|CCD Imaging Tips|
This section is devoted to passing along some things that I have learned in my experiences with taking CCD images. I started shooting them in 1995, and have worked with the ST6, ST7, ST8e, ST10xe and ST11000 cameras on telescopes including a refractor, a Dall Kirkham Cassegrain, a Ritchey-Chretien Cassegrain and now a Takahashi Astrograph.
Three software components I find essential to finding, shooting, and processing objects are The Sky, MaxIm DL", and Adobe Photoshop.
CCD imaging is the definitive learning curve experience. There are a number of things that can go wrong at any given time, and often more than one at once. The counter side is that it is an endeavor which richly rewards your patience. When things work its truly fun! Hopefully the following suggestions can cut down on your learning curve.
Shoot High! --- I try to take all my shots within 20 degrees either side of the meridian to minimize atmospheric effects. You will get acceptable results down lower but the amount of sky background noise will increase proportionately, and have to be dealt with in processing. Another trick is when shooting the Blue filter portion of a long LRGB sequence is to plan the B shot when the object is near the meridian, since the blue exposure suffers most as the object drops down in the sky.
Focus Hard! --- Sharp focusing is extremely important for getting good results. The best way to achieve it quickly, and consistently is get yourself one of the automatic focusers with their attendant software. I have an automatic focuser built into my Tak 250-C Astrograph, and use Focus Max software. It focuses perfectly in less than a minute from 165' away. And its fun to watch it go through its routine on screen!
Find a Good Guide Star! --- A good, bright guide star is worth its weight in gold. It can occasionally be frustrating to find one in the field of the secondary guide camera on the SBIG models. A good method is to go to "The Sky" and zoom up the object to telescope size. Using the Orientation-Rotate tool rotate the object in "The Sky" until you find a good guide star in the area right above the image frame. Then rotate the camera into the proper position to pick up the guide star in the guider field. A good guide star is one you can take 1 second guiding shots on through the blue filter.
I have found excellent results using a 50mm Hutech refractor with a Starlightexpress Lodestar camera attached. This allows you to simply go to the object itself, and start guiding on a star that will be readily available in the wide field of the system.
Freeze That Camera! --- Get your camera as cold as you can when you shoot . Watch the power level (don't go above about 85%). The SBIG water pump works well in the warmer climates. sub-tropical sites like mine. The image clarity is worth it.
Shoot & Stack! --- Use a series of shorter exposures stacked together vs a single long one. I find 5-10 minute exposures provide a sufficient dynamic range to capture a lot of detail. Long images always run the risk of uninvited visitors - like an asteroid or an an airplane leaving a prominent track across your image. On a shorter one you just throw it out.
Use White Stars to Get Your Color Right! --- Each camera, and telescope system is different in its requirements for the ratios of Red, Green and Blue exposures. Grey carding is all right for color filter calibration, but white star calibration is hard to beat, and maybe easier. On page 45 of the DEC. 1998 issue of Sky & Telescope is a chart showing the name and location of 22 "sun-like" stars. Simply take short exposures (to avoid saturation) of one of these stars using the same length of exposure through Red, Green and Blue filters. Measure the resulting photometric count on screen with MaximDL for each of the three images. This will give you the ratios for your exposures in Red, Green and Blue. Be sure to select a star near the zenith since calibrating down in "the soup" of the atmosphere will be accurate (usually requiring a longer Blue exposure) - but it won't be good for shooting higher up in the sky where you want to shoot most of your shots.
Again, there is a simpler way - buy the Astrodon RGB color filter set. The filters are beautifully pre-balanced to within a hair of being an exact match with each other.
Planets Call for Lots of Stacked Exposures! --- I don't think you can beat the Phillips Toucam camera for shooting planets. Its cheap, and very easy to use. One of the problems with using astronomical CCD cameras on the planets is that you have to quickly rotate through the Red, Green, and Blue exposures. The sheer length of this exercise means you will be shooting through various levels of seeing, and while the planet is rotating. (Jupiter especially can be a problem unless you can get all 3 images in less than 1 1/2 minutes). With the Toucam you get all 3 colors at once in 1/25th of a second, and by taking multiple shots you wind up with a lot of images. Select out the ones when the seeing was best, and stack them for great results.
Tee Shirts Make Great Flat Frames! --- Use a tee shirt or a white cloth pulled snug across the front end of the telescope tube for twilight flats. If the telescope is over 10" double the cloth thickness. This is the easiest flat field method I have ever tried, and it produces very high quality flats. I usually target a flat bright surface - like a nearby wall.
If you have a permanent site consider buying a flat field light panel, which just fits over the top of the scope. I use a small regulator that is used by tatoo shops for finding just the right amount of light for good flats.
Average Those Dark Frames! --- Shoot at least 3, and average them. The more the better for cleaner images. I like to average 5 or more.
Resist "Clipping" Your Images Early! --- Use the histogram during processing but avoid early clipping of the L,R,G or B images. Before moving to the next step always save the histogram in an open position. In other words don't clip off any of the histogram's slopping line that shows the data. I like to close it off well to the outside of the left side, and just past the last data on the right side. Remember, once clipped the data is gone for good.
Use the Double "L" Method of LRGB Processing! --- This is Rob Gendler's excellent technique for providing richer, less noisy final images. Go to his website to get the directions.
Polish the B/W "L" Component! --- In the LRGB method nothing is as important as a high resolution, smooth B/W image that will be used as the "L" component. Take your time, and try to get everything you can in the B/W before combining it with the color components. Its the cornerstone of your finished image.
Watch Your Parameters! --- You can get pretty caught up in the minutia of an image during the processing steps. Always watch to be sure you don't miss the forest for the trees. Stars should generally be white, and the background should be dark grey towards light black. Individual star colors (except green) are desirable, but there should be some white ones in almost any image. Also, the more colors you are seeing in the normal processing steps of the full image the more confidence you can have in the total color accuracy of the image.
Weird Results Check List --- Here are some phenomena you want to be familiar with:
1. Seagull shaped star images Youve probably got a wire hanging up somewhere.
2. Frosted images They probably are. Recharge your desiccant.
3. "Black holes" in your stars Your focus has slipped.