On 25 Jun 2010, at 21:20, Joe Bauman wrote:
OK, It's time for me to get down to doing something I've never succeeded at before, taking flats. Could any of you experienced astrophotographers fill me in on some essentials? First, I assume you have to take a series of 'em for each image you get; in fact, flats for luminosity and flats for r, g and b. Is that so? How many do you take? Second, how can you take them when it's dark outside and you want to slew to another target? Please give me a few hints. Many thanks, Joe
Short question requiring a pretty long answer. Making flats takes a lot less time than darks. Darks have to be the same chip temperature and exposure length as the light images whereas flats also need to be at the same temperature but only need to be long enough exposure to reach about half the saturation point of your camera. As an example my camera saturates at 50,000 or 65,000 depending on what binning I'm using. So I try to shoot flats that are about 25,000 to 30,000. With the light source I use that's usually about 2 to 4 seconds. Flats (and darks) must be shot with the chip at the same temperature and the same binning as the lights. You need a separate flat for each filter you use to make the lights. So if you're shooting R, G, B and luminance you need a flat for each of those. Shoot several flats through each filter (I do 15 each though I've heard of others who shoot more). Then median combine each set into a master flat. That way you end up with master R, G, B and luminance flats. Although it takes a lot longer to make master darks they can be reused for quite some time assuming they were made at the same exposure, temperature and binning as the lights. Flats can be reused too but ONLY if the scope is not refocused and the camera is not moved (rotated) or removed. If focus is changed significantly or if the camera is moved or removed you have to make all new flats. That's the reason I leave the camera securely bolted to the scope all the time. I've left what can be the most problematic part of making flats for last and that's the physical set up of the equipment. In my case with a fixed observatory it's pretty straight forward. I wait until it's dark, hang a piece of white cardboard half a meter or so in front of the scope, illuminated the cardboard from the other side of the room with a diffuse light source and fire way. How in the world that is done with a portable setup is beyond me. I know Cindy and Jerry Foote came up with a rather interesting device made of baby diapers (to diffuse dawn/dusk sky light) that fits over the front of their scopes. I've also heard of light boxes that fit over the front of the scope. Maybe those here that have done portable astroimaging can chime in on how they do it. So, let's see how many of what you need to shoot. Let's say you're going to be shooting some spectacular Tyleresque image with the chip cooled to -10, the binning set to 2x2 and a series of 5 minute exposures using red, green, blue and clear filters. To make the master dark: 15 - 5 minute exposure darks, -10 degrees, binned 2x2. All 15 darks are then median combined into a master dark. To make the master flats: 15 - X" (X equals whatever exposure is need to expose the chip half way to saturation) exposure flats, -10, 2x2, shot through the clear filter. All 15 are then median combined into a master clear flat. 15 - X" exposure flats, -10, 2x2, shot through the red filter. All 15 are then median combined into a master red flat. 15 - X" exposure flats, -10, 2x2, shot through the green filter. All 15 are then median combined into a master green flat. 15 - X" exposure flats, -10, 2x2, shot through the blue filter. All 15 are then median combined into a master blue flat. I guess it looks a bit complicated and time consuming (I know it did to me when I first started thinking about doing color imaging). Maybe that's why so few people does serious astroimaging. But fortunately Jerry Foote was there to guide me along (thanks again Jerry). It always helps to have a mentor. As with so many things once you've got a system down it's really not that complicated. As for time consuming, the flats are not that bad since they are so short. But, yeah, the darks do take a lot of time if your light frames are any longer than a minute or two. Happily, since the camera does not even have to be on the scope to make darks you could just put the camera in a dark room some cloudy night, cool it down, set the binning and instruct the software to take 15 darks and go have dinner. I hope this helps. If you've got more questions let me know. Or maybe we could talk it over at an upcoming SLAS function. And now (appropriately enough) I'm going to turn all the darks and flats I shot last night into masters. Boringly yours, patrick