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Caltech Submillimeter Observatory Office,
111 Nowelo St., Hilo, Hawai'i 96720 Voice: (808) 935-1909 Fax: (808) 961-6273 |
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| SHARC
Submillimeter High Angular Resolution Camera |
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Caltech Submillimeter Observatory Office,
111 Nowelo St., Hilo, Hawai'i 96720 Voice: (808) 935-1909 Fax: (808) 961-6273 |
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| Weather | Tau225 | Chance | Map 2' x 2' | Map 10' by 10' | Point Source |
| Excellent | 0.05 | 28% | 500 mJy hr1/2 | 2.5 Jy hr1/2 | 35 mJy hr1/2 |
| Good | 0.07 | 47% | 1.5 Jy hr1/2 | 8 Jy hr1/2 | 80 mJy hr1/2 |
| Moderate | 0.10 | 66% | 3 Jy hr1/2 | 15 Jy hr1/2 | 150 mJy hr1/2 |
| Poor | 0.15 | 82% | 10 Jy hr1/2 | 50 Jy hr1/2 | 1 Jy hr1/2 |
1 sigma Sensitivity vs Time and Weather
A memo regarding the calculation of observing time written specifically for SHARC is available in PostScript format.
For Mapping
SHARC can make maps up to 10' wide and 30' high with good sensitivity. In good weather (Tau at 225GHz of 0.05), a restored map has a sensitivity of approximately 4 Jy per root-second of on-source time per pixel. Maps are typically heavily oversampled & multicovered, so after binning to ~5" resolution, sensitivities over a 1' square field will be ~1 Jy/pixel in a map which takes ~10 minutes of telescope time. Keep in mind that SHARC is sensitive to surface brightness. When you calculate fluxes, try to be realistic about the flux density. For instance, a 100 Jy IRAS point source of 60'' diameter at 100 microns will have a surface brightness of only 250 mJy at 350 microns!
For Point Sources
You may use the approximate SHARC NEFD plot (below) to estimate observation time needed to reach a certain noise limit. However, note that the NEFD shown in the plot refers to pure on-source time (i.e. time the telescope is literally pointed on source). Because of the need for chopping, nodding, and the associated overheads, the observing time will be ca. 4 times the on-source time, for which reason the NEFD value you should use to estimate observation time is a factor of 2 higher than what is indicated on the plot. That is, in weather of tau225< 0.04 the appropriate value is ~2Jys1/2 instead of the plotted ~1Jys1/2.
SHARC NEFD plot.
Multiply by 2 to estimate observing time
(see note above).
SHARC is optimized to operate at either 350 or 450 microns; the filter transmission curves (below) match the atmospheric windows. Because the peak atmospheric transmission at short wavelengths rarely ever reaches 50%, it is advised to plan observations assuming Tau at 225GHz is less than 0.08, which is the case only 20% of the time. For a very rough estimate of the NEFD for varying Tau, download the following graph (below).
SHARC filter transmission curves
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Also, it's not worth tracking sources below 30 degrees elevation (2 airmasses) - at that point, if Tau=0.08, the transmission is only 2%. The table at left indicates the amount of time for which a source is observable. Although this is just a rough guideline, the 4 Jy/sqrt(Hz) number is quoted at ~20% transmission, so 2% is more like 40 Jy/sqrt(Hz). For reference, Tau at 350 microns is roughly 25 times that at 225 GHz. |
SHARC's beam is approximately a diffraction-limited gaussian with a width of ~9'' at 350 microns and ~12'' at 450 microns. The array itself is comprised of rectangular bolometers, 5''x10'', stacked linearly to form an array 10'' in Azimuth by 2' in Zenith. In mapping mode, the array is raster-scanned in Azimuth & Zenith to build up a map, all the while using the chopping secondary mirror to perform rapid (~4 Hz) sky subtraction.
Since SHARC needs some expertise for efficient use t is advised that you contact a CSO member with familiarity with the instrument who can observe with you during your run. In fact, collaboration is welcomed for this sort of endeavor.Finally, since SHARC requires good pointing & calibration, you should propose to observe at times when planets such as Mars and Saturn are visible. Failure to do this may make your errors in pointing & absolute flux calibration larger than they would otherwise be. There are other calibration sources in the sky besides planets, but the uncertainties associated with them are larger. Being careful about flux calibration requires that you observe a calibration source several times throughout the night, so plan on spending 25% of your observing time looking at calibrators.