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5.3 Observing with the Echelle-CCD System

Before you arrive at Kitt Peak, you should discuss with your staff contact the instrumental requirements of your scientific program, and choose the gratings that will best satisfy the needs of the program. Your instrument assistant will install the appropriate gratings in the spectrograph, and align and focus the dewar for the wavelength region you have requested. The procedures to align and focus the CCD are described in the previous section of this manual.

Once the CCD has been aligned and focused, the required calibration exposures should be made. These calibration data include zero (bias), comparison, flat field, and, possibly, dark frames. An initial set of calibrations can be made just before dark, and additional images secured at convenient times during the night. Be sure to repeat all of the calibrations each night as small changes of dewar position may occur when the dewar is filled with LN2.

Before any data is obtained the observer must decide whether on-chip summation of pixels will be used. If the spectra are several pixels wide, or the full resolution of the instrument is not needed, the charge from adjacent pixels on the CCD may be added together during the read-out operation. Especially for low-light level observations, on-chip summation improves the final signal-to-noise of the data. The counts from several pixels are added, but the read-out noise is equivalent to only one pixel. On-chip summation will increase the dark current per equivalent pixel, but for most applications the dark current will still be less than 5 ADUs per hour.

The TI CCDs exibit some charge transfer defect which evidence as assymetric wings on comparison lines. This may not be a problem for stellar spectra where the lines are measured from a continuum of hundreds of ADUs or more, but for weak spectra or faint emission lines, the charge can become seriously smeared. Performance can be somewhat improved by 'preflashing' the CCD up to a level of ~20 ADUs. This can be done using the LED lights in the dewar shutter and setting the preflash time in the detpars task (§6.3).. When the UV camera is in use, the dewar shutter is removed but several LEDs are mounted between the dewar and UV camera for preflashing. Use of this technique can reduce distortion of line profiles and bring out features otherwise lost due to poor charge transfer.

Comments are given below on the various types of pictures:

A DC offset is added to the overall signal from the CCD to insure no negative data. This 'pedestal' level (a few hundred ADUs), formerly known as 'bias' has no particular significance other than it must be very stable. Obtain 20 - 30 per night. Can be spaced throughout the night or split between beginning and end of night. Zero exposures are important for the TI CCDs where preflash is generally used, but may not be necessary for some of the TEK CCDs where the bias level is very uniform. In this case one could use the overscan or the region on each side of the spectrum to subtract the bias level.
Dark integrations are probably unnecessary for most applications. You may want to obtain a couple 10 or 20 minute darks just to check on dark current and look for any hot pixels. The dark current is sometimes high if the chip has been exposed to too much light, or if power has been interrupted to a cold chip. The spectrum may be moved to avoid any bad regions on the chip.
Obtain a total of at least 10 times the number of ADU's compared to the count rate of your most intense spectrum. For example if your spectrum has an intensity of 5000 ADUs, obtain at least 10 FLATS (preferable more) with 5000 ADUs each. To avoid non-linearities with TI CCDs, no spectrum (OBJECT or FLAT) should exceed 8000 ADUs. On-chip summing will extend these limits somewhat, but caution is advised. Values for other CCDs may be found in table 8. These exposures should be obtained at the beginning of the night so that reductions can proceed as the data is obtained. Additional calibrations can be obtained during the night if the gratings are moved or the dewar refilled. For flat field exposures, the same decker that is used for stellar exposures may be used as long as it is wider than the stellar image.
Obtain thorium argon exposures as needed for your scientific program. The instrument does suffer from some flexure, so if accurate wavelength calibration is required, comparison spectra must be taken while the telescope is pointed at the object. If you aren't interested in a careful wavelength calibration, comparison pictures can be skipped entirely. They are useful however, for monitoring the spectrograph resolution and applying a wavelength scale during subsequent reductions.
This is the command for obtaining spectra of astronomical sources. The same decker used for FLATS may be used with the object kept centered in it with the guider.

Observers are encouraged to use Exabyte or DAT tapes to record raw and reduced data. There is no one-half inch tape drive available at the 4 m anymore. An automatic data archiving system is functioning to record all images obtained. This system is intended for long term storage and not as a readily accessible backup medium. Observers are therefore encouraged to make two copies of their raw data or at least ftp a set to their home computer. Tapes should be labeled ``computer grade'' or ``data grade'' as many problems have been experienced with tapes of lower quality. Tapes may also be purchased on the mountain or downtown.

Subsequent data reduction will be much easier if the logsheets are filled out carefully. Please leave the originals in the notebook; copies can be made in the console room to take home.

next previous contents
Next: Data Reductions
Previous: Alignment and Focusing of CCD
Fri June 9 08:00:00 MST 1995