Hi Joe, Now that you are retired are you coming to SPOC this evening? On 26 Apr 2008, at 16:47, Joe Bauman wrote:
I have two questions.
With folks here like Jerry and Tyler I'll preface my comments by saying anything I say is subject to correction by the pros. :)
Is it possible that the blue filter is insensitive to compensate for the blueness of the atmosphere?
I *think* it's just that CCDs are not sensitive in the blue end of the spectrum.
Also, could you detail how the flats are made?
If you come to SPOC we can go into more detail but I'll attempt a brief overview below.
I had the impression that a new one had to be made for every different setup you use...
Correct. Every time you make a significant change in focus, move the camera or add/subtract anything to/from the light stream a new flat must be made (that's the major reason I have the camera so securely bolted to the scope and rarely move or remove it).
Also, what was the illumination source?
I've got a very dim nightlight on one side of the observatory that illuminates a piece of white cardboard I hang from the ceiling on the other side of the observatory.
I had thought that a sheet, etc., stretched over the telescope at dawn would do it...
Yes, I have seen that done. In fact I think Cindy and Jerry use diaper cloth stretched over the front of their scopes. But I'm tend to be impatient and prefer not to have to wait until dawn. With my setup I can shoot flats any time it's dark outside (even if it's raining). BTW, some have also used an illuminated box they put over the front of the scope. I think they call them light boxes. Ok, now how I do it (thanks to Jerry for helping me learn this): First the scope must be focused the same as it was/will be for astro imaging (another reason for my not removing the camera or messing with the focus between sessions). The chip in the camera must also be chilled to the temperature used for astro imaging. Point the scope at the white light source. Insert the filter you want to start with in the light train. Starting with a short exposure take a series of increasingly longer exposures, measuring each until they average out to no more than half of what it takes to max out the CCD chip. In the case of the ST-10, it maxes out at 50,000 to 65,000 so I tend to make flats of around 20,000. Once you have determined the exposure, shoot and keep 15 shots (I've read of people taking many more than that but Jerry assured me 15 is sufficient). At this point some people will then subtract darks (darks are a whole 'nother thing and usually take much more time than flats to make) from each of the 15 exposures made in the previous step but I'm told that for flats of a couple of minutes or less you probably do not have to do that. Once you are satisfied with what you have you median combine them (another thing I learned from Jerry is you median combine them, NOT add or average combine them). Once you've combined the 15 into one, that one is your master flat for that particular filter which can be used and reused until someting in the light train gets moved. Having finished with that master flat you then repeat the process for every filter your are going to use plus another time for an unfiltered flat. Given all of the above, you can see why so few people really try to do pretty pictures. As I've always said, no matter how much time and money I spend on making pretty pictures the next issue of Sky & Telescope will always have better pictures than mine. So I stick mostly to astrometry and photometry. Hope to see you at SPOC this evening, patrick