Quote:
Originally Posted by Amaranthus
I use the trick of covering the scope aperture with a white t-shirt, and then pointing at the inside of my garage (about 10 m away from my concrete pad) with the lights turned on. Seems to work well, and it means I can do them at night, right before or after my subs session.
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sounds like a practical approach if you have a nearby light source - thanks for the idea.
Quote:
Originally Posted by RickS
Ray,
You could be right about the FPN and sky glow. I haven't really looked deeply into the maths yet either.
I do have a Gerd Neumann EL panel but I've pretty much stopped using it since I found that dawn/dusk flats were giving great results. If I get a chance I'll try a bunch of EL flats and see what I learn...
Cheers,
Rick.
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Hi Rick. Trying to get my head around it now
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Quote:
Originally Posted by gregbradley
I don't personally believe modern CCDs vary too much in sensitivity across the sensor.
Perhaps there is some variance with different coloured lights but again its not the main thing you are trying to correct.
Its usually dust bunnies and vignetting from your scope you are trying to correct.
Some scopes have hot spots/bright centres plus vignetting. This is becoming more common with the use of correctors on RC, Dall Kirkhams etc.
They become more of a challenge for flats I find.
I also find high quality darks and bias essential for a good flat correction on these types of scopes.
APOs are a piece of cake, its the ones with correctors that are tough.
Greg.
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Hi Greg.
Agree, modern chips probably have less than 1% non-uniformity - for daylight imaging that is inconsequential. But for broadband astro, the main source of light is the sky background. If you have even very slight non-uniformity, it will be applied to the bright background sky light and then that bright noise will get mixed up with the much dimmer target. Even small sensitivity non-uniformity can introduce significant sky noise after you stretch to bring up the signal. eg if you have a sky that generates 1000 electrons in a sub and your target is much dimmer at only 10 electrons in a sub, then 1% of the sky will amount to 10 electrons - ie the sky non-uniformity noise will have as much power as the full the target signal - even minor non-uniformity is not inconsequential when you are trying to surface a dim signal buried under sky radiation.
The noise reduction action of flat calibration is in addition to the correction of vignetting and dust bunnies and it requires much higher quality flats. If you haven't already read it, discussed in
http://www.narrowbandimaging.com/inc...3_expanded.pdf
Regards ray