Calibration Lamps

Updated on June 15, 2021, 9:22 am

Hydra CTIO Calibration Lamps

In addition to other types of calibration data (dome flats, sky flats, milky flats), Hydra uses a system of calibration lamps located in the chimney of the Blanco telescope. This system currently provides the following lamps.

1. Quartz (bright)
2. "Penray" (very bright)
3. Etalon (fairly bright)
4. HeNeAr (dim)
5. Th-Ar (dim)

NOTE: All these lamps (especially He) have finite lifetimes, so please don't leave them turned on unnecessarily for long periods.


 

Quartz:

The Quartz lamp has no spectral features and can be used for producing flat fields. Unfortunately, the uniformity of the illumination changes with telescope position, making the flats not as useful as dome flats for this purpose, and useless for the fiber-to-fiber throughput measurement needed for good sky subtraction. Where the quartz flats (also known as pflats, for projector flats) are especially useful is providing a trace of the fibers in the field for any given configuration. They are bright and can generally be taken in under a minute of exposure.

"Penray":

This refers to a set of 4 lamps, three of which are Oriel pencil lamps of Ar, Ne and Xe. (Oriel P/Ns 6030, 6032 and 6033). The fourth is a high intensity He lamp, Oriel type #65020. These lamps are very bright and in most cases can give a good calibration exposure in a few seconds. On request, any combination of these lamps can be enabled by Observer Support via switches in the Cass. cage. Individual lamps cannot be selected or deselected from the Hydra console. Thus, if you ask to have only He and Ar turned on, when you select the Penray lamps, it will be an He-Ar source. The relative intensities of the He, Ne and Ar lines which are seen with the Penray lamps are quite diferent from those produced by the HeNeAr lamp because the gasses are at different temperatures, pressures and states of ionization.

Etalon:

This is a collimated quartz lamp illuminating a fused silica Fabry-Perot etalon 0.5mm thick. It produces regularly spaced fringes of high visibility. 3000th order occurs at approximately 4900Å. The fringes are separated by roughly 1.6Å at this wavelength These fringes are quite strong over the spectral range from 5000-8000Å and are useful to some extent above and below these wavelengths. The fringe width is around 1/25 of the separation.

The etalon is potentially very useful for echelle spectroscopy. High resolution reference spectra can be taken rapidly with it and can be calibrated with the penray or Th-Ar lamps. Any drift in the wavelength of the etalon fringes can be quickly detected with a penray exposure. Drift is primarily caused by thermal expansion of the etalon substrate and should be less than 1 ppm/degree C. It is thus unlikely that the wavelengths of the fringes drift by more than 1/20 fringe over the temperature ranges seen on Cerro Tololo.

HeNeAr:

This is a standard HeNeAr lamp well known to astronomers familiar with the R/C spectrograph. We have a good atlas of the spectral lines produced by this lamp. It is quite dim because of the need to illuminate the entire Hydra field, so it is almost never used.

Th-Ar:

A standard Th-Ar lamp, identical to the one used in the echelle spectrograph. As in the case of the HeNeAr lamp, the Th-Ar lamp is dim but well understood. An accurate atlas of the lines will be found in the 4M console room. We recommend that observers take long Th-Ar exposures in the afternoon, but rely on the etalon as much as possible for nighttime wavelength calibration.


 

Click here for an opto-mechanical layout of the calibration lamp system. Light from four of the five calibration lamps is generated in a box located in the Cass. cage of the telescope. A mirror inside this box directs the light from the selected lamp through a liquid-filled light pipe into the chimney where it is projected onto a holographic diffusing screen located on an arm which swings into the beam when the calibration lamps are in use. The penray lamps are located in the chimney and swing automatically into the optical path when used. This location insures that they will be very bright.

A second, larger, spherical mirror swings into the beam at the same time. Light from the diffuser screen is then projected by this spherical mirror onto the Hydra focal plane.

Lamps are selected by buttons on the Hydra GUI. When a lamp button is pressed, the mirrors will move into position and the selected lamp(s) will turn on. When no lamp is selected, the mirrors automatically swing out of the beam. The lamps can also be controlled directly from the TCS operator's console.

Click here for a catalog of available reference spectra and line lists.