Hi Joe, On 08 Jan 2009, at 22:39, Joe Bauman wrote:
Hi Patrick, Thanks for doing this. I'd like a kit.
Ok, I'll get one out to you with a followup email.
I have shot darks at the time of the exposure, sometimes one per exposure, sometimes one per set of three or four exposures, but I've never combined darks.
A single dark may contain imperfections (like random noise and cosmic ray hits) that are then transferred to the light image. Shooting several (Jerry recommends at least 15 and I know of others that shoot many more) and then median combining (as opposed to adding or averaging) into a master dark keeps the good parts of the darks but tends to remove the bad. Happily it's very easy to make darks. The camera does not even need to be attached to the telescope (although as I'll cover in a minute, it's a good idea keep the camera attached to the scope). Just put the camera in a dark room, chill the chip to the temperature you shoot light images at (I use -10 year round as I can get that low even on warm summer nights) and have the software shoot 15 (or more) darks with the same exposure lengths as the lights you have taken or plan on taking. Darks can be archived. I typically use mine for a month or so and make new ones on nights when it's cloudy (though some insist on new darks every night).
Also I have yet to shoot a flat, though I know I need to do that. The problem with flats, as far as I am concerned, is that you have to use the same angle, focus, etc., as with the images you're correcting.
Yes. While flats typically take much less time to make they are more difficult and can not be reused if anything in the optical train is moved (which is why I leave the camera bolted securely to the telescope).
But in the field it's not possible to shoot a bunch of pics of one target, shoot flats, then go to another target -- at least, that's my understanding.
It is possible. But I can see where it's a pain. I've seen posts from folks on the Software Bisque lists that do it all the time. But it's far easier to do in a fixed observatory.
I think I need to shoot flats in the morning, using the latest target, but not a setting for an earlier target. Is that right?
Waiting until dawn and shooting the dawn sky is one way to do it. Cindy and Jerry do it by covering the front of the scope with cloth diaper material (really!) and shooting through that. I do it by pointing the scope at a blank sheet of white cardboard and then reflecting light off of another piece of white cardboard which then reflects off the first. Like the darks, I shoot the flats at the same temperature as the light images. You also need to shoot them through the same filter as is used for the light image (hence for color shots that means a separate flat for the clear light image and for each of the 3 filtered light images). The big difference from darks for flats is the exposure time. For darks the exposure is the same as for the light image. But for flats you only expose long enough to get about half way to saturating the chip. The purpose of the flat, BTW, is to image the out-of-focus various dust bunnies that exist in virtually every optical train (on the corrector, mirrors, filter, CCD chip window, CCD chip). When applied to the light image the flat removes the shadows caused by the bunnies. And as with darks, you'll shoot at least 15 flats to be made into each of the 4 master flats. So, how many total? 15 darks for one master dark. 15 flats for the clear master flat. 15 flats for the green master flat. 15 flats for the blue master flat. 15 flats for the read master flat. -- 75 Total I should add that some people also make master darks for each of their flats and since the exposures for each color flat is different, that adds another grundle darks. And of course, none of this has anything to do with the light images. Looking through Sky & Telescope it's easy to find images where the photographer spent many hours over many nights shooting each of the clear, green, blue and red images which were combined to make the finished product. This may look pretty complicated. Well, yes, in the beginning it is. But as with most things complicated it's really quite straight forward once you've done it a few times and even easier still when you let the software do most of the work. But it's still very involved and, as I've said many times, no matter how hard you work at getting the perfect image, it seems the next issue of Sky & Telescope always has a better one. :) So maybe the above illustrates why virtually all of my images are black and white data images... patrick