Help file for CIRSI Exposure Time Simulator

Contents

Important Notes and Known Bugs

  • Red Leak problem During the Dec 97/Jan 98 INT/WHT observing runs it was discovered that the broad band filters had substantial red leaks
  • During period Dec 97 to Jul 98 we used a 12bit ADC; ie data values ranged from 0-4095. A gain of 25e- per Data Number(DN) is assumed.
  • Initial set of blocking filters The initial set of blocking glass(Hoya-UV22) gave 52% throughput in H.
  • New blocking filters New blocking filters shall be added to CIRSI in Oct 98. These new filters have predicted peak throughput of 90% in H compared with 52% for the blocking filters used in May/June 1998.
  • 14bit ADC For the Oct/Nov 1998 run we are using a 14bit ADC. ie valid DN are 0-16383. A gain of 5.4 is assumed.

Introduction

The CIRSI exposure time calculator is aimed to provide potential users with information to aid proposal preparation and to aid observers in determining whether the system throughput is what we expect. Please report any obvious errors to rgm@ast.cam.ac.uk..

signal_cirsi is modified version of the program signal written by Chris Benn.

How to get the Fortran source

If the WWW version is sufficient for your purposes, the Fortran source code or the binary may be of more practical value. The fortran source resides in ~rgm/soft/signal/signal_cirsi.f
  • The binary executable resides at IOA in ~rgm/data/cass50a/bin/signal_cirsi
  • To recompile the source, copy the source to a suitable location and type "make signal_cirsi". The default make options should operate and generate you a suitable binary.
  • How to run signal_cirsi
    • To select the cirsi simulation options type I, for Instrument, at the prompt after you have started signal_cirsi.
    • Until the original version of signal, you need to type C, for compute to calculate the expected count rate and signal to noise.

Hardware Version

 The CIRSI hardware has gone through a number of changes between observing runs:

  • Dec 97/Jan 98 Configuration

  • May 98/Jun 98 Configuration

  • Post Jun 98 Enhancements

Telescope
Telescope and Focus Diameter
(m) 		um/" "/pixel fov
(single array) 		Contiguous field
for 4 exposures 		Exposures
per degree
WHT
Prime f/2.8 		4.2m 57.0 0.32 5.5' x 5.5' 20.9' x 20.9' 30.9
INT
Prime f/3.3 		2.5m 40.49 0.46 7.80' x 7.80' 29.6' x 29.6' 16.4
OCIW DuPont
Cass f/7.5 		2.5m 92.6 0.20 3.4' x 3.4' 13.0' x 13.0' 85.2
Notes:
  1. Assumes 10% overlap on all exposures.

Measured and Expected Throughput

Below are tabulated separately the actual measured system throughputs and expected throughputs based on improvements to the blocking filters. The zeropoint is given as the number of detected photons per second from a zero magnitude star on the alpha-Lyrae system.

The current version of signal_cirsi uses the H band throughput and sky background estimates measured at the INT on May 6th 1998.

  • The throughput measurements and background estimate are here .
  • Read the material here as well.
CIRSI Measured Zero Point Information
Telescope and Focus Date measured or
predicted 		Waveband Zeropoint
WHT Prime June 98 measured H 2.9e09
INT Prime May 98 measured H 0.9e09
CIRSI Predicted Zero Point Information
Telescope and Focus Date measured or
predicted 		Waveband Zeropoint
WHT Prime Sep 98 predicted z 1.1e10
" Sep 98 predicted J 8.5e09
" Sep 98 predicted H 5.0e09
INT Prime Sep 98 predicted z 4.7e09
" Sep 98 predicted J 2.9e09
" Sep 98 predicted H 1.5e09
DuPont Cass Sep 98 predicted J 3.7e09
Sep 98 predicted H 2.2e09
Sep 98 predicted Ks 1.3e09
Notes:
  1. Improvements between May/Jun 98 and Sep 98 are based on the replacement of the Hoya-UV22 blocking glass which had a throughput of 53% with a custom blocking filter make by OCLI with an expected peak transmission of 90%. This represents a gain in system throughput of 1.7.
  2. DuPont figure assumes a further factor of 1.50 improvement based on the estimated 60% throughput of the INT corrector and the assumption that the 1:1 to reimaging optics have 90% transmission.
  3. The Z band zeropoints have been computed assuming that the CIRSI QE is twice that of a LaPalma thinned Tek chip. Z is not available at the DuPont since the tip-tilt dichrioc wavelength split is at 9000ang.
  4. The J and K zeropoints are computed based on the relative zeropoints provided by ESO for SOFI on the NTT(private communication).
    • J: 1.2 e10 e-/s
    • H: 7. e9 e-/s
    • K: 4. e9 e-/s
    • Thus the relative zeropoints for a zero mag star on the alpha-Lyrae system are: J:H:K = 1.7:1.0:0.6

