| Diag. is In/Out.. TOP |
This button inserts and withdraws the TUB diagonal mirror.
For spectroscopy, targets are always acquired on axis, and the diagonal mirror, when in the "In"
position, reflects the on-axis beam to the guide TV. In this position the diagonal blocks all light
to the entrance slit, so must be moved out of the way for data-taking. When in the "Out" position,
the reflective surface around the slit is exposed to the guide TV via a periscope. Guiding
must always be done with the diagonal out, using the image reflected from the slit surface. (Either
the target itself or a star within the same TV f.o.v. is used for guiding.)
When acquiring on the diagonal, it is moved to the "In" position while the target is sought.
Once the target is found and approximately centered on the guide TV, the diagonal is sent to the
"Out" position and the target is seen directly on the reflective slit surface. N.B. There is a small
shift between the diagonal and the aperture, as well as a flip in the image orientation on the guide TV.
Targets may be acquired directly on the slit surface with the diagonal mirror in the "Out" position,
but acquiring on the diagonal is generally preferred as it reflects more light and provides a larger field
of view.
See a Nickel Lightpath for a diagram
of the lightpaths and diagonal configurations for spectroscopy and direct imaging.
Use of the diagonal for direct imaging is described
elsewhere.
|
|
