Astronomical Spectroscopic Techniques

Contents 1.Optics (1): Stops, Pupils, Field Optics and Cameras 2.Basic Electromagnetics –Math –Maxwell's equations –Waves and Wave Equation 3.Optics (2): Diffraction Grating –Fresnell vs Fraunhofer diffraction –Diffraction limit –Diffraction and Interference for single or N slits –Blazed Gratings 4.Electronic Devices in Optical and Infrared Astronomy –Absorption of Light by Semiconductors –Charge Storage, Charge-Coupling, Clocking –CCD Constructions and its Practical Operations –Infrared detectors 5.Application to Spectroscope

1. Stops, Pupils, Field Optics and Cameras

1-1. Stops A stop is something in the optical system that limits the diameter of the beam of light. Aperture Stop: Like an iris in your eye. It physically limits the solid angle of rays passing through the system from an on-axis object point. Exit Stop: Limits the size of the field of view.

Field-of-View The field-of-view (FOV) is the range of angles from which the incident radiation can be collected by the detector. The FOV is determined by a combination of the focal length of the lens ( f ) and the size of the field stop (D F.S. ). The focal length is the distance from the center of the lens to the point where all of the incident light coming from a source at infinity will be focused. If the source is at infinity, the incident rays of radiation will be nearly parallel. The lens will refract them all to the same point, namely the focal point of the lens.

As can be seen in the figure below, the diameter of the field stop, D F.S. affects the FOV. Similarly, the instantaneous field-of-view (IFOV) will be affected by the size of the individual detecting element, d. The IFOV and FOV can be calculated using trigonometry: and

Quiz Calculate the slit width of the spectrograph at the SNU optical observatory in arcsec. For reference, the diameter and the focal length of the telescope are 600 mm and 4,200 mm, and the physical width of the slits are 72  m and 18  m. Instrument developer usually design to sample of the astronomical data with a half size of the seeing disk size. If you have a CCD with the pixel size of 20  m, calculate the best focal length for this detector. For reference, the typical seeing size at SNU is 2 arcsec.

1-2. Pupil Entrance pupil: The image of the aperture stop in object space Exit pupil: The image of the aperture stop in image space All the light transmitted by the optical system must pass through the entrance and exit pupils. Chief Ray: Any ray that passes through the center of the aperture stop. It will also pass through the center of the entrance and exit pupils. Different chief rays will correspond to different object and image points.

1-3. Vignetting As we move off-axis, all the rays from a point in the object plane may not make it through the optical system. For example, due to an undersized mirror, represented by "A", not all the rays from point P make it through the entrance pupil. This phenomena is called vignetting. object planeimage plane entrance pupil exit pupil A P

Simple Camera Simple optical/infrared imaging systems will contain four major elements: 1.Relay lens 2.Lyotstop 3.Filters 4.Detector (explain later separately) Primary lens or mirrorrelay lens or mirror Telescope image planeFilter Lyot stop at pupil Detector

Simple Camera 2 Relay lens: reimages telescope focal plane onto the detector focal plane. Reimaging f chosen to match physical size of the pixels or detectors Lyotstop: a stop on which the secondary (or primary for a refractor) is imaged by the relay lens. For thermal IR systems, this stop is a cold baffle that prevents unwanted thermal radiation Primary lens or mirrorrelay lens or mirror Telescope image planeFilter Lyot stop at pupil Detector

K-band H-band J-band Focal point Toward the telescope Offner Optics Optical Layout dichroic mirrors