Cassini’s Optics

I have always enjoyed learning about the history of astronomy, it is a science whose roots can be traced continuously back to the dawn of human history.

One of my Facebook friends is a bit of an old telescope nut, even more so than myself, regularly posting photos of historic observatories and in particular old refactors. I too have a soft spot for these historic instruments, going out of may way to visit Greenwich Observatory in London, to drive up Mt. Hamilton to see the beautiful old refractor at Lick Observatory, or flying across the country to see one of William Herchel’s telescopes on display at the Smithsonian.

Ovidiu Cotcas recently posted a link to a fun research paper analyzing the telescope optics of Cassinni’s telescopes. These instruments were state of the art in the mid-1600’s, a period when the first telescopes were being used to provide the first good look at astronomical objects, revolutionizing our understanding of the universe. Only five decades after Galileo astronomers across Europe were attempting to build ever better instruments to provide views of the planets that had only recently been nothing but moving lights in the heavens. These early telescopes showed that planets were worlds, opening a whole new realm to observation and study.

Paris Observatory XVIII Century
Paris Observatory in the times of Cassini during the late 1600’s showing the very long focal length refracting telescopes of the day. Credit: Wikimedia Commons
Prior to the invention of the achromatic doublet in 1758 the main limitation of refracting telescopes was chromatic error. A single lens is also a prism, focusing the different colors of light at different focal lengths. The only solution to this was to make objective lenses with very long focal lengths. Today’s telescopes use compound lenses of two or three elements in the objective with different types of glass. This combination of lenses can be cleverly arranged to cancel out chromatic error resulting in an achromatic lens.

The long focal lengths of those first singlet lens telescopes appear absurd by modern standards, huge instruments with long tubes suspended from masts or with the objective lenses mounted upon tall towers while the observer and eyepiece were at ground level. Telescopes were thirty or even a hundred feet long. Unlike today’s convention of referring to a telescope’s aperture, telescopes were referred to by focal length. Cassini’s primary instruments had focal lengths of between 17 and 40 feet, with one having the incredible focal length of 150ft! As familiar as I am with using small telescopes I shudder at the challenges of aligning and aiming such an instrument, much less tracking a target across the sky.

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Saturn from Above

A spectacular image of Saturn from above. Even better, the image was assembled by an amateur astronomer, Gordan Ugarkovic, working with Cassini imagery taken on October 10th. Click on the image to zoom in, then zoom in some more! You can see exquisite data in the polar cloud-tops and in the rings. Keep an eye out for the shepherd moons at the edges of the various rings…

Saturn from Above
This portrait looking down on Saturn and its rings was created from images obtained by NASA’s Cassini spacecraft on Oct. 10, 2013. It was made by amateur image processor and Cassini fan Gordan Ugarkovic. This image has not been geometrically corrected for shifts in the spacecraft perspective and still has some camera artifacts.The mosaic was created from 12 image footprints with red, blue and green filters from Cassini’s imaging science subsystem. Ugarkovic used full color sets for 11 of the footprints and red and blue images for one footprint. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic

Saturnine Storm

Saturn Storm
The huge storm churning through the atmosphere in Saturn's northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA's Cassini spacecraft, image credit: NASA/JPL-Caltech/Space Science Institute

Cassini Press Release

This picture, captured on Feb. 25, 2011, was taken about 12 weeks after the storm began, and the clouds by this time had formed a tail that wrapped around the planet. Some of the clouds moved south and got caught up in a current that flows to the east (to the right) relative to the storm head. This tail, which appears as slightly blue clouds south and west (left) of the storm head, can be seen encountering the storm head in this view.

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Titan before Rings

A world shrouded in hydrocarbon smog, where there are rivers and lakes collecting methane rain. Despite numerous flybys of the Cassini spacecraft and landing of Huygens probe on the surface, Titan remains a very mysterious world. A thick atmosphere and exotic chemistries create conditions that might even harbor some form of life.

Taken from this angle, the view looks toward the side of Titan that always faces away from Saturn. Keep in mind that Titan is 5,150 kilometers (3,200 miles across), much smaller than the Earth, but quite a bit larger than our Moon. The image was taken with the Cassini spacecraft narrow-angle camera on Aug. 9, 2011 using a spectral filter sensitive to wavelengths of near-infrared light centered at 938 nanometers. The view was acquired at a distance of approximately 1.4 million kilometers (870,000 miles) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 35 degrees. Image scale is 8 kilometers (5 miles) per pixel.

Titan before Rings
Saturn’s large moon, Titan, with edge-on rings seen behind, image taken 9Aug2011, acquired by the Cassini narrow angle camera and a 938nm infrared filter, credit NASA/JPL-Caltech/Space Science Institute