FORS Filter page
The FORS instruments can accommodate
- Broadband filters in the collimated beam: There are
3 wheels in the
collimated beams with 7 positions available per wheel. A number of
positions are usually occupied by grisms and (in FORS1) the Wollaston
prism. Broadband filters have a free diameter of 138mm and are bayonet
mounted.
- Interference filters in the converging beam of the
camera: There are 2
wheels in the converging beam with together 8 available
positions. Interference filters have a size of 115mm and are
bayonet mounted.
FORS1 Standard Filters
Presently the broadband filters listed in the following table are
available for FORS1. In addition, the interference filters listed
below can be requested.
| Filter name+ESO number |
eff. wavelength (nm) |
FWHM (nm) |
| u_HIGH+112 |
361 |
50.5 (1) |
| b_HIGH+113 |
440 |
103.5 (2) |
| v_HIGH+114 |
557 |
123.5 |
| g_HIGH+115 |
470 |
160.5 |
| U_BESS+33 (3) |
336 |
36.0 |
| B_BESS+34 (3) |
429 |
88.0 |
| V_BESS+35 |
554 |
111.5 |
| R_BESS+36 |
657 |
150.0 |
| I_BESS+37 |
768 |
138.0 |
| u_GUNN+38 (3) |
359 |
33.5 |
| v_GUNN+39 (3) |
398 |
46.0 |
| g_GUNN+40 (4) |
506 |
79.5 |
| r_GUNN+41 (3) |
653 |
81.5 |
| z_GUNN+42 (3) |
910 |
130.5 |
| GG375+30 (5) |
n/a (edge filter) |
n/a |
| GG435+31 (5) |
n/a (edge filter) |
n/a |
| OG590+72 (5) |
n/a (edge filter) |
n/a |
(1) red leak <5*10^-6
(2) red leak <1*10^-5
(3) Available on request. These filters do not form part of the standard instrument configuration. They can be inserted in the instrument as needed in the same manner as for interference filters.
(4) this filter is located in one of the interference filter wheels,
as it is designed as an interference filter
(5) these are intended as order separation filters for spectroscopy
FORS2 Standard Filters
Presently the broadband filters listed in the following table are
available
for FORS2. In addition, the interference filters listed
below can be requested.
| Filter name+ESO number |
eff. wavelength (nm) |
FWHM (nm) |
| U_SPECIAL+73 |
362 |
29.0 (1) |
| B_BESS+74 |
429 |
88.0 (2) |
| V_BESS+75 |
554 |
111.5 |
| R_SPECIAL+76 |
655 |
165.0 |
| I_BESS+77 |
768 |
138.0 |
| z_GUNN+78 |
910 |
130.5 |
| GG375+80 (3) |
n/a (edge filter) |
n/a |
| GG435+81 (3) |
n/a (edge filter) |
n/a |
| OG590+32 (3) |
n/a (edge filter) |
n/a |
| FILT_465_250+82 (3) |
465 |
250 |
(1) red leak <10^-5
(2) red leak <4*10^-4
(3) these are intended as order separation filters for spectroscopy
Interference Filters for FORS1 and FORS2
The following table lists all presently available interference filters.
They are used for FORS1 and for FORS2. Their characteristics are given
with the FORS SR and HR collimators: central wavelength, peak
transmission
and FWHM, all in nm. Due to their location in the converging beam, the
filter characteristics depend on the collimator used. The filter
bandwidths
are wider, the central wavelength is blue-shifted, and the peak
transmission is lower than in a parallel beam. With the SR collimator,
the
effect is larger than with the HR collimator. The filters are centered
on
important emission lines and on 5 and 10 % longer wavelengths.
| Line |
filter name and ESO nr |
centr. wavelength |
T0 |
W0 |
| Collimator |
|
SR |
HR |
SR |
HR |
SR |
HR |
| |
OII+44 |
371.7 |
372.9 |
0.45 |
0.48 |
7.3 |
6.9 |
| [OII] 372.7 |
OII/4000+45 |
377.6 |
378.8 |
0.37 |
0.40 |
6.5 |
6.1 |
| |
OII/8000+46 |
381.4 |
382.6 |
0.43 |
0.47 |
6.5 |
6.1 |
| |
HeII+47 |
468.4 |
469.1 |
0.79 |
0.82 |
6.6 |
6.4 |
| HeII 468.6 |
HeII/3000+48 |
472.6 |
473.4 |
0.76 |
0.79 |
5.8 |
5.6 |
| |
HeII/6500+49 |
478.1 |
478.9 |
0.78 |
0.81 |
6.8 |
6.6 |
| |
OIII+50 |
500.1 |
500.9 |
0.76 |
0.80 |
5.7 |
5.5 |
| [OIII] 500.7 |
OIII/3000+51 |
504.5 |
505.3 |
0.76 |
0.80 |
5.9 |
5.7 |
| |
OIII/6000+52 |
510.5 |
511.3 |
0.74 |
0.78 |
6.1 |
5.9 |
| |
HeI+53 |
586.6 |
587.6 |
0.79 |
0.84 |
6.0 |
5.7 |
| HeI 587.6 |
HeI/2500+54 |
592.0 |
593.0 |
0.77 |
0.81 |
6.8 |
6.5 |
| |
HeI/5000+55 |
597.5 |
598.5 |
0.85 |
0.89 |
7.4 |
7.2 |
| |
OI+56 |
629.5 |
630.6 |
0.75 |
0.79 |
7.2 |
6.9 |
| [OI] 630.0 |
OI/2500+57 |
635.4 |
636.4 |
0.75 |
0.81 |
5.9 |
5.5 |
| |
OI/4500+58 |
640.4 |
641.4 |
