Quick 'flats' question - I'm about to do them!!

[swag72]

So, I've been imaging again this evening and have decided that I'll try some flats. JCJC has said that the way he does them is to use a pc screen on white, same ISO and aperture as I've been using, with the camera on AV and take about 20 shots.

Just before I go out I have a question - Do I have to change the focus and focus on the screen or leave the focus as it was at the DSO?

Is there anything else I need to be away of? Like does the screen need to be parallel to the sensor?

Thanks.

[steppenwolf]

It is IMPERATIVE that the focus remains untouched from the imaging session! The screen should be perpendicular to the telescope/lens and close to it, even touching it would be good.

[swag72]

Excellent Steve - Thank you. Will 20 be enough for over 2 hours of subs in total?

[steppenwolf]

Anything over 16 will be fine.

[swag72]

So just 16 will stop the awful vignette that I've experienced?

[steppenwolf]

They will certainly help! Remember that these FLAT images must be correctly exposed to be of any use - between 30% and 50% fully saturated. At a guess, your flat images will be under 1/2 a second in length.

[swag72]

How on earth do you get a correct exposure of an out of focus all white image? On all white I would normally over expose by about 2 stops to get a proper white, instead of 18% grey - What am I looking for as an end result? 18% grey or white?

[steppenwolf]

Here is a typical flat that I have just grabbed from my archive. OMG, look at that dust :-

[swag72]

I've just taken loads of flats - Can't get ANYTHING to look like yours! The correct exposure gave me 1/800 sec, but there's lines over the screen and no sign of any vignette at all. The image just gets brighter as I lower the shutter speed. I don't get this at all - Where's the vignette that you have got? Why can't I get it?

I knew there was a reason I avoided them - They're IMPOSSIBLE!!

[steppenwolf]

I knew there was a reason I avoided them - They're IMPOSSIBLE!!

Nah, it just seems that way! Try stacking the ones you have taken and then have a look at the stacked version. As a matter of interest, my exposures were 432ms (1/432 sec)

[RobH]

Here is a typical flat that I have just grabbed from my archive. OMG, look at that dust :-

Looks pretty clean compared to mine :D

[swag72]

So I need to stack them seperately to the rest of the stack? I thought I just chuck it all into DSS, come back in a couple of hours and then the fun starts!

[RobH]

You need to stack them and make a master flat, and the flats should really be bias subtracted too.

Steve's book is well worth getting as it explains loads of stuff that you'll need to know.

Have a look at my website under 'primers and tutorials'....I've done a big section on image calibration which will hopefully make things a bit clearer. The site address is in my sig.

Cheers

Rob

[Photosbykev]

Here is a typical flat that I have just grabbed from my archive. OMG, look at that dust :-

my car windscreen is cleaner than that !!!

this stretched flat is from my Canon 5D MkII which I clean at least once a month lol

Swag: you are correct to get a good white in normal photography you need to overexpose the image from the indicated value by 1-2 stops BUT for flats just use AV mode with no compensation as a start. Once you have a batch of them you can process them in DSS and then look at the resultant master flat image which will show the vignetting.

[steppenwolf]

my car windscreen is cleaner than that !!!

I think my dog's bed is cleaner than that - talk about 'airing your dirty linen in public', I am just soooo embarrassed

[swag72]

I have Steve's book and it is all starting to make a bit more sense now! But - this has led to another question. I stack my flats in DSS and it gives me a master flat file. I save this to use at a later date as a tiff file? - When do I put it into the stacking process? Why I ask this is that my subs are all in raw, and when I have treid to add a tiff to a stack where there are raw files, it says that the different files are not compatible and it won't work.

Sorry that I am asking such silly questions - I am such a newb! I can't wait to know the answers of these questions so that I can help folks myself!

[jgs001]

You don't need to worry about stacking the flats separately in DSS... Just load them as flats, alongside the darks and lights. DSS will do the hard work for you.

I've had odd issues with flats using camera lenses with a laptop screen. I think the screen is bright enough, the exposure comes out fast enough to reveal the screen refresh rates, and that has caused me issues with them. I think, dimming the brightness on the laptop screen is the way to go to extend the flat exposure time, do not adjust your camera...

[swag72]

Hi John - Thanks for that - When you take flats, what speed are you looking at? Mine seemed to be OK at 1/200s. That was set on AV and shutter speed sdjusted to get a 'correct' exposure.

[jgs001]

I don't actually recall... with the scope it's pretty slow... If they look ok, and you're not getting the line/screen refresh effects... I guess you'll be good.