I showed my mom the observatory one night, and she was truly amazed. I quickly imaged a few galaxies for her, and as they popped up on the computer screen, mom couldn't believe what she was seeing. She still wonders how a CCD camera works without film. I attempted to explain the camera uses a CCD chip, which is cooled to -30 degrees Celsius below ambient, and about subtracted dark frames, but I don't think she cared all that much about it. She liked the "pretty pictures", though.
Now, I do not profess to know a whole lot about this CCD imaging stuff. Heck, I am sure they're lots of people out there who know all about pixel size, Nyquist sampling, signal-to-noise ratios, flat fielding, and the like. Me, no. I just like to image. Fortunately, the Celestron Fastar/Pixcel combination has got to be the easiest CCD system on the market. So, with that said, I thought I spend some time writing about what I do at Darklight Observatory. That is, how I personally go about imaging deep sky objects.
A typical night at Darklight starts about an hour before sunset. As the shadow of my house hits the observatory, I go out and crank the roof off. This gives the instruments about an hour and a half to cool down. Letting the scope cool down and approach ambient temperature is very important, especially for a Schmidt Cassegrain. A steady scope temperature means less refocusing as the night progresses. However, as the air temperature usually falls throughout the night, I have to keep a close eye on focus. I have a digital thermometer mounted inside the observatory. It shows inside and outside temperature. If the outside temperature holds steady, I can be relatively assured the scope focus can be left alone. Of course, I routinely check it throughout the night using the peak method in CCDOPS.
While the building and telescope is cooling down, I power up the CCD camera, IBM 380ED Thinkpad, and CD player (gotta have tunes). I usually set the camera TE cooler to -10 degrees Celsius, and lower it five or ten degrees every fifteen minutes or so. I have heard this can help reduce chip frosting. As twilight nears an end, I set about calibrating the Astro-Physics 900 mount. I generally leave the CCD camera attached to the front of my Celestron Fastar 8, in the f/1.95 imaging mode. I simply cover the dew shield with a plastic shower cap when not in use. Before the AP900 can be used for goto or tracking, it must be calibrated on a known star. I do this by setting the camera up in the automatically updated dim focus mode, with an exposure time of .5 seconds. I then aim the scope at a known bright star (such as Altair or Formahault ). I keep my finderscope well aligned, so the star will generally appear on the computer screen. Using the slow motion controls of the mount, I center the star on the screen, and set the mount to North Polar align. The scope then automatically slews to Polaris, and the mount is calibrated. It will now slew to any object I choose, and track.
By this time, the scope has usually cooled down and leveled off, and the CCD chip is very cold. I usually try to get the chip as cold as possible, keeping the power level around 85 to 90 percent. So far, the coldest I have been is about -39 degrees Celsius. I hope to go colder as winter nears. Next, I check the focus.
I had, in the past, fiddled with focusing masks (Hartmann type). With the PixCel 237 used in the Fastar mode, it is difficult to use masks; the camera is in the way. I discovered the best way to focus is to use the peak value method in CCDOPS. In this utility, I pick a medium brightness star (say fifth or seventh magnitude). Generally, I'll slew to my first object to image, and pick a star there. Since my scope remains more or less permanently set up, the focus is usually close to start with. (I have a focusing micrometer with a digital readout. This makes it easy to return to a particular point of focus, depending on whether I am imaging at f/6.3 or f/1.95, etc.).
To focus, I switch to planet mode, and set the exposure for two seconds. This allows the averaging out of atmospheric turbulence. This turbulence can cause the peak readout to fluctuate if shorter exposures are used. After taking the initial exposure, I pick a suitable star to focus on, and check its peak value. I can control the focus motor on my scope via the hand controller of the AP900, so I can sit at the computer and make adjustments. Everyone always says to enter focus counter clockwise with a Schmidt Cassegrain. I have never really noticed a difference whether I do or not. After focus is achieved, I calibrate the autoguiding capability of the CCD camera. This calibration makes it possible for the camera to make minor adjustments in the positioning of the mount. Specifically, I use it to automatically move the mount between the individual Track and Accumulate exposures. By using the Relays>Both setting, the software will make corrections so as to return the registration star to where it was at the begining of the previous exposure. Generally, I'll aim the scope at about 0 degrees near the meridian to perform calibration. Then, if I choose to image an object at a different declination, I simply insert the appropriate angle in the Track and Accumulate declination box.
