Filter Notes

The formula for mapping the keywords fil1pos and fil2pos to named filter positions is as follows:

fil1pos = the position number as listed in the below table.

fil2pos = the position number listed in the below table + 12*(fil1pos); e.g.

NIRSPEC-1 +    open: fil2pos = 0 + 12*(0) =  0  (NIRSPEC-1-open)
NIRSPEC-1 +    thin: fil2pos = 0 + 12*(1) = 12  (NIRSPEC-1)
NIRSPEC-1 + AO-stop: fil2pos = 0 + 12*(2) = 24  (NIRSPEC-1-AO)

NIRSPEC-7 +    open: fil2pos = 6 + 12*(0) =  6  (NIRSPEC-7-open)
NIRSPEC-7 +    thin: fil2pos = 6 + 12*(1) = 18  (NIRSPEC-7)
NIRSPEC-7 + AO-stop: fil2pos = 6 + 12*(2) = 30  (NIRSPEC-7-AO)

Converting from keywords fil1pos and fil2pos can be tricky. This is because if one had the BLANK filter in (fil1pos=11), then switched to NIRSPEC-1+thin blocker (fil2pos=12), the fil1pos keyword will still read "11" even though the BLANK filter is no longer in the beam. For this reason, it is important to note the keyword filname.

Position	Motor Steps	Filter 1	Filter 2
0	0	OPEN	NIRSPEC-1¹
1	9000	THIN	NIRSPEC-2
2	18000	AO-STOP	NIRSPEC-3
3	27000	L-PRIME	NIRSPEC-4
4	36000	M-PRIME	NIRSPEC-5
5	45000	KL²	NIRSPEC-6
6	54000	HeI	NIRSPEC-7²
7	63000	Pa-Beta	AO-STOP³
8	72000	FeII	CO
9	81000	H2	K
10	90000	M-wide	K-PRIME
11	99000	BLANK¹	OPEN

Footnotes

Primary initialization position
Secondary initialization position
AO-STOP replaced Br-Gamma in September 2009

Other Notes

Use the THIN blocker in combination with Nirspec-1 through Nirspec-6. The THIN blocker can be used with Nirspec-7, but it will cut off the long-wavelength end of the Nirspec-7 passband. Without the THIN blocker, spectra taken in Nirspec-7 will cause persistence on the SPEC detector within a few seconds.
The THIN position in Wheel 1 is a thin piece of clear PK50 glass used to block thermal leaks in NIRSPEC-1 through NIRSPEC-6.
The OPEN position in each wheel is Lyot stop.
The BLANK position in Wheel 1 is a solid piece of metal. This "filter" choice is the only way to block the light path with a cold element, and thus is the only way to take a true dark. Note that this position CANNOT be used with NIRSPEC behind AO, as the software locks out moves of Wheel 1 in AO-mode.