Both film and CCD Camera astrophotography can require exposures long enough that inaccuracies in the scope’s drive system will cause the scope to drift slightly during the exposure. This drift blurs the
photographic image and thus deteriorates the photograph. In years past, astrophotographers using film would constantly manually correct the scope’s position by observing a star in an illuminated reticle
eyepiece in a guide scope or with an off-axis guider. With film requiring exposures of a half-hour to more than an hour, this became a very tedious process.
Celestron and Meade LX SCTs have an input jack that accepts electronic position correction signals from a dedicated CCD camera called an autoguiding camera. In this setup you replace the illuminated reticle
eyepiece on a guide scope or an off-axis guider, with an autoguiding CCD camera and connect the camera to the autoguiding input of the scope (the AUX
jack on the U2K). Note that the astrophotographs themselves are taken by a separate film or CCD camera attached to the scope itself, not related to or connected to the autoguiding camera.
The autoguiding CCD cameras discussed below use small CCD chips to keep costs low, and thus are not really intended for astrophotography. Also to keep costs low, sufficient electronics and controls are built
into the camera system to execute the autoguiding operation – they do not
require a computer. (But three of the four autoguiders below are also able to take astrophotographic images, and just like almost any other CCD camera they do
require a computer for that function.)
In the table below are four commonly used inexpensive autoguiding cameras – three were made by Meade and one by the Santa Barbara Instruments Group (SBIG).
None of these are sold any longer but are often found in the used equipment market:
In reviewing this table note the following:
1. All three Meade Pictors (the 208XT / 216XT is shown here on the right) used the same Texas
Instruments CCD chip while the SBIG uses a different TI chip. The SBIG’s chip is smaller and has a larger pixel size: both attributes are less desirable in this application.
2. The Pictor 201XT is not cooled while the others are. Cooling reduces the noise level in the camera and thus allows the autoguider to
track on dimmer (lower magnitude) stars, a definite advantage.
3. The 201XT (shown here on the left) cannot be used for
astrophotography while the other three can, but in all cases the CCD chip is really too small for that, and most folks who start with one of these for astrophotography eventually trade up to a better CCD
camera. The fact that the 216XT can produce 16-bit images (65,536 gray-scale levels rather than 256) is not an advantage for an autoguider.
Thus, on a price vs. performance basis the Pictor 208XT appears to be the winner, and it is indeed a popular autoguider. But note that even though the ST-4 (shown here on the
right) is almost twice as expensive as the 208XT, many amateur astrophotographers believe it works better, and because it has a control panel, it is easier to use than the Pictors’ single-button short/medium/long button
-press menu system.
For more information on these autoguiders check the following links:
Guido Pasi’s Meade Pictor 201XT Problems and Solutions
James Janusz’ Instructions and Tips for Using the ST-4 Autoguider
Pictor entries in the MAPUG Topical Archive
Sky & Telescope’s Review of the 201XT
Note that there are a few CCD cameras for astrophotography that include an inherent autoguiding
function. The SBIG ST-7 and ST-8 mount the TC-211 chip used in the ST-4, as a second CCD for autoguiding next to the main imaging CCD chip, and many of the Starlight Express cameras can
autoguide from their CCD chip while they image. They are worth considering if you plan to concentrate
on CCD photography. If you plan to concentrate on film photography, then of course their extra cost will be wasted.
Back to Guiding an SCT