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Old 18-04-2011, 05:02 PM
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Logieberra (Logan)
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Flats with a White T-Shirt

Arvo guys

I have been thinking about flats for months now. They have scared me off for some reason. It's time to 'just do it'.

I am using a Nikon D40 SLR (same CCD as the QHY8 I believe).

I realise that I can purchase a whiz-bang light box for $100+, or build my own, but I am also aware that people use white t-shirts effectively (pulled taught across the dew shield).

I am also aware that the focus and camera orientation must remain the same during image capture (at night), and when taking the flats the next day (or whenever the conditions are ideal!)

Let's talk about the t-shirts option. If you have, or continue to use the white t-shirt method - please chime in.

If you think your know how to do it, or only use light boxes... please reserve your comments.



What I would like to know:
  • time of the day to shoot the flats;
  • ideal conditions
  • where to aim;
  • how many explosures do I take;
  • exposure length;
  • what's this ADU business; and
  • any other issues to consider / suggestions?
If I can walk away from this post, information in-hand, and start using flats I will be very pleased! Looking forward to it!


Cheers,

Logie

Last edited by Logieberra; 19-04-2011 at 08:25 AM.
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  #2  
Old 18-04-2011, 05:52 PM
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Octane (Humayun)
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Logan,

The ISO of the flat lights and flat darks should be the lowest your system is natively capable of operating at. I believe in the Nikon's this is ISO-200.

If you're using a refractor, drape the shirt over the front, and, simply point the scope at a laptop screen set to white in Photoshop in a full screen document.

Ignore ADU counts, blah, blah, blah. All you want is even illumination across the frame -- keep checking your histogram, and ensure it peaks somewhere between 1/3rd to 2/3rds of the way across the X-axis.

Take enough to give you an SNR of 4-5 (16-25). Then, put the cover on the scope, cover your viewfinder on the camera, and, take an equivalent bunch of flat dark frames at the same ISO and exposure duration.

That's all there is to it.

H
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Old 18-04-2011, 06:04 PM
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Tee shirts, white anything that is smooth will work fine, also just the clear blue skies after Sunset will do the job as well, just point the scope straight up, and try to get the frames as close to each other as possible.

Leon
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Old 18-04-2011, 07:35 PM
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tlgerdes (Trevor)
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White t-shirts make me look Fat not Flat.

On the serious side, I currently shoot the evening or morning sky for my flats. Have also used a computer monitor on occasion.
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Old 18-04-2011, 08:21 PM
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Drape the T shirt over the front of the scope with no creases. I hold it in place with a strip of elastic around the outside of the scope.
The scope needs to be aimed at a relatively even light source. This can be blue sky if you like but you need to keep the scope in the shade. If the sun is shining on the T shirt it risks a partial shadow and uneven illumination.
When I did this for a DSLR I just let the camera auto expose the image. It will be a very fast exposure and really doesn't need a dark frame to be subtracted. You might like to subtract an offset frame if that is part of you reduction process.
I use a different technique now as I have an observatory. I still use a T shirt but aim it at a white board on the inside of my observatory that is evenly illuminated either by overcast skies with the lid still on the observatory or 2 lights aimed at the wall.
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Old 18-04-2011, 10:38 PM
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Mine came out with a reversed out of focus AC/DC
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Old 19-04-2011, 06:33 AM
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I also use my laptop screen to take flats. With the t-shirt held in place with an elastic band I use Al's virtual light box to control the light level.
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Old 19-04-2011, 08:29 AM
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Logieberra (Logan)
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Gentlemen

Thanks for the steady flow of info.

I will print this page and give it a try over Easter. I'll try the different capture settings, light sources etc, take pics and post my results. Your comments will be appreciated.

This thread will definitely help newcomers J

Cheers,

Logie
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Old 19-04-2011, 10:12 AM
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Quote:
Originally Posted by tlgerdes View Post
White t-shirts make me look Fat not Flat.
You need to wear a dark then
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Old 19-04-2011, 12:19 PM
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I got sick of having to wait for the sun and the white tee shirt thing, that is why I built myself a lightbox. Much more convenient. Now I have built about 70 of them
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Old 19-04-2011, 01:51 PM
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Quote:
Originally Posted by strongmanmike View Post
You need to wear a dark then
I was thinking more along the lines of a Bathinov mask printed t-shirt, the stripes would help to camouflage the the uneven histogram.
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Old 19-04-2011, 02:43 PM
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strongmanmike (Michael)
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Quote:
Originally Posted by Exfso View Post
I got sick of having to wait for the sun and the white tee shirt thing, that is why I built myself a lightbox. Much more convenient. Now I have built about 70 of them
Totally agree, having to rely on a narrow window of sky light and hope it doesn't get cloudy before you can take the flats or you want to change or move your optical configuration before dawn etc...a light box or panel is deffinitely the way to go or a light panel on your observatory wall - this is good for remote/automated observatories too so you can always take flats

Relying on the sky to cooperate just seems so silly to me

Mike
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Old 19-04-2011, 03:51 PM
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I improvised one night at home. Pointed the scope at a blank wall and manually fired a flash with a wide beam at the wall during a 5 second exposure. The reflected light was nice and even. Took me about 3 goes to get the histogram right, but it worked well.

Don't really recommend that one at a dark site with others around.
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Old 19-04-2011, 04:53 PM
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Octane (Humayun)
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How come? You'd be a hit! Not to mention getting hit!

Quote:
Originally Posted by troypiggo View Post
Don't really recommend that one at a dark site with others around.
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Old 19-04-2011, 05:48 PM
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Put a red gel over the flash maybe?
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Old 20-04-2011, 11:02 PM
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Flats under an overcast sky

No need to wait for a clear sky: you can shoot tee shirt flats on an overcast day, too, as long as you're pointed at a uniform area of sky. And you can shoot any time of day. At dusk just before beginning an imaging run is a great opportunity to shoot flats. But as mentioned before, make sure that sunlight isn't falling directly onto the shirt: use a card or something to shade it if necessary.

If shooting with a DSLR, just use the camera's aperture-priority exposure setting.

As a rule of thumb I usually shoot 20 flats.

-Joe
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Old 21-04-2011, 08:37 AM
Hagar (Doug)
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A light box or panel seems to be the way to go these days. It does allow flats to be taken anytime, day or night and a box like Peter makes with a dimmer fitted allows very precise and uniform flats at almost any exposure time you wish to use.

Flats can be the hardest frame you will use. They are the hardest to get just right and even with a light box/ panel it can take quite a bit of experimentation to get them just right but once sussed out they become very repeatable and even.

For the cost I would never consider going back to tee shirt or sky flats again. More work and effort for what usually becomes sub standard and will have to do flats, than it's worth.
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