SALTICAM Specifications

NEW! Real count rates at last. See Observer Specifics subsection.

If you are familiar with the instrument, jump to Observer Specifics. If not, go to Basic Properties.

Basic Properties

Image Quality	See SALTICAM Optical Design page. 0.3 arcsec (EE50), combined with SALT 0.6 arcsec (EE50), to give 0.67 arcsec image quality, independent of seeing. EE80 shall be no more than 0.5 arcsec. Distortion shall be less than 1 per cent. The mean plate scale shall be 107 micron/arcsec or 9.35 arcsec/mm within 1 per cent.
Science Field of View	8 arcmin in diameter
Guide Star Field of View	10 arcmin in diameter
Wavelength range	320 – 950 nm
Filters	8 position filter unit: UBVRI + ND + clear filters + short wavelength interference filters at 340 nm (FWHM 35 nm) and 380 nm (FWHM 40 nm) supplied
CCD chips	E2V Technologies 44-82
Format	2048 x 4102 x 15 micron square pixels per chip
Imaging area per chip	30.7 x 61.5 mm² imaging area per chip
Readout capabilities	2 readout amplifiers per chip
Mosaicing	2 x 1 mini-mosaic
CTE	better than 99.99%
Full well	164 and 172 k e-/pix (for CCDs SALT-01 and SALT-02 respectively)
Dark current	less than 1e-/pix/hr at 160 K
Readout noise	less than 3.0 e-/pix at 100 kHz (10.0 usec/pix) (slow readout)
CCD Controller	SDSU II (Leach) from Astronomical Research Camera Inc.
Sensitivity	Thinned, back-illuminated. Deep depletion silicon. Astro Broad Band anti-reflection coating.

Instrument Efficiency

"Typical" instrument and system efficiencies are shown in Fig. 1 and were calculated for the on-axis field position using:

Optics:
(i) No absorption in any of the lens material or cryostat window (CaF₂, BaF₂, fused Si); Absorption by Sylgard 184 at two doublet interfaces;
(ii) Reflection at 10 air-glass interfaces using the Spectrum Thin Films BBAR coating;
(iii) Reflection at 2 air-glass interfaces using a single layer of MgF2 coating (see the 3310AE0001 Optical Design Issue 2.7.doc for details).
The reflection or absorption in any filter is not included.
CCDs: Quantum efficiency as delivered.

Fig.1 shows SALTICAM efficiency as the product of the optics and the CCD curves. For reference, PFIS performance taken from Fig. 5 of the PFIS PDR Instrument Description Document is also shown.

: Figure 1. Instrument Efficiencies

Fig. 2 shows overall efficiency based on:

Atmosphere: The standard atmospheric extinction curve for Sutherland at a zenith distance of 37 degrees.
SALT + Fold: This is the minimum throughput taken from the System specification and includes reflectivity of the SALT Primary Mirror and the spherical aberration corrector (SAC), the SAC central obscuration, four per cent light losses at the four surfaces of the ADC, and the reflectivity of the fold mirror using the Livermore coating performance as supplied by David Buckley.
Total: In the bottom panel of Fig. 1 is the product of the SALTICAM, Atmosphere and SALT+Fold curves.

: Figure 2. Overall Efficiency

Observer Specifics

Cosmetics:

Delivered quantum efficiency for each chip is shown below:

Wavelength	Spectral Response (QE)
(nm)	CCD SALT-01	CCD SALT-02
350	41	49
400	80	71
500	81	76
650	78	73
900	48	45

Cosmetics:

Delivered cosmetics for each chip are shown below:

Defects	CCD SALT-01	CCD SALT-02
Column defects (black or white)	5	0
White spots	25	0
Total spots (black or white)	51	11
Traps	2	1

Gain:

Gain is user selectable and dependent on selected readout speed:

For this readout speed	Observer specifies gain using the word	Actual e/ADU
Fast	Faint	1.55
Fast	Bright	4.50
Slow	Faint	1.0
Slow	Bright	2.5

Prebinning:

1 x 1 to 9 x 9, independently in each direction

Readout speed:

Frame transfer architecture: 0.10 sec frame transfer time 100-333 kHz (10-3.0 usec/pix). Observer specifies readout speed as "FAST" or "SLOW".

Readout times:

Mode	Prebin	Observer Specifies	Readout Speed (usec/pix)	Readout Noise (e-/pix)	Readout Time (sec)
Full Frame	2x2	Slow	10.0	3.3	11.2
Full Frame	2x2	Fast	4.0	5	4.6
Frame Transfer	2x2	Slow	10.0	3.3	5.7
Frame Transfer	2x2	Fast	4.0	5	2.4

Minimum exposure times:

The table shows the minimum exposure times for slot mode and frame transfer mode for all the valid binning parameters:

Prebin	Slot Mode (sec)	Frame Transfer (sec)
1x1	0.70	15.90
2x2	0.30	4.70
3x3	0.20	2.80
4x4	0.15	2.00
5x5	-	1.70
6x6	0.08	1.40
7x7	-	1.30
8x8	0.07	1.10
9x9	0.05	1.10

Windowing:

Up to 10 windows (prefer not to specify for P-V phase)

Fastest windowed
photometry:

0.1 sec/sample with no dead time

Count Rates And Signal-To-Noise For A Star With U=B=V=R=I=20

Real Count Rates

Using data obtained in the measurement of SALT and RSS efficiency, photon rates for standard stars observed with SALTICAM through its UBVRI filters were transformed to those appropriate for a star with U=B=V=R=I=20. The results are shown in the table below. Technical details are described in the text after the table.

