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 B rQrtz 4400 HgXeK rH 4400
R300  
B1200  
R1200

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 B rQrtz 4400 HgXeK rH 4400
R300  
B1200  
R1200

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).

Grating 60s flat field plot max exp time exp time for S/N~100 exp time for S/N~100 typical recommended
B400
Bright Quartz 4400
300
240s @ 4000Å
90s @5000Å
5 x 120s
R300
Bright Quart z 7600
120
30s @ 6500Å
30s @ 8500Å
5 x 90s
B1200
Bright Qua rtz 4500
1200
3600s @ 4000Å
240s @ 5000Å
5 x 300s
R1200
Bright Quar tz 7300
300
120s @ 6500Å
120s @ 7500Å
5 x 300s
R830  
?
? @ 7500Å
? @ 8500Å
R830  
?
? @ 7500Å
? @ 8500Å

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.