3 Preparing for Observation

 3.1 What CCD Data Do You Need?
 3.2 Detector Pre-processing
 3.3 Flat Fields
 3.4 1-D Spectrum Tracing or 2-D Frame De-rotation
 3.5 Wavelength Calibration
 3.6 Flux Calibration

In this section some notes on what to be aware of prior to an observing run are given. In outline: to successfully extract and calibrate spectra a complete set of reference frames should be obtained at the telescope; it’s always easier to take more calibrations at the telescope than to cope with poor data at the terminal.

You might want to refer to the more extensive discussion of CCD data calibration in UGCRI[14], the section entitled, ‘How Many and What Calibration Frames Do You Need?’

3.1 What CCD Data Do You Need?

This is what you need to attempt ‘textbook’ data preparation:

Bias frames
Zero-second exposures taken with no signal light entering the instrument, but with any pre-flash used for the object exposures.
Dark frames
Long exposures taken with the shutter closed. Typically, the exposure time used is similar to that selected for the object frames.
Flat-field frames
Exposures taken with a suitable continuum lamp (usually Tungsten) as light source.
Arc frames
Exposures taken with an arc lamp (usually Thorium-Argon) as light source, to be used for wavelength reference.
Object frames
Exposures taken with a target object or reference object as the ‘light source’.

The arc and object frames will be processed by the data reduction software. The bias, dark, and flat-field frames are used in the preparation of the CCD data.

Whether you actually need all these data frames, and how many of each you need is to a large extent determined by what you hope to achieve. In general, you will need a set of calibration frames for each night of observing.

3.2 Detector Pre-processing

In order to remove detector-related effects a complete set of bias, dark, and flat-field frames should be obtained. It is important to bracket the science data exposures with sets of CCD reference frames. A post-observation review of these will reveal any image shifts. Having bracket frames allows the data to be processed even in the event that some time-dependent variation is found (as long as it is a small, slowly varying effect).

See §4.1 for outline details of CCD data preparation.

In some cases it may be possible to not use any bias frames. Instead, a median value for the bias level is obtained by inspecting the overscan region in some or all of the object/arc frames. You can use fewer bias frames when you have a high signal-to-detector-noise ratio.

For exposure times limited by cosmic-ray event counts, the dark current in most CCD cameras is not a significant factor. The simplest way to decide whether to take dark frames is to take one of exposure time similar to that you are using for object frames, and check the signal level.

3.3 Flat Fields

You may not need to flat field; flats taken with modern CCDs can be uniform in response to only a few percent. Flat fields are only needed if the signal-to-noise ratio you require is high. The precise figures will depend upon several factors, and should be estimated for each observation.

You should try to match the configuration of telescope and instrument in the flat-field and object exposures as closely as possible. If there might be an image shift between flat fields taken before observing and the science data, the best procedure is to take flats bracketing each science exposure in the same way as you would take arcs. Consult the instrument manual for your spectrograph to decide which strategy to use.

See §4.5 for details of the purpose and use of flat-field frames.

3.4 1-D Spectrum Tracing or 2-D Frame De-rotation

If the CCD is not perfectly aligned with the dispersion axis of the spectrometer then the data will be recorded at an angle to the grid of dectector pixels. This can be corrected by tracing the spectrum (for 1-D data) or de-rotating the frame (for 2-D data).

It may be worth having several possible frames for 1-D spectrum tracing—in case some of them are badly contaminated by cosmic rays and so difficult to use.

See §4.4 for more details on 1-D spectrum tracing.

3.5 Wavelength Calibration

As with the CCD characterisation frames, two wavelength-scale reference (arc) frames should be taken bracketing the science exposures if wavelength scales are required (e.g. for radial-velocity measurements). Normally you should find no significant difference between the two extracted arc spectra; however, if you only take one arc exposure and some shift does occur you won’t be able to correct for it. Using both arc spectra, a time-weighted mean wavelength scale can be produced and applied to the science data.

See §4.7 for more details.

3.6 Flux Calibration

To get the best results when flux-calibrated spectra are wanted, select reference stars as close as possible on the sky to your target objects. You want the conditions of the target and reference exposures—both instrument configuration and air mass through which the observations are made—to be as similar as possible.

Refer to §4.8 for more information on flux calibration.