Glossary

Acronyms:

1. IMACS - Inamori-Magellan Areal Camera and Spectrograph; wide-field imager and multi-object

    spectrograph mounted on the Magellan Baade Telescope located at the Las Campanas Observatory

    in Chile.

2. CCD - charge-coupled device; circuit consisting of capacitors that respond to light, which result

    in a distribution of electrons that is interpreted as an image.

3. XCS - XMM Cluster Survey; X-ray galaxy cluster survey using data from the ESA's XMM Newton satellite.

Terminology:

1. grating - used to diffract light and produce spectra; the number of lines per unit area determines  

    the properties of the spectra such as wavelength range; see table for specifics on grating used

    with IMACS; used 200 grism; 10 Angstrom resolution.

2. object spectrum - spectrum created by diffracting light from a celestial body; this spectrum is

    indicative of certain properties of the object such as composition and velocity (red shift).

3. calibration spectrum - spectrum created by using light emitted from a lamp containing a noble

    gas, such as Ne, He, or Ar; used to calibrate spectrograph; want an average of 10,000 counts on the 

    CCD; the Ne lamp is brighter than the He and Ar lamps; use one Ne and two He and Ar; take a series of 

    ~5 second exposures for the bright lines and ~20 second exposures for the dim lines (bright lines will 

    saturate).

4. bias frame - zerosecond dark exposure (see dark exposure); used to determine the pattern of 

    low-voltage inherent on the CCD chip.

5. mask - sheet used for restricting the amount of light that enters a spectrograph from a telescope;

    individual slits are placed in the pathways of photons from targeted celestial bodies to allow this

    light to pass through; see here for instructions; the primary goal for this project is to get redshifts of 

    galaxies in XCS clusters; ideally, we want 25-50 galaxies per cluster; in reality, each mask gets us 

    between 10 and 20 galaxies (crowding); the secondary goal is to fill the CCD with redshifts for galaxies 

    with similar colors to those in the cluster; the third goal is to get redshifts for X-ray point sources.

6. slit - rectangular opening on the mask through which photons from a targeted celestial object

    pass; determines the spectral resolution; a larger slit will allow more wavelengths to pass

    through; a smaller slit will allow for greater resolution but results in light reduction; also affects

    the signal-to-noise ratio; width of 1 arcsecond, length between 10 and 12 arcseconds.

7. spectral map - computer file containing information used to transform data from CCD coordinates

    to representations of wavelength vs. slit position; use calibration spectrum (see number 3).

8. sky subtraction - identifying ambient light in an image of a celestial body and removing it so that

    only light from the targeted celestial body remains; we have long slits to collect sky around each galaxy 

    and have defined between 6 and 7 sky slits per mask.

9. bad pixel map - computer file with the locations of known bad pixels on the CCD chip.

10. order - one of the resulting spectra that occurs with the diffraction of light; as the magnitude

      of the order increases (as the order moves away from the center order), the spectrum becomes

      more elongated.

11. Nod & Shuffle - process involving the frequent changing of the telescope between a celestial

      object and the night sky; allows for more accurate sky subtraction.

12. dark exposure (dark) - CCD images taken with the shutter closed; used to determine the

      amount of "background noise" caused by electrons on the CCD related to the temperature of the

      chip.

13. photo-z (photometric/color redshift) - alternate to measuring redshift via spectroscopy; utilizes 

      broadband photometry; pros include shorter exposure times and imaging a larger area of the sky.

14. broadband photometry - somewhat similar to spectroscopy; uses a greater range of wavelengths than 

      spectroscopy, which allows for more photons to reach the CCD chip and fainter objects to be detected.

15. sky lumpiness - the random pattern of ambient light in the sky due to the uneven distribution of 

      matter throughout the universe; an area in which there is more matter will be brighter than an area in 

      which there is less matter; limits the precision and accuracy of sky subtraction.

16. flux - rate at which photons from a celestial object reach (or pass through) a unit area; unit is 

      erg/s/cm^2.