Spectroscopy: Saturation and Sky Counts

IMPORTANT UPDATE: A new, more efficient, controller was commissioned in May 2007. This means that currently, because of memory limitations on the acquisition machine, the MAXIMUM exposure time possible with UIST is 240 seconds.

On the Quest for Background Limited Performance (without saturating on source)...

It is very difficult to obtain background limited data with the IJ, short-J, long-J, short-H, and long-H grisms. With the 4-pixel (0.48") wide slit the required exposure times are 10 minutes or greater. A target line that happens to coincide with a sky line could be "background limited", though this of course depends on the strength of the OH line.

The table below gives sky counts and backgound limited exposures for the longer wavelength grisms and the 4-pixel wide slit. Note, however, that these change with wavelength. With the HK grism (and the 1-2.5 micron short-/long- grisms) the "longest possible" exposure time is recommended for best performance (for recommendations on exposure times please have a look at the section on preparing observations for UIST spectroscopy).

In the thermal, the "maximum possible exposure times" simply give the time to saturation on sky lines or the thermal background - they are not recommended exposure times . For other slits scale the sky counts and exposure times by the slit width (for smaller slits decrease the sky counts and increase the exposure time).

In the above the time to reach background limited performance is equal to RN*RN/(sky counts * G) . RN is the readnoise (we assume 15 electrons -- expected for a longish exposure time) and G is the gain (to convert counts to electrons); the sky counts used is that given in column 2.

Note regarding the IFU: Per-pixel on the array, the IFU is equivalent to a 2-pixel slit, so the numbers in the last column in the above table should be doubled!

Saturation on Source

Saturation on source is only potentially a problem for very bright targets with the low-resolution gratings (and widest slits); e.g. for the HK grism saturation occurs in the shortest exposure time (1 second) for targets with K < 3.

Below we list the brightest point source that can be observed in a given exposure time. The figures were derived from observations of bright standards in median seeing. Obviously, wider slits and/or better seeing will affect the magnitude limit; a 7-pixel slit will potentially transmit twice the signal from the source (and the sky), so the magnitudes below would be increased by 0.75mags.

In summary, point sources that are fainter than 8th magnitude can be observed with 240sec exposures with all grisms except those in the L and M-bands.