Section 3 - At the Telescope with a CCD Camera
Page 40
Another aspect of the Focus command and its various modes is the Camera Resolution
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setting in the Camera Setup command. Briefly, the Resolution setting allows trading off image
resolution (pixel size) and image capture time while field of view is preserved. High resolution
with smaller pixels takes longer to digitize and download than Low resolution with larger
pixels. The cameras support High, Medium, Low and Auto resolution modes. The Auto mode
is optimized for the Focus command. It automatically switches between Low resolution for Full
frame mode to provide fast image acquisition, and High resolution for Planet mode to achieve
critical focus. While Auto resolution is selected all images acquired using the GRAB command
will be high resolution.
3.9.3. Track and Accumulate
An automatic Track and Accumulate mode (SBIG patented) is available in CCDOPS which
simplifies image acquisition for the typical amateur with an accurate modern drive. These
drives, employing PEC or PPEC technology and accurate gears, only need adjustment every 30
to 120 seconds. With Track and Accumulate the software takes multiple exposures and
automatically co-registers and co-adds them. The individual exposures are short enough such
that drive errors are not objectionable and the accumulated image has enough integrated
exposure to yield a good signal to noise ratio.
Operationally the camera will take an exposure, determine the position of a preselected
star, co-register and co-add the image to the previous image, and then start the cycle over again.
The software even allows making telescope corrections between images to keep the object
positioned in the field of view. The resulting exposure is almost as good as a single long
exposure, depending on the exposure used and sky conditions. The great sensitivity of the
CCD virtually guarantees that there will be a usable guide star within the imaging CCD's field
of view. This feature provides dramatic performance for the amateur, enabling long exposures
with minimal setup!
3.9.4. Autoguiding and Self Guiding
The CCDOPS software allows the ST-7XE, ST-8XE, ST-9XE, ST-10XE, ST-10XME and ST-
2000XM cameras to be used as autoguiders and self-guiders through the commands in the
Track menu. While these systems are not stand-alone like the ST-4, but require a host
computer, they can accurately guide long duration astrophotographs and CCD images with
equal or superior accuracy. Their sensitivity is much greater than an ST-4, and the computer
display makes them easier to use.
When functioning as an autoguider, the CCD camera repeatedly takes images of a guide
star, measures the star's position to a fraction of a pixel accuracy, and corrects the telescope's
position through the hand controller. While autoguiding alleviates the user of the tedious task
of staring through an eyepiece for hours at a time, it is by no means a cure to telescope drive
performance. All the things that were important for good manually guided exposures still exist,
including a good polar alignment, rigid tubes that are free of flexure and a fairly good stable
mount and drive corrector. Remember that the function of an auto guider is to correct for the
small drive errors and long term drift, not to slew the telescope.
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The Resolution setting in the Camera Setup command combines pixels before they are digitized.
This is referred to as on-chip binning and offers increases in frame digitization rates.
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