Narrow-Band, Special, and Order-Sorting Filters

Alan T. Tokunaga Institute for Astronomy University of Hawaii

05 July 2004

Narrow-band and special filters.

A production run of a set of narrow-band and special filters was started in 1999. The specified center, cut-on, and cut-off filter wavelengths (µm) are given below.

**Narrow-band, 1.5% FWHM**
name	center	cut-on	cut-off
He I_A	1.083	1.075	1.091
Pa-gamma	1.094	1.086	1.102
J-continuum	1.207	1.198	1.216
Pa-beta	1.282	1.272	1.292
H-continuum	1.570	1.558	1.582
[FeII	1.644	1.632	1.656
He I_B	2.059	2.043	2.073
H2 v=1-0	2.122	2.106	2.138
Br-gamma	2.166	2.150	2.182
He 1_C	2.189	2.173	2.205
K-continuum	2.270	2.253	2.287
H2 v=2-1	2.248	2.231	2.265
hydrocarbon	3.295	3.270	3.320
Br-alpha cont	3.990	3.960	4.020
Br-alpha	4.052	4.022	4.082

**Broad-band**
name	center	cut-on	cut-off
Z	1.033	0.996	1.069
CO (2-0 bh)	2.316	2.292	2.340
H2O ice	3.050	2.974	3.126

**Special**
name	center	cut-on	cut-off
Grism_1	1.300	1.000	1.600
Grism_2	1.950	1.400	2.500
CH4_s	1.570	1.520	1.620
CH4_l	1.690	1.640	1.740
HK Notch	1.89	1.48	2.30

Note: "HK Notch" is a 1.47-2.4 µm filter with notch at 1.8 µm that blocks out the atmospheric water band. This filter is designed for deep imaging. The half-power points are at 1.48, 1.80, 1.98, and 2.30 µm.

The manufacturing specifications are the same as for the Mauna Kea near-infrared filters except for the items given below:

For narrow-band filters: 65% peak transmission (goal >70%). Bandwidth ≤1.5%.
For broad-band filters and special filters: >80% average transmission (goal >90%) .
Half-power points: ±0.2% for narrow-bands; ±0.5% for broad-bands; ±1.0% for special filters.
Roll-off spec: <0.5% slope for narrow-bands; <2.5% for broad-bands; <3.0% for special filters.
Substrate flatness prior to coating is ≤ λ/10 peak-to-valley at 0.63 µm on a best effort basis.

For the 1999 production run a long-wavelength blocker was also specified to suppress long-wavelength leaks. It was anticipated that achieving the out-of band blocking would prove to be very difficult, and this was the case. After delivery of the initial set of filters, some of the filters were found to have leaks near 4 µm.

Order-sorting filters.

Order-sorting filters described here are used in cross-dispersed spectrographs where the first order of the cross-disperser is centered at 6.6 µm. The filters for the spectrograph are placed before or after the slit. In order to avoid a shift in focus when changing filters, the optical thickness of each filter was specified to be fixed.

The order-sorting filters and their wavelength ranges (in µm) are given below:

**Order-Sorting Filters**
Order sorter 1	4.40-6.00
Order sorter 2	2.90-4.25
Order sorter 3	1.92-2.54
Order sorter 4	1.47-1.80
Order sorter 5	1.17-1.37
Order sorter 6	1.03-1.17
Order sorter 7	0.91-1.00

Manufacturing Specifications for order-sorting filters.

Area used: 92% clear aperture (For example, for the 25.4 mm filter the area used will be 23.4 mm.)
Thickness: all filters for a given project to be parfocal (same optical thickness). Therefore if the fused silica filter is 5 mm, then the Si substrate is 2.1 mm etc.
Thickness tolerance: fused silica ±40 µm; Ge, Si ±15 µm; other materials according to index.
The thickness spec and tolerance are required to ensure that the focus does not change when changing the filters. These filters are not located at a pupil image, thus the optical thickness is required to be constant.
Surface curvature <1.5 fringe for fused silica, <0.25 fringe for Ge, <0.35 fringe for Si, etc.
Irregularity peak-to-valley <1/2 wave (0.63 µm) for fused silica, <1/4 wave for Ge, Si, etc.
Filter properties specified at 77K.
Peak transmission: >80% .
50% transmission tolerance: ±1%.
Out-of-band blocking: <0.01% (0.3-5.6 µm).
Roll-off tolerance: [ λ(90%) - λ(10%) ] / λ(50%) <2%.
Free of pinhole defects.

Note: The adopted optical thickness can be adjusted if it helps in manufacture or in meeting specs, provided all filters are not thicker than specified and all filters can be handled with reasonable precautions.