The PFIP filter wheel has 7 positions and can hold round filters of diameter 125mm. The set of standard broadband filters is shown in the table below. Other filters may be mounted in PFIP using special adapters and the full list of such filters is available in the filter database, along with measured transmission curves.

PFIP Standard Broadband Filters
Filter Name	ING Number	CW/FWHM (A)	Size (mm)	Thickness (mm)	Comments
RGO U 5	7	3711/426	125	4.1	Max. Transmission ~35%
RGO U 9	213	3622/579	125	10.0	Liquid
Harris B 5	29	4292/1062	125	4.0	-
Harris B 6	205	4295/1050	125	4.0	-
Harris V 5	34	5437/957	125	3.8	-
Harris R 5	40	6408/1562	125	4.0	-
Harris I 5	45	8096/-	125	4.0	Transmission Tail To Near IR
RGO Z 4	23	8734/-	125	4.0	Transmission Tail To Near IR

The PFIP filter wheel is located in a converging beam and hence the filter thickness introduces a focus offset. The table below shows the latest measurements of the focus offsets for the standard PFIP filters.

PFIP Standard Broadband Filters
Filter Name	ING Number	Focus offset (mm)	Last measurement
RGO U 5	7	Unknown	-
RGO U 9	213	2.54	2007-10-10
Harris B 5	29	1.21	?
Harris B 6	205	1.27	2007-10-10
Harris V 5	34	1.20	2005-10-24
Harris R 5	40	1.20	2005-10-24
Harris I 5	45	1.25	2008-10-21
RGO Z 4	23	1.20	?

Filter Adaptors

The adaptors available to hold filters smaller than the standard circular 125mm diameter are as follows:

• 6 adaptors to hold TAURUS 76mm circular filters.
  
• 1 adaptor to hold 63.4mm circular filters.
• 5 adaptors to hold TAURUS 50mm circular filters
  
• 6 adaptors to hold 50mm square filters
  

Important notes:

1. Be aware that when using filters smaller than ~125mm diameter they will vignette the field of view. The unvignetted FOV is ~7'x7' for the 50mm square filters approximately.

2. When interference filters are used in a converging beam (as is the case for PFIP, f/2.8), their transmission curve changes with respect to the transmission curve they have in a collimated beam: there is a wavelength blueshift, a broadening of the transmission curve and a depression of the peak transmitance. The following formula can be used to determine the central wavelength shift of a filter when used in a converging beam (valid for maximum incidence angles of 15 deg):

   l' - lo = (lo/2) * ((sqrt(n^2 - (sin(t))^2) / n) - 1),

   where:
   

   l' = Central wavelength of the filter when used in the converging beam.
   lo = Central wavelength of the filter when used in a collimated beam with an incident angle of zero deg.
   n = Effective refractive index of the filter (2.1 is a representative value for an ING interference filter).
   t = Maximum incidence angle of the light on the filter (10.1 deg for PFIP)
   

For more information about filters see Calculation of Central Wavelength.