WIYN / Bench Spectrograph

Physical Spectrograph Layout: Dimensions, Distances, and Obstructions

This page documents the physical layout of the Bench Spectrograph, starting with the existing system (circa 2005 and earlier). This is based on work done as part of the Bench Upgrade project, graduate seminars at UW, (see this page), and the SparsePak instrument project.

The geometry of the spectrograph optical elements are based on (i) MAB's perusal of ZEMAX layouts from C. Harmer (NB: another design from D. Vaughnn sent to MAB indicates a faster collimator with a 914.4mm f.l.); (ii) at-spectrograph measurements by Bershady, D. Harmer, and C. Harmer; and (iii) measurements of the VPH configurations by D. Harmer, reported via telecon.

The geometry of the obstructions are taken from MAB's know of the fiber feed. Additional information is still needed to define the filter stop, the foot mount, and the camera casing.

[Dimensions] [Distances] [Obstructions]


Optical element Radius or
half-width (mm)
Distance to
next element (mm)
Next element
fiber slit 38.1 1021
800 upgrade
on-axis collimator
off-axis collimator
collimator1 117.5
TDB upgrade
TBD upgrade
all gratings or
fold-flat (low-densithy VPH)
echelle grating spatial   x   spectral
103   x   203
1016 (805-1016)camera objective
3300 l/mm VPH grating2 spatial   x   spectral
105   x   240
356 camera objective
low-order SR gratings3 spatial   x   spectral
101.5   x   115
390 camera objective
fold-flat4,5 spatial   x   spectral
107.5 x 130
533 VPH grating
740 l/mm VPH grating3,5 spatial   x   spectral
100   x 105.75
206 camera objective
camera objective (red)6,7 103.0 533.5 detector
camera objective (Simmons) ??? ??? detector
detector 24.6 x 24.6
[1] Collimator clear aperture measured by C. Harmer and M. Bershady, Feb 17, 2003
[2] VPH grating sizes from VPH grating summmary.
[3] low-order SR gratings have masks/stops. Above dimensions (from Hydra Manual) are too small; usable size is somewhat larger. A trace made on Feb 17, 2003 by MAB will be used to update these numbers.
[4] flat dimensions from D. Harmer (04 Jan 2006 email).
[5] grating-camera and flat-grating distances for 740 l/mm VPH configurations are from D. Harmer via Dec'05 telecon. Final, optimized distances may be shorter.
[6] Camera objective clear aperture revised Feb 18, 2003 after inspection of optical model by C. Harmer and M. Bershady. The first elememnt ("objective") is the limiting stop.
[7] Camera focal length for BSC has chromatic dependence.

Distances between critical elements

Distances (mm)

collimator focal length: 1021
800 upgrade (effective)
collimator-grating distance: 1838.9 original, all gratings
xxx upgrade, all gratings or fold-flat
grating-camera distance:
non-echelle (&thetacc = 30o)
grating-camera distance:
echelle (&thetacc = 11o )
1016 (805-1016)
grating-camera distance:
direct VPH (&thetacc<70o, &alpha>55o)
folded VPH configuration: (&alpha<55o)
   fold-flat--grating distance: 533
   grating-camera distance: 206
camera focal length:
(physical: objective to CCD)
camera objective: 134.4
fastest unvignetted, on-axis beam for camera: f/1.98
[1] grating-camera and flat-grating distances for VPH configurations are from D. Harmer via Dec'05 telecon.
[6] camera focal length for BSC has chromatic dependence.
[2] minimum unfolded user-angle (&alpha) is not yet finalized


Known Obstructions:

1. Fiber Feed.   This is currently on-axis, in the beam, at the pupil.
  • width (transverse to slit dimension):   12.8 mm
  • height (parallel to slit dimension):     +55.5 mm / -190.7,
    where the + direction is "down" towards the table, to higher fiber numbers; and the - direction is "up" away from the table, to lower fiber numbers.
  • length (along the beam):     217 mm = 181 mm between focus and grating (down stream) + 36 mm between focus and collimator (up stream).
    NB: Since this is a curved piece with a snout, there is no single number which can characterize the length. However, a number which should be representative is the axis length, which is given. The numbers are for the original toes, i.e. blue MOS, red MOS and DensePak; the SparsePak length is slightly shorter (up stream length is 28.6 mm).
At this, the fiber feed obstruction is included the Bench Simulator.

