Testing the MOSFIRE wavelength solution: comparison of arc lamp wavelength solution to sky lines

Tuan Do

In order to determine the accuracy of the MOSFIRE wavelength solution, I compared the solution determined using Ne and Ar lamps with that of the expected positions of OH lines.  This experiment tests a specific night and configuration, so is not meant to be comprehensive.

The dataset comes from observations of the Galactic center on 2012-05-29 and were reduced with an unofficial development version of the MOSFIRE pipeline that was modified to use the arcs to compute the wavelength solution.

One of the reduced, rectified slits without sky subtraction. The rectification is based on using the Ne and Ar lamp solution.

The sky spectrum was extracted from a position on the slit and I compared the location of sky lines with that of their vacuum wavelengths to determine the possible systematic offsets.

The sky lines results were:

Night sky spectrum. The labels on each peak is the offset in Angstroms from the reference wavelength of that sky line.

The fit for each individual line and their difference:

fit line: 20007.762697, closest line: 20008.023500, difference: 0.260803
fit line: 20275.868131, closest line: 20275.940900, difference: 0.072769
fit line: 20412.682539, closest line: 20412.719200, difference: 0.036661
fit line: 20563.573483, closest line: 20563.607200, difference: 0.033717
fit line: 20728.986568, closest line: 20729.032000, difference: 0.045432
fit line: 21507.039490, closest line: 21507.187500, difference: 0.148010
fit line: 21802.241253, closest line: 21802.275700, difference: 0.034447
fit line: 21955.506557, closest line: 21955.685700, difference: 0.179143
fit line: 22125.429306, closest line: 22125.448400, difference: 0.019094
fit line: 22312.470517, closest line: 22312.820400, difference: 0.349883
overall average difference: 0.117996, stdev: 0.107980
difference in velocity: 1.637010 km/s, stdev: 1.498054 km/s

The conclusion is that for this set of configuration and night, the wavelength solution between the arc lamps and the sky is better than about 2 km/s.

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