come in different types, some in tubes and some
in struts and wrappers. Struts and wrappers come apart and pack in little spaces, and therefore win in transportation. But
tubes win out in viewing because they’re dark inside with thermal insulation.
But the tubes are harder to get around. The tube of our old twenty-four
incher (Delphinium) is thirty inches in diameter and twelve feet long, and it doesn’t pack in little spaces. (But it
sleeps two, end to end, and you can’t roll out of bed.) And although it’s hard to get it around, the tube makes
it easy to set up the telescope. The box goes in the rocker, and the tube goes in the box. Struts take longer to put together
(some assembly required), but they have the big advantage in making it easy to get the telescopes out for public use. And,
after all, that’s what matters.
struts require wrappers, or shrouds, to keep out the turbulence of the air near the ground, and to keep out stray light. And
therein lies a problem. Most shrouded telescopes wipe out to their shrouds because of thermal conductivity. The upper surfaces
of the shrouds cool off by exchanging infrared radiation with the night sky, while the bellies or lower surfaces, stay warm
by exchanging infrared radiation with the ground. Then, if the wrappers or shrouds are not thermally opaque, they transfer
these temperatures to the air column inside and seriously disturb the viewing.
It is a usual thing at star parties, even on good nights when the atmospheric turbulence
is low, to see oversized star images in these shrouded telescopes. And when you throw the eyepiece out of focus, you can see
the eddies of the cold air coming down from the ceiling and the warm air coming up from the floor. This internal turbulence
can be largely controlled by wrapping the shroud in some lightweight, thermally insulating, wrapper like a Space Blanket.
The collar that holds the diagonal, and any light shield beyond it, have the same problem. They should be thermally opaque.
ago in Alberta, Canada, I was asked to check an eighteen incher f/6 in struts and wrapper, and with a lightweight extra collar
beyond the eyepiece to control stray light. The problem was in the telescope, not in the night sky. One third of the problem
was in the extra collar, and another third was in the shroud. Only one third was in the eyepiece collar and the rest.
the sixties the telescopes one saw at the star
parties were small, like sheep grazing on the knolls. But now we see eighteen and twenty-inchers running over the hills like
cattle. And most of these larger telescopes are in struts and wrappers. Now these larger telescopes are much more beset by
turbulence problems than are the smaller ones. Because if you consider the large aperture as made up of many smaller apertures,
you can see two things. First, the effect of the turbulence on the smaller apertures is twofold. It blurs them a little, and
it makes them dance. And second, in the image from the full aperture, they dance out of step. It is this dancing out of step
that blurs the image in these larger telescopes. And that is why it is so important that the shrouds and the collars on the
larger telescopes with struts and wrappers should be thermally opaque.
And that is also why, when the atmospheric turbulence is obstructive, we use aperture masks on
our larger telescopes to give us unobstructed reflectors with smaller apertures and longer focal ratios.