set of flats and chose the best result.

Cheers

Ant

[ollypenrice]

As above, the only significance in exposure time arises from the effects of a mechanical shutter where one is used. The wipe of the shutter can put a gradient into very short flats. With no mechanical shutter in the story it doesn't matter at all. Always calibrate flats by subtracting a master bias which makes a perfectly good dark on these short exposures.

Olly

[Ouroboros]

I've only started taking flats recently. So I am still experimenting. I read somewhere that it's fine to take flats on an automatic setting like Aperture priority and let the camera decide the exposure. The question I have about that is what happens if you take Dark Flats? Aperture priority would result in much longer exposures when taking a Dark. So maybe it's better to do it in manual and go for something close to 1/3 saturation as explained earlier.

[ollypenrice]

I've only started taking flats recently. So I am still experimenting. I read somewhere that it's fine to take flats on an automatic setting like Aperture priority and let the camera decide the exposure. The question I have about that is what happens if you take Dark Flats? Aperture priority would result in much longer exposures when taking a Dark. So maybe it's better to do it in manual and go for something close to 1/3 saturation as explained earlier.

The only dark flats you need are biases. Make a master bias (shortest exposure possible, all light scrupulously excluded, take 50 of them and combine them in average or sigma, it really won't matter much.) This works because, for short exposures, there will be no significant difference between a bias and a slightly longer dark taken at the same setting as your flats. SO it may be called a master bias, which it is, but it will also work perfectly as a dark for flats. This is a massive time and tedium saver as well!

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

[carl20171]

Thanks for the replies. It is an Atik 490 so no shutter. As for bias I thought a dark already has a bias in it so I could use a dark as a bias also but not the other way around, using a bias for dark. wouldn't the longer exposure of the dark make more electrical current built up in the pixel over time changing the data compared to a very short exposure bias?

[Ouroboros]

The only dark flats you need are biases. Make a master bias (shortest exposure possible, all light scrupulously excluded, take 50 of them and combine them in average or sigma, it really won't matter much.) This works because, for short exposures, there will be no significant difference between a bias and a slightly longer dark taken at the same setting as your flats. SO it may be called a master bias, which it is, but it will also work perfectly as a dark for flats. This is a massive time and tedium saver as well!

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

I'm wondering how you "make" a master bias. I usually add all the RAW files into DSS and let it do what it does. I assume one can extract the master bias file for later use, or is there some other way of making just the master bias?

Secondly how to use a master bias in DSS? I assume you would just add the one master bias frame rather than the 20 or so RAW bias exposures.

[D4N]

The default settings in DSS will create master calibration frames.

You just need to select these instead of the individual frames the next time you stack.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

[Ouroboros]

OK, thanks. Yes, I guessed right.

[ollypenrice]

Thanks for the replies. It is an Atik 490 so no shutter. As for bias I thought a dark already has a bias in it so I could use a dark as a bias also but not the other way around, using a bias for dark. wouldn't the longer exposure of the dark make more electrical current built up in the pixel over time changing the data compared to a very short exposure bias?

A dark does contain the bias offset. You are right, but that's not the point here.

The difference between a bias and a very short dark is a statistical zero and not worth the effort. Trust me. The most experienced imagers regard a master bias as no different from a dedicated dark-for-flats. My flats have been dark calibrated this way for years.

Olly

[carl20171]

Thanks, I now have my darks bias and flat all in 3 folders. I used maxum dl to create a master of each. What to do now is the question. I think I open my flat master in maxum dl and then open the calibrate window and calibrate the flat with the bias master and forget about the darks. Does that sound correct. Then I would have a bias corrected flat. Then each time I stack I would just correct with the one bias corrected flat master?

[D4N]

Not sure if I get what you mean there but you still need a dark as the flat doesn't contain the correct dark noise for calibrating your light frame.

You need the three master calibration frames each time you want to calibrate images.

Unless of course you don't want to correct for dark noise.

One thing you can do is calibrate images without stacking them. Then you can stack these calibrated images with no further calibration so it is faster to change the stacking parameters or add more data.

You just need to change the DSS settings to save intermediate files.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

[carl20171]

Yes I think you answered my question. I don't take the 3 masters and make a master of them leaving 1 master master to correct with. I use the 3 separate masters to calibrate the light frame. Is that correct?

[D4N]

Yes that's right.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.