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LICK OBSERVATORY
MOUNT HAMILTON
CALIFORNIA
Shane 3-meter Reflector
2006 August 9

ABOUT THE IODINE CELL:

Developed by Geoff Marcy and Paul Butler in collaboration with Steve Vogt, the iodine cell is a sealed glass tube filled with iodine gas, positioned in the telescope light path. It is similar in shape and size to a soda can, and is wrapped in blue insulating foam and silver foil (and of course, duct tape!). Heating wires maintain an internal temperature of 122° F.

GEOFF MARCY CONTINUES:

" White starlight enters the iodine cell, but emerges with thousands of specific colors (wavelengths) removed, absorbed by the iodine molecules. These thousands of discreet shades are removed predominantly from the green light, leaving the red and blue to pass through relatively unscathed. Thus the lower part of the beam, emerging from the iodine cell, appears slightly pink in color. The iodine absorption now superimposed on the incoming starlight sets the wavelength scale for the stellar spectrum, like tick marks on a ruler, allowing the Doppler shift to be measured with a precision of one meter per second."

ABOUT THE ILLUSTRATION:
 
This is a photo-illustration composite. An actual photograph of the iodine cell is overlaid with a digital illustration (generated in Photoshop) depicting a conical beam of starlight beam which enters the photograph at the upper right corner on its way from the telescope. The gradually narrowing beam passes through the iodine cell, then comes to a focus on the mirrored aperture plate. Most of the beam—now rendered pink/violet by its passage through the iodine gas—continues through the aperture plate into the spectrograph, but a small fraction is reflected diagonally up to a mirror on the right where it used to guide the telescope.   

IN MORE DETAIL:

The slowly converging white starlight beam enters the cell from above and emerges, somewhat pink in color, as it comes to a focus on the aperture plate. The majority of the light passes through one of a number of small apertures on the highly polished plate and continues its journey (unseen here) to the Hamilton Spectrograph, which is located in a large room behind this wall. The remaining fraction of light that does not enter the aperture is used to guide the giant telescope above. This diminished and subdued pink beam is then reflected at the focal point by the slightly tilted aperture plate mirror. It begins to diverge as it bounces diagonally up toward a square black-backed mirror tangent to the cell lip on the right. Although out of view in this image, the beam then passes horizontally to the right through a filter wheel and is refocused by a telescope guide camera. This camera automatically guides the telescope to keep the starlight centered upon the spectrograph aperture.

When astronomers are observing a star, the "slit room" is completely dark and the starlight beam is invisible. (Like laser pointers, the star beam can only be seen when it is reflected by particulates in the air, such as smoke, fog, or steam.) If the iodine cell, surrounding optics, and starlight beam were visible to the human eye during routine observing, this is how they would appear. This scientifically accurate light path illustration is based on experimental test photographs made on 2006 August 9 and 13 with the telescope pointed at the bright star Vega. It was subsequently determined that generating an actual Vega light beam photograph suitable for publication is not possible due to technical limitations. However, sufficient information was derived from the test photographs and from visual assessment to create an overlay illustration that precisely describes the shape, density, color, and position of the actual light beam traveling through the cell. 

Because this image depicts highly refined optical components and delicate starlight, only white light was used to illuminate the cell interior during the photographic exposure. No false or incorrect light colors have been artificially introduced which might mislead or generate confusion for the lay viewer who may be unfamiliar with the natural colors of focused star light as it enters and emerges from the cell. It is the objective of the photographer to remain as faithful to scientific accuracy as possible and to still produce a visual representation of an otherwise dark and invisible subject.

This photo illustration is available with or without the starlight beam and hand size reference. It has been cropped from a slightly larger field of view, which is also available on request.

The photographer thanks UCO / Lick Observatory astronomers and staff for their invaluable guidance, collaboration, and contributions to the production of this image series.

Special thanks are extended to Professor Debra Fischer on whose telescope program nights the photographic experiments and sessions were performed. The success of this endeavor was in large part a function of Debra's generous and thoughtful input and participation.

COPYRIGHT

All images and text are property of Laurie Hatch Photography; unauthorized use is a violation of copyright law. You are welcome to email me with your useage requests.

LINKS:

California and Carnegie Planet Search ~ CCPS

University of California Observatories ~ UCO

Lick Observatory ~ Mount Hamilton

WebElements.com ~ Iodine

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