The primary goal of a tunable filter is to provide a monochromatic field over as large a detector area as possible. With a tunable filter, however, the field of view is not strictly monochromatic. The effect is most acute at high orders of interference.
Wavelengths are longest at the centre and get bluer the further one moves off-axis. For instruments such as TTF, the wavelength as a function of off-axis angle q (see Eq. 1.2). This change in wavelength, compared to the central wavelength when q = 0º,can be written as
where qsky is the angular distance on the sky away from the central axis of the etalon. For OSIRIS, the wavelength changes are:
fcoll
| Wavelengh
Change
dλ/λ
(Θsky = 1 arcmin)
0.08%
dλ/λ
(Θsky = 2 arcmin)
0.34%
dλ/λ
(Θsky = 3 arcmin)
0.76%
dλ/λ
(Θsky = 4 arcmin)
1.36%
Thus, we can see that an FPF is not truly monochromatic across an eight–arcminute–diameter field of view even if the etalon is not tilted. For a filed view with |dl/l| less than a certain tolerance, the angular diameter is proportional to fccll and the solid angle is proportional to f2coll Following
the definition given by Bland-Hawthorn & Jones the monochromatic
field will be the size of the Jacquinot spot, the central region
of the ring pattern. In this region the wavelength changes less
than
Where N is the effective finesse of the etalon. Then, the monochromatic field is a region which subtent a angle j that can be written as
For a particular etalon, the size of the Jacquinot spot depends on order m alone. The above equations shows how the spot covers increasingly larger areas on the detector as the filter is used at lower orders of interference. The absolute wavelength change across the detector remains the same, independently of order. However, its effect relative to the bandpass diminishes as m decreases.
Monochromatic field for the BOTF at wavelength =372.7nm, working in order 11 and a tilt of 0 degrees. Last update August 5, 2005, by Héctor Castañeda
|
the etalon bandwidth which verify:
