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Red helps maintain the human eye’s dark adaptation, Blue is the color to which
most CCD cameras are least sensitive, and
Dim maintains the color display but
reduces the overall intensity to about one-eighth normal.
• In
red mode, all menus, dialog boxes, etc., are switched to red.
Monochrome images are displayed red, but
color images are displayed
in their normal colors. This allows the user to inspect a color image
without switching the entire screen back to normal colors. Red light helps
maintain the human eye’s dark adaptation because it does not as readily
destroy the “visual purple” photosensitive pigment of the scotopic vision
system (“rod” cells).
• In
blue mode, all menus, dialog boxes, and monochrome images are
shown in blue. As before, color images are displayed in their normal colors.
The blue mode is useful when imaging with CCD cameras because it is
the band in which most CCD cameras are least sensitive.
• In
dim mode, all menus, dialog boxes, and images (both monochrome
and color) are displayed at approximately one-eighth normal brightness in
order to reduce ambient light conditions in the observatory.
7.3. Observatory Control Window
The Observatory Control Window provides comprehensive control and status for
any other astro-imaging accessory you might be interfacing with your computer,
including:
• Telescope mount control with auto-center
• Focuser control with autofocus
• Rotator control with FOV slaving
• Dome control with telescope slaving
• A simple but powerful planetarium with image overlays, FOV overlays
• Extensive searchable catalogs
The Observatory Control Window is resizable using the mouse.
8. tips
8.1. Polar Alignment
Good telescope mount polar alignment is of critical importance for long-exposure
CCD imaging. Inaccurate polar alignment leads to image movement over time
(even with motor drives running and engaged), which limits the amount of time an
exposure can be taken before the stars begin to streak and blur.
If your equatorial mount uses a polar axis finder scope, we highly recommend uti-
lizing it for polar alignment. If not, a technique known as the “drift method” of polar
alignment has been used for many years, and can achieve an extremely accurate
polar alignment. Unfortunately it is very time consuming, since the drift of a star
over time must be observed. The basic idea is to let the telescope mount track
while watching a star to see which way the star drifts. Note the direction of the drift,
and correct by moving the mount in the appropriate direction.
To perform the drift method of polar alignment:
1. Do a rough polar alignment by pointing the R.A. axis of the mount at Polaris
(the North Star).
2. Find a bright star near the meridian (the imaginary line running north-
to-south through zenith) and near the celestial equator (zero degrees
declination). Point the telescope at this star, and center it in an illuminated
reticle eyepiece (available from Orion). If you don’t have an illuminated reticle
eyepiece, use your highest- magnification eyepiece.
3. Determine which way is north and south in the eyepiece by moving the
telescope tube slightly north and south.
4. Now, let the mount’s motor drive run for about five minutes. The star will
begin to drift north or south. Ignore any east-to-west movement.
5. If the star drifts north, the telescope mount is pointing too far west. If the star
drifts south, the telescope mount is pointing too far east. Determine which
way the star drifted and make the appropriate correction to the azimuth
position of the mount. Rotate the entire mount (and tripod) slightly east or
west as needed or use the azimuth adjustment knobs (if your mount has
them) to make fine adjustments to the mount’s position.
6. Next, point the telescope at a bright star near the eastern horizon and near
the celestial equator (Dec. = 0).
7. Let the telescope track for at least five minutes, and the star should begin to
drift north or south.
8. If the star drifts south, the telescope mount is pointed too low. If the star
drifts north the telescope mount is pointed too high. Observe the drift and
make the appropriate correction to the mount’s altitude (or latitude); most
mounts have some sort of fine adjustment for this.
Repeat the entire procedure until the star does not drift significantly north or south
in the eyepiece. When this is accomplished, you are very accurately polar aligned,
and should be able to produce good (unguided) images of up to several minutes
long, assuming the mount’s drives track well with little periodic error.
8.2. Choosing a Site for Astro-Imaging
Once you have a focused image, you may find your image shifting and washed
out. This can be caused by many environmental factors. Poor seeing (movement
of molecules in the air, such as heat rising) and poor transparency (moisture,
smoke, or other sky contaminants) will all serve to reduce image quality. That is
why most major astronomical telescopes are on high mountains in thin air, to get
above much of the transparency and seeing problems. Also, wind will move your
telescope and affect images. Your eyes viewing through an eyepiece can change
