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Taking Images (part 1)

Initial Thoughts Second Thoughts More Thoughts

Initial Thoughts

(Please note that as I change the way I do things, I don't change already existing web pages. Instead, I add new pages - hence "Second Thoughts")

Setting up the Telescope

First, you have to set up and align the telescope. Needless to say, to try to get the maximum resolution, you will want as stable a platform as is possible. While I had some pretty good results with the scope sitting on a shelving unit next to the window, putting it on a more substantial bar stool made a noticeable improvement, and putting it on a real tripod made an even more dramatic one.

While pointing the telescope out of a window may be a practical necessity if you don't have a laptop computer, be aware that in cold weather huge gusts of warm air will flow out, potentially ruining the seeing. Make the room as close to the outside temperature as you can get it.

Gamma Aries- seeing
2 frames of Gamma Aries taken 2 seconds apart. The second clearly shows the results of a thermal updraft.

I have heard it said many times that, for visual use or short exposure photography, there is no reason to polar align the ETX, since the AutoStar knows how to track in alt/az mode. I'm afraid that I have to disagree with this. It has been my experience that the drive motors on the ETX cause a slight vibration in the view while they are running. When viewing Saturn's Cassini Division or the diffraction rings around a star you can distinctly see them disappear from view while the motors "purr" and then snap back into view for the brief period when the motors are quiet in between corrections. I don't know if the Dec motor causes more trouble, or if it's just that only one motor is running, but for whatever reason, I find the view much more steady if the scope is equatorially mounted.

Setting up the Software

I use the QuickPict software that came with my Color QuickCam. It has some quirks, but it's the only way I know to get full 24-bit images from the camera. I use 16-bit mode to focus, but find the image to be much sharper in 24-bit mode. In an attempt to have the camera do as little massaging of the data as possible, I set all of the camera adjustments to mid-scale, or 128 and turn off auto-exposure, color control and the low-light filter. The exception is the Brightness slider, which I use to control exposure. Fortunately, someone else has done the work and determined the correlation between this setting and exposure time, and posted it here.

Taking the Shots

First, find your target visually. For simplicity's sake, I'm going to assume it's a bright one (otherwise, check here.) Center it in a high power eyepiece and then let the telescope's drive settle. I find it takes a couple of minutes for the ETX drive to calm down and run smoothly after slewing. Now comes the hard part: replacing the eyepiece with the QuickCam. The problem is that the image will almost certainly not even be close to focused, and this means that the light will be spread all over the frame, making it too dim to find and center the object. I dealt with this by going out on a night with a full Moon and pointing the telescope at the center of it. For each of my eyepiece/barlow combinations, I focused on the moon visually and then inserted the QuickCam or QuickCam/barlow pair. Since the Moon is so big and bright, I was able to then focus the QuickCam and note how far and in what direction I had to turn the focus knob. Now, when I'm trying to center a target in the QuickCam, I can set an approximate focus from my notes, which is usually close enough to find the target. Also, set the Brightness level up higher than you think you'll need - putting the software in "thousand of colors" 16-bit mode will make the screen updates faster.

Now focus. I generally spend 15 minutes to a half-hour setting the focus for a single series of exposures. I think a clothespin (see Equipment) or the equivalent is necessary to do this properly.

I now usually take a test exposure to check on the brightness setting by putting the QuickPict software into "Single Capture" mode and capturing a single image. Because QuickPict seems to use almost all of the computer's resources to read the QuickCam, I find it useful to exit the program and bring up PhotoShop to make sure the output levels are what I'm looking for. Currently. I'm working under the impression that it's more important to use short exposures than it is to make full use of the camera's dynamic range, so I generally set the images to be pretty dim. I could be completely wrong.

Now take the shots. I start up QuickPict again and put it into "Auto Capture" mode, set the interval to 2 seconds, and give it a prefix and directory for the series of pictures that it's going to take. Then I put a black piece of cardboard in front of the telescope and start capturing frames. After 1 or 2 "dark frames" are taken, I move the cardboard out of the way and take the rest of the frames. Lately I've been taking from 50 to 100 frames for each image, the idea being that the more that I take, the more sharp ones I'll end up with.

An interesting thing to note is that it is not always necessary to have the telescope tracking at all, and because of vibrations may be better not to, just letting the target drift. Since an object will drift a maximum of 15 arcsec per second (on the celestial equator, less near the poles), an exposure of 1/30 sec will only show it moving .5 arcsec at most. Since this is well below the 1.2 arcsec Dawe's limit of an ETX-90, and about 1/2 pixel with a Color QuickCam at prime focus, it should not be noticeable. In practice, I have found this to be the case, but also find it difficult to quickly and accurately slew the target back to the correct place for a "drift" over and over again. I usually end up losing it out-of-frame.

Centering Dim Objects

Dim objects can be a real pain. It seems to me that there are 3 options, the hardest one being the one that I currently use (just 'cause I'm too lazy to go buy the things I'd need for the others.) Here's what I currently do:

This requires an AutoStar. First make sure that your drive is trained very well and has very little slop (you may have to take it apart and do the ScopeTronix tune up.) Also, it needs to be aligned very carefully. Now find your target visually and sync your AutoStar to it. Next, locate a relatively bright nearby star, the closer the better, and slew over to it. Press "goto" and the ETX should slew back to the target. Do this a few (OK, a whole lot) of times, re-syncing if necessary, until you are confident that the scope will accurately slew back to the target. It matters a lot that you slew to the star the same way each time, so the backlash is also the same. Now slew back to the star and focus as described above. Hit "goto" and start taking images. Believe it or not, this actually works.

Or, if you're smarter than I am, you'll just buy a 1.25" visual back for your ETX and put the QuickCam in it and an eyepiece in the eyepiece holder (or vice versa) and set it up with setscrews and spacers and things so the eyepiece is in focus at the same time as the QuickCam. Now just focus everything on a bright star and then find your object in the eyepiece and flip the mirror.

Or... You could buy a small refractor and mount it to the side of your ETX to use as a high-power finder.


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