Airmass

The airmass can be computed from the Skycalc page: http://imagiware.com/astro/airmass.html .

Seeing

The seeing parameter is given in arcseconds and corresponds to the full-width at half-maximum of the seeing disk.

Photometry Aperture

signal_cirsi assumes the photometric aperture diameter is twice the seeing(FWHM). It also assumes(I think) that all the flux is within this aperture.
  • To simulate a 1sec diameter aperture use a FWHM of 0.5"
  • To simulate an aperture of size 1 arcsec^2, use FWHM of 0.56"

    Sky Background

    The values currently set in the simulation are the following: 

        Broad Band Filter        Sky magnitude  (mag/arcsec**2)  
    
    z                        18.8 (from LaPalma)
    J                        16.0 (from ESO)            
    H                        14.0     "         
    Ks                       12.5     "

    Note: Sky brightness can vary substantially during the night and with season.

    Filter

    At the INT and WHT CIRSI does not function usefully longward of the H band, and this can only be used in the wavelength range 0.9 to 1.8 microns (z to H). At the Dupont, Ks will be available be z shall not. CIRSI has a large selection of broad and narrow band filters. The filter response curves and specifications are here

    Exposure Time

    Do not confuse exposure time and total observation time, the latter being a sum of exposure time and overheads in the telescope and instrument. Further information on this will follow. On the WHT and INT, assume 50% unless told otherwise.

    Example Output

    Note: These examples are are based on early estimates of the CIRSI system throughput when non-optimum blocking filters were in use.

  • Telescope: INT
  • Seeing(FWHM): 1"
  • Target magnitude: 0.0;
  • Aperture radius: 1"
  • Exposure time: 1sec 
     I Instrument = INT CIRSI                 U Tel. geometric area (sq.m)     4.41
 G Grating                       none       Telescope/instr. throughput    0.14
 D Detector                     CIRSI       Detector efficiency            0.60
 B Band                         H           Pixel size (microns)          18.5
 M Apparent magnitude             0.0       Readout noise (electrons)     40.0
 P mag (0) or mag/arcsec**2 (1)   0         Photons/s/A/sq.cm mag=0       92
 T Integration time (sec)         1.0     W Effective bandwidth (A)     2300.00
 F FWHM (object*seeing)           1.0       Empirical/theoretical          1.0
 S Slit width (arcsec)            0.0     K Sky brightness mag/sq.arcsec  14.00
 A Airmass                        1.0     E Extinction per airmass (mag)   0.11

 Detected photons       from object =     8.08943E+08
 Detected photons/pixel from sky    =     423.939
 Detected photons/arcsec from sky    =     2031.97
 Detected photons from sky in aperture with diameter 2*FWHM   =     6383.63
 Signal-to-noise     8.08943E+08/    28442.5 =     28441.4

Scale in X direction:    0.46 "/pixel; 1024 pixels =    467.73 "
Scale in Y direction:    0.46 "/pixel; 1024 pixels =    467.73 "
CIRSI  gain =25.0 e-/ADU approx.; readnoise = 40 electrons

  • Telescope: INT
  • Seeing(FWHM): 1"
  • Target magnitude: 18.0;
  • Aperture radius: 1"
  • Exposure time: 300sec 

