FORS Filter page
FORS can accommodate
- Broadband filters in the collimated beam: There are3 wheels in thecollimated beams with 7 positions available per wheel. A number ofpositions are usually occupied by grisms and the Wollastonprism. Broadband filters have a free diameter of 138mm and are bayonetmounted.
- Interference filters in the converging beam of thecamera: There are 2wheels in the converging beam with together 8 availablepositions. Interference filters have a size of 115mm and arebayonet 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 |
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Interference Filters
The following table lists all presently available interference filters.Their characteristics are givenwith the FORS SR and HR collimators: central wavelength, peaktransmissionand FWHM, all in nm. Due to their location in the converging beam, thefilter characteristics depend on the collimator used. The filterbandwidthsare wider, the central wavelength is blue-shifted, and the peaktransmission is lower than in a parallel beam. With the SR collimator,theeffect is larger than with the HR collimator. The filters are centeredonimportant 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 havebeen measured within the instruments. The results are available asASCII tables or ps-plots via this link.
Non-standard filter policy
Users may propose to use their own filters with FORS in theirapplicationsfor 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 mountswhich will be sent to the users on request. To assemble the filtersinto the mount is under the full responsibility of the users.
- Filters supplied by users shall fulfill the followingrequirements:
- Broadband filters are used in the collimated beam nearthe pupil. Theirdiameter shall not be smaller than 138mm to avoid vignetting whichwould beequivalent to a reduction of the main mirror diameter.
- Interference filters (115mm) are used in the convergingbeam of the camera. Their spectral resolution can not exceed 100 (SRmode) or 400 (HR mode).More narrow interference filters will be convolved to lower resolutionandlow peak transmission due to the dependance of the transmission fromthe incident beam angle.
- More technical specifications as required to purchase thefilterswill be send to the users on request.
- The filters have to be send to Paranal about 6 weeks inadvance for the final optical tests within the instrument and thesoftware 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 perinch (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 ofattenuation 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 theconverging beam of FORS and the central wavelength will shift bluewardsin respect to the measured filter curves for a normal incident.Accordingly filters from other instruments are typically shifted inwavelength if used with FORS. Filters have to be purchased in mostcases for the FORSes such that the central wavelength is right for theFORS instruments.