Speaking About Telescopes; I Have a Question

Reader:  Speaking about telescopes, I have a question you might be able to answer. I recently looked at some Internet photos of the Hobby-Eberly Telescope at McDonald Observatory, which is in Texas. The photos of the primary mirror which is a segmented mirror in the shape of a hexagon do not show any opening in the center. It appears to be pretty solid, except for the segmented pieces. Question:  How do they get light to the Cassegrain focus, which should be behind the primary mirror? Is there some trick they use which is not obvious? Is this telescope not a Cassegrain scope? Your “About Me” page says you worked at astronomical observatories. Thanks for any light you can shed on this (no pun intended).

Tony:  You asked the right person this question, I suppose!  As it turns out, I worked at the Hobby-Eberly Telescope (we referred to it as the HET) at McDonald Observatory. While there, I was their on-site optical engineer for that scope. So, I definitely know the correct answer to your question.

The HET is not a Cassegrain telescope. It is a Prime Focus type of telescope. Therefore, there is no secondary mirror at all. There are mirrors up near the Prime Focus, but their purpose is to reimage the pupil of the scope on a corrector surface (also a mirror.) More on this shortly. The primary is segmented as you said, incorporating 91 individual mirror segments, each of which is a hexagon shape, one meter across the flats. So, the resulting primary is enormous, as you might imagine. That’s what makes the HET a wonderfully useful telescope for doing spectroscopy of faint objects, which is its main reason for existence!

The figure of each of the mirror segments is spherical, and they all have the same radius of curvature, so when they are adjusted correctly, they all fit into the same global sphere. Of course, the resulting large spherical mirror will suffer from spherical aberration. This aberration is corrected very nicely by a “corrector mirror” which has an aspherical surface. Its figure is “just right” to cancel the spherical aberration. This is located up close to Prime Focus. The image at Prime Focus is analyzed by a spectrograph, which is how data is acquired by the astronomers, for whatever research they happen to be doing. When I worked at HET, there was a low-resolution spectrograph located in the Tracker Assembly right at Prime Focus, and two other systems, a medium-resolution and a high-resolution spectrograph located in the basement below the scope. Light was fed to these spectrographs by means of fiber optics originating at Prime Focus and terminating at the appropriate spectrograph optics below. I don’t know if that configuration has been changed, now. I believe the HET is being used for a new research project.

Hope this helps you get a better understanding of the Hobby-Eberly Telescope. Thanks for your question.