Quote:
Originally Posted by naskies
Hrmm actually since you're using a DSLR you probably don't need the T-shirt/towel. It's used mainly to create a diffuse light source (if you're using an artificial light source), or to block out enough light so that a CCD camera can take a >= 1 sec exposure - many CCDs have problems with ultra-short exposures, but DSLRs don't.
Just wait say a few mins before/after dawn, point the scope at a featureless patch of sky at say 45 deg alt directly opposite the sun. Turn sidereal tracking off (or use the hand controller and slew east slowly), and take lots of flat frames. Combining a decent stack of flat frames will cancel out the stars (but avoid bright constellation stars).
Light boxes can work great, but they're a bit trickier to do right. Given the choice, I prefer to take sky flats - but if I'm packing up at say 1.30 am because clouds have rolled in, the light box is the only way  |
I've got an old sheet I'm trying with tonight, but the light is fading fast. It's another reason to use a lightbox, plus if I want to change the configuration during the night, I can take some flat frames at that point.
I can see though, given the extra time and hassle involved with the flats, that I'm most likely going to pick one configuration for the night and go with it. It's going to be bad enough to have to take flats for each ISO I use. Another bonus to the lightbox - I should be able to consistently predict the exposure duration for each ISO, and therefore easily build the dark flats library.
Quote:
Originally Posted by naskies
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That's interesting - and vaguely familiar - I think I've read it before. It's another reason I'd like to use a light box instead.
Quote:
Originally Posted by sil
Those circles are internal reflections from the optical path, in your case its a reflection from the inside surface of the corrector.
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I suspect you may be right. If so, it's somewhat annoying.