0.77 |
0.83 |
6.3 |
6.0 |
| |
H_Alpha+83 |
656.3 |
657.4 |
0.70 |
0.76 |
6.1 |
5.7 |
| H alpha 656.3 |
H_Alpha/2500+60 |
660.4 |
661.5 |
0.77 |
0.83 |
6.4 |
6.1 |
| (2) |
H_Alpha/4500+61 |
666.5 |
667.6 |
0.72 |
0.77 |
6.5 |
6.1 |
| |
SII+62 |
672.8 |
673.9 |
0.77 |
0.82 |
6.6 |
6.3 |
| [SII] 672.4 |
SII/2000+63 |
677.4 |
678.5 |
0.77 |
0.82 |
6.8 |
6.5 |
| |
SII/4500+64 |
683.2 |
684.3 |
0.72 |
0.78 |
6.4 |
6.0 |
| |
SIII+65 |
952.3 |
953.9 |
0.68 |
0.80 |
5.9 |
5.2 |
| [SIII] 953.2 |
SIII/1500+66 |
957.2 |
958.8 |
0.72 |
0.84 |
6.3 |
5.6 |
| |
SIII/3000+67 |
962.1 |
963.7 |
0.70 |
0.83 |
5.9 |
5.2 |
| "special 485" (1) |
FILT_485_37+68 |
485 |
|
0.89 |
|
37 |
|
| "special 691" (1) |
FILT_691_55+69 |
691 |
|
0.93 |
|
55 |
|
| "special 815" (1) |
FILT_815_13+70 |
815 |
|
0.90 |
|
13 |
|
| "special 834" (1) |
FILT_834_48+71 |
834 |
|
0.90 |
|
48 |
|
| "special z" |
z_SPECIAL+43 |
915 |
|
0.94 |
|
20 |
|
| "special z" |
FILT_917_6+88 |
917 |
|
0.85 |
|
6 |
|
| |
FILT_500_5+85 |
500 |
|
0.81 |
|
5 |
|
| [OIII] 500.7 |
FILT_503_5+86 |
503 |
|
0.83 |
|
5 |
|
| |
FILT_530_25+84 |
530 |
|
0.85 |
|
25 |
|
(1) night sky suppression filter
Measured Measured filters transmission curves
The broad band filter curves were measured on the optical bench.
The transmission curves for many of the FORS interference filters have
been measured within the instruments. The results are available as
ASCII tables or ps-plots via
this
link.
Non-standard filter policy
Users may propose to use their own filters with FORS in their
applications
for observing time. The following conditions apply:
- User supplied filters may only be used in Visitor Mode.
- ESO has now purchased a limited amount of filter mounts
which will be sent to the users on request. To assemble the filters
into the mount is under the full responsibility of the users.
- Filters supplied by users shall fulfill the following
requirements:
- Broadband filters are used in the collimated beam near
the pupil. Their
diameter shall not be smaller than 138mm to avoid vignetting which
would be
equivalent to a reduction of the main mirror diameter.
- Interference filters (115mm) are used in the converging
beam of the camera. Their spectral resolution can not exceed 100 (SR
mode) or 400 (HR mode).
More narrow interference filters will be convolved to lower resolution
and
low peak transmission due to the dependance of the transmission from
the incident beam angle.
- More technical specifications as required to purchase the
filters
will be send to the users on request.
- The filters have to be send to Paranal about 6 weeks in
advance for the final optical tests within the instrument and the
software configuration.
example for interference filter specification:
| Filter type: |
(at least) tri-cavity interference filter |
| CWL: |
486.1nm+-1nm |
| FWHM: |
6.0nm+-1nm |
| Minimum transmission: |
80% |
| Used in: |
normal incidence |
| Incident beam: |
half cone angle of incident beam: 8.5 degrees |
| Size: |
115mm+-0.25mm |
| Max thickness: |
7.0mm+-0.25mm |
| Minmum clear aperture: |
110mm diameter |
| Operating temperature: |
0...+15C (nominal: 12C) |
| Storage temperature: |
-10C...+30C |
| Optical quality: |
image quality, transmitted wave front TWF: lambda/4 per
inch (at central wave length), parallelism <30arcsec,Scratch-Dig:
60-40, minimum ghost, TWF of lambda/2 per 25mm |
| A/R coatings: |
anti-reflection coat all exterior surfaces,
reflectivity <1.5 % per surface |
| Out-of-band blocking: |
1E-4 UV to 1200nm |
| Edge: |
black-painted, no metal ring |
| Documentation: |
spectrophotometric trace of transmission in %T and of
attenuation regions in OD, if possible data files in ascii or similar. |
central wave lengths shifts - contributed filters:
Please note that the interference filters are installed into the
converging beam of FORS and the central wavelength will shift bluewards
in respect to the measured filter curves for a normal incident.
Accordingly filters from other instruments are typically shifted in
wavelength if used with FORS. Filters have to be purchased in most
cases for the FORSes such that the central wavelength is right for the
FORS instruments.
