WIYN / Bench Spectrograph

Throughput Budget

This is currently in progress, but has been updated as of April 08 2003. Updates include downgrading telescope and collimator reflectance from 91% to 89% per surface, adding final model vignetting values for spectrograph, adding final aperture corrections (telecsope and CCD focal planes), and updating the CCD systems to note the value includes detector and dewar reflection losses. A final update on the telescope reflectance will be forthcoming from JG. Otherwise this is a completed excercise.

We are using a good zeropoint measurment taken with SparsePak and the echelle in order 8, as analyzed by Westfall. The throughput measurement was made Feige 34 as observed on March 25, 2002 through fiber 52 of SparsePak and is converted to the on-axis and spatially-off axis (slit-edge) values using the vignetting function defined by dome flats, both for the central wavelength near 6687A. Note that in this echelle setup the peak efficiency is slightly redwards due to the blaze function. The efficiency on-axis is at 94% of this peak. Some small corrections for seeing effects still need to be done to account for light lost in the telescope focal plane (of order 4%) and in the spectrograph CCD focal plane due to a finite extraction aperture (of order 5%). The throughput is calculated taking into account the effective telescope aperture (i.e., including the secondary obstruction). We use a collecting area of 7.986 m², i.e. a 3.5m primary with a 17.1% central obstruction (in area).

Vignetting estimates come from Crawford's geometric spectrograph model using an excellent analytic approximation to the laboratory measurement of the EE as a function of f/. (We adopted a Sersic function -- see notes on input beam profile.) This model matches the observed vignetting profile to better than 10%; it should contain all significant geometric obstructions in the spectrograph, including the foot, finite collimator, grating, and camera objective. (For SparsePak, there is no vignetting from the toes and filter. The camera enclosure vignetting should be minimal or non-existant for the camera back-distace used in this setup. Based on a visual inspection of a bright f/5 beam in the systems [D. Harmer, C. Harmer, and M. Bershady, Feb 12 2003], there are no other obstructions in the system. C. harmer and M. Bershady determined that the first camera element [objective] is the limiting stop in the camera.) Filter, grating, and CCD efficiencies comes from the Hydra/Bench manual.

TABLE A1.   BENCH SPECTROGRAPH THROUGHPUT BUDGET
Setup:	Echelle, order 8, cwl=669nm, BSC, SPK
COMPONENT	ESTIMATE QUALITY	ON-AXIS	OFF-AXIS
Top-End "Feed"
atmosphere
transmission	reasonable estimate: 1.12 airmass at 6687A	0.90
telescope (3 mirrors)
reflectance	rough estimate / variable: assume 0.88-0.89 per surface	0.69
fiber
throughput	good estimate in lab	0.88
``slit losses''	high-fidelity aperture correction	0.91
Top-End subtotal		0.50
Spectrograph		On-axis	Off-axis
toes
filter transmission	good estimate? (X19)	0.90
vignetting	good estimate from model	1.0	1.0
collimator
reflectance	ok estimate	0.89
vignetting	good estimate from model	0.98	0.89
pupil obstructions (foot)
vignetting	good estimate from model	0.93	0.92
grating
efficiency	peak from Hydra Manual times theoretical blaze fnc for 6687A	0.32   (0.50x0.63)
vignetting	good estimate from model	0.93	0.86
camera
transmission	unknown	TCam
vignetting	good estimate from model	0.81	0.54
ccd system
window & det. QE	Hydra Manual	0.80
Spectrograph subtotal		0.14 TCam	0.078 TCam
Spectral Extraction	high fidelity measurement	0.975
Total		0.069 TCam	0.038 TCam
Measurement		0.054	0.028
Implied TCam		0.78	0.74