The diagonal mirror allows you to send light to the guide camera
for either field acquisition or autoguiding the telescope. Different
diagonal mirrors are used for different instruments. The available
options for the diagonal mirror are:
Direct Imaging Diagonal
Direct imaging uses a diagonal mirror with an on-axis hole. Light hitting the solid part of the mirror is relayed to the guide camera via two flat "trombone" mirrors (Figure 1). Light goes through the hole to the science camera. Typically the telescope is pointed to an object with the guide camera "on-axis" (as selected in the POCO telescope control software) and acquired using the guide camera. The telescope is then moved a preset distance via POCO to put the object on the center of the science camera. At this point a guide star can be identified in the off-axis guide camera field of view.
Spectrograph (Solid) Diagonal
When the diagonal mirror is in, all light coming through the telescope is sent to the guide camera for field acquisition. Once the object is acquired, the diagonal mirror is moved out of the way to send all the light to the spectrograph. A periscope lens allows viewing of the slit with the guide camera. The field of view with the periscope is about half what it is when the diagonal is in position, limiting the chances of finding useable guide star in the field if you are unable to guide off the slit.
Mounted on the back of the solid diagonal mirror is a white card. When the diagonal is in this white card reflects light from the arc lamps into the spectrograph (see Figure 3).
Pellicle Beam Splitter
A pellicle beam splitter is available for use with the eyepiece or other user supplied instruments. The pellicle sends 5% of the light to the guider and 95% of the light to the science instrument.
Folding Flat "Trombone" Mirrors
The primary function of the folding flats (or relay mirrors or trombone mirrors) is to fold the light path around inside the TUB from the diagonal mirror to the guider. A secondary function of the folding flats is to allow compensation for various positions of the focal plane of the telescope which are required by different instruments. By moving the secondary mirror in or out, the telescope may be focused over a wide range which corresponds to about 12 inches of movement of the image plane. Because the guider is looking at the incoming beam from the telescope, it is necesessary to be able to make a compensating adjustment of the path-length to the guider so that the guider and the instrument may both be in focus simultaneously. This is done by altering the position of the folding flats, thereby shortening or lengthening the light path to the guider. The determination of the proper position for the flats for a given instrument is usually just done once when that instrument is first mounted at the telescope, and the correct position for the flats is recorded and thereafter reset by the dome crew whenever that instrument is remounted.
Despite the god-like reliability of the dome crew at making these adjustments, on very rare occasions they do forget. The symptom will be that the guider is out of focus when the instrument is in focus. The thing to adjust is the position of the trombone mirrors. This can be done in two ways, either manually or via the naux_fe software. We recommend using the software because you can look at the guider image while adjusting the mirrors, where that isn't possible when adjusting them manually.
However, if you must adjust them manually, you need go to the dome and open the access door on the TUB. The two folding flat mirrors are mounted on a common stage. When you look in through the access door you will see a small black handle with a red button in the middle just in front of you in between the two folding flats. If you grasp the handle and press the button the stage will disengage from its fine-adjustment screw and you can slide it quickly and freely in or out. You can then make fine touchup adjustments by turning the screw with a knurled wheel which is at the outside end of the screw, out of sight just above the inside of the access door. If you know your instrument is in focus, just move the stage until stars are focused on the guider.
Figure 1: Direct Imaging Diagonal Mirror (click for full-size image)
Figure 2: Spectrograph Diagonal Mirror (click for full-size image)
Figure 2: Diagonal Mirror Light Paths (click for full-size image)