How did Johannes Hevelius' long telescope work? Why all the round holes?

Question

The drawing below, found in Wikimedia and at lib.harvard.edu is of a very long tubed aerial telescope. I believe it is taken from his 1673 work Machinae coelestis.

I've always wondered about the purposes of all those (20+) square blocks with round holes, spaced evenly from one end to the other.

I've included two cropped views from Harvard University, Houghton Library, pga_typ_620_73_451_fig_aa (found here) the first of which I've zoomed and enhanced the contrast both to better appreciate the cute bird, and to try to read the annotations; there seems to be a 772 or 722 next to one of the blocks near the top left.

Question: What was the function of all of these blocks? Is it optical, or mechanical, both, neither? When I was young I thought there were lenses in each one, but I'm assuming now that at least the central circular holes are completely open.

I think it is pretty easy to guess either one, but presumably with such detailed annotation there is text to go along with this image somewhere and it is likely that we can know Hevelius's intention. I just don't know where to read the text.

I believe that the text is here but it's 1,000 pages long and in a language that I can't understand.

Answer 1

In general, the less the support structure weighs, the easier it is to move the whole thing around. Home-user telescopes usually have a solid body because it's by far the cheapest and simplest way to get a small rigid structure.

Large observatory 'scopes often have a minimal framework supporting the secondary (the far element in a reflector); here the primary refractor is supported with a similar structure.

Most certainly the support blocks do not contain lenses. For one thing, just imagine trying to align and focus a 12 or 15-element system! For another, producing the lenses for telescopes of that size was incredibly tedious back then (and now, really).

There are a couple great chapters in "The Age Of Wonder" by Richard Holmes about a couple of the early astronomers & scope builders in Great Britain for those who are interested.

Answer 2

Machinae Coelestis chapter XXI discusses the long telescope in detail. Fortunately C. L. Prince's 1882 English translation is online via HathiTrust. The square boards with round holes are baffles to keep the eyepiece field free of unfocused skylight, with structural reinforcement as a secondary benefit.

From pages 41-42 of the translation, apparently referring to Figure Z in the Latin:

On the other side of the upright plank (a a), as may be seen at A and D, i.e., on the side opposite to the supports and braces (c), I placed square boards (d), also at right angles, about a foot square, having a large hole bored through them...not only with the view of giving additional support and strength to the planks, but principally...entirely to exclude all light from the outside, which might fall on the lenses or dazzle the eye of the observer.

From pages 51-52:

You will perhaps exclaim, "...in what way, I ask, is the entire instrument and its intermediate parts to be closed up? Will the instrument you have described perform its duty, being quite open on all sides?" I reply that the instrument is certainly not a perfectly closed tube, yet nevertheless it does the duty of one quite as well as if it were really covered in on every side; for since the square perforated boards are not more than three or four feet apart, they totally exclude light from the lenses and from the eye of the observer, so that when you apply the eye to...the first hole, you will see nothing but the tube in complete obscurity, and at the end a perfectly round hole; for all the perforated [boards] are completely blackened on the side turned towards the observer, so that you catch nothing but the tube with its veil of blackness.

Hevelius further explains that the holes near the observer are narrower than those near the objective, but not so narrow that a slight bend would induce vignetting. He does not mention a lens in any of these holes.

Modern refractors, in addition to a closed tube with a blackened inner surface, may employ multiple baffle rings to hide that surface from the eyepiece. Here is such a baffle assembly before installation in a DIY refractor:

Source: Dick Parker

Answer 3

Machinae coelestis has two volumes. You've linked to volume 2, but this drawing is in volume 1 on page 410.

The annotation isn't '772' but 'M'.

hanc vero M cui lens objectiva inserta est

"the objective lens has been inserted at M"

I don't know if there are lenses in all the other blocks. There's at least 10 pages of description. The text needs extensive editing before Google Translate can make sense of it (most of the 'f' in the text need to be replaced with 's'), and even then half of it is gibberish.

Hevelius' 150-foot telescope is called a Keplerian telescope in various places, suggesting it used 2 or maybe 3 lenses.

Perhaps the blocks allowed Hevelius to easily move the lens around (by inserting it into different blocks)? You can often adjust the length of a refractor telescope (they are built with two or more tubes that can slide inside each other).