QSI WSG 500 Series User Manual - page 15
Filter Thickness and the Integrated Guider Port
The Integrated Guider Port on the WSG cover is designed to bring the guide camera image to
focus at the same focal depth as the main sensor. Adding additional glass to an optical path,
such as the glass in a filter, reduces the effective focus position by roughly 1/3 the thickness of
the glass. For instance, adding a 3mm thick filter will reduce the optical back focus by
approximately 1mm.
No two filters are exactly the same thickness, but in most situations, the small variations in filter
thickness will not require refocusing the guide camera. Because the pick-off prism is
positioned in front of the filter wheel, the focus position of the guide camera
will not change
when filters of different thicknesses are positioned in front of the main sensor,
however
the
focus of the main sensor will change slightly
. If changing a filter requires adjusting the
focus on your telescope, you may also want to refocus the guide camera. Note however, as
mentioned earlier in this section, guiding works very well even if when the guide stars are
slightly out of focus.
Note:
It’s best to use filters that are all approximately the same thickness with the
WSG. Once the guide camera has been focused with a filter in the path to the
main sensor, the guide camera focus will not require adjustment when changing
filters. In practical terms, it is usually best to standardize on filters from one
manufacturer as they will usually be made from the same glass substrate.
Other sources of small changes in focus should not require adjusting the guide camera focus.
Once you’ve focused your guide camera, the guide camera sensor will be positioned at the
same focal depth as the main sensor. For instance, If changes in temperature require you to
refocus your telescope, adjusting the focus on the main sensor will automatically bring the
guide camera to the correct focus position as well.
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