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 NOAO Perkin-Elmer Lambda 9 Spectrophotometer with an ~f/13 beam. For further information contact: Dick Joyce ( or Brent Hansey (

With the installation of the Dark Energy Camera on the Blanco 4-m telescope at Cerro Tololo, the Mosaic2 imager is no longer in use at that site. The Mosaic2 filters have been shipped to Tucson and are now included in the Mosaic filter listing. It is possible to cover the central field of the pODI instrument now in use on the WIYN 3.5-m with a Mosaic filter. Please select one of the CTIO filters, rather than the KPNO filter for use with pODI, if it is available.



Filters may be loaned to non-NOAO organizations, provided that they are not pre-committed for NOAO 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.



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



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*.



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

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


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.


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