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Old 24-08-2012, 05:17 PM
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JB80 (Jarrod)
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Taking flats for dslr widefields?

Hi all,

I was thinking about taking some flats next time I use the dslr but I'm wondering just how useful they will be and the best way to go about taking them with just a camera lens?
I have done a couple of tests one using the computer screen and one using a twilight sky but I'm not certain I'm on the right track.
I have attached the photos below.

If I can take successful flats are they going to help with the gradient in my photos, like where it is darker in the corners of the picture and lighter in the core type of thing?

Cheers

Jarrod.
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  #2  
Old 24-08-2012, 09:43 PM
Garbz (Chris)
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Twilight sky would be a good source. Computer screen less so (have a look at the colour cast at the top of the frame compared to the bottom, not to mention backlights on displays are not uniform).

You need to match the flat to the lens and the orientation. It's easy for me since I mount the camera the same way every time and I've been using the same set of flats for multiple shoots. What I did is took a photo of the wall of my house through the telescope at a time and place where there was no light gradient (bathroom was the best since it's all white).

But if you prep your frames early like this rather than shooting right after or during your lightframes then they will do nothing for dust removal. Worse still if there's dust in your flats which isn't in your lights then you'll introduce nasty looking bright blotches in your images.
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Old 25-08-2012, 10:46 AM
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Hi Jarrod, as chris has said twighlight is the best, however I have used computer monitor before by making the room dark and placing a piece of A4 copy paper over the monitor, which spreads the light more evenly. It's not ideal but it does work.
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Old 25-08-2012, 11:15 AM
LAW (Murphy)
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I was checking out Brady Haran's YouTube channel and I was surprised to learn that even the big overservatories he was visiting in La Palma preferred "sky flats" over "dome flats"
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Old 28-08-2012, 06:36 PM
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JB80 (Jarrod)
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Thanks guys for the replies, sorry for the late response as it's been a bit hectic here the last few days.

Quote:
You need to match the flat to the lens and the orientation. It's easy for me since I mount the camera the same way every time and I've been using the same set of flats for multiple shoots. What I did is took a photo of the wall of my house through the telescope at a time and place where there was no light gradient (bathroom was the best since it's all white).
Chris,
Matching it to the lens I get but by orientation do you mean the way it's pointing?
If I do say an hour of shooting on the Polarie then the orientation has changed over that hour so it's best to leave it in that position once it's finished?
Also another what may be a silly question, when you use the wall is this with the lights switched on or in the dark?

Quote:
But if you prep your frames early like this rather than shooting right after or during your lightframes then they will do nothing for dust removal. Worse still if there's dust in your flats which isn't in your lights then you'll introduce nasty looking bright blotches in your images.
I was only taking these to see if I was on the right track more than anything so when it comes time to take some in anger I have a vague idea what to expect. Would you say I should be aiming for something more like the blue one than the grey one?

Quote:
Hi Jarrod, as chris has said twighlight is the best, however I have used computer monitor before by making the room dark and placing a piece of A4 copy paper over the monitor, which spreads the light more evenly. It's not ideal but it does work.
Hi Rex,
That is something else I can try, it makes a touch more sense than shooting the screen flat out.
I'm open to all ideas.

Definitely a lot of reading does say sky flats are better, the only thing that is a bit tricky I found that there is maybe not a huge window before stars start to show up, especially if shooting at 18mm.
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Old 31-08-2012, 02:49 PM
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Quote:
Originally Posted by JB80 View Post
the only thing that is a bit tricky I found that there is maybe not a huge window before stars start to show up, especially if shooting at 18mm.
I've read that the easiest way to combat that is to dither (move your mount very slightly between each flat exposure), that way when you median combine your flats the program will throw the stars out because they don't line up within all of the images and it thinks they are hot pixels. Mind you I have never tried this myself but apparently it works.
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Old 03-09-2012, 06:00 PM
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JB80 (Jarrod)
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Thanks for the tip, it's something I will keep in mind.
I just wish I could get a break to give it a go. It can't always be cloudy can it?
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