Real Count Rate For A Star With U=B=V=R=I=20

Filter	FWHM (Ang)	Photon Rates in 1 sec from 20th Mag Point Source (photons)	Photon Rates/Sec/Square Arcsec from Sky (No Moon) (photons)	Effective Noise for Images Spread over a Diameter of 3 Arcsec (photons)	Signal-To-Noise
U	700	135	13.5	16	8
B	1000	980	98.0	41	24
V	900	1160	290	57	20
R	1500	1160	555	71	16
I	1500	840	920	86	10

Input to the above measurements and computations:

Photon rates were derived from "burst primary" measurements obtained during the process of measuring SALT and RSS throughput. It was found that the typical effective number of segments illuminated for an arbitrary tracker position is 50 (compared to the expected 73 for a centred pupil and point source on axis in the science field). Centred tracker observations will thus have more photons and extreme offcentre tracker will have less.

Dark sky count rates are shown in the table in the theoretical calculations in the next subsection below, using measurements made at Sutherland by Dr. J. Menzies.

In view of the image quality problems, it is hard to know what number to use for the size of point source images. It was eventually decided that a diameter of 3 arcsec was as good an estimate as any.

The calculations assume 2x2 prebinning. However, apart from some small contribution in the U band, readout noise is negligible compared to photon noise from the sky.

Values for fainter targets and/or different exposure times can be calculated by scaling the point source counts to those appropriate for the fainter magnitudes or longer exposure times, and recalculating the noise as the square root of the sum of the photons from star and sky.

The count rates are typically a factor two smaller than the theoretical ones below. Part of the reason is the fact that only 50 illuminated segments were assumed above, whereas 73 were assumed in the calculations below. Another factor is that the primary mirror reflectivity is in poor condition due to dust and dirt on the mirror segments. Measurements have shown 30 per cent of the light is lost because of this. A full reconciliation of the observed count rates with those expected is in progress.

"Theoretical" As-Designed Count Rates

These calculations were performed during the design phase of SALTICAM. The real count rates above should be used. This section retained temporarily.

Using the overall “typical” efficiency shown in the bottom panel of Fig. 1, the area of each SALT mirror (8660 cm2) and count rates for point sources were calculated and are shown in the third column of the table below. Dark sky count rates are shown in the righthand column from measurements made at Sutherland by Dr. J. Menzies.

The photon rates are THEORETICAL. They will be updated with actual rates as soon as they are known. Note that these are “monochromatic” magnitudes.

Users can calculate signal-to-noise ratios by:

integrating across the appropriate filter (multiplying by the full width half maximum of the filter at its peak transmission is a good start)
calculating the number of square arcsec to include in the point source measurement and its associated sky subtraction
calculating the sky contribution by multiplying this number by that in the righthand column of the table below, and then calculating the sky noise
calculating the readout noise contribution

The PI apologises that users have to do these by hand. Ultimately the Exposure Time Calculator will be available.

Do NOT Use These Count Rates: See Real Count Rates Above

Wavelength (nm)	Photon Rates Per Sec (U=B=V=R=I=20)
	Point Source	Sutherland Sky (No Moon) /square arcsec
	(photon/sec/Ang)	(Magnitudes)	(photons/sec/Ang)
360	0.6	22.5	0.06
440	2.5	22.5	0.25
550	2.0	21.5	0.50
640	1.5	20.8	0.72
790	1.1	19.9	1.21

Filter	FWHM (Ang)	Photon Rates in 1 sec from Point Source (photons)	Photon Rates in 1 sec from Sky (No Moon) (photons)	Effective Noise (photons)	Signal-To-Noise
U	700	420	132	39	10
B	1000	2500	785	65	38
V	900	1800	1410	65	27
R	1500	2250	3393	82	27
I	1500	1650	5702	91	18

Natural seeing at Sutherland has median EE80 of 1.7 arcsec. When combined with the SALT image quality requirement of EE80 of 0.9 arcsec and allowing for modest degradation (0.5 arcsec EE80) by SALTICAM, median image quality on the detector will have EE80 of about 2.0 arcsec, corresponding to pi square arcsec. Assuming one second exposures and 2 x 2 prebinning (30 micron or 0.28 arcsec pix: ~40 pix will be used in the measurement), count rates and signal-to-noise in 2 x 2 prebinned images through UBVRI filters for 3.14 square arcsec can then be calculated. The results are shown in the Table above.

List of TBC Issues

These “To Be Confirmed” items will be resolved when the limitations of the Leach CCD controller as well as the CCD chips are defined. The numbers given above are therefore current best estimates, must be considered as provisional and may improve or get worse. As an example, those familiar with ULTRACAM’s likely high time resolution capability will be curious as to why the SALTICAM detectors will take 0.1 sec to perform a frame transfer. This results simply from the larger size of the CCDs to be used in SALTICAM, compared to the E2V CCD 47-20s in ULTRACAM. Indications from E2V are that the CCDs can probably be clocked at a faster rate, but this remains to be verified.