The Mauna Kea Observatories Near-Infrared Filter Set

Alan T. Tokunaga
Institute for Astronomy
University of Hawaii

01 May 2005


  1. Rationale for the filter set
  2. The Mauna Kea near-infrared filter set
  3. Photometric standards based on the filter set.
  4. The Y filter
  5. 5. The 2004 production run
  6. 6. Manufacturing Specifications

1. Rationale for the filter set

The "Mauna Kea Observatories Near-Infrared Filter Set" is described in detail in the following papers. The filter set was optimized to be insensitive to atmospheric effects for observatories at altitudes higher than 2 km. These filters have been adopted by observatories at Mauna Kea and elsewhere.

2. The Mauna Kea near-infrared filter set

The specified center, cut-on, and cut-off filter wavelengths (micrometers) are given below. The cut-on and cut-off are the wavelengths where the transmission is 50% of the peak.

Broad-band Filter Wavelengths
λ center cut-on cut-off
J 1.250 1.170 1.330
H 1.635 1.490 1.780
K' 2.120 1.950 2.290
K_s 2.150 1.990 2.310
K 2.200 2.030 2.370
L' 3.770 3.420 4.120
M' 4.680 4.570 4.790

The filter profiles for the 1998 production run may be obtained at: Mauna Kea Filters

3. Photometric standards based on the filter set.

Standard star magnitudes and color transformations obtained at the UKIRT with these filters are given at: IZJHKL'M' Standard Star Magnitudes. Color transformations obtained at the IRTF are given at: Characterization of New Mauna Kea IR Filter Set .

4. The Y filter

For the 2004 production run of these filters I have added the Y filter since this filter is important in the identification of brown dwarfs. This filter has been been designated as "Z" in the past. However, I recommend using the label "Y" to avoid confusion with the "Z" filters commonly used with CCDs at <1 micron.

A discussion of the Y filter can be found in L.A. Hillenbrand et al. 2002, PASP, 114, 708. A detailed justification for the filter bandpass and central wavelength adopted here is given by the WFCAM project.

The Y Filter
λ center cut-on cut-off
Y 1.02 0.97 1.07

5. The 2004 production run

We are fabricating the Y,J,H,Ks,L', and M' filters in 2004. We are not producing the K and K' filters for the reason that the Ks filter was used by the 2MASS survey and this filter has good performance at both low and high altitudes. By producing only the Ks filter we can maximize the number of orders and therefore reduce the cost per filter.

6. Manufacturing Specifications

The manufacturing specifications are given below. These specifications can be used for ordering filters. One should also require the production of a witness sample that is scanned at 77 K. The vendor should provide a plot showing the measured change in wavelength with temperature.

  1. Out of band transmission <0.0001 out to 5.6 µm (in other words OD 4 required to 5.6 µm).

  2. All parameters for 65 K; cold filter scans of witness samples to be provided with prediction of wavelength shift with temperature.[a]

  3. >80% average transmission (goal >90%).

  4. Peak transmission of broad band filters to have a ripple of less than ±5% of average transmission between 80% points.

  5. Cut-on and cut-off: ±0.5%.[b]

  6. Roll-off spec: %slope ≤2.5%.

    %slope = [λ(90% trans.) - λ(10% trans.)] / λ(10% trans.) *100.

  7. Substrate flatness.

    For Adaptive Optics:

    Substrate flatness <0.0138*lambda/(n-1), where n is the index of refraction of the substrate (compatible with AO systems). For example, for n=1.5, lambda=2200 nm, the substrate flatness should be <61 nm. For n=3.4, lambda=2200 nm, the substrate flatness should be <13 nm, where lambda = wavelength of the filter.[c]

    For non-Adaptive Optics:

    For ≤65 mm size, 1/2 fringe at 0.656 microns across the filter diameter or diagonal. For >65 mm size, 1 fringe at 0.656 microns across the filter diameter or diagonal. (If tested in reflection, 1 fringe = 1/2 wave.)

  8. Scratch/Dig: 40/20.

  9. Entire coated area shall be free of pinhole defects (best effort).

  10. Substrate surfaces parallel to 5 arcsec or better to suppress ghost images.

  11. Filter to be designed for a tilt of 5 degrees to suppress ghost images.

  12. Size: Up to TBD mm clear aperture. Dimensional tolerance +-0.1mm. Note: may be round or square.

  13. Maximum thickness: 0.1*size.

  14. Must be single piece filters (cemented filters are not acceptable).

  15. No radioactive materials such as thorium to be used (may cause spurious noise spikes in the detector).


  1. We expect the filters to be used at 50-77 K. The filters should be designed for 65K.

  2. The cut-on and cut-off are the wavelengths where the transmission is 50% of the peak. This is also referred to as the "half-power points".

  3. This is a flatness spec, not a roughness spec. The filters will bend after coating, but the front and back surfaces will remain parallel.

  4. Environmental conditions: Operation in vacuum at temperature ≤77K. Laboratory handling and storage at room temperature, pressure, and humidity (15-25 deg. C; 5%-90% humidity). Require long-term storage with no degradation.

  5. Coating durability: Filters should pass the scotch tape test for adhesion and strokes with pencil erasure for abrasion resistance.