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User-supplied Filters

GMOS users may supply their own filters for queue observations. The policy for doing so and the specifcations for the filters needed are given below.


  1. The users should indicate clearly in their PhaseI proposal that they intend to supply their own filter(s).
  2. The users should indicate clearly in their PhaseI proposal whether they already have the filter(s) needed, or whether they will be purchased at some later time.
  3. The users should ship the filters to Gemini ahead of their observations. The filters need to arrive at Gemini a minimum of two weeks before they are first needed for use in the instrument.
  4. The users should supply with the filters measured transmission curves as a function of wavelength.
  5. The users are encouraged to leave the filters in the instrument for the full semester, in order to increase the probability that observations will be obtained for their programs.
  6. Since filters for GMOS are of a size that is unlikely to be useful in other instruments, Gemini also asks that the users supplying their own filters consider letting the filters stay in GMOS for use by the community.


GMOS Filter Specifications

GMOS filters should be designed for use in a parallel beam of light. 

Substrate:  BK7 or similar 
Operating temperature range: -10 to 20 C
Substrate diameter: 160 +0 -0.2 mm
Substrate thickness1: 10.0 +0 mm 
Mechanical thickness1: 12.0 +0 mm (3 and 4.12 degree holder)
Coated aperture: > 150 mm
Flatness2: Better than lambda/4 (p-v) at 633 nm over any 100 mm diameter patch
within the clear diameter
Parallelism: < 30 arcseconds 
Cosmetic quality: 60-40 scratch dig or better (Mil-O-13830), no visible pinholes 
Angle of incidence3:

Original (3.0 degrees)

Updated (4.12 degrees) 

(Drawings can be found here.)


1 A thinner subtrate thickness can be compensated for through the use of rubber o-rings in the holder and/or the incorporation of metal banding on the filters. Gemini unfortunately cannot provide metal banding. An example of this metal banding with thinner subtrate is shown in the photo below. Substrate thickness could be increased up to the maximum mechanical specification with prior Gemini approval. 

2 Gemini filters are made to the following specification: Transmitted wavefront errors which contribute to wavefront deformations greater than the 'power' term (which would generate a defocus in the camera) to be less than lambda/4 (p-v) at 633 nm over any 100 mm diameter patch within the clear diameter. It is recommended that user supplied filters are made to the same specifications. Relaxing this specification impacts the delivered image quality. 

3 There are two angle of incidence tilts used in GMOS, an original specification of 3 degrees and a change in 2009 to 4.12 degrees. Our recommendation is to ulitize the 4.12-degree angle of incidence as this significantly reduces ghosting. 


Example of metal banding with thinner substrate

Example of metal banding with thinner substrate 


Contact information

After approval of their program for queue observations with Gemini, the users should contact their assigned contact scientist regarding how to proceed with supplying their own filters. The contact scientist will put the users in contact with the appropriate Gemini Staff members.