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Adjusting the Pick-Off Mirror (FW8G-STXL)
The STXL guider assembly has three black knobs that the user can adjust externally without
opening the cover. The knob by itself on one side is a lock knob that, when tightened clockwise, will
press against the Mirror and CCD slides and lock them both down. Of the pair of knobs on the other
side of the assembly, the knob closest to the centerline adjusts the CCD focus slide, and the one
farther to the side adjusts the mirror slide. They are easily set for optimal operation as follows:
First, mount the camera/wheel/guider assembly to the
telescope. Point the telescope at a nearby wall or flat field
screen illuminated by ambient light or a penlight of some
kind. Total darkness is not necessary. Start up the imaging
CCD in focus mode with an exposure adequate to get a
uniform, not saturated field. A count level of 5,000 to
50,000 counts should be fine. The CCD does not need to be
focused on a star field first. Loosen the lock knob on the
side of the guider assembly so the slides are free to move
(about a turn). Next, while watching the image updating on
the computer screen, rotate the mirror slide knob counter-
clockwise, which will push the mirror closer to the optical
axis. There is no danger of the screw falling out: when you
reach the limit of its travel it will jam up. You should eventually see a shadow start to appear at the
bottom of the image, which is the shadow of the mirror vignetting the imaging CCD. This is
undesirable. Adjust the image brightness in focus mode so that the contrast is not too stretched, and
then back out the mirror by rotating the knob clockwise until the shadow is fully off the screen. This
adjustment depends on telescope choice and F/number, which is why we cannot preset it at the
factory. Also, it is important to get the mirror as far in as you can without vignetting – popular
Schmidt-Cassegrain telescopes have limited exit apertures and can barely illuminate a guide chip off-
axis. [TIP: Once this knob has been positioned we recommend putting some tape over it so you don't
accidentally grab the wrong knob in the dark and change the adjustment – Its easy to do! Black
electrical tape is best since it can be removed in the future with the least mess.]
Adjusting the CCD Focus (FW8G-STXL)
Once the mirror slide is adjusted, point the telescope at the sky and focus the imaging CCD on a star
field using the telescope focuser controls. If you plan on using filters do this with a filter installed.
Next, view the field hitting the guide CCD by starting it up in focus mode with an exposure of 1 to 3
seconds, and focus that image by turning the more central knob on the assembly, the CCD slide, back
and forth. Finding stars initially can be a bit tough, so go to an open cluster or crowded area to
facilitate this. Once you see stars, focusing on them is easy due to the action of the built-in focal
reducer. A critical focus is not required for the guider. If you change filter thicknesses over the
imaging CCD the guider focus may need to be adjusted, but with reasonably matched filters this is not
necessary. You will probably see a gradient in the sky background due to the apertures upstream of
the assembly limiting the field of view, but this does not affect guiding. It merely causes stars farther
off-axis to be dimmer, if it is occurring.
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