SAAO - South African Astronomical Observatory

Grating Spectrograph with SITe CCD

The grating spectrograph is available only on the 1.9-m telescope.  It employs a SITe CCD chip, using an f/2.2 camera with an 86 mm beam. Most spectrograph functions are automated and are operated from the warm room. Grating angle adjustment, slit width adjustment, comparison beam filters and arc lamp insertion are not automated. The star is acquired viewing the sky via a 45° mirror, which is then moved to permit light to reach the slit area, which is viewed via a transfer lens. Using the CCD acquisition and field viewing camera in good seeing with no moon, stars as faint as V = 20 can be seen in field viewing mode. Provided an object can be adequately centred in field viewing mode, it is not necessary to see it on the slit.

Spectrograph Specifications

Scale of slit:                      6 arcsec/mm
Slit to detector reduction: 8.2 (perpendicular to dispersion)
Normal slit width: 200-250 microns (1.2-1.5 arcsec)
TV acquisition field: 4' x 3.5'
Comparison spectrum: Cu/Ar. Cu/Ne is also available
Flat field illumination: Quartz-Iodine lamp
Star filters: Neutral densities up to 10 magnitudes,
BG39, GG495. Transmission curves for the
neutral density filters are available
on request.

The SITe CCD is effectively 266 x 1798 pixels in size, and is usable over wavelengths ranging from 0.35 µm. Pixel size is 15 µm. Between 0.6 µm and 1.0 µm, fringing effects become progressively more severe with increasing wavelength, and flat fields should be obtained at the exact grating angle used for an observation. The short exposure times required for dome flats make it possible to obtain flat fields during the night without losing too large a percentage of observing time, but it is still best to stick to the same grating and angle throughout the night if possible.

There is no direct indication of throughput so care is required in centring on faint objects. The field viewing system is mounted on XY slides, and allows autoguiding on stars bright enough to be in the Guide Star Catalogue, but not brighter than 9th magnitude.  Guide star positions can be determined using software available at the telescope.

Gratings

As of February 2012, the damaged gratings 4, 5, 6 and 8 have been replaced with new gratings of identical specification.

Grating

Lines (mm-1)

Order

Dispersion (Å/mm)

Useful range (Å)

Blaze (Å)

Resolution (Å)

4

1200

1

50

800

4600

1

5

1200

1

50

800

6800

1

2

20

350

3400

0.5

6

600

1

100

1600

4600

2

7

300

1

210

4200

4600

5

8

400

1

155

2300

7800

4

2

75

1150

3900

2

9

830

1

65

960

7800

1.5

2

30

480

3900

0.7

10

1200

1

50

800

10000

1

11

600

1

100

1600

10000

2

12

300

1

10000

Gratings 7, 8, and 9 are direct replacements for 1, 2, and 3 used in earlier years.

Data backup and archiving

Observers can backup their data using DAT tapes available at the Sutherland hostel (these must be paid for by the observer). There is a DAT drive in the 1.9-m telescope.  Observers are required to make an extra copy of their data while in Sutherland to be deposited in the SAAO archive in Cape Town (observers are not charged for these duplicate tapes). Alternatively, observers can arrange with IT staff to write their data to DVD, and copy their data to their laptop and/or home institute via ftp.

Manuals

Further information can be found in the manuals on this page.