ING Banner
Home > Astronomy > Central Wavelength Shift Calculator

Central Wavelength Shift Calculator

This tool can be used to calculate the shift of the central wavelength passed by a filter, when at least some of the light is incident at an angle to the normal. The shift is calculated for one of the following: converging beam incident on filter; ray of light (or collimated beam) incident at a specific angle to the normal; convering beam incident at an angle to the normal. Ambient temperature can also be considered.

ACAM allows filters mounted in either the focal plane or the pupil plane, which has different implications for the transmitted values of the filter central wavelengths. See narrow-band imaging with ACAM for more information.

Focal station
Field of view (arcmin)
ACAM (WHT Cass.)PFIP (WHT Prime)PAUCam (WHT Prime)WFC (INT Prime)

Reference values
Focal stationPlaneBeamDistance
from center
Focal ratio (F)Angle of
incidence* (°)
INT PrimeFocalConverging03.290.00
INT PrimeFocalConverging173.341.72

WHT Cassegrain
FocalConverging 010.950.00
WHT Cassegrain
FocalConverging 410.950.24
WHT Cassegrain
PupilCollimated 010.950.00
WHT Cassegrain
PupilCollimated 410.957.72

WHT PrimeFocalConverging02.810.00
WHT PrimeFocalConverging202.902.99
WHT PrimeFocalConverging303.054.91

Select , or alternatively provide F:

*: Non-telecentricity leads to the change of the angle of incident light with the distance to the center of the field of view.

Filter Angle of incidence (AoI, incident angle to the normal, maximum 10°): °
(leave blank for a range of wavelength shifts and AoI).

Check the ING filter database and provide:

1. Nominal central wavelength (or wavelength of peak transmission): Å
Provide a value for a collimated beam, i.e. not for a particular F ratio

2. Refractive index:
Effective refractive index of the total filter (glass + coating), generally varying from 1.6 to 2.6 (n=2.1 is an average)

3. Temperature.
Nominal temperature at which central wavelength of the filter is specified:  °
Temperature coefficient at nominal central wavelength:  Å/°C
(typical values are 0.15Å/°C at 4000Å or 0.20Å/°C at 8000Å)
Ambient temperature:  °
(T=9° is a typical average)

See "Calculation of Central Wavelength" for more information.

© 2015 Isaac Newton Group of Telescopes
Developed by Javier Méndez

Top | Back

Contact:  (Astronomy Organisation)
Last modified: 17 March 2016