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Optical and near-IR filters at the NOT

The optical filter set consists of 60mm diameter round filters, 51mm square filters, and some 75mm square, 95mm round, and 100mm round filters. Most of the 51mm square filters belong to the Instituto de Astrofísica de Andalusía (IAA) and are at the NOT as part of the ALFOSC contract. There is one 13mm diameter NaD narrow band filter for use with SOFIN as ordersorter allowing long slit echelle spectroscopy to be done. The optical set is interchangable between instruments. The near-IR set can only be used inside NOTCam.

There is also the possibility to use filters available at the ING, some of which are complementary to our filters (e.g., NOT has a collection of 51-mm square redshifted Hbeta and OIII filters, and ING has a collection of 51-mm square redshifted Halpha filters). If you are interested in using any filter from the ING, visit the ING filters web page for contact information. You should also contact the NOT staff before your observing run to see if the filter can be fitted and/or needs a special adapter.

The table below shows the various instruments in which the optical filters can be mounted; note that in some cases there is vignetting as in the table. In ALFOSC, filters are used in a parallel beam. Due to the large ratio of the focal lengths of the telescope and ALFOSC's collimator, the angles at which the light passes through the filters changes considerably from one side of the field to the other, causing wavelength shifts. For narrowband filters these wavelength shifts can be more than the FWHM and such filters should be used in the FASU (Filter And Shutter Unit), which is always mounted on the telescope in front of ALFOSC.

Filter sizes for the various optical instruments at the NOT. For each size the number of available filter holders are given.
Instrument51mm s60mm r 75mm s90mm r 100mm rFWHM <= 15nmFOV / Vignetting
ALFOSC 3 8 no no no no FOV=6.5' square; 51mm filter needs aligning
yes FOV=4.9' square; needs aligning by 15 degrees
yes FOV=5.8' diameter
2 6 7 yes FOV=6.5' square
7 yes FOV=7.7' square
yes FOV=5.5' diameter
StanCam 9 10 no no no yes FOV=3.0'

In the optical filter lists, the columns give respectively the NOT identification number, the size in mm (diameter or side) and r for round, s for square filters, the name, the central wavelength (centre of gravity of the transmission curve), the FWHM, the maximum transmission, and the marking on the edge of the filter. The filter lists also contain some abbreviations: rlb means red leak blocked; rs means the filter is red shifted from a well known emission line. Some transmission curves are measured using ALFOSC, but nearly all filters have had their transmission measured on a Cary measuring machine belonging to the ING, La Palma. The wavelength range of the Cary is 190 to 900nm, so red leaks beyond 900nm are not detected. For the special HiRAC filters we do have the manufacturers transmission curves, showing that there are no transmission features between 900 and 1200nm stronger than 0.01%. Anything beyond 1200nm is not seen by the CCD.

The 5 HiRAC special UBVRi filters have one side indicated with an up arrow marked on the edge of each filter. This side has been polished to lambda/12 instead of the more usual lambda/4, and is coated with a special multi--layer which is reflective over the whole visible range except in the pass band of the filter. This has been done to provide light for the adaptive optics of HiRAC, and these filters should only be used in HiRAC. Due to their high cost and sensitive coating, extra care should be taken in handling them.

There are two Beam-shaping diffusers for ALFOSC to do high-precision photometry providing about 5 or 9 arcsec FWHM images.

There are a set of plastic rings to be used together with the 95mm round filterholders for FASU in order to mount ING TAURUS filters, which are 76mm round and about 14 mm thick. At most 4 such filters can be mounted at the same time. The set of rings is found in the drawer "FASU rings". Mount such that the filter protrudes in the upward direction when viewing the filterholder the way it is mounted in FASU, and mount them only in FASU B, which is the uppermost wheel.

NOT filters should never be handled by visiting astronomers but only by NOT staff.

  • Staff instructions for cleaning filters.
  • Staff instructions for installing filters.
  • Back to top Last modified: April 19 2023