Things that Annoy me!

All languages evolve - an inevitable consequence of life. But should all words change their meanings? This is a little collection of things that annoy me. Technical terms which mean something quite definite being used in a broad and often incorrect way. Certainly misleading.

Yes, this page is written on purpose with a black background and red text - just to annoy you.

Serrurier Truss

What exactly is a "Serrurier Truss" and why don't most amateurs have one when they claim they do?

All tubes will bend a little when they are moved. Engineers can easily calculate the amount of deformation and know when it becomes serious enough to misalign the telescope optics. When the Palomar 200-inch was being designed, it was shown that existing tube designs weren't adequate for this new monster - the optics would become misaligned and affect its performance. Mark Serrurier was given the task of finding a solution. The truss tube he came up with (and now bears his name) is an elegant example in lateral thinking.

As I've said, all tubes will bend - there's no way around it. What was done was not to try to stop the tube from sagging, but arranged so the top and mirror ends sag by the same amount and so remain parallel, thus ensuring that the optics remain collimated no matter where in the sky they are pointed. The triangular trusses used in the design were not new, they have appeared in structures for the past few thousand years. What is special about the Serrurier truss is the back-to-back triangular trusses which keep the optics in alignment when the tube sags. Amateurs usually make only the top half of a Serrurier Truss and thus don't qualify for the name - it is nothing more than a simple truss tube which they have made.

A picture which shows the layout of my Serrurier truss telescope is here, or go to my telescope page to see more of it.

Prime Focus

When an amateur astronomer tells me he takes "prime focus" photographs I'm usually impressed at the effort to which he's gone. But when I ask him how he converted his telescope to prime focus operation he looks at me as if I'm from Mars. Why?

If you look in a beginner's astronomy book, somwhere near the front they usually define the types of telescopes - Newtonian, Cassegrain, etc. They also define a setup called Prime Focus. It's different to the others. How has this term become perverted into meaning "the focus of every telescope"?

The basic telescope types

The definition of "prime focus" is basically the "first" focus. A refractor without a star diagonal is prime focus; remove the diagonal from a Newtonian and stick your head in the middle of the tube and it's prime focus (tilt the mirror and look out the side and it's at the Herschellian focus). Large, professional telescopes usually have a prime focus camera where the photography is done - the Hale 200-inch, the AAT, etc. But put a diagonal in front of a paraboloidal mirror and it is a Newtonian, and you take images at the Newtonian focus. If you have a Cassegrain then it's the Cassegrain focus where you observe. Add a flat along the train and it becomes the coudé focus.

It's all rather straightforward and obvious, I think, but people see that David Malin takes his pictures at prime focus and so should they. Celestron (and other commercial places) are now advertising that their images were taken at the prime focus of an SCT. It annoys me greatly.


But that's the way the language evolves. I'm already prepared to accept that Cassegrainian has become the slightly easier to say Cassegrain. That seemed to have happened around the 1930's. Most references in ATM book I refer to the Cassegrainian, but by book 3 there are none. Texereau still calls it a Cassegrainian.

Technical words with accurate descriptions I feel should be used correctly or else confusion occurs. You'll also notice that Newtonian, Herschellian, Cassegrain and Gregorian have capital letters because they are named after people. On the other hand, coudé has a small letter because it is a description of a design by nobody in particular (it's French for "elbowed" or "bent"). I could go on... in fact, I probably will.


The cross-section of a Newtonian telescope mirror is parabolic, but the description of the mirror as a whole is a paraboloid. The former is a two-dimensional curve, while the latter is the description of a three-dimensional figure. If you have a parabolic primary mirror then I wouldn't expect it to focus much light from its infinitely thin width. My mirrors are paraboloids (or at least as closely as I can make them).

Decimal Point

Have you ever heard of a "decimal point"? Sure you have. But where is it today?

Throughout the history of typesetting there has been a decimal point separate from the "full stop" (or "period"). But in the early days of computer typsetting there wasn't the necessary character and so the full stop was substituted. Today it has changed and we again have almost the full complement of strange characters available. Of course some stalwarts have kept to using the decimal point, but these are in the minority. I've kept it in my writings because LaTeX (a "scientific" typsetting languauge which you either love or loathe) was able to do it. HTML also allows it as character #183. It looks like this "·". You'll find it throughout my pages where necessary. Can I convince anybody else to use it?


We all know what a Dobsonian telescope is, don't we? Wrong! It's not any telescope on an alt-azimuth mounting, and it's probably not one with a fast mirror. John Dobson made his telescopes long - typically f/6 or slower - in order to obtain the best possible performance, not the typical f/4 light-buckets frequently seen these days. Indeed, it could also be argued that he didn't even invent Dobsonian technology. For example, Texereau advocated a simple alt-azimuth mount back in the 1930s. What Dobson did was merge and evolve several ideas and then take it to the masses.

Dobson's need was for a large, cheap telescope to show as many people as possible the wonders of the heavens. His vow of poverty kept him away from high technology because of its cost and so he sought cheap solutions. Porthole glass - and thin too - for the mirror and plywood, teflon and formica for an alt-azimuth mounting. It seemed a backward step in the evolution of telescopes and when he submitted an article to Sky & Telescope magazine they told him so. But you sometimes need to take a step back before you can go forward.

And the rest is history, as they say, but to be honest the evolved designs we see today made of titanium steel and driven by computer can hardly be reconciled with the Dobson philosophy. Certainly they have their roots with Dobson, but they have lost his plan and so should be called what they are - just alt-azimuth mounted Newtonians.


The biggest annoyance of all - most especially this year (1999) - is when is the end of the millennium and the start of the next.

It's all quite easy to understand, really. The definition of a millennium is simple - 1000 years. The calendar system we use had its first year beginning January 1, year 1. Add 1000 years to that and you come up the end of the first millennium as December 31, 1000 and the end of the second millennium as December 31, 2000. Therefore, the start of the third millennium is January 1, 2001.

Those 9s turning over to 0s may look attractive (and worth celebrating) but it isn't the start of the new millennium. The same arguments have happend each 100 years - in 1900 and before in 1800 the newspapers of the day would be inundated with arguments for and against when was the start of the new century. Invariably, the argument was settled by the editor saying "Enough!" and ceasing to publish any more letters on the subject.

For more information see the excellent USNO Millennium page, which also offers a cute Java millennium countdown clock.

(Of course I'm not too proud to go to BOTH parties!!!)

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Page last updated 2009/09/06
Steven Lee