     I Instrument = INT CIRSI                 U Tel. geometric area (sq.m)     4.41
 G Grating                       none       Telescope/instr. throughput    0.14
 D Detector                     CIRSI       Detector efficiency            0.60
 B Band                         H           Pixel size (microns)          18.5
 M Apparent magnitude            19.0       Readout noise (electrons)     40.0
 P mag (0) or mag/arcsec**2 (1)   0         Photons/s/A/sq.cm mag=0       92
 T Integration time (sec)       300.0     W Effective bandwidth (A)     2300.00
 F FWHM (object*seeing)           1.0       Empirical/theoretical          1.0
 S Slit width (arcsec)            0.0     K Sky brightness mag/sq.arcsec  14.00
 A Airmass                        1.0     E Extinction per airmass (mag)   0.11

 Detected photons       from object =     6095.92
 Detected photons/pixel from sky    =     127182.
 Detected photons/arcsec from sky    =     609592.
 Detected photons from sky in aperture with diameter 2*FWHM   =     1.91509E+06
 Signal-to-noise     6095.92/    1394.73 =     4.37068

Scale in X direction:    0.46 "/pixel; 1024 pixels =    467.73 "
Scale in Y direction:    0.46 "/pixel; 1024 pixels =    467.73 "
CIRSI  gain =25.0 e-/ADU approx.; readnoise = 40 electrons

  • Telescope: INT
  • Seeing(FWHM): 1"
  • Target magnitude: 19.9
  • Aperture radius: 1"
  • Exposure time: 300sec 

    
 I Instrument = INT CIRSI                 U Tel. geometric area (sq.m)     4.41
 G Grating                       none       Telescope/instr. throughput    0.14
 D Detector                     CIRSI       Detector efficiency            0.60
 B Band                         H           Pixel size (microns)          18.5
 M Apparent magnitude            19.9       Readout noise (electrons)     40.0
 P mag (0) or mag/arcsec**2 (1)   0         Photons/s/A/sq.cm mag=0       92
 T Integration time (sec)       300.0     W Effective bandwidth (A)     2300.00
 F FWHM (object*seeing)           1.0       Empirical/theoretical          1.0
 S Slit width (arcsec)            0.0     K Sky brightness mag/sq.arcsec  14.00
 A Airmass                        1.0     E Extinction per airmass (mag)   0.11

 Detected photons       from object =     2660.97
 Detected photons/pixel from sky    =     127182.
 Detected photons/arcsec from sky    =     609592.
 Detected photons from sky in aperture with diameter 2*FWHM   =     1.91509E+06
 Signal-to-noise     2660.97/    1393.50 =     1.90956

Scale in X direction:    0.46 "/pixel; 1024 pixels =    467.73 "
Scale in Y direction:    0.46 "/pixel; 1024 pixels =    467.73 "
CIRSI  gain =25.0 e-/ADU approx.; readnoise = 40 electrons

  • Telescope: WNT
  • Seeing(FWHM): 1"
  • Target magnitude: 19.9;
  • Aperture radius: 1"
  • Exposure time: 300sec 

         
 I Instrument = WHT CIRSI                 U Tel. geometric area (sq.m)    12.47
 G Grating                       none       Telescope/instr. throughput    0.09
 D Detector                     CIRSI       Detector efficiency            0.60
 B Band                         H           Pixel size (microns)          18.5
 M Apparent magnitude            21.5       Readout noise (electrons)     40.0
 P mag (0) or mag/arcsec**2 (1)   0         Photons/s/A/sq.cm mag=0       92
 T Integration time (sec)      5000.0     W Effective bandwidth (A)     2300.00
 F FWHM (object*seeing)           1.0       Empirical/theoretical          1.0
 S Slit width (arcsec)            0.0     K Sky brightness mag/sq.arcsec  14.00
 A Airmass                        1.0     E Extinction per airmass (mag)   0.11

 Detected photons       from object =     17759.5
 Detected photons/pixel from sky    =     1.86997E+06
 Detected photons/arcsec from sky    =     1.77596E+07
 Detected photons from sky in aperture with diameter 2*FWHM   =     5.57933E+07
 Signal-to-noise     17759.5/    7473.87 =     2.37622

Scale in X direction:    0.32 "/pixel; 1024 pixels =    332.28 "
Scale in Y direction:    0.32 "/pixel; 1024 pixels =    332.28 "
CIRSI  gain =25.0 e-/ADU approx.; readnoise = 40 electrons

    Published by Richard McMahon rgm@ast.cam.ac.uk
    Last modified: Tue Nov 10 02:07:53 1998