Summary characteristics for CASIMIR at first flights
September 2004
Receivers:
CASIMIR will first fly with two input bands contained in a single cryostat. Observations will be possible with only one band at a time. The receivers are double-sideband with temperatures of approximately 0.5 K/GHz. Boldface in the table below highlights the bands for which LO sources are presently available. Observations in the other bands may be possible but are mainly limited by LO availability.
|
Band |
Frequencies |
Trec (DSB) |
LO now available |
|
550
GHz |
500-600 GHz |
100 K |
Yes |
|
750 GHz |
700-800 GHz |
350 K |
No |
|
1000 GHz |
920-1070 GHz |
500 K |
No |
|
1100 GHz |
1000-1100 GHz |
500 K |
No |
|
1200 GHz |
1110-1260 GHz |
600 K |
Yes |
|
1400 GHz |
~1.4 THz |
1000 K |
No |
Spectrometers:
We expect to have both a moderate-resolution wide-bandwidth analog correlator and a high-resolution moderate-bandwidth digital correlator on board. The digital correlator has four fixed-frequency 1 GHz samplers for 500 MHz sub-bands. There are 224 lags for each 500 MHz sub-band, but lags can be redistributed behind the samplers for higher spectral resolution within a smaller number of 500 MHz sub-bands. The table below does not yet include the number of lags that overlap between sub-bands.
|
Bandwidth |
Type |
Lags |
Resolution |
Center in IF |
|
3.5 GHz |
Analog |
128 |
33 MHz |
6.00 GHz |
|
2 GHz |
Digital |
896 |
2.2 MHz |
6.00 GHz |
|
1 GHz |
Digital |
896 |
1.1 MHz |
5.50 GHz |
|
500 MHz |
Digital |
896 |
0.6 MHz |
5.75 GHz |
Representative
spot frequency summary:
|
Frequency |
Trec |
Beam FWHM |
Max/Min Bandwidth |
Max/Min Resolution |
|
550 GHz |
100 K |
51 |
1910/272 km/s |
18/0.3 km/s |
|
700 GHz |
300 K |
40 |
1500/214 km/s |
14/0.2 km/s |
|
1200 GHz |
600 K |
23 |
875/125 km/s |
8/0.1 km/s |
Atmospheric
absorption:
Absorption for specific lines is listed in the reference materials at http://www.astro.umd.edu/~harris/casimir. Also see summary from the original proposal, or check with Jonas. The JPL line list at http://spec.jpl.nasa.gov/ covers CASIMIR’s frequency range.
Optics:
|
DTel |
2.5 m (underilluminated 2.7 m) |
|
DSec |
0.352 m |
|
hMB |
0.80 (theoretical for 11 dB edge taper) |
|
hA |
0.77 (theoretical for 11 dB edge taper) |
|
hOhmic |
0.95 at 240 K |
|
Chopping secondary |
±4 arcmin max. throw |
|
Elevation range |
20 to 60 degrees unvignetted |
|
f-ratios |
f/19.6 system, f/1.28 primary |
|
DSec/DTel |
0.141 |
Signal-to-noise
ratio calculation:
The general expression for the signal-to-noise ratio for a small line source is:
where:
S/N is the signal-to-noise ratio
f is the observing frequency
c is the speed of light
Tsou is the source temperature
Dv is the equivalent velocity width
f is the areal filling factor
Wsou is the source solid angle
hA is the aperture efficiency including scattering and ohmic losses
tz is the atmospheric optical depth at zenith
AM is the source airmass, 1/cos(z), where z is the zenith angle
Trec is the single-sideband receiver temperature
Texcess is the single-sideband excess temperature from loss, scattering, and spillover
Tatm is the physical temperature of the atmosphere
t is the integration time