Install other filters

 To complete the setup of the observing system, observation filters may now be installed. Narrow-band ``blocking'' filters are used to limit the wavelength span of light passed into the Fabry-Perot. This keeps strong emission lines from adjacent orders from being imaged along with the emission line of interest.

The choice of filters is clearly dependent upon the scientific project being undertaken. Prior to your run, you should consult the Palomar web site for a list of the available filters, their transmission curves, etc. For Galactic and extragalactic work at H$\alpha$ and [NII] wavelengths, a set of 20Å-wide filters has been purchased for the Fabry-Perot; you will find them in a wooden box in the 60-inch observing floor equipment closet. (If you are observing at the 200-inch telescope, this box will probably have been brought over from the 60-inch dome.) Another notable set of narrow-band filters can be found in a gray briefcase, usually kept in the first-floor equipment room at the 60-inch telescope or at Caltech. This filter set was originally purchased for emission-line studies of Galactic nebulae and contains narrow-band filters at a number of important emission lines, along with corresponding continuum filters. Consult one of the contacts listed in Appendix D for the current location of this filter set.

• 60-inch
  At the 60-inch telescope, there are 12 3-inch square slots in the standard filter wheel. Accounting for the Neon filter and a blank slot for testing purposes, this leaves 10 slots for use by observing filters. Adaptors to mount 2-inch square and round filters in the 3-inch square cells are available in the 60-inch observing floor equipment closet, but note that significant vignetting will then occur with the Fabry-Perot.

• 200-inch
  At the 200-inch telescope, the filters are installed in the F/9 guider base. As described above, a pair of special 3-inch square filter holders have been designed for this filter wheel. The result is that you can currently install only a single observation filter in addition to the calibration filter. You should plan your observations accordingly. Although changing filters in the middle of the night is feasible, keep in mind that flats should be taken in each configuration.

Install the additional filters in the same manner as the calibration filter (see Section 3.7 above). First clean any dust off the filters, using dry nitrogen or a lens brush. Handle the filters with care, to avoid damaging their coatings, since they cost about $1000 each! Also, the filter wheel is located near the focal plane in the optical path, so any defects on the filters will be clearly visible in the data.

If you need to use a filter at the 60-inch telescope that is not mounted in a holder, you will find a number of unused 3-inch square holders in the 60-inch observing floor equipment closet. When installing a filter in a holder, be sure that the filter is held tightly and doesn't move around in the holder. Please do not remove existing filters from their holders unless absolutely necessary, i.e., to place one in a special holder for use at the 200-inch telescope. Be particularly careful when installing a filter in one of these special holders, as the small white screws contact the filter surface directly, allowing ample opportunity for scratching the filter with a slipping screwdriver.

Install each filter in the filter wheel with its reflective surface oriented upwards, i.e., toward the sky. Be careful not to damage the filter when tightening the holder screws at the 200-inch telescope. Write down the slot number in which each filter is placed; this list will be used to update the list on the control computer.

When the filters are installed, replace the metal access cover at the 60-inch telescope, tightening the screws hand-tight. The cover for the filter wheel access hole at the 200-inch telescope consists of a piece of dark plastic. This should be securely affixed with a large quantity of black tape. Use additional black tape to cover any light leaks.