FORS Filter page
FORS 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 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.
FORS2 Standard Filters
The following filters form the Standard Instrument Configuration. Response curves are given in the User Manual.
| 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 | R_SPECIAL+76 | 655 | 165.0 |
| I_BESS+77 | 768 | 138.0 | ||
| z_GUNN+78 | 910 | 130.5 | ||
| GG435+81 (3) | n/a (edge filter) | n/a | ||
| OG590+32 (3) | n/a (edge filter) | n/a | ||
|
(1) red leak <5*10^-6 (2) red leak <1*10^-5 (3) these are intended as order separation filters for spectroscopy | ||||
Interference Filters
The following table lists all presently available interference filters. 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 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. |