If the above calculation is correct, we need to shorten the
total back distance of the camera mount from 1490 mm to 1500 - 383 mm = 1117
mm. This is a shortening of 373 mm or 14 inches. If this shortening were
achieved, the camera could be swung up to 90 deg (camera-collimator
angle). With a longer table, e.g., 5x10 ft, this would allow the camera to be
swung around past 90 deg to open up to camera-collimator angles of almost 120
deg, or &alpha as low as 30 deg.
This is highly desirable from a science perspective since it would maximize
efficiency by eliminating one reflection and making the most compact
spectrograph geometry. This would also be desirable from an operations
perspective since it would minimize the number of configuration changes. With
the camera length shortened, it is conceivable that the first turret could be
moved down the table (away from the edge) even further and still maintain the
echelle configuration. This would allow for even larger camera-collimator
angles in VPH mode, and conceivably this could eliminate the need for a flat
and 2nd turret all together.
Knocking off 14 inches from the camera mount seems very challenging,
but I think it may be possible. There are two places where we can
conceivably reduce length. The purpose of the following discussion
is to vett these suggestions and determine what can really be
done with a reasonable amount of effort.
(i) Rear pad placement. The primary notion is to change the location of
the rear pad on the camera mounting rails so that this pad end only 32 inches
from the front of the rail 48 inch. Currently the read pad ends close to the
end of the rail -- far beyond the load-point of the dewar mount. This
back-distance seems un-necessary, and it is what is currently limiting the
length of the over-all "camera mount" (camera+dewar). If the pad can feasibly
be mounted farther forward along the rails (ending 32 inches from the front),
this more than accomplishes the needed change in length. In this mode,
the dewar-azimuth actuation mechanism and the end of the dewar will
extend behind the pad, but will still be supported by the steel
rail.
At large &thetacc, the end of the rails will
hang off the end of the bench, but the rear pad will not.
The key thing is that the full weight of the dewar itself will still sit
on the pad; the weight that sits beyond the pad is minimal (and in fact is
no different whether the rail sticks off the table or not). What is critical to
determine is:
- whether the pad can be moved this far forward both
in terms of any conflicts with existing CCD dewar mount and stability (I don't
think stability is an issue but I do not have enough detailed information
to determine if there are mechanical conflicts);
- how the locking mechanism will work in this configuration (this should be
straight forward); and
- how to maintain the rear handle for physically swinging
the bench arm (I think this should be straightforward).
It may be necessary to declare a "no-fly zone" around the back-side of the Bench
for large &thetacc configurations so there is no risk of hitting
the rails and dewar-az control (we may want to build a simple, removable
shield for the latter to give it some mechanical isolation from
an inadvertent collision).
Pictures of camera mount in SR-mode (&thetacc=30) illustrating fron
and back pads and handle:
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front view: front pad |
read view: "far" side rear pad |
rear view: locking side rear pad |
end view: handle |
full side view "far" side |
full side view locking side |
(ii) Front rail location relative to camera objective. Another place
where a few inches may be had is in the front of the rail.
-
- Is all the extra rail at the front end
needed for the camera changes?
- How much less rail could be used to still accomplish the
camera-dewar separation, removal and exchange of BSC and Simons
cameras?
Reminder of why the camera front is mounted back from front end of the
rails:
(Courtesy of K. Westfall's notes from working with D. Harmer.)
The components on the mount are actually the camera and the dewar.
The dewar is used for both cameras. To change the cameras, the camera
is separated from the dewar (at the place where the black part joins
the white part). The camera is then slid away from the dewar, with
the dewar position on the rails held fixed. (The position of the
dewar is fixed so that some settings in preparing the spectrograph for
observations do not need to be redetermined each time the spectrograph
is setup.) Once the camera is moved forward, it is clear to be removed
from the rails. So the extra railing at the front of the camera is
needed to slide the camera away from the dewar.
This
picture (courtesy K. Westfall) shows the front of the camera objective (top
right) and camera mount (upper right) sitting on the front-end of the twin 48
inch steel rails (middle). Note the front of the camera is nearly flush with
the end of the rails, but the camera mount (which attaches to the rails is set
back some 6-8 inches (check this number with
D. Harmer). To the upper left is the VPH grating mount with the 740
l/mm grating. At the bottom is the long, rectangular positioning boom. [Click
on image for larger version.]