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FAPOL commisioning report

NOTE The default orientation of the Calcites has been changed in November 2008. The Calcites are now mounted "vertically" by default. I.e. the ordinary component is the lower one, the extraordinary component the upper one. The old setup ("Horizontal Mount", ordinary to the left, extraordinary to the right) is still available, but only if especially requested by the observers.

Imaging mode

Preliminary tests indicates that the transmissions of the 1/2 wave plate + calcite are about:

57 % in U 
75 % in B 
77 % in V 
79 % in R 
81 % in I 
in comparison to the flux with the filter only. Most of the attenuation is in the calcite plate.

Spectroscopic mode

Internal ALFOSC focus

Both images of the slitlet were found to be sharp with an internal ALFOSC focus set to 850 (22/2-2007). The width of the so-called 1.8" slitlet was measured to be close to 1.5" (8 pixels). With the internal focus at 850 the telescope focus for normal imaging (no calcite, no retarder) was at 23130 (for no filter). The focusing was done using the focus pyramid and the I-band filter and correcting back the I filter offset.

Slit alignment

For spectropolarimetry it is very hard to get a perfect alignment of the slit, since it has to be aligned with respect to the calcite plate by eye. Currently, the slit is pretty well aligned with the calcite, while the calcite is not entirely perfectly aligned with the CCD. The two images of the slit will therefore follow the lines of the CCD, but one image of the slit is about 1 to 1.5 pixels shifted relative to the other. This should be possible to calibrate well with the arc lamp spectra.

This is an example of the He arc lamp taken with Gr#6 and the 1.8" polarimetric slitlet.

Calibration spectra

In order to take arc lamps you have to take the retarder plate out of the beam, and then put in the calibration lamp mirror and turn on the arc lamp.

For spectral flats the best solution is to use the blue halogen lamp mounted on the inside of the baffle cover. Then the flats can be taken in all orientations of the retarder plate - if wanted. We have done this, but data reduction will show whether this is strictly needed.

Dispersion in calcite

Because of the dispersion in the calcite, the ordinary component (left) will have its spectrum well aligned along the columns of the CCD, while the extraordinary component (right) will be slightly misaligned with the separation between the two components the largest in the blue end of the spectrum.