CGS4 Exposure Times

Since the installation of the long focal length camera in August 1997, many observers have requested that optimum exposure times be placed in the CGS4 web pages and in the manual. This is not a trivial task, as the optimum exposure time is dependent on many factors such as resolution, wavelength, object brightness and weather conditions. In this article, we provide a guide on how to select appropriate exposure times given the above factors.

NB. Generally, the maximum possible exposure time is the optimum exposure time, as overheads are reduced to a minimum. However, you should discuss this with your support scientist before and during your run.

150 l/mm Grating

Approximate maximum exposure times (sec) for the 150 l/mm grating (J, H, K)

Assume a 1-pixel wide slit and that the light falls on one row of the array. Typically light falls over three rows and these exposure times can be increased by about 30%. Half these times when using the 2-pixel wide slit.

Approximate maximum exposure times (sec) for the 150 l/mm grating (L, M)

This table is for 1-pixel wide slit and the long camera. Half exposures for 2-pixel wide slit.

		FULL ARRAY, NORMAL WELL		SUBARRAY, DEEP WELL*
Wavelength (µm)	Notes;	Sky + tel emission; Max exposure (sec)	0.12s exposure Brightest observable star	Sky + tel emission Max. exposure (sec)	0.016s exposure. Brightest observable star
3.0		20		40
3.31	CH4 Q;	3		6
3.2-3.5	CH4 v-r lines	6	L ~ 1.5	12	L ~ -1.5
3.8		4	L' ~ 1.0	8	L' ~ -2.0
4.1		2		4
4.7		0.2	M ~ 0.5	0.4	M ~ -2.5
5.2		0.12		0.25

* These figures refer to the 256 x 32 subarray. There is also a slightly larger 256 x 48 subarray with a minimum exposure time of 0.023 sec. Therefore the brightest observable magnitude is 0.4m fainter when using this array.

150 l/mm grating and background limited exposures.

This medium-to-high resolution grating enables one to work in between OH lines in many regions in the 1.1 to 2.3 micron spectrum (OH line emission is not a factor beyond 2.3 microns).

In simple terms, in order to be background limited in the non-thermal regime (<2.3 microns), the sky noise must be greater than the array read noise. For multiple non-destructive reads (NDR), the read noise is approximately 23 electrons. A typical value for the continuum background (from the telescope, sky and long wavelength leaks) using the 1-pixel wide slit in J, H and K is 30 counts in 100 seconds, and with a gain of six, corresponds to 180 electrons and a sky noise of approximately 13.5 electrons. Therefore, in most cases, an exposure time of approximately 300 seconds is required for the sky noise between OH lines to equal the read noise. For longer exposures than this the array is background limited and best s/n is achieved. With the two pixel wide slit (which still gives high enough resolution to work between many OH line pairs) the exposure time must be greater than about 200 seconds. Beyond about 2.2 microns, the background increases rapidly as the thermal background from the sky and telescope begin to increase, and the background-limited exposure time drops rapidly.

The drawbacks to using such long exposures are variations in the sky background and OH line intensities, OH line saturation (at H and K), and increasing likelihood of spikes on individual or small groups of detectors. If the critical wavelengths are well clear of the OH lines, then you probably don't have to worry about their approximate 5-10 minute variation timescales or strength for OH variations, but if you are close to one than these can become problems (but see the two paragraphs below for ways to minimise these). Add to this that you will probably need to oversample your spectra, your time on source will become at least 600 seconds before you nod to sky. If you are using the 1-pixel wide slit and 2x2 sampling, it will be 20 minutes before you can nod the telescope. In addition to the dangers of sky variations these long times mean that, although in principle maximum sensitivity is achieved, a lot of time is wasted if something goes wrong.

For extended sources, nodding to sky is required, and the brightness of the source and stability of the sky background on that night will effectively determine the time between nods and hence the exposure time; these may be considerably less than the above ideals.

For spectra obtained while nodding along the slit, subtraction of the negative spectrum from the positive spectrum will remove most of the sky and OH fluctuations because both vary slowly across the rows of the array. When observing faint and compact sources it is always advisable to nod a small number of rows along the slit (e.g., much less than the canonical 30 rows), so that the cancellation of sky and OH residuals is as accurate as possible. Remaining residuals can be removed by polyfitting techniques, using blank sky rows adjacent to the rows of interest, but doing this will increase the noise in the final spectrum.

The frequency of spikes is difficult to judge and their effect difficult to assess, because spikes sometimes are severe, sometimes are only somewhat above noise levels, sometimes effect only one pixel, sometimes effect a few adjacent pixels, and their frequency may vary. Clearly they are more likely to affect observations of an extended source than a pointlike source.  Empirically they do not appear to be a serious problem when observing point sources with exposures of a few hundred seconds.

40 l/mm Grating

Approximate maximum exposure times (sec) for the 40 l/mm grating (J, H, K)

Assume a 1-pixel wide slit and that the light falls on one row of the array. Typically light falls over three rows and these exposure times can be increased by about 30%. Half these times when using the 2-pixel wide slit.

¹ The strongest OH line in the band will be saturated using this exposure time (on a good night).

Approximate maximum exposure times (sec) for the 40 l/mm grating (L, M)

This table is for 1-pixel slit and the long camera. Half exposures for 2-pixel wide slit.

* These figures refer to the 256 x 32 subarray. There is also a slightly larger 256 x 48 subarray with a minimum exposure time of 0.023 sec. Therefore the brightest observable magnitude is 0.4m fainter when using this array.

40 l/mm grating and background limited exposures.

In most cases the 40 l/mm grating is background limited at much shorter exposure times at all wavelengths than the 150 l/mm grating due mainly to its lower resolution, which ensures that an OH line is present in almost every resolution element.

Typical background-limited exposure times are about 30 seconds at H and K (less than 2.3um), giving 2 minutes between nods with 2x2 sampling. In the J band the OH lines are weaker and exposures of ~75 seconds are required to reach the background limit (5 minutes between nods). The same concerns and optimal procedures regarding OH fluctuations as discussed for the 150 l/mm grating apply here, except that spikes are less of a problem because super-long exposures are not needed.

The Echelle

Optimum exposure times for the echelle are difficult to predict due to the small wavelength coverage of this grating, the exact wavelength of a particular observation and the changes in sky transmission with wavelength. The best course of action is to consult with your support scientist or with; Paul Hirst before your CGS4 run.