Preparing a HERMES Proposal

HERMES proposals

All HERMES detectors are from the E2V CCD231-84 family, grade-0 cosmetic quality (the highest available quality), and are 4096 x 4112, 15um pixel devices. All CCDs are housed in cryostats operating at about a temperature of 170K.

The "blue" and "green" channel detectors are both 16 micron, standard silicon devices with broadband and mid band coatings. The "red" channel detector is a 40 micron, deep depletion device with fringe suppression and an ER1 coating. The "IR" channel detector is a 100 micron bulk silicon device with fringe suppression and a "Multi-9" coating.

CCD Readout Amplifiers 

Each detector is controlled with an AAO2 CCD controller, and permit readout from one, two or four detector outputs at various readout rates with windowing and binning options.

Y (spatial direction)

Top Left Top Right
Bottom Left Bottom Right

 X (spectral direction)

Using two Left amplifiers or two Right amplifiers readout the detector in the spatial direction avoiding splitting the data in the spectral direction. Using two Top amplifiers or two Bottom readout the detector in the spectral direction, splitting a single fibre spectrum into two. Using all four amplifiers reads the detector in 4 quadrants.

CCD Readout modes

The table below gives the gain, readout noise and readout times for the various readout modes using  a single top left amplifier. The values for other amplifers are very similar.  The level of dark current in the four CCDs is between 1.5 - 3.0e/pixel/hour.

CCD Mode Readout Time (sec) Gain (e/ADU) Read Noise (e)
BLUE

FAST

NORMAL

SLOW

144

282

420

2.6

1.8

1.2

4.1

3.2

2.9

GREEN

FAST

NORMAL

SLOW

144

282

420

3.0

2.0

1.4

4.4

3.1

2.6

RED

FAST

NORMAL

SLOW

144

282

420

3.1

1.9

1.4

4.9

3.1

2.9

IR

FAST

NORMAL

SLOW

144

282

420

2.7

1.5

0.7

4.4

3.5

3.0

 

Each CCD can be binned independently in the Y (spatial) direction and up to 2x in the X (spectral) direction for all readout modes. The readout times for binned data using two amplifiers with FAST readout are as given below. Double the readout time for single amplifer mode and halve the readout time if four amplifiers are to be used.

X Y TIME
1 1 71s
2 1 52s
1 2 36s
2 2 26s

 

 

 

 

At minimum each conguration observed with HERMES requires a Flat and Arc exposure.  Flats need to be taken only once for a given conguration, provided the slit unit has not been moved. More frequent arcs (e.g. before and after a science frame) can be taken for higher precision wavelength calibration requirements and twilight and/or dome flats might be needed for detailed throuput calibration.

HERMES wavelength calibration uses 2 ThXe lamps installed within 2dF top end ring. For Flats, there are a selection of quartz lamps within 2dF, however the 75W HERMES lamps are recommended for HERMES flat exposures. The exposure times provided below gives suficient flux for wavelength accuracy within 0.1 pixels and flat field counts to achieve a signal-to-noise of 100 per resolution element. For different lamp setups the exposure times should be adjusted.

Channel Nominal resolution Exposure Time (sec) Slit Mask mode Exposure Time (sec)
Flat Arc Flat Arc
Blue 80 180 160 300
Green 80 180 160 300
Red 45 180 90 300
IR 60 180 120 300

The values provided are approximate and only given for guidance. Always ensure the calibrations contain sufficient flux for the specific science requirements.

The throughput of the AAT+2dF+HERMES system was determined by measuring the detector counts over a range of standard star observations. The over all flux transformation equations (from star to detector) are given below for each channel:

Blue Channel:               e- per resolution element per hour = 10(-0.4(0.993*B - 24.05))
Green Channel:             e- per resolution element per hour = 10^(-0.4(1.18*V - 26.25))
Red Channel:                e- per resolution element per hour = 10^(-0.4(1.07*R - 24.98))
IR Channel:                   e- per resolution element per hour = 10^(-0.4(0.89*I - 22.33))

Note that in the above transformations, B, V, R, I refer to the Johnson B,V, R and I filters which are the nearest bands to the four HERMES channels, and is based on the median seeing conditions at Siding Spring Observatory. The Table below gives the corresponding magnitude limit to achieve 104 electrons (signal-to-noise = 100) per resolution element for a 1 hour exposure time.

 

Channel

Magnitude

Blue

B = 14.2

Green

V = 13.8

Red

R = 14.0

IR

IR = 13.8

 

The use of the high-resolution mask reduces the flux by a factor of two, raising the limiting magnitude by 0.7mag from those given in the above table.