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Struggling with flats - they dont seem to work for me :(

Simms

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Simms

Simms

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So taking some advice a while back from Olly, I have now started taking a set of flats with every evenings imaging session (I usually only image a single target in each session). I do not move the camera (Canon DSLR) and also make sure I keep ISO settings the same - I expose a 3rd of a stop under 0EV. However, when adding them to a DSS project they just dont seem to work very well - sure they take a bit of the vignetting out of the image, but not enough to make them worthwhile.. Can someone post a JPG of one of their flats taken with a DSLR so I can make sure I am nailing the exposure? This is really annoying me now as the vignetting is quite apparent in the light subs due to crappy light pollution.

Thanks.

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Psychobilly

Psychobilly

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I had similar problems in the past and resorted to doing some experiments with bracketed flats and applying each exposure set to a test image and seeing which worked the best...

IIRC the results were posted "somewhere" I don't go anymore... I'll see if I can dig them out but I lost a HDD last week and they might have been on there...

Peter...

I'm restoring a backup from an external drive to a new internal one so will have a look when its finished... failing that I'll look at the EV setting for the flats I have been using

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JCJC

JCJC

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Not sure why they are mot working. I used to just put the camera on AV mode with the ISO and everything else the same (aperture if you're using a lens) take a shot and remember the exposure time. Switch the camera back to manual and change the exposure to the same as the AV shot. Then snap a few (maybe 20) and chuck them into DSS. This method seemed to work ok to correct my images. Can't post an example until I'm on a pc I'm afraid.

Cheers,

Jordan

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Ant

Ant

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I had exactly the same issue a while back...

I started taking three different sets at different ADU levels. 9000, 19000 and 29000.

I then re ran the stack with each set of flats and chose the one that worked best. I never really really got to the bottom why...

That was with my dSLR.

Cheers

Ant

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MikeWilson

MikeWilson

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Don't worry about ev, exposure, aperture, whatever....

Switch camera to Av and snap away at a smoothly illuminated surface :( leave shutter speed to the DSLR and it will give you nicely illuminated flats.

Also consider an EL panel or use a laptop screen to get a consistent light source.

Clear skies,

Mike

Sent from my mobile using TapaTalk (so please excuse bad grammar & spelling!) :-)

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ollypenrice

ollypenrice

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So taking some advice a while back from Olly, I have now started taking a set of flats with every evenings imaging session (I usually only image a single target in each session). I do not move the camera (Canon DSLR) and also make sure I keep ISO settings the same - I expose a 3rd of a stop under 0EV. However, when adding them to a DSS project they just dont seem to work very well - sure they take a bit of the vignetting out of the image, but not enough to make them worthwhile.. Can someone post a JPG of one of their flats taken with a DSLR so I can make sure I am nailing the exposure? This is really annoying me now as the vignetting is quite apparent in the light subs due to crappy light pollution.

Thanks.

Uh-oh, I risk ending up in the doghouse here!! No, seriously, all astro imagers need flats. But most of us have to learn to get them to behave. I certainly did.

I don't use a DSLR at night so I'd try following the previous suggestions but keeping the same ISO setting cannot be necessary. All you are trying to do is photograph the true lightpath of your system, warts and all. (There should be nothing to photograph but there will be...)

I need a different technique in my mono CCD from my one shot colour because my OSC flat needs debayering but I have no idea how/if that applies to DSLR.

Olly

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Psychobilly

Psychobilly

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Why should you end up int he doghouse Olly...

The important thing is for people to find out what works for them... I have seen flats over- and under-correcting the effects of this are noticeable on dust bunnies as well as the vignetting...

Checking back it seems I have settled on flats at around +1.0 to +1.3EV ... I have found shooting Flats in Av mode can casue "problems" as the camerra will sometimes take flats at two different exposure times if the light level is in between EV values I tend set the camera to work in 1/3EV steps - DSS will only use flats that have the same exposure duration so will not include all the flats when creating the master flat...

Peter...

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Simms

Simms

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OK, just finished an imaging run on the Owl Cluster again, and took flats using the Laptop method outlined above - didn`t move the camera and shot the flats at 0EV.

Not knocking you at all Olly - your advice is priceless and I am just trying to get rid of that damn vignetting - so no doghouse for you.

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Simms

Simms

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...and work they have; that is a pristine image, faultless. Perfect background sky, pinpoint focus, lovely star colour. Don't let it languish on this board, get it up on the DS imaging one where it belongs!

Olly

Cheers Olly, might just do that! Thanks for the advice all!

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