Re: [Utah-astronomy] couple of questions regarding CCD imaging
How do you achieve focus with a CCD camera since there is no live focus mode?
As Patrick and Joe note, all cameras come with some driver software that displays the current camera view with an option to change the length of exposure. Generally, again as Patrick and Joe note, it boils down to three cases: 1) There is bright star (mag. > 8.0) in the DSO image frame (occurs infrequently) Focus on the bright star with a short exposure time (e.g. 5 - 15 secs). The DSO will not exposure. Turn your focus knob in and out focus taking repeated images. When the bright star is a pinpoint, then you are at focus. Now up your integration time to expose the DSO. 2) There is no bright star in the DSO image frame but there is a faint star (mag. 11 to mag. 9) in the DSO image frame (most frequent case). Use the procedure above, but find _one of the faintest stars_ in the image field. When you are out of focus, the faintest star disappears. When the faintest star is out of focus it is a blurred disk, the star's disk brightness decreases, and the star merges with the background brightness of the image's field. You can see this effect with a telescope or binoculars by rolling the scope in or out of focus and watching what happens to the visibility of the faintest stars in the eyepiece. Now here's where motorized focuser controls come into play, if you are using a short focal length optical tube. Images are less forgiving to being out of focus as compared to visual observing. (The mind's wetware compensates for the blurred image.) Short focal length tubes have a very thin focus play - sometimes measured in less than one millimeter. Your hand cannot turn the focus knob accurately enough to get an ultra precise focus. The gear reduction ratio on a motorized focus controller can turn the number in 10ths of a millimeter and really gets the focus adjustment set exactly. 3) There is no faint star, just the DSO in the image (occurs infrequently). Slew off center until you find a faint star. Use the case no. 2 procedure to focus, then slew back to the target. Hope that additional explanation helps. Clear Skies - Kurt
Maybe I missed something, and I really a novice but I got one real crummy picture last night by 3:20 AM. SBIG Cameras Use the free program CCDOPS. CCDOPS can be downloaded from SBIG's website. It follows all the ASCOMS protocal's and I am guessing it works with other camera's. CCDOPS has a focus function that keeps taking frames while you change the focus. The frames are displayed repeatedly on the screen and the value of the brightness is also displayed. When you get close you see the stars bloom and the numbers advance. When you pass the focus the numbers decrease. It is a real simple procedure to operate, but getting the focus is challenging. My Crayford Type(Moonlite for SCT) will be here this week and I can't wait to mess with it. Steve From: "Canopus56" <canopus56@yahoo.com> To: "Utah Astronomy List Serv" <utah-astronomy@mailman.xmission.com> Sent: Tuesday, May 31, 2011 5:41:49 PM Subject: Re: [Utah-astronomy] couple of questions regarding CCD imaging
How do you achieve focus with a CCD camera since there is no live focus mode?
As Patrick and Joe note, all cameras come with some driver software that displays the current camera view with an option to change the length of exposure. Generally, again as Patrick and Joe note, it boils down to three cases: 1) There is bright star (mag. > 8.0) in the DSO image frame (occurs infrequently) Focus on the bright star with a short exposure time (e.g. 5 - 15 secs). The DSO will not exposure. Turn your focus knob in and out focus taking repeated images. When the bright star is a pinpoint, then you are at focus. Now up your integration time to expose the DSO. 2) There is no bright star in the DSO image frame but there is a faint star (mag. 11 to mag. 9) in the DSO image frame (most frequent case). Use the procedure above, but find _one of the faintest stars_ in the image field. When you are out of focus, the faintest star disappears. When the faintest star is out of focus it is a blurred disk, the star's disk brightness decreases, and the star merges with the background brightness of the image's field. You can see this effect with a telescope or binoculars by rolling the scope in or out of focus and watching what happens to the visibility of the faintest stars in the eyepiece. Now here's where motorized focuser controls come into play, if you are using a short focal length optical tube. Images are less forgiving to being out of focus as compared to visual observing. (The mind's wetware compensates for the blurred image.) Short focal length tubes have a very thin focus play - sometimes measured in less than one millimeter. Your hand cannot turn the focus knob accurately enough to get an ultra precise focus. The gear reduction ratio on a motorized focus controller can turn the number in 10ths of a millimeter and really gets the focus adjustment set exactly. 3) There is no faint star, just the DSO in the image (occurs infrequently). Slew off center until you find a faint star. Use the case no. 2 procedure to focus, then slew back to the target. Hope that additional explanation helps. Clear Skies - Kurt _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php
On 31 May 2011, at 20:53, gazebo4sale@comcast.net wrote:
Maybe I missed something, and I really a novice but I got one real crummy picture last night by 3:20 AM.
Not to worry. It takes a while to figure out what to do. Heck, even when you know what you're doing it can still take a while. A great example is when several of us gathered in Bob Moore's office in downtown SLC some time ago and remotely used a 2 meter scope on Maui to take pictures. We were there a few hours and in the end we had one nice picture (see page two of http://www.slas.us/Nova/2010/NOVAMA2010.PDF ). Afterwards I heard a few people were disappointed as they had thought we were just going to shoot one image after another. It just does not work that way, at least not if you're trying to make pictures that are nice to look at. That could be partly why impatient people like me go with taking data images. You should see the ones I'm churning out as I enter this. Focus isn't quite right. Targets are not very well centered. Some of the stars are way overexposed and blooming. But every one contains useful scientific data. And every now and then you end up with yucky pictures that lead to discoveries: http://www.slas.us/Nova/2008/NOVAJF2008.PDF Ah, but there I go again, proselytizing for data taking. :)
CCDOPS has a focus function that keeps taking frames while you change the focus. The frames are displayed repeatedly on the screen and the value of the brightness is also displayed. When you get close you see the stars bloom and the numbers advance. When you pass the focus the numbers decrease.
That's the basic idea, but if the stars are blooming you probably ought to be using fainter stars. In my experience the fainter stars show problems with focus much better than bright ones.
It is a real simple procedure to operate, but getting the focus is challenging.
Welcome to astroimaging. :) Ok, back to my very ugly but very useful data images... Carpe Noctem! patrick
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Patrick Wiggins