Final commissioning of the Loral 1200 x 800 CCD (hereinafter Loral 1K CCD) plus 1.5-m spectrograph combination was carried out during two engineering nights on November 15-16, 1995. A previous engineering run in May (reported on pages 25-26 of NOAO Newsletter No. 43, see below) had allowed an initial evaluation of the image quality, throughput, fringing characteristics and flexure of the system at the telescope. The encouraging results of that engineering night were amply confirmed on the two commissioning nights in November -- the performance of the Loral 1K CCD is superior in all aspects to that of the front-illuminated GEC CCD used for so many years with the 1.5-m spectrograph, offering 40% greater wavelength coverage (at the same resolution) and >= 2-3 times the quantum efficiency.
Within the next few months we will attempt to have a new user manual for the 1.5-m spectrograph + Loral 1K CCD available on the CTIO WWW pages. Twelve gratings are available which give the following wavelength resolution and coverage with the Loral 1K:
FWHM Wavelength
Gtg l/mm blaze $ Resolution Coverage
--------------------------------------------------
13 150 5000 17.2 Å 6900 Å
11 * 158 8000 16.4 6550
09 300 4000 8.6 3450
32 300 6750 8.6 3450
22 * 300 10000 8.6 3450
58 400 8000 6.5 2590
16 527 5500 4.8 1965
26 600 4000 4.3 1725
35 600 6750 4.3 1725
56 600 11000 4.3 1725
47 831 8000 3.1 1245
36 & 1200 7500 2.2 860
$ Blaze is first order Littrow blaze. Effective blaze
wavelength when used in the 1.5-m spectrograph is 0.89
of the Littrow value.
* silver coated does not reflect light below ~4000Å
& Cannot be tilted far enough to be used in II order
The Loral CCD has 15 micron pixels and a slit width of 143 microns
(2.6 arcsec) projects to 2 pixels. In practice, however, the focus of
arc lines is observed to range between 2-3 pixels (FWHM) due to the camera
optics and charge diffusion within the CCD (for more details, see the
article in Newsletter No. 43). The wavelength resolution figures in the
above table are calculated for a FWHM of 3 pixels.
Efficiency measurements of the total telescope/spectrograph/CCD combination were obtained with gratings 09 and 32 during the November engineering nights. The results are given in the following table in terms of the percentage of photons striking the telescope primary mirror which are eventually detected by the CCD. Note that these numbers are still not definitive due to uncertainties in the precise value of the CCD gain on the engineering night; however they should be accurate in a relative sense. We hope to be able to provide final numbers within the next 6 months.
Grating
Wavelength 09 32
-----------------------------------
3500 7.5 %
4000 13.5
4500 16.5
5000 17.0
5500 16.0
6000 13.5
6500 10.0 14.8 %
7000 13.6
7500 11.8
8000 8.5
8500 6.5
9000 5.0
9500 2.9
The upper right amplifier does not perform satisfactorily, therefore the CDD
is read in single-channel mode (through the lower left amplifier). As part
of the commissioning process, a reduction of 10 microseconds per pixel was
achieved. The resulting gains, read noises, and readout times are:
1/Gain Read Noise Read Time
(e-/ADU) (e-) (seconds)
-----------------------------------
4.11 7.71 15.6
2.87 7.11 19.7
2.05 6.50 25.8
1.42 6.13 34.0
0.96 5.88 46.3
Full well of the Loral 1K CCD is 118,000 e-. Over this range, the CCD
delivers excellent linearity (gain variation = 0.26% peak-to-peak).
As reported in Newsletter No. 43 (see below), the Loral 1K CCD fringes with substantial amplitude at wavelengths redward of 7500Å. However, thanks to the lack of significant flexure in the 1.5-m spectrograph and camera, it is possible to remove nearly all of the fringing using normal dome flats.
The accompanying figure (above) shows spectra of two type II supernovae obtained with the 1.5-m + Loral 1K commbination. These spectra are the sum of separate blue and red observations obtained at ~8.5 Å resolution with gratings 09 and 32. Total integration time for SN 1995ad was 30 minutes in the blue and 30 minutes in the red, while SN 1995V was observed for 90 minutes in the blue and 45 minutes in the red. Approximate magnitudes measured from the spectra are B = 16.8 and V = 15.7 for SN 1995ad and B = 18.7 and V = 17.6 for SN 1995W. Note the lack of obvious residual fringing at red wavelengths. In the case of SN 1995ad, a signal-to-noise value of 35:1 was obtained at a wavelength of 9250Å where the CCD fringing amplitude reaches 20%. (The signal-to-noise in the these spectra is limited more by photon statistics than any residual fringing.) With the successful commissioning of the Loral 1K CCD which is controlled by an Arcon, the last VEB controller in service on Tololo has now been officially retired! Que en paz descansen...
