Abu filters

There are eight filters in the Abu wheel, of which the first two are a blank and a standard J filter intended for engineering tests and alignment. The six science filters are:

FilterCentreWidthRange (all microns)
H2 Q(3) 2.4245 0.0338 2.4076 - 2.4414
"PAH" 3.299 0.074 3.262 - 3.336
L 3.514 0.618 3.205 - 3.823
L' 3.821 0.602 3.520 - 4.122
Brackett Alpha 4.051 0.054 4.024 - 4.078
Narrow M 4.668 0.162 4.586 - 4.749

Summary Curves for the six science filters are given in the summary chart (GIF or JPEG), and are available as individual higher resolution (GIF) plots: H2, PAH, L, L', Brackett alpha, narrow M.

There is one qualification, which is that the front of the vacuum system is closed by a sapphire window which has slightly lower transmission at longer wavelengths than was expected (as measured on the IR spectrophotometer at NOAO). The transmission curve drops noticeably below 100% only after 2.5 microns. The sapphire window plot is also shown with the relevant filter curves in this plot. The only filter noticeably affected is the narrow M.

The individual filter curves were made by digitizing the paper charts obtained with the filters, the first five from Barr Associates and the last (narrow M) from Optical Coating Laboratory, Inc. The actual scans are available as TIFF files: H2, PAH, L, L', Brackett alpha, narrow M. Digitization by DataThief.

All except narrow M were traced from the 77K curves in the original plots: the narrow M only had a room temperature trace, so it has been shifted in the graphs and in the table by -1.6% to allow for the temperature change. There is still some open discussion about this exact value (-1.6)!

The digitized numbers are also available for any use you might have. Note that the x axis is in wave numbers not microns, and the curve for the narrow M is prior to the -1.6% adjustment. H2, PAH, L, L', Brackett alpha, narrow M.

[ Back to South Pole home page ]

Posted: 05Oct98
Nigel Sharp