Most, if not all, of my exposures with the Fastar to date have been 60 or 120 seconds in duration. These two lengths of time are the building blocks of Track and Accumulate (TA) images produced with the scope. I usually try a five or ten minute TA shot, made up of these shorter images. Before I begin to image in earnest, I take one preliminary 60 second image to "test the waters" so to speak. I use this image to check for critical focus, and especially to measure the "seeing" conditions for the night. After downloading the preliminary image, I'll active the crosshairs mode of CCDOPS and check the stellar diameter. My best stellar diameters so far have been about 5 arc seconds. So, with this in mind, I check the preliminary image and compare the stellar size. If they seem bloated (measure far above 5) and I am sure the focus is good, I know I have bad seeing conditions. Usually, I am aware of this earlier. Bad seeing makes focusing difficult, as the peak value will fluctuate wildly. If the seeing turns out to be bad, I will probably have a short night. But if things look good, I get down to serious imaging.
The rest of my night consists of slewing though my list of objects. Depending on the overall brightness of the object, I will take a 60 or 120 second image. If it is a fainter object requiring a higher signal to noise ratio, I will use TA to image. The beauty behind TA is that it allows you to take ten one minute exposures, instead of one ten minute exposure. The results are nearly the same. In fact, I have noticed that TA shots often look better than single long exposures. For example, a ten minute exposure may reach the sky fog limit. Ten one minute shots will not. Also, some very bright objects (such as the Great Nebula in Orion) can easily be overexposed by single long shots. Many very short images makes for a better image.
As I mentioned earlier, TA images begin with a single exposure. This exposure is best when at least 60 to 120 seconds long, but it cannot be any longer than the scope can easily track without producing star trails. For my old fork mount, this was only about 20 seconds! But, the AP900 easily allows longer exposures. After the first image is taken, I pick a field star to register the other images on. CCDOPS will then automatically align all other images on this single star. In this way, the software "stacks" the images one atop the other, creating the final image. There can be a few problems with this. First, if your tracking error is great, the final image will be reduced in size. For example, the PixCel 237 creates high resolution images measuring 480x640 pixels. If the final tracking error is 15 pixels (that is, the register star moved 15 pixels in RA or DEC) the final image will measure say 465 x 625. Or something like that. My average tracking error over ten or fifteen minutes is less than an a few pixels. Fortunately, TA has a neat options to counter these problems. If you have a dual-axis type, CCD ready mount, you can train CCDOPS to move the register star back to where it was at the beginning of the exposure.
Along about two A.M., if I've had a good night, I roll the roof back, and take a few flat field images. I do this by imaging a white piece of plywood that I lay below the scope, on the floor. I then bounce a 50 watt incandescent lamp off the roof of the observatory, thereby creating a diffuse lighting. I expose for about .04 seconds to achieve an exposure of about 2000 counts. These flats are combined with the Track and Accumulate track log files to create a special flat for each individual image. I am convinced that flat field images are generally unnecessary for the short exposures afforded by the Fastar system. However, I usually save the track log information for future processing.
The next morning, I view the images. I will generally run a few image processing routines over the raw files to see what's there, hidden in the file. Most nebulae and some galaxies can be automatically enhanced by using CCDOPS Utility>Scale>Auto-Log function. I occasionally run a Utility>Sharpen>Medium routine over the image. Also, I have been known to use Adobe Photoshop to adjust the brightness and contrast of an image to reveal faint details. I have also had limited success with Photoshop's unsharp masking. Lately, I have experimented with producing mosaics of large, extended objects using some of Photoshop's more specialized tools.
That's about it. Simple, eh?
The most useful printed material I have read concerning CCD cameras and imagery came from the following sources.
Art and Science of CCD Astronomy
Sky and Telescope Magazine
Choosing and Using a CCD Camera
Bill McLaughlin's Simple Guide to LRGB