Calibration Data before 2012-02-09

4.2. Wavelength Calibration

Arc lamps of He, Ne, and Ar are normally installed; we have several other lamps (H, Xe, Kr and Hg) that can be installed upon request. In the blue channel, only He provides particularly useful lines; the red channel includes lines of all 3 species. The following table includes links to typical lamp spectra for some of the standard grating setups:

Grating	30s wavecal plot	30s flatfield plot	30s wavecal plot
B400	HeNeAr 4400	B rQrtz 4400	HgXeK rH 4400
R300	HeNeA r 7700	BrQrt z 7600
B1200	HeNeAr 4200 HeNeAr 4500 HeNeAr 4800	BrQrtz 4200 BrQrtz 4500 BrQrtz 4800
R1200	HeNeAr 6600 HeNeAr 7300	BrQrtz 6600 BrQrtz 7300

These figures in the above table show the number of counts received

Grating	30s wavecal plot	30s flatfield plot	30s wavecal plot
B400	HeNeAr 4400	B rQrtz 4400	HgXeK rH 4400
R300	HeNeA r 7700	BrQrt z 7600
B1200	HeNeAr 4200 HeNeAr 4500 HeNeAr 4800	BrQrtz 4200 BrQrtz 4500 BrQrtz 4800
R1200	HeNeAr 6600 HeNeAr 7300	BrQrtz 6600 BrQrtz 7300

These figures in the above table show the number of counts received from each of the lamps in a 30 second exposure with the 1.5 arcsec slit. The following table gives recommended exposure times for lamps also with the 1.5 arcsec slit:

lamp	Recommended exposure time
He	60s
Ne	30s
Ar	150s
Hg	120s
H	120s

Based on this, a minimum of 60s exposure is recommended for He and Ar; Ne should not be exposed for longer than 30s if the brightest lines are in the selected spectral range. For other slits, the exposure times should be modified according to the inverse ratio of the slitsize to the 1.5 arcsec slit.

There is some small amount of internal motion within the spectrograph as the instrument is rotated on the image rotator, leading to shifts in the wavelength direction of about one pixel. Accurate wavelength calibration will require accounting for this effect. Users have the option of taking calibration lamp data at the rotator angle of each object, but in many cases, it should be possible to measure the shift using observations of night sky lines.

4.3. Flat Fielding

Flat fielding is generally done using the Bright Quartz lamp. The following table lists links to typical spectra for some of the standard grating setups. The lamp is moderately red, so moderately short exposures are sufficient for the red channel (e.g. 60 and 240 s for R300 and R1200, respectively). In the blue channel, there is a strong wavelength dependence; 240s will provide good flat fields longer than ~4500Å, but significantly longer exposures are needed at shorter wavelengths. (Dec 06: we are currently investigating ways to improve this situation).

The plots and exposure times are for the 1.5 arcsec slit; for other slits, the exposure times should be modified according to the inverse r atio of the slitsize to the 1.5 arcsec slit.