Spectra

Taking spectra is very similar to taking images. The main difference is that a slower frame rate is required to fill the detector wells optimally, and the positioning of the telescope is more critical because of the narrow slit width.

1. Before leaving camera mode, measure the slit position on the detector so that you will know where to place sources. Turn on Cont. Acq., show Buf0, set the slit to 15'', and choose your favorite filter. Record the locations of the left and right edges of the 15'' slit. Switch to the slit you plan to use for the observations, and record the detector column in the center of the slit image. With this technique, you can place faint sources on the desired pixel using camera mode, then know that they will be in the slit. There are several precautions to take, however. GSM drift will change the position of the slit on the detector, so use the slit edges to periodically monitor and correct for this effect. Also, be aware that the position of the slit depends on the filter because of slight wedges in each filter. If you get into the habit of referring desired source positions to one edge of the 15'' slit, then both GSM and filter problems will be avoided.

2. Move the telescope to place the source in the center of the slit. If the slit is bright enough, you can show the Diff buffer and see the source through the slit -- the 4'' slit is especially useful for double-checking the position because it is wide enough so that sources and the surrounding dark sky can be easily seen. The autoguider is essential in accurately placing and keeping sources in the narrow slits. If you're not using it, frequent checking of the pointing, and especially the accuracy of nodding, is a must.

3. In the SCACQ main window, set the Instrument Mode button group to LORES or HIRES (the following procedures are similar for both mode). The filter and slit may both turn, so set them back to their desired positions. The 8-14 um filter is usually used for the spectroscopy modes.

4. In SCDISP, show Buf0, set the units to mV, and activate the photometry mode. Turn on Cont. Acq., and measure the background. In the Integration Menu, select a ROM table and chop frequency to bring the brightest part of the detector to between 10 and 15 volts. In LoRes mode, the proper setting will depend on wavelength and slit size. For example, several bright telluric water lines between 11 and 13 um are much brighter than the average signal below 8 um. Explore the full range of your desired spectral wavelength to be sure the detector will not saturate. The newest SC-10 detector (AsM4-3, installed May 1996), has a switchable capacitance multiplexer. This means that a different unit cell capacitance can be selected for high backgrounds (Camera and LoRes modes) and low backgrounds (HiRes modes). This allows the wells to fill up in HiRes mode without excessively long integration times. The desired mode is selected via the ROM tables: 0 -- 11 for the medium background mode and 12--15 for low background. As of this writing, the effects of these modes on the HiRes sensitivity and the best parameters to use have not been determined under actual observing conditions.

5. Check all the Mode-Dependent parameters in the Integration Menu to be sure they are properly set. The phase should match the Camera-mode setting (usually either 0 or 180 degrees). The Bandwidth should be about 3 times the group rate. The Gain should be kept at 10 unless the signal is less than about 3000 mV at the slowest possible frame rate so that the detector wells remain as full as possible.

6. The rest of the observing is very much like for Camera mode.