To those here that shoot tri-color CCD images: Is there a rule of thumb for how much time to be spent imaging through each filter (red, green, blue, clear)? patrick
Hi Patrick, I'm probably not much help because I'm still learning. But I generally shoot the same exposure for each of the three colors. Then I think the clear should be longer, maybe three times the exposure for the others. The best way to get an idea is to look in S&T and find one of the amateur views, and the caption will list the exposure times. Good luck, Joe PS: What are you shooting? Patrick Wiggins <paw@wirelessbeehive.com> wrote: To those here that shoot tri-color CCD images: Is there a rule of thumb for how much time to be spent imaging through each filter (red, green, blue, clear)? patrick _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://gallery.utahastronomy.com Visit the Wiki: http://www.utahastronomy.com --------------------------------- Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now.
Doesn't it depend on the particular response curve of the chip you are using? Some are much less blue sensitive than others, IIRC. Joe and Patrick, by "clear", I assume you are referring to the luminance exposure? And isn't an IR blocking filter typically used for this exposure? I've been kind of wowed by the "tricolor" images I've seen lately that use narrowband filters in place of color filters. I'm sure the exposures are much, much longer due to the vastly decreased throughput, but the resulting images are striking, as well as having some scientific value. On Sat, Apr 26, 2008 at 12:17 AM, Joe Bauman <josephmbauman@yahoo.com> wrote:
Hi Patrick, I'm probably not much help because I'm still learning. But I generally shoot the same exposure for each of the three colors. Then I think the clear should be longer, maybe three times the exposure for the others. The best way to get an idea is to look in S&T and find one of the amateur views, and the caption will list the exposure times. Good luck, Joe
PS: What are you shooting?
Patrick Wiggins <paw@wirelessbeehive.com> wrote: To those here that shoot tri-color CCD images:
Is there a rule of thumb for how much time to be spent imaging through each filter (red, green, blue, clear)?
patrick
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://gallery.utahastronomy.com Visit the Wiki: http://www.utahastronomy.com
--------------------------------- Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://gallery.utahastronomy.com Visit the Wiki: http://www.utahastronomy.com
I tend to agree, you have to really want to take color pictures to go the tricolor route. But the one-shot color cameras apparently can't come close, from what I've read. On Sat, Apr 26, 2008 at 1:12 AM, Patrick Wiggins <paw@wirelessbeehive.com> wrote:
Morning,
On 26 Apr 2008, at 00:32, Chuck Hards wrote:
Joe and Patrick, by "clear", I assume you are referring to the luminance exposure?
Correct.
And isn't an IR blocking filter typically used for this exposure?
Really? Ummm, this is already getting complicated. :)
Chuck, in your earlier message on the subject you wondered about possible blue insensitivity of some cameras. Well, this morning after finishing my routine minor planet work I set to making flat frames for each of my filters. For those who may not know, flats are used to remove the effects of dust in the optical train and are made by pointing the scope/camera at a uniformly illuminated sheet of white cardboard and shooting a whole bunch of pictures through each of the filters. Those pictures are then combined into one master flat for each filter. With no filter I was able to get enough signal to make a good flat in just under 5". For IR it took 10". Red was 15", green (actually a "visual" filter) 90". And then I tried blue. After 360" (6 minutes) I still only had about 1/4 of what I was able to get unfiltered in 5". So I think it's safe to say the SBIG ST-10 is not very sensitive in the blue. BTW, for those that might be interested, for last night's/this morning's galaxy imaging experiment, I settled on M-63. I shot 10 one minute exposures through each of the 5 filters plus another 10 one minute exposures with no filter. Next step it to apply the dark and flat frames to each and then register them all so they all line up nicely. Once that's done I should have 60 frames which, if I understand this stuff correctly, should enable me to add some (all?) together and form a color "pretty picture" of M-63. If anyone is interested I'd be happy to make the data available to others so they can try their hand at it. Cheers, patrick
Thank you for the great info, Patrick. I have two questions. Is it possible that the blue filter is insensitive to compensate for the blueness of the atmosphere? Also, could you detail how the flats are made? I had the impression that a new one had to be made for every different setup you use, such as one for a 60-second exposure with a certain focus, another for a 30-second exposure, etc. So I have never done it. Also, what was the illumination source? I had thought that a sheet, etc., stretched over the telescope at dawn would do it, but I really don't know. Thanks, Joe Patrick Wiggins <paw@wirelessbeehive.com> wrote: Chuck, in your earlier message on the subject you wondered about possible blue insensitivity of some cameras. Well, this morning after finishing my routine minor planet work I set to making flat frames for each of my filters. For those who may not know, flats are used to remove the effects of dust in the optical train and are made by pointing the scope/camera at a uniformly illuminated sheet of white cardboard and shooting a whole bunch of pictures through each of the filters. Those pictures are then combined into one master flat for each filter. With no filter I was able to get enough signal to make a good flat in just under 5". For IR it took 10". Red was 15", green (actually a "visual" filter) 90". And then I tried blue. After 360" (6 minutes) I still only had about 1/4 of what I was able to get unfiltered in 5". So I think it's safe to say the SBIG ST-10 is not very sensitive in the blue. BTW, for those that might be interested, for last night's/this morning's galaxy imaging experiment, I settled on M-63. I shot 10 one minute exposures through each of the 5 filters plus another 10 one minute exposures with no filter. Next step it to apply the dark and flat frames to each and then register them all so they all line up nicely. Once that's done I should have 60 frames which, if I understand this stuff correctly, should enable me to add some (all?) together and form a color "pretty picture" of M-63. If anyone is interested I'd be happy to make the data available to others so they can try their hand at it. Cheers, patrick _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://gallery.utahastronomy.com Visit the Wiki: http://www.utahastronomy.com --------------------------------- Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now.
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
Hi Joe, Thanks for the thoughts. I had a look at S&T and also did a bit of looking on the web but I could not find any common thread. I've no real desire to get big into pretty pictures but I'm between projects and with the skies looking ok I thought I'd give it a go. I think I'll find a pretty face on spiral galaxy and shoot 5 or 10 one minute exposures in each color and then mix and match the results to see what happens. patrick On 26 Apr 2008, at 00:17, Joe Bauman wrote:
Hi Patrick, I'm probably not much help because I'm still learning. But I generally shoot the same exposure for each of the three colors. Then I think the clear should be longer, maybe three times the exposure for the others. The best way to get an idea is to look in S&T and find one of the amateur views, and the caption will list the exposure times. Good luck, Joe
PS: What are you shooting?
participants (3)
-
Chuck Hards -
Joe Bauman -
Patrick Wiggins