-- mphillips@noao.edu, sheathcote@noao.edu, rsmith@noao.edu
Gratings, Resolution & Coverage:
Gtg l/mm blaze GEC Loral Å/pix Cover Å/pix Cover 13 150 5000 8.4 4800 5.73 6820 11 * 158 8000 8.0 4600 5.45 6530 09 300 4000 4.21 2400 2.87 3410 32 300 6750 4.21 2400 2.87 3410 22 * 300 10000 4.21 2400 2.87 3410 58 400 8000 3.15 1800 2.15 2560 16 527 5500 2.36 1350 1.61 1920 26 600 4000 2.11 1200 1.44 1705 35 600 6750 2.11 1200 1.44 1705 56 600 11000 2.11 1200 1.44 1705 47 831 8000 1.5 860 1.02 1220 36 & 1200 7500 1.05 600 0.72 850 % Blaze is first order Littrow blaze. Effective blaze wavelength when used in the 1.5-m spectrograph is 0.89 of the Littrow value. * silver coated does not reflect light below ~4000Å & Cannot be tilted far enough to be used in II order
The GEC CCD has 22 micron pixels; a slit width of 210 microns (3.8 arcsec) projects to 2 pixels. With this device there is no evidence that the resolution of the spectrograph is limited by either the camera optics or the MTF of the detector. The measured FWHM of comparison lines corresponds very closely to the projected width of the spectrograph slit down to the Nyquist sampling limit, and 2 pix FWHM resolution is routinely achieved. There is little variation of image quality with position on the chip, or with wavelength.
The Loral CCD has 15 micron, pixels and is 1.4 times longer than the GEC; a slit width of 143 microns (2.6 arcsec) projects to 2 pixels. Because of the finer sampling and larger size of this CCD it is expected that the camera optics will somewhat limit the resolution, especially at the extreme edges of the field. In addition at KPNO they have been unable to get images better than ~3 pix FWHM with their Loral chips. This has been attributed to diffusion of photoelectrons within the CCD. This effect is greatest at blue wavelengths since higher energy photons are absorbed closer to the surface of the CCD.
The following table shows the measured FWHM of arc lines obtained for a single tilt of grating 32 for a slit width 110.5 microns (2 arcsec) showing the dependance on position on the chip (and wavelength). Values are given for the center of the slit (Y=200) and at the two extreme edges (Y=130, 270).
FWHM (pix) as a function of position ==================================== Line X Y (Å) (pix) (pix) 130 200 270 ==================================== 4471 112 | 3.12 2.78 2.82 4764 215 | 2.96 2.86 2.70 5015 303 | 2.89 2.69 2.70 5876 602 | 2.65 2.42 2.46 6678 879 | 2.52 2.13 2.25 6965 978 | 2.47 2.04 2.47 7384 1121 | 2.62 2.13 2.46 7635 1207 | 2.97 2.13 3.14 ====================================In general, although the lines are wider than the projected slit width, and there is some variation with position, the resolution with a slit width of 2-3 arcsec is better than or comparable to what would be obtained with the GEC CCD and the same grating.
The graph shows the FWHM as a function of slit width for spectral lines at the center of the CCD (in X and Y). Curves are shown for 3 wavelengths 3888Å, 6678Å and 9224Å
QE:
GEC Loral 1K
3000 20 25
3500 19 48
4000 17 65
5000 22 83
6000 35 93
7000 45 91
8000 30 83
9000 14 59
10000 3 10
System efficiency:The following are the measured system efficiencies (percentage of photons striking the telescope primary mirror which are eventually detected by the CCD) for the 1.5-m spectrograph with the GEC CCD using gratings 11 and 13
GEC Loral 1K
11 13 11 13
3000 0.0 1.1 0.0 1.4
3500 0.2 1.6 0.5 4.0
4000 0.9 2.0 3.4 7.6
5000 2.2 2.7 8.3 10.2
6000 7.0 4.9 18.6 13.0
7000 6.4 3.6 12.9 7.3
8000 3.6 2.0 9.9 5.5
9000 1.5 0.7 6.3 2.9
10000 0.4 0.0 1.3 0.0
The numbers for the Loral were estimated by scaling by the ratio of the QE's
given above.Unfortunately the engineering night had heavy cirrus / thick cloud. Therefore we do not have any measurements of the absolute sensitivity. However, observations of standard stars confirm that the sensitivity peaks at approximately 6000Å, where the QE curve peaks, and that there is significant sensitivity down to the atmospheric cutoff at 3000Å.
