Palomar Double Spectrograph Throughput Curves

This page illustrates the throughput of various configurations of the Palomar Double Spectrograph.  Curves are shown for both the red and blue channels.  The throughput changes depending on the gratings, the grating angles, and the dichroic filter.  The next section describes the procedure used to measure throughput, and the following section presents the throughput curves.

Please click on any plot to see a high-resolution encapsulated PostScript (EPS) version of the same plot.

Evan Kirby calculated the throughput curves and wrote this page.  Thanks to Jeff Hickey and Kevin Rykoski for obtaining the measurements.  Special thanks to the DBSP observers in 2010 who sacrificed some dark time to make these measurements possible.


  1. Arc lamps were obtained through a narrow slit.  Generally, the FeAr lamp was used for the blue side, and the HeNeAr lamps were used for the red side.  However, sometimes the same lamps was used for both sides.
  2. A convenient spectrophotometric standard was selected from the list of Stone, R. P. S. 1977, ApJ, 218, 767, Massey, P., Strobel, K., Barnes, J. V., & Anderson, E. 1988, ApJ, 328, 315, Massey, P., & Gronwall, C. 1990, ApJ, 358, 344, or Hamuy et al. 1994, PASP, 106, 566.
  3. Some exposures were obtained through the 10 arcsec-wide slit on a clear night.  The number of exposures and exposure time varied depending on the magnitude of the standard star.
  4. The NOAO/twodspec/longslit/doslit procedure was used to extract and sky-subtract the stellar spectra.  The extraction windows were generously wide to keep as much flux as possible.
  5. The spectra were wavelength-calibrated using the NOAO Spectral Atlas Central database of emission line spectra for calibration lamps.
  6. If applicable, one-dimensional spectra were summed.  For the following equation, texp is the sum of exposure times from individual frames.
  7. The AB magnitudes of the standard star, taken from the above references, were converted to the expected number of photons per wavelength bin: Nphotons, where A = 1.76 105 cm2, which is the collecting area of the Palomar primary mirror, and h is Planck's constant.  Both Nphotons and mAB are functions of wavelength.
  8. The total sky-to-detector throughput is given by the quotient of the measured number of photons and the expected number of photons.
  9. The extinction due to the atmosphere was corrected according to the Palomar extinction curve (Hayes & Latham, ApJ, 197, 593).  These magnitudes are tabulated for an airmass of 1.  Therefore, the extinction magnitudes were multiplied by the airmass of the standard star at the time of observation.
  10. The reflectivity losses due to two mirror reflections (primary and secondary) were corrected according to the reflectivity coefficients of aluminum (Weaver & Frederikse, Handbook of Chemistry and Physics, 12-123).  The coefficients were renormalized to 86% at 6700 .

Throughput Curves

The following table gives throughput curves in graphical (jpg) and tabular (dat) form for different combinations of gratings and dichroic filters.  In each table cell, the letter B or R indicates that the curve is valid for the blue or red channel.  The following number is the grating tilt in degrees.  Click on jpg to see the graph of the throughput curve.  Click on dat to see the ASCII table with wavelength and precentage throughput.  The curves show the sky-to-detector, telescope+spectrograph, and spectrograph-only throughput measurements (see steps 8-10 above).  Finally, the date of the throughput measurement is given in parentheses.  Some spectrograph configurations have more than one throughput measurement.  Differences between them can give a rough idea of the uncertainty of the measurements.

Large fluctuations in throughput within a small spectral range are not real.  These can be caused by incompletely excised telluric absorption bands or bad traces near the ends of the CCD.

Grating / Dichroic
R 21.0: jpg dat (2010-04-08)
R 21.0: jpg dat (2010-05-07)
R 21.0: jpg dat (2010-06-08)
R 21.2: jpg dat (2010-05-21)

R 24.8: jpg dat (2010-08-12) anomalously high throughput R 24.6: jpg dat (2010-06-09)
B 22.9: jpg dat (2010-06-08)
B 26.9: jpg dat (2010-05-12) spurious fluctuations
B 26.9: jpg dat (2010-05-14)
B 27.1: jpg dat (2010-08-12)
B 27.3: jpg dat (2010-04-08)
B 27.3: jpg dat (2010-05-07)
B 27.4: jpg dat (2010-05-21)
B 28.4: jpg dat (2010-06-09)
B 29.2: jpg dat (2010-09-11)
R 29.5: jpg dat (2010-05-12)
R 30.3: jpg dat (2010-05-14)
R 30.9: jpg dat (2010-09-11)
B 36.8: jpg dat (2010-08-17) B 41.7: jpg dat (2010-05-09)
R 40.8: jpg dat (2010-08-17) R 44.5: jpg dat (2010-05-09)