WIYN / Bench Rebuild:
Bench Layout working page

 

v1.0

This page contains technical notes, issues, concepts, and numbers for the process of designing the new Bench layout.
Requirements Problem Option 1 Option 2 Preliminary conclusion

Requirements

The Bench Spectrograph must have a layout (in the current and upgraded systems) that allows for all of the following configurations:
  • Echelle configuration with camera-collimator angles of &thetacc = 11-13 deg. (Goal: accommodate &thetacc down to 9 deg)
  • VPH configurations for all user angles (&alpha) from 10 to 72 (goal: 5 to 75).
  • Any single VPH grating should be able to be used continuously in one setup (i.e., folded vs. unfolded). This implies folded and unfolded modes will have a user-angle overlap of 10-20 deg.
  • The &alpha range of direct configurations for VPH gratings should be maximized.
  • Existing surface-relief (SR) gratings with &thetacc of 20-45 deg to tune blaze function about nominal &thetacc = 30.

The Problem

Current system: D. Harmer reports the camera mount allows for &alpha>55 deg for direct VPH configurations. For folded VPH configurations, the maximum angle is &alpha = 45 degrees. (This places the second turret at 90 deg from the first turret, along the bench width.)
To check:
(a) can 45 degree fold be achieved in the current system?
(b) what is expected reflectivity of current flat at 45 degrees?
This leaves a gap of 10 degrees (45<&alpha<55) that is unaccessible for VPH configurations on the Bench. This is prime territory. The VPH spectral resolution depends on user angle (&alpha), independent of wavelength. For the Bench with a 800mm collimator f.l. and the red Hydra cable, the resolution can be estimated as a linear function of &alpha : R = 4000 + 400 x ( &alpha - 25 ) for 25 < &alpha < 65. The general approximation is
R = [ 8000 + 800 x ( &alpha - 25 ) ] / &phi,
where &phi is the fiber diameter in arcsec. (Strictly, R is proportional to tan ( &alpha ) for a Littrow grating.) The change in resolution between 45 and 55 deg is 33%. This also locks out a range of wavelengths for a given grating, but this can, in principle, be compensated by adding other gratings (this is not desirable).

Upgraded system: I estimate the situation is largely unchanged assuming the first turret is still placed at the end of the current optical bench (to accommodate echelle configuration). The maximum folded angle is still &alpha = 45 degrees (flat folds at 90 deg). The minimum un-folded angle is also almost unchanged: while the grating mount moves down along the edge, the angle increases by 14 degrees. These two effects roughly cancel. The detailed calculation is as follows.

Currently:
  • location of grating mount is 100 mm from edge of bench in width dimension. [confirm with D. Harmer]
  • 1.5m table width (this is the actual width of a "5 ft" Newport bench).
  • with &alpha = 55 max for current bench yields 1490 mm total used (and needed) back distance. of camera mount from grating mount. [confirm with D. Harmer]

Make the following assumptions about the layout in the upgraded system (based on the latest, near-final optical design from C. Harmer, and M. Bershady's assessment of optimal collimator-grating, i.e., first-turret, distance).

Upgraded System:

  • fibers feed collimator parallel to table edge (as in current system).
  • 14 deg off-axis system (C. Harmer's final optical design).
  • first grating-mount has 1170 mm back-distance from collimator (M. Bershady's assessment).
  • total camera back-distance at minimum un-folded angle. is unchanged from current system (i.e., 1490 mm).

  • first grating-mount is then shifted 283 mm along width of bench due to 14 deg wedge.
  • with grating mount 383 mm from edge of bench, this yields a maximum angle of 41.4 + 14 deg = 55.4 deg.

Summmary: A solution must be found in current and upgraded systems to accommodate VPH configurations with 45 < &alpha < 55. This solution should permit a 10-20 deg overlap in &alpha in folded and unfolded VPH configurations.

Suggested Solution Option 1: Use fold-flats

Use fold-flat to accommodate &alpha up to at least 65 deg. To keep the camera at angles that do not fold back on the collimator requires two things, which each raise an issue:

(i) flat angles up to 65 deg (this yields a camera angle parallel with the collimated beam for &alpha = 65)

How good is reflectivity of fold flat for angles up to 65 deg?

(ii) 2nd turret location "behind" the flat (1st) turret -- relative to the collimator. For 50 deg, assuming a flat-VPH grating distance of 533 mm, the back distance is 533 cos(30) = 461 mm. This is a lot.

If the first turret is placed 461 mm from the table end, will this allow for the echelle configuration with a camera angle of only 3 deg (14-11) w.r.t. the table edge?
How much shorter would the camera mount have to be in this case?

One solution that would not require modifying the existing camera mount is to get a longer table. A 5x10 ft table gives ample room for this extra back-distance of the 2nd turret.

Suggested Solution Option 2: Shorten camera mount

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:

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:

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.]

Preliminary conclusion

  • We should assume that one way or the other we will need a 5x10 ft bench and we should start the process of looking for a suitable unit.

  • We should immediately embark upon a study to determine the feasibility and level of effort to significantly shorten the effective length of the existing camera + dewar mount.

last updated: 05 Jan, 2006 (mab@astro.wisc.edu)