RON & Dark Current:
So far only the lower left amplifier has been comissioned and the CCD is being read in single channel mode.
The following table shows the gain (e-/ADU), RON (e-), and readout time (s) for the currently available gain settings.
Arcon3.9 == Loral 1K (1200*800)
i
n Bin1x1 Maximum
d DCS Delay Read_Noise 1/Gain -> Read_Noise Read Linear
e time (ADU) (e-/ADU) (e-) Time Signal
x (us) LL UR LL UR LL UR (s) (ADU)
---- ----- ---------- --------- ---------- ---- -----
1: 1 5 3 2.5 0 4.11 0 10.3 0 25.1 21900
2: 2 7 3 2.6 0 2.87 0 7.6 0 29.1 31400
3: 3 10 3 3.6 0 2.05 0 7.2 0 35.3 43900
4: 4 14 3 4.9 0 1.42 0 6.9 0 43.4 63400
5: 5 20 3 6.7 0 0.96 6.5 0 55.6 65534
Full well (~90,000 e- is reached before the ADC saturates at the higher
gains (more e-/ADU). The Non-linearity (peak-to-peak gain variation) is
believed to be less than 2% for levels below full well / ADC saturation.Currently the dark current is very high ~15e-/pixel/hour. However, it is expected that this will be reduced to a few e-/pixel/hour by opperating the CCD in MPP mode and by running it at a lower temperature. Fringing:
The Loral 1K CCD fringes with substantial amplitude at wavelengths redward of 7500Å. Press here if you realy want to be horrified . The fringes run approximately perpendicular to the dispersion. The peak-to-peak amplitude and fringe spacing are given in the following table and shown in the accompanying graph:
Wavelength Amplitude Spacing (Å) (%) (Å) ======================================= 7500 2.5 38 7750 2.8 40 8000 4.5 40 8250 8.4 46 8500 11.3 38 8750 14.6 42 9000 16.0 54 9300 20.6 54 9500 19.6 56 9750 17.0 66 10000 11.4 50 10500 7.4 60 =======================================At least in flat field frames the fringe amplitude does not depend on slit width. Nor does it depend (to first order) on the position where the light of a particular wavelength falls.
We do not yet know how well fringing can be corrected by flat fielding techniques. Given the above amplitudes it seams likely that for wavelengths shortward of about 8000Å fringing is unimportant or easily correctable. Redward of this it is likely that it will be necessary to obtain quartz flats for each object and take great care in flatfielding the data. Even then, observations requiring high S/N at wavelengths redward of 8000Å may not be possible with this CCD.
Flexure:
The following table shows the displacements (pixels) due to flexure parallel and perpendicular to the dispersion as a function of Hour Angle and Declination.
Parallel to Dispersion
===============================================================================
| HOUR ANGLE
DEC | WEST EAST
===============================================================================
+30 | 0.0h
|
| +0.10
------|------------------------------------------------------------------------
0 | 3.0h 0.0h 3.0h
|
| +0.35 -0.16 -0.23
------|------------------------------------------------------------------------
-30 | 4.5h 3.0h 1.5h 0.0h 1.5h 3.0h 4.5h
|
| +0.75 +0.42 +0.14 0.00 -0.09 -0.10 -0.06
------|------------------------------------------------------------------------
-60 | 5.0h 0.0h 5.0h
|
| +0.79 +0.13 -0.06
------|------------------------------------------------------------------------
-90 | 0.0h
|
| +0.68
===============================================================================
Perpendicular to Dispersion
===============================================================================
| HOUR ANGLE
DEC | WEST EAST
===============================================================================
+30 | 0.0h
|
| -0.02
------|------------------------------------------------------------------------
0 | 3.0h 0.0h 3.0h
|
| -0.66 +0.08 +0.34
------|------------------------------------------------------------------------
-30 | 4.5h 3.0h 1.5h 0.0h 1.5h 3.0h 4.5h
|
| -0.75 -0.61 -0.29 0.00 +0.26 +0.34 +0.37
------|------------------------------------------------------------------------
-60 | 5.0h 0.0h 5.0h
|
| -0.81 -0.10 +0.40
------|------------------------------------------------------------------------
-90 | 0.0h
|
| -0.49
===============================================================================