IRCAL Filters

Basic specifications and characteristics for the available filters are listed below. Click on a filter name to get a transmission curve.  Not all filters are available in IRCAL at all times; be sure to specify ahead of time what filters you need for your run. See this page in the IRCAL manual for the current filter wheel population.

Name	Nominal Central Wavelength (µm)	Measured Central Wavelength (µm)	FWHM (µm)	Cut-on Wavelength (µm)	Cut-Off Wavelength (µm)	Peak Transmission (percent)	Notes	Source
J	1.238	1.238	0.267	1.104	1.371	82.6		IR Labs/Barr
H	1.656	1.657	0.297	1.508	1.806	85.2		IR Labs/Barr
K	2.196	2.200	0.414	1.993	2.406	75.3		IR Labs/Barr
KS	2.150		0.320			?	1,4	? Barr or OCLI
CH4-1.2	1.183	1.188	0.040	1.167	1.208	59.8	2	CTI
H cont	1.570	1.577	0.023	1.566	1.590	75.0	3,5	?
[Fe II]	1.644		0.016			?	1	?
H2-2.122	2.125	2.125	0.021	2.115	2.136	82.2		IR Labs/Barr
BrG-2.16	2.167	2.168	0.021	2.158	2.179	78.2		IR Labs/Barr
2.2/0.04	2.192	2.189	0.046	2.166	2.213	79.1	5	?
K cont	2.270	2.290	0.031	2.274	2.305	?	5	?
CH4-2.4	2.356	2.309	0.140	2.239	2.379	81.7		CTI

Some other curves you might find useful:  PICNIC detector QE, Lick Atmosphere

Notes:
(1) Specified, not measured.
(2) This filter needs to be crossed with the J filter for blocking; the numbers listed here are for the COMBINATION of J*CH4-1.2.
(3) Recently measured by Marshall Perrin (with assistance from Edmundo Angeles of Berkeley Chemistry). Nominal values were 1.570,0.020, so the filter appears to be shifted almost a third of its FWHM to the red from its nominal center. (But see the following paragraph, too!)
(4) The provided filter trace is for a stock Barr Ks filter, which may or may not correspond to the filter actually in IRCAL. No one today seems to remember where the IRCAL filter was purchased from.
(5) Recently measured at UC Santa Cruz by Scott Severson and Bill Brown. These were measured WARM; we expect the central wavelength to shift between room temp and 77K operating temp, but haven't yet quantified this. The wavelengths listed here are for the warm measurements!

  Most of the transmission curves are re-digitized from printed plots supplied by the vendors. There are thus small errors introduced throughout the data set; if you care about accuracy at a level below a percent or so, beware! The exceptions are the three filters scanned at UCSC and marked with '5' in the Notes column above, which were taken directly from the digital trace output.

Which filter wheel is which?

The filter wheel called "Filter Wheel 2" is actually the first one that photons encounter, closer to the telescope. "Filter Wheel 1" is closer to the detector.