Important Filter Information

Filter Information README File

This directory contains information for Mosaic, 4x4-inch, 2x2-inch KPNO filters and filters used at the WIYN Observatory 3.5-m and 0.9-m telescopes. Order separating filters for the various spectrographs, as well as those installed in infrared instruments are included, even though these filters are generally dedicated to a particular instrument. 1x1-inch filters are no longer supported. All measurements have been done with the NOIRLAB (NOAO) Perkin-Elmer Lambda 9 Spectrophotometer with an ~f/13 beam. For further information contact: Dick Joyce (

Borrowing Filters

Filters may be loaned to non-NOIRLab organizations, provided that they are not pre-committed for NOIRLab use during that time. Borrowers will need to fill out a Filter Loan Form and assume responsibility for the value of the filters.


The General 4-inch and 2-inch filter lists tabulate all the available filters by size (sorted by wavelength). These are the preferred lists for browsing. USERS SHOULD BE AWARE THAT ALL THE OLDER (LATE 1970'S TO 1980'S) 2X2-INCH AND 4X4-INCH FILTERS CAN HAVE SIGNIFICANT RED LEAKS. Examine the data closely before using one of these filters or request that the filter be rescanned. Unless noted otherwise, all data is for an f/13 beam.

Digital Data & Plots

The various data/plot directories contain digital transmission data and plots.

  • Mosaic Imager--Data for 5.25-inch Mosaic filters
  • KPNO 4-inch Filters--Data and plots for 4-inch filters, including the Liquid CuSO4 U and Hale-Bopp filters
  • KPNO 2-inch Filters--Data for 2-inch filters (some plots)
  • RCSP Order Separation Filters--Data and plots for RCSP order separation filters
  • Echelle Filters--Data for Pre- and post-slit Echelle filters
  • Phoenix Filters--Data and plots for the Phoenix order separating filters
  • NEWFIRM Filters--Link to the NEWFIRM Home Page
  • FLAMINGOS Filters--Data for the FLAMINGOS filters
  • GCAM Filters--Plots of order separators for GoldCam spectrometer
  • WIYN Filters--Data and plots for 4x4-inch WIYN Imager filters
  • pODI Filter--Data and plots of the 428x445 mm ODI filters
  • WIYN Bench Filters--Data and plots for the WIYN Bench Spectrograph filters
  • WHIRC Filters--Data and plots for the WHIRC filters
  • WIYN 0.9-m Filters--Data and plots for the 4x4-inch filters used with the WIYN 0.9-m Imager

2x2-Inch H-Alpha Set

Two sets of 2x2-inch H-alpha filters were purchased in the 1980s. Because of age and deteriorating condition, the better of the two filters from sets A and B were combined into a single set. Even the better filter may show some signs of deterioration. SINCE MANY OF THESE FILTERS ARE SUB-STANDARD, OBSERVERS SHOULD EXAMINE THEM BEFORE USE. Some of these filters also have a clear edge that requires masking to eliminate light leaks.

Each filter of the combined set was rescanned on 7/23/96 and is updated in the 2inchdata directory. These filters are listed as ha*.

Cautions for Narrow Band Filters

Observers using filters with passbands of less than 100 Angstroms should be aware of the following when selecting filters from these lists.

  • Central wavelength (CWL) shifts with focal ratio:

    The CWLs in the lists are measured with the KPNO Lambda-9 spectrometer that works at ~f/13. Use of these filters in a faster beam will shift the effective CWL to shorter wavelengths. When using these filters at other focal ratios, the following shifts are typical:

    4m Mosaic at Prime Focus (f/3.1):  -11 to -17 Angstroms
    WIYN(f/6.3):  -4 to -6 Angstroms
    2.1-m and WIYN 0.9-m (f/7.5):  -3 to -5 Angstroms
  • Central wavelength (CWL) shifts with temperature:

    There also will be a shift to the BLUE if the temperature of the observing environment is COOLER than the temperature in the laboratory of the filter vendor. Similarly, a shift occurs due to the temperature difference between our lab where the filter plot data were obtained and the observing environment. A typical shift is 1 Angstrom to the BLUE for every 10 degrees Fahrenheit of COOLING (5.5 degrees C.). Thus, if we measure the filter in our 74 F lab, when used at 44 F on Kitt Peak, the filter curve will shift approximately 3 Angstroms to the blue. 
  • Bandpass is a function of the focal ratio:

    Narrowband filters in a fast beam will also experience both a broadening of the bandpass and a reduction of the peak transmission. An optical filter in a f/3 beam will broaden to approximately 30 Angstroms FWHM (~0.5%), independent of its intrinsic bandwidth.


G. Jacoby and J. De Veny

Updated by R. Joyce 3/13/2013


Updated on June 30, 2022, 12:42 pm