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AAO Diffraction Gratings

The AAO owns a set of 22 diffraction gratings which have been used with 2dF, the RGO spectrograph and SPIRAL on the AAT, and with FLAIR on the UKST. The current list of gratings is below. Grating numbers have been assigned in order of acquisition. Three gratings (150ECH, 600H and a second 600U) have been unmounted and are not available.



Name Number Blaze (A) Status
300B 14 4200 Current
300B 19 4200 Current
250B 4 4300 Current, but 300B has higher efficiency
270R 3 7600 Current
316R 22 7500 Current, but frequently in use by 6dF*
600U 18 3500 Current, but not recommended lambda > 3500A
600V 8 5000 Current
600V 17 5000 Current
600V 21 5000 Available for 6dF only
600R 9 7500 Current
1200B 6 4300 Current
1200B 20 4300 Current
1200V 1 5000 Available for 6dF only
1200V 11 5000 Current
1200V 15 5000 Current
1200R 2 7500 Current
1200R 16 7500 Current
1200I 12 10000 Current, but recommended for order II only 
1200J 10 12000 Not available - on loan to the UK**
NB Blaze angles given are the nominal Littrow values.
*  The 316R is being used for the 6dF Galaxy Survey, and is usually at the Schmidt. Please contact Paul Cass before use (ideally 1 week) to avoid grating conflicts.
**Contact your AAO Support astronomer if you have any concerns. For most purposes the 1200I will perform as well as the 1200J.

Wavelength Parameters

Calculators to estimate wavelength centre, coverage and dispersion for a given grating angle are available for 2dF and the RGO spectrograph. Note that slightly different parameters are used for the RGO at the telescope, but the differences are within the estimate error. For exact values, arc frames should be used.

Grating Efficiencies

The efficiency curves of the older gratings were measured in 1986 (AAO Manual 19). New measurements in the V band were taken in 1999. They showed little change, with ~5% loss in V for most older gratings. The data from the old measurements are presented here, with new measurements of the 300B grating (14) and the 316R (22).

Low Dispersion Grating Efficiencies

300l/mm grating comparison

Postscript version of comparison plot

Detailed plots showing 2dF / RGO 25cm and RGO 82cm measurements and higher orders:

250B:  gif, postscript, data: 25cm, 82cm
270R:  gif, postscript, data: 25cm, 82cm, Order II 25cm, 82cm
300B:  data: 25cm
316R:  data: 25cm


Medium Dispersion Grating Efficiencies

600l/mm grating comparison

Postscript version of comparison plot

Detailed plots showing 2dF / RGO 25cm and RGO 82cm measurements and higher orders:

250B:  gif, postscript, data: 25cm, 82cm
270R:  gif, postscript, data: 25cm, 82cm, Order II 25cm, 82cm
300B:  data: 25cm
316R:  data: 25cm


High Dispersion Grating Efficiencies

1200l/mm grating comparison

Postscript version of comparison plot

Comparison of Order II measurements: gif, postscript

Detailed plots showing 2dF / RGO 25cm and RGO 82cm measurements and higher orders:

1200B:  gif, postscript, data: 25cm, 82cm
1200V:  gif, postscript, data: 25cm, 82cm
1200R:  gif, postscript, data: 25cm, 82cm, Order II 25cm, 82cm
1200I:   gif, postscript, data: 25cm, 82cm, Order II 25cm, 82cm, Order III 82cm
1200J:  gif, postscript, data: 25cm, 82cm, Order II 25cm, 82cm, Order III 82cm
Note that the 1200J is not currently available.




Appendix - Details of Measurement Procedure

1985/86 Measurements

A laboratory experiment was setup by R. Robinson and P. Gray in 1985. Measurements were undertaken by H. Johnston and R. Stathakis. Results were presented in AAO UM 19.

The angle between the incoming light and the detector position was set at 45o to simulate the 25cm RGO camera and 20o for the 82cm camera (this should have been 15o but was restricted by the physical limitations of the setup). 2dF has a similar angle to the 25cm RGO camera (40o).

The light source was a 12V quartz halogen lamp. Light passed through an order sorting filter and a double monochromator system composed of two 125 mm focal length Oriel 7240 grating monochromators. The second monochromator was necessary to avoid problems of scattered light. For wavelengths above 4000A an exit slit of 0.35 mm was used, giving a bandwidth of 30A. Because of the low light levels below 4000A, the exit slit was widened to 1 mm.

An iris diaphram restricted light to a 1 cm diameter beam which passed through a 40 mm diamter 100 mm focal length fused silicon double convex collimator lens. The light passed through a polaroid filter, was reflected by an aluminium flat mirror or the grating mounted on a turntable, and refocused using a 50 mm diameter 75 mm focal length fused silicon double convex lens onto a 1 cm diameter EG & G silicon photo diode (type UV-44B).

The wavelength range was divided into three regions (U: 3600 - 4600A; V: 4600 - 7000A; I: 7000A - 11400A). Polaroids used were HNP'B in U, HN 32 in V and HR for I. In overlap regions measurements usually agreed to 1-2% but occasionally disagreed by up to 4%. Observations were made at steps of at most 200A, starting near the nominal blaze wavelength and progressing in both directions until an efficiency of 30% or less was attained or the measurement range was exceeded. At each position the reference mirror was measured, then the grating and finally the mirror again. Mirror measurements normally agreed to within 1-2%. Grating measurements were normalised by the mirror measurements. The absolute reflectivity of the mirror flat was measured and grating curves were corrected to an absolute scale.

New measurements of 300B (14) and 316R (22).

Two spectrophotometric standards were observed through the 300B grating and then the 250B grating on the RGO 25cm camera in 1995. The weather was stable but not photometric, and absolute efficiencies of the other components of the spectrograph are not well known, so this test measured only the relative efficiencies of the 250B and 300B gratings. The absolute curve for the 300B grating was estimated by scaling by the 1985/6 measurements of the 250B grating. Spot measurements in FLAIR and with the RGO in photometric weather are consistent with the efficiency curve.

Observations were taken by Ian Lewis and Karl Glazebrook with the 300B, 316R and 270R gratings on 2dF in 1999 covering 4400A to 8000A. The relative efficiencies were reported in the AAO Newsletter 90, August 1999. After scaling by the 1985/6 measurements of the 270R grating, the 300B observations agreed well with the 1997 figures for wavelengths redward of 5200A. The 316R figures agree well with test laboratory measurements.

The final 300B measurements are combined from the 1995 and 1999 observations.