What ISO is best for shooting Flats and Bias frames?

[Adrian23]

Just a quick question everyone - should I be shooting my flat and bias frames at the same ISO setting as my subs and darks, or is there a more general ISO at which to shoot these calibration frames?

Adrian

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[Astro_Gaz]

same as your light frames mate

[tingting44]

BIAS shoot same ISO as your lights

FLATS shoot at your lowest ISO (mine is ISO100)

[Astro_Gaz]

flat frames should also be same ISO as lights 

the below is taken from DSS FAQ

Light, Dark, Flat, Bias... What are they and how to create them?
(if you are wondering how all the files are used during the calibration process you can have a look here)

Light Frames
The Light Frames are the images that contains the real information: images of galaxies, nebula...
This is what you want to stack.

Dark Frames and Dark Flat Frames
The Dark Frames are used to remove the dark signal from the light frames (or the flat frames for the Dark Flat frames).
With DSLRs and CCD Camera, the CMOS or CCD is generating a dark signal depending of the exposure time, temperature and ISO speed (DSLR only).
To remove the dark signal from the light frames you use a dark frame that contains only the dark signal.

The best way to create the dark frames is to shoot pictures in the dark (hence the name) by covering the lens.
The dark frames must be created with the exposure time, temperature and ISO speed of the light frames (resp. flat frames).
Since the temperature is important try to shoot dark frames at the end or during your imaging session.

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master dark or master dark flat.

Bias Frames (aka Offset Frames)
The Bias/Offset Frames are used to remove the CCD or CMOS chip readout signal from the light frames.
Each CCD or CMOS chip is generating a readout signal which is a signal created by the electronic just by reading the content of the chip.

It's very easy to create bias/offset frames: just take the shortest possible exposure (it may be 1/4000s or 1/8000s depending on your camera) in the dark by covering the lens.
The bias frames must be create with the ISO speed of the light frames. The temperature is not important.

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master bias/offset frame

Flat Frames
The Flat Frames are used to correct the vignetting and uneven field illumination created by dust or smudges in your optical train.

To create good flat frames it is very important to not remove your camera from your telescope before taking them (including not changing the focus).
You can use a lot of different methods (including using a flatbox) but I found that the simplest way is to put a white T shirt in front of your telescope and  smooth out the folds. Then shoot something luminous (a flash, a bright white light, the sky at dawn...) and let the camera decide of the exposure time (Av mode), 

The flat frames should be created with the ISO speed of the light frames. The temperature is not important.

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master flat frame.

 

link to original http://deepskystacker.free.fr/english/faq.htm

[tingting44]

My bad...... I swear I read to take flats at the lowest ISO, I've only done flats twice so far, but from now on will indeed do the flats at the same ISO as the lights

[Astro_Gaz]

My bad...... I swear I read to take flats at the lowest ISO, I've only done flats twice so far, but from now on will indeed do the flats at the same ISO as the lights

lol

[Adrian23]

Thanks everyone - looks like all calibration types are done at the same ISO as the lights. Well that's nice and easy. Wish I could say the same for dew prevention. I spent just under an hour of imaging on lights of m27 last night plus all the rest of the calibration frames on top of that just to find out that most of my lights were near useless due to fogging by dew.

Does anyone have any experience of using dew heaters? Do they do what you hope they will do?

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[laser_jock99]

There's no explaination in there as to why flats should be at the same ISO as the other calibration & light frames?

Personnally I'd go for a lower ISO for the flats as techincally there should be less camera  generated noise - though in reallity it probably doesn't make a lot of difference as more flats will average noise out.

If like me you're frequently tempted to shoot at ISO 3200 or higher- then definately turn the camera gain down for the flats.

[Astro_Gaz]

There's no explaination in there as to why flats should be at the same ISO as the other calibration & light frames?

Personnally I'd go for a lower ISO for the flats as techincally there should be less camera  generated noise - though in reallity it probably doesn't make a lot of difference as more flats will average noise out.

If like me you're frequently tempted to shoot at ISO 3200 or higher- then definately turn the camera gain down for the flats.

apart from the instruction from DSS FAQ? they stare to keep the ISO the same and the software asks the same when before stacking, if your using DSS to stack id stick to what they suggest, if stacking with other software then whatever floats ya boat  

[Scott]

apart from the instruction from DSS FAQ? they stare to keep the ISO the same and the software asks the same when before stacking, if your using DSS to stack id stick to what they suggest, if stacking with other software then whatever floats ya boat

I use dss and shoot flats at 100 iso. No problems.

I certainly wouldn't argue with you Gareth, your images stand up for themselves. I'm merely saying that 100 works for me

[Scott]

My bad...... I swear I read to take flats at the lowest ISO, I've only done flats twice so far, but from now on will indeed do the flats at the same ISO as the lights

As above, I (and I'm guessing many others) use dss and shoot flats at 100 iso. No problems. Whether or not there is a technical reason for not doing this, I don't know but if it ain't broke, I'm not gunna fix it

[Astro_Gaz]

I use dss and shoot flats at 100 iso. No problems.

I certainly wouldn't argue with you Gareth, your images stand up for themselves. I'm merely saying that 100 works for me

wasn't arguing bro, im far too old in the tooth to argue with a fellow astronomer

[Astro_Gaz]

auspom, just noticed your a fellow stokie, where abouts you from mate?

[jgs001]

Interesting... flats at 100, the rest at lights ISO... I've only ever used the same ISO on all the calibration frames... dropping the ISO might sort out some oddities I get on flats using a laptop with camera lenses though

[wxsatuser]

My bad...... I swear I read to take flats at the lowest ISO, I've only done flats twice so far, but from now on will indeed do the flats at the same ISO as the lights

Your not wrong as such.

Flats should be at a low ISO but DSS expects them to be at the same as the lights.

You can do flats later as long as nothing changes in the setup.

Temperature does'nt matter and of course the exposure will not be the same as lights.

Edited July 25, 2014 by wxsatuser

[Scott]

auspom, just noticed your a fellow stokie, where abouts you from mate?

Alsagers Bank Gareth,

I'm still waiting to get along to one one of the North Staffs Imaging nights

[Astro_Gaz]

Alsagers Bank Gareth,

I'm still waiting to get along to one one of the North Staffs Imaging nights

cool come along were a friendly bunch, were having a BBQ meteor shower evening for the perseids cant wait 

[dph1nm]

Remember to take a bias (or a 'dark flat')  at the same ISO as the flat, otherwise the flat-fielding won't work.

NigelM

[Michael1971]

On 25/07/2014 at 14:51, dph1nm said:

Remember to take a bias (or a 'dark flat')  at the same ISO as the flat, otherwise the flat-fielding won't work.

NigelM

Nigel, If I use dithering and only flats and no darks and bias, then flat fielding won't work?

Did I understand that correctly?

You had me puzzled there..

 

Thanks,

Mike