2. Fiber-Feed Mount.   This is a thick, semi-circular mount that is significantly outside of the current beam, since it is at the pupil (as per visual inspection by D. harmer, C. Harmer and M. Bershady, Feb 17, 2003). Because of the cable hardware on top, this mechanical hardware will become an issue for vignetting if a field-lens is used to push the pupil back significantly. At this time, it does not appear that a field lens is feasible in the upgraded system, and it is likely we will move to an off-axis collimator. Hence this element is unlikely to be important, and currently is not included in the Bench Simulator.

3. Toes and Filters.   The attached figure shows the geometry for what should be the critical stops in the toes. This analysis makes the simplifying assumption that all light emanates from the center of the fibers. Each cable has "toes" at the end of the fit, which is a multi-chambered extension to hold a slit mask and up to three filters (Hydra and Densepak), or two filters (SparsePak). The SparsePak toes are shorter. The DensePak and Hydra toes have internal chamber apertures that widen with distance from the slit end. The SparsePak toes have chamber that are larger, and are constant. In the adopted, simplified model where all rays emanate from the center of each fiber, the limiting stop turns out, in all cases, to be the last aperture. If one considers ray-bundles exiting from the edge of the fibers then on the Hydra and DensePak toes, the first two chambers provide the limiting stop. The present analysis does not take this into consideration, and the issue should perhaps be revisted in the future if the present model does not provide an accurate description of the observed data. At this time, the overall vigenetting model appears to be accurate to better than 10%.
        Currently, the limiting stop obscures rays faster than f/5.7 in the perpendicular dimension and f/5.65 in the parallel dimension (for edge fibers only) for DensePak and Hydra cables; the limiting stop obscures rays faster than f/4.1 in the perpendicular dimension and f/2.1 in the parallel dimension (for edge fibers only) for SparsePak. Orientations here are w.r.t. the slit.
       Measurements to define the stop from both interference and glass order-blocking filter were completed on Feb 17, 2003 and added to this figure in Oct 2004. We also observed that the filters, when placed at their respective distances, did not vignette an f/5 beam output from a thick, coherent bundle (this was done during tests with D. Harmer, C. Harmer, and M. Bershady). This indicates the above concerns about additional vignetting from fiber edges are likely unimportant. Direct calculation using the numbers in this figure shows that either the interference or glass filter placed in the first filter slot is a more open stop than the end of either the SparsePak or other feeds. This assumes the filters are inserted with the filter-aperture stop facing the fiber.
       Summary: Limiting stops for this component are "S" and "O" in this figure.

4. Camera Mount.   The attached figure shows the geometry for what should be the critical dimensions for determining the limiting stops of the camera. The vignetting from these elements will depend on the camera-collimator angle and the back-distance of the camera (camera-grating distance). Depending on the camera-collimator angle and the angle of the bevel in the front side of the camera mount, one of two cross-sections of the camera mount will provide the limiting stop: A-A or B-B in this figure.
        This is the highest-priority element to add to the Bench Simulator because it will determine the optimum back distance of the camera. This in turn will place constraints on the layout of the upgraded Bench. The specific constraint will come from the maximum back-distance needed to optimize configurations with the echelle with an 11 deg. camera-collimator angle. Note that if toe/filter vignetting is added (item 3 above), this will modify the results here since the vignetting is in the same plane as the camera vignetting.

Final note:    Order of the above in light path is: 3, 4, 1 & 2.

modified: Oct 28 '04, MAB