Flat field exposures

[ebonezer]

Hi all,

I'm feeling my way in this game and I've got myself very confused about flat field exposures. I do get why they are taken and used but some sources tell me to take the flats at the same ISO as the lights while others say take them at the lowest ISO available because you don't want to introduce noise and others favour slightly blurring the flats by 2pixels before combining.  Another option seems to be combining flats with dark flats all at the same exposure and ISO settings.   Which is right/better??

Advice from experienced imagers out there will be appreciated.

Cheers,

Eb

(ManchesterUK)

[red dwalf]

i`ll watch this thread also with interest, i`ve been taking my flats at the same settings as the lights but ajusting the exposure of course to get a ADU reading of about 25000.

[Owmuchonomy]

I always take flats at the same ISO as my lights. Do NOT touch anything from the imaging setup and take 20 or so flats using Aperture Priority mode if it's a DSLR and the histogram somewhere around 30 to 45%. YOu will need a uniform white light source such as a light box or white t shirt stretched across your objective and pointed at the morning sky.

[Tinker1947]

A copy and paste from the DSS Help file...

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

[lensman57]

or the orientation of the scope changes

Hi all,

I'm feeling my way in this game and I've got myself very confused about flat field exposures. I do get why they are taken and used but some sources tell me to take the flats at the same ISO as the lights while others say take them at the lowest ISO available because you don't want to introduce noise and others favour slightly blurring the flats by 2pixels before combining.  Another option seems to be combining flats with dark flats all at the same exposure and ISO settings.   Which is right/better??

Advice from experienced imagers out there will be appreciated.

Cheers,

Eb

(ManchesterUK)

What is the point of altering the ISO? You are doing AP and not daytime photography. Your calibration frames must be taken at the same iSO setting as your lights, these include, Bias, Flats and Darks. They must also be taken to within a couple of degrees of C to each other. Your flats contain noise and this is ISO and temp related so they must correspond to the same ISO and temp as the other frames. Flats must also be taken at exactly the same focus position as your lights otherwise they are not effective. Most hobby telescopes including some quite expensive EDs  have less than ideal focusing mechanisms that move about and are prone to focus shift if the temp or the orientation of the telescope changes. You must also make sure that the illumination source for the flat lighting is as neutral as possible. I have an array of flat panels and daytime balanced lightboxes (  from the time of my photography years with film and transparency) and you'd be surprised to find out how much colour bias there is in these so called flat panels, if the colour bias is too much it will cause over correction in the field. Hope you find this helpful.

Regards,

A.G

[ebonezer]

Hi all,

thanks for all your inputs. I did omit to mention I am using a Canon 60D unmodified.  Another fly in the oinment is the temperature.  I use MagicLantern in the Canon and this reports the sensor temperature which can range from 12dgs after the camera has cooled down during mount set-up (last night at the start of a session) to 32dgs after 40mins or so with 23dgs reached after about 10mins.  Temperatures up to 45dgs were common during  our warm summer nights.

Things don't get easier do they!  I foresee a lot of experimentation happening in the future.

Thanks again,

Eb

[lensman57]

Hi all,

thanks for all your inputs. I did omit to mention I am using a Canon 60D unmodified.  Another fly in the oinment is the temperature.  I use MagicLantern in the Canon and this reports the sensor temperature which can range from 12dgs after the camera has cooled down during mount set-up (last night at the start of a session) to 32dgs after 40mins or so with 23dgs reached after about 10mins.  Temperatures up to 45dgs were common during  our warm summer nights.

Things don't get easier do they!  I foresee a lot of experimentation happening in the future.

Thanks again,

Eb

That is why the temp is important. Good luck.

A.G

[sharkmelley]

In my experience temperature doesn't matter for flats because thermal noise is immaterial at the typical short exposures used.  I also take them at the lowest ISO to get them as noise-free as possible - remember their purpose is to record dust vignetting and dust particles in the imaging train as well as the varying individual pixel  responses to light, none of which are affected by ISO.

On the other hand, darks and bias frames must be done at the same ISO as imaging and temperature is important for darks (unless your processing can auto-scale the darks to your lights by noise minimisation).

Mark

[Astrosurf]

Why not try two sets of both: one with the same ISO and one with a lower ISO? Then stack each and compare.

Alexxx

[dph1nm]

I also take them at the lowest ISO to get them as noise-free as possible

ISO doesn't affect the signal-to-noise, which is what you are really trying to minimise. Low ISO does allow you to get more photons in a single flat exposure before you saturate (i.e. you can take longer flat exposures), but you achieve the same result at high ISO by taking more, shorter flats (as long as the total exposure time is the same).

Personally I would stick to to the same ISO as the lights, just in case the camera responds differently in some unexpected way at a different ISO. You may also need different bias (or flat-dark) frames if you change ISO, which is just one more thing to have to remember!

I also would not blur flats in any way, as one thing they are supposed to do is remove pixel-pixel variations in sensitivity, which won't happen if you blur.

NigelM

[ebonezer]

Hi all,

thanks again for the advice.  I'm in agreement with Mark but the concensus is to shoot flats at the same ISO as lights. One of my problems is that being new to all this I don't know what to expect and my images are crawling in noise - I'm shooting  NGC7000 as a reference which is conveniently placed for me -with an unmodded Canon 60D with a UHC clip filter and I can get similar results using different combinations of ISOs stacked in DSS, IRIS and Regim. Very confusing.

Cheers to you all.

Eb