CFHT, Instruments, Spectroscopy, Gecko: The Coudé f/4 Spectrograph

Throughput Measurements
for various Gecko configurations



Before taking a look at the numbers in the following table, please remember a few things:

  1. Data for these measurements have generally been obtained under photometric conditions. However, for some nights, there might have been some hardly detectable small cirrus clouds, which would then affect the throughput values.
  2. Vega was used for some measurements because its magnitude is zero. When stars other than Vega have been used, we applied a correction for the magnitude using the UBVRI magnitude closest to the observed wavelength. However, this doesn't mean that the magnitude of the star for the wavelength observed is equal to the broad-band one. This can also affect the final throughput measurement.
  3. Imperfect guiding or declination oscillations can lower the throughput.
  4. The choice of filter or grism can significantly change the throughput.
  5. The theoretical response of the different orders must be taken into account when using a value in the table to find the throughput value for another (close) wavelength.
  6. The CCD QE must also be considered.
  7. Using the UV optics for Gecko can change the throughput.
  8. The throughput numbers below are the average number of electrons (not ADUs) you can expect to get per second when you sum all the pixels of your spectra in the spatial dimension, for a Magnitude=0 star.

There are throughput measurements for the following configurations:




Throughput measurements for Gecko + CAFE + EEV1, and the "main" optical fiber.

Wavelength (Å) Order Gecko
Optics
Filter or
Grism
THROUGHPUT
(e-/col/sec)
Notes
8660 6 RED 1814 17840 Vega
7773 7 RED 1704 39400 Omi Her and Vega
7700 7 RED 1704 36810 Vega
6707 8 RED 1626 76000-90000 Omi Her and Vega
6565 9 RED 1617 14750 Vega
6428 9 RED 1617 36030 Vega
6286 9 RED RED 47390 Vega
6150 9 RED RED 76200 Omi Her and Vega
5957 9 RED 1521 97800 Omi Her and Vega
5890 9 RED 1519 78360  
5890 9 RED 1521 94533 Vega
5786 10 RED 1521 42000 Omi Her and Vega
5657 10 RED RED 64400 Vega
5143 11 RED Blue 78000 Vega
4860 12 RED Blue 17030 Vega
4352 13 RED Blue 29410 Vega
4040 14 UV UV 6500-7200 July 2001; 2 stars, 2 nights, 2 measurements
3950 15 RED UV 23630 Vega
3947 14 UV UV 22500 July 2001; 2 stars, 4 measurements, 3 nights
3930 14 UV UV 10000-14000 July 2001; 2 stars, 2 measurements, 2 nights
3772 15 UV UV 5264 HR39, Sep 14 2000
3635 16 UV UV 350-600 July 2001; 3 stars, 2 nights, 3 measurements
3536 16 UV UV 2242 HR39, Sep 14 2000
3328 17 UV UV 949 HR39, Sep 14 2000
3130 18 UV UV 168 HR39, Sep 14 2000



Throughput measurements for Gecko + CAFE + EEV1, and the "spare" optical fiber.

Wavelength (Å) Order Gecko
Optics
Filter or
Grism
THROUGHPUT
(e-/col/sec)
Notes
8082 center of 7 RED RED 11000 2 stars
7281 8 RED 1711 150002000 3 stars
7281 8 RED filter 15000-30000? 2 stars, 2 results?
7072 center of 8 RED RED 20900 2 stars
6707 8 RED RED 75000 2 stars
6675 8 RED RED 42000 1 star + cirrus
6645 8 RED 1617 33000-45000 2 stars
6600 8 RED RED 40000 1 star + cirrus?
6562 8 RED RED 35000  
6335 9 RED RED 70400 2 stars
6330 9 RED 1604 40000 4 stars
6330 9 RED 1604 34777 Xi Ori
6330 9 RED RED 29201 Xi Ori
6286 center of 9 RED RED 29500 3 stars
6150 9 RED RED 35000-45000 3 stars
5657 center of 10 RED BLUE 37900 3 stars
5143 center of 11 RED BLUE 44000 1 star
4861 12 RED BLUE 14600 2 stars
4714 center of 12 RED UV 19700 2 stars
4455 12 RED BLUE 23000  
4352 center of 13 RED UV 18100 2 stars
4041 center of 14 RED UV 19400 3 stars
3950 14 RED UV 11000 3 stars



Throughput measurements for Gecko + CAFE + MIT2, and the "main" optical fiber.

Wavelength (Å) Order Gecko
Optics
Filter or
Grism
THROUGHPUT
(e-/col/sec)
Notes
4100 14 RED UV 3100 2 stars
5142 11 RED BLUE 30000 3 stars
6565 8 RED 1617 43300 1 star
7281 8 RED 1711 19000 2 stars
8082 7 RED RED 17000 2 stars



Throughput measurements for Gecko + UV mirror train + EEV1

Wavelength (Å) Order Gecko
Optics
Filter or
Grism
THROUGHPUT
(e-/col/sec)
Notes
3078 18 UV UV 400 4 stars + cirrus



Data reduction procedure:

  1. Take spectral data and biases (or darks)
  2. Combine the biases or darks
  3. Subtract the average bias or dark from the object frame
  4. Do NOT flat field the data
  5. Extract the spectra; use apall and carefully select the aperture to include all the slices
  6. Get the mean signal in the ADU for the whole spectra; use imstat on the main portion of the spectra (cut the edges if there is vignetting)
  7. Get the mean signal in e-/col/sec by multiplying the number of ADU by the gain, and dividing by the exposure time
  8. Correct for the magnitude of the star to normalize to magnitude 0
  9. The airmass correction is not computed; data was taken with reasonable airmasses.

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