A night on Mount Wilson.
I enjoyed reading about Robert Naeye's visit to Mount Wilson's 100-inch reflector (S&T: Sept. 2016, p. 22). He mentions the novel relay lens system to facilitate viewing through this historic telescope. However, he didn't go into detail about its design and function. So I was curious about its optical configuration and how it converts the system from f/16 to f/11.3.
Adelaide, South Australia
Astronomer Laszlo Sturmann replies: As shown at right, the telescope is used in its Nasmyth configuration and the relay optics are on the outside. Think of the Meade objective as a very long-focal-length "eyepiece" that, just like any other eyepiece, forms a real image at the exit pupil of the telescope. Usually, the exit pupil of an amateur telescope is just a few millimeters across. But for the combined 100-inch telescope and Meade objective, the exit pupil is about 90 mm in diameter. If the telescope is focused to infinity, the light passing through the exit pupil is collimated. So we added a second refractor objective, from Explore Scientific, to take in that big, collimated beam and send it to a conventional eyepiece.
I had no idea that the 100-inch telescope is now available for half- and full-night rentals, much as the 60-inch has been. A full night's rental for these telescopes costs $5,000 and $1,700, respectively, but both scopes represent a huge bargain if the fee is split among members of a visiting group. I wish I lived in California!
Somerset, New Jersey
It was surprising to learn that many deep-sky objects fared so poorly through Mount Wilson's famous 100-inch telescope. In fact, Naeye notes that he'd had better views of many of these with smaller-sized instruments under darker skies. This only shows that a large aperture is no remedy for light pollution.
Then there were the scope's relatively poor views of Uranus and Neptune, even though he rated the seeing at 1 to 1 1/2 arcseconds. With its large aperture, the telescope was probably looking through a column of air that contained numerous image-distorting convection cells, each an estimated 10 to 20 cm across. In fact, it's often claimed that, when the seeing is less than ideal, smaller apertures are affected by fewer convection cells (or perhaps only one) and thus can provide a more stable image compared to that from a larger instrument.
Please note: Some tables or figures were omitted from this article.