flats - when to take?

[Joshua Bunn (Joshua)]

Hi,

I understand flat field frames need to be of the exact imaging setup as the light frames and should be taken at the end or start of an observing sesion, but. If capturing data over several nights, should i take flats at the end of every nights imaging for that particular target, and combine all of them (from the separate nights)?

thanks
Josh

[leinad (Dan)]

Flat fields expose the irregularities of the imaging system, ie an illuminated frame shows vignetting, dust donuts, specs, etc..

So, regularly yes. Combined no. Why? Because then you'd be adding the above where it all doesn't exist on the present exposure conditions.

Eg, if you over stretched your images; and the calibration with your current flat fields didn't remove a new dust don't you can see, then it warrants you need new flats to remove from the calibration. Unless you have some nifty processing tricks

[rmuhlack (Richard)]

You could take flats and darks on every night, and then use those flats/darks to calibrate the light frames from that night before all the light frames across all the nights for that object are registered and then stacked. DSS can do this (if you load each nights frames into a separate 'group') as i'm sure most other astro image programs can.

However, (unless the weather has been very dusty or similar) I would generally find it unnecessary to take flats on the same imaging setup on consecutive nights if everything has been left set up and is just as it was the previous night. I also only take flats against a twilight sky, and sometimes it isn't practical to take twilight flats every day (it might be cloudy, or i might not be home at sunset/twilight that evening etc). Especially with widefields with camera lenses, in practice I will often only take a new set of flats if I can see obvious dust marks etc in my images, in which case I will take them at the next available opportunity.

[multiweb (Marc)]

I take flats every single time I image. I take them right after my imaging session with an EL sheet. At the same temperature. I also take 'dark flats' that I subtract from the 'light flats' to scale them. I do the dark flats at the same temperature and same duration as the light flats. For my flats regardless of the camera or filter I aim for an ADU between 9000 and 10000. I do about 80 lights and 100 darks.

[Joshua Bunn (Joshua)]

thank you guy's for the replies, I thought i had email notification for this thread - but i hadn't.

Ok, so ideally take flats at the end of every night for the images taken that night, and make a master flat out of them - and do this for each nights imaging to apply to the light frames taken on that night only. And do not combine flats from different nights of imaging due to the chance of changing dust particles.

Does this make sense?

thanks
Josh

[Joshua Bunn (Joshua)]

Want to check this is the way i ideally should be taking flats?

Josh

[rmuhlack (Richard)]

that is the ideal yes. i guess i'm just lazy (and/or time poor_ for not taking flats every single time. I would definitely take them at the next available opportunity if i realised that by previous 'master flat' was no longer suitable.

I take my flats at twilight. Point the scope or camera lens at the sky (away from the sunrise/sunset) where there are no clouds and essentially 'even' illumination across the field. i then setup Backyard EOS to take 40-50 AV Flats (set the camera to Av and then it meters the frame to get the appropriate shutter speed for the correct flat illumination)

[Joshua Bunn (Joshua)]

OK, Thankyou Richard.

CS
Josh