Star Ware: The Amateur Astronomer's Guide to Choosing, Buying ...

258 Star Ware
Ideally, it should have a locking thumbscrew, just to make sure that everything
stays put. Also, it must not shift when that thumbscrew is locked down. Even a
slight movement will blur the image, leading to wasted time and ever-growing
frustration.
Even with the finest focuser, some focus shift can result as the night wears
on and the temperature drops. This problem can be especially noticeable in
catadioptric telescopes and multiple-element apochromatic refractors, in which
different optics contract at different rates.
The telescope’s mount must also be extremely sturdy and outfitted with an
accurate clock drive (one that has very little periodic error). Remember, any
inaccuracies that you can detect visually, either due to your telescope or its
mounting, will only be magnified by the CCD.
The CCD cameras listed in Table 7.14 image in black-and-white only (save
for two—the MX5C and the MX7C—from Starlight Xpress), yet color CCD photography
is possible due to a variation on the tricolor photographic technique
introduced by James Maxwell in 1861. Three separate exposures are made with
red, green, and blue filters over the camera; these are then combined electronically
to produce a true-color image. Some CCD cameras have optional built-in
color filter wheels, while others require a separate attachment.
Several companies market CCD cameras for today’s amateur astronomer.
Some of the most highly regarded are manufactured by Santa Barbara Instrument
Group, or SBIG for short. Built around Texas Instruments’ popular
TC255 CCD, the SBIG ST-5C includes a feature that SBIG calls “Track and
Accumulate.” This allows the camera to spend most of its time imaging, but
once in a while, it switches briefly to autotrack mode to make sure the telescope
is tracking properly, thereby freeing the observer from the tedious task
of manually checking that the instrument is still on target. This is a great
advance in CCD design, because it eliminates the need either to track the sky
manually or to purchase a separate autoguider.
The ST-237A’s small pixel size produces very-high-resolution images when
used in Fastar mode with a Celestron Schmidt-Cassegrain telescope (see chapter
5 for more details). Using SBIG’s Camera Lens Adapter, the ST-237A can
also be attached to standard camera lenses with focal lengths from 50 to 400
mm for additional versatility. Like the ST-5C, the ST-237 features Track and
Accumulate autoguiding. The ST-237A also has an internal mechanical shutter
as well as an electronic shutter for easier exposure control, a feature usually
not found on cameras in this price range. An optional color filter wheel can
also be mounted inside the CCD head for tricolor imaging.
As impressive as the ST-5C and ST-237 are, SBIG’s ST-7 through ST-10
imagers are in a whole different league. Built around much larger CCD chips
from Kodak, all four include a separate TC-211 CCD chip that provides fulltime
autoguiding. Although the Track and Accumulate flip-flop method works
fine, it can cause a skipping (jerking) motion if the telescope is slightly off in
polar alignment. But with a separate chip devoted only to autoguiding on a
field star, that problem is eliminated entirely. The ST-5C through ST-10E models
all connect to a Windows-based computer through the PC’s parallel port,
while Macintosh computers require an additional SCSI / Parallel adapter (note