Makee: Keck HIRES Observing Strategies (as November 2000)

  • Binning, Gain, and Dual Amp Readout:    With HIRES, binning 1x2 (column x row or dispersion x spatial) is usually used. MAKEE works with 1x1 or 1x2 HIRES binning. It should also work adequately with 1x4, but the wavelength calibration will fail for 2x1, 2x2, or 2x4 binning (this may change in the future).

    For faint objects, use the high gain setting (fewer electrons per digital number). This will sample the readout noise better, although it limits your dynamic range somewhat. Low gain is sometimes used for very bright objects. However, you should probably stick to the same gain during your run.

    For HIRES, the electrons per digital number (eperdn) is set according to ./makee/eperdnHIRES_higain.dat (eperdn=2.45) or ./makee/eperdnESI_logain.dat (eperdn=4.80). The readout noise is set according to ./makee/ronoiseHIRES.dat (ronoise=6.0 electrons). You can override these values by editting these files, or giving "eperdn=" or "ronoise=" on the MAKEE command line.

  • Flat field (quartz lamp):    Ideally the total quartz lamp counts at any position in your spectral range should be much higher than the count rates in your object spectrum. This may require several exposures to be taken for each setup. Usually you will take exposures of several seconds each in the red with the ng3 lamp filter, or several seconds in the blue with the ug5 filter.

    The alignment of the cross disperser angle (XDANGL) between the flat image and the object image is very important. makee determines the slit length using the flat field orders, so any misalignment can severely decrease the effective length of your slit. This can be especially bad if you have overlapping orders in the blue. A change of 0.001 XDANGL units is about 1 row (unbinned). The makee XDANGL default tolerance is about 0.002 units, although in practice the program can be run with larger differences in XDANGL.

  • ThAr (arc lamp):    You will want to take at least one ThAr lamp exposure for each setup. In the red, beyond 5000 Angstroms, there are enough night sky lines to get an accurate shift from the arclamp wavelength scale to the object exposure wavelengths. So it is reasonable to take the arc lamp in the afternoon before or after observing. However, many people choose to take an arc lamp immediately before or after your object exposure, and this is the recommended strategy.

    It is especially important to take an arclamp exposure next to the object exposure when using the UV cross disperser, since there are no usuable night sky lines to obtain an accurate wavelength scale shift. In later releases of makee (1999 or later), the program can "average" the wavelength scales of two arc lamp exposures taken immediately before and after the object exposure. This is the preferred strategy when using the UV cross disperser.

    A reasonable ThAr exposure time is 1 second with the clear lamp filter. If too many lines are saturating (more than 4 or 5 in any order), you may consider using the ng3 lamp filter. In the red, beyond about 7000 Angstroms, there are many very bright Argon lines which will saturate and bleed down columns. This is normal and the makee programs are designed to ignore these lines.

  • Bias (short dark):    It is good to take at least one short dark exposure (usually 1 or 0 seconds) with the shutter closed and lamps off. This is not critical, since the bias pattern does not usually change significantly over months or even years. If a short dark is not available, makee will use an archived bias pattern. Please note, that recently (late 1999), some subtle changes in the bias pattern may have occurred, so it is a good strategy to take at least one short dark during your observing run.

    Only the column-to-column bias pattern is used (row-to-row variations are ignored), so makee will obtain the bias pattern by median-ing each column. This takes care of any cosmic rays, so it is only necessary to take one good short dark exposure.

  • Trace "star":    You need at least one bright point source exposure in order to define the echelle order pattern on the CCD. If you have a bright object with no large absorption lines, you can use the object itself to define the trace function. Otherwise, you should take a bright star exposure for each setup at some time during the run. This need not be next to the object exposure.

    You can also use a pinhole quartz exposure to define the trace function. This avoids having to take a star exposure and should work just as well as a star. Just use the same setup as your flat field exposures and then put in the "D5" decker. You can also probably go 2 or 3 times longer in exposure time to insure you have enough counts in the blue (at least several hundred at any column position across the entire order.)

  • Standard stars:    Accurate flux calibration is generally very difficult or impossible with echelle spectrographs. In the case of HIRES, there appears to be an elevation dependence on the response function across orders (although this has not been definitively shown). Unless your standard star is taken at the same place at the same time, your star response function probably cannot be accurately transferred to your object. Nevertheless, it is probably a good idea to take standard star exposures-- it may help you estimate the flux level and it and it may be possible possible later to take out this elevation dependence.

    Here is a list of "good" HIRES standard stars: standard star list . A 300 second exposure is usually adequate for these stars. For more on flux calibration, see: flux calibration.

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