Is It Diagonal, Is It Secondary

leon · #1 28-07-2006, 03:23 PM

Hi All

Firstly don't get me wrong, this is not a gripe of any kind.

Some weeks ago i posted a thread, where i spoke about the secondary mirror and called it a Diagonal, and was corrected, and some didn't know what i was talking about, ( you don't have a diagonal in a telescope) they muttered...

Well we did, (that is us older folk on this site)

When we, us older people built, or purchased telescopes, it was always called a diagonal, and not a secondary as used today buy all you young blokes.
The secondary was actually the main mirror, and the diagonal was the other one at the other end of the tube.

And it's quite interesting to read some of the threads, where some say diagonal, and some say secondary.

This just means that we have heaps of old buggers on this site just like me...

Just thought i would get another interesting thread happening, have a nice day.

Now i'm off to Snake Valley to spend the night with Ken and some of the members of this site, for the Meteor Showers.

Leon

matt · #2 28-07-2006, 03:34 PM

Oh dear...

methinks another olde can o' worms has been opened!!!

leon · #3 28-07-2006, 03:44 PM

Yea, isn't that good Matt..

asimov (John) · #4 28-07-2006, 03:52 PM

I always used to call it the 'diagonal mirror' Now I just call it whatever the term is popular nowadays (secondary mirror).

I've always called the primary mirror, 'the primary mirror'.

acropolite (Phil) · #5 28-07-2006, 03:58 PM

I suspect that over time the term has been hijacked and used for the third mirror (diagonal) assembly used with SCT's.

matt · #6 28-07-2006, 04:00 PM

Who knows Leon... who knows

I'm just casting my mind back 24 hours ... to the "advise" "advice" debate.

I reckon we should set aside one day a month where everyone and anyone can just rant their head off about whatever nitpicking point they want to make without fear of retribution. Just let it fly. Say whatever you want. Whatever comes into your head coz you got nothin' else to worry about or offer.

And then .... get... over.... it!!!!!!!!

Just kidding. I'm gonna go set up the 9.25 now and hopefully grab more semi-OK images tonight of Joops.

You guys have a nice night at SV

asimov (John) · #7 28-07-2006, 04:29 PM

If they used the term 'star diagonal' there would be no confusion.

Lester · #8 28-07-2006, 04:45 PM

Hi Leon,

I have been around for a while in this hobby. In my opinion when talking about diagonal in telescopes= secondary for Newtonian because it is set at 45%. Other types eg. SCT, and cassegrain type telescopes don't have a diagonal (the secondary is not set an an angle) so it can only be call a secondary.

Any way more importantly have a good one at Snake Valley. Wish I was a bit closer to those kind of get togethers.

Starkler (Geoff) · #9 29-07-2006, 09:31 AM

So if the 45 degree mirror in a newtonian is called a diagonal, what would you call that 90 degree mirror adapter that refractor and sct users put their eyepieces in ?

Astroman (Andrew Wall) · #10 29-07-2006, 09:50 AM

Obviously there is some difference between the two and as like alot of the words in the english language they have different meanings. Like with a cassergrain type of telescope the secondary is not a diagonal so therefore it has the meaning secondary (second mirror). With a typical newtonain the secondary is actually diagonal, so we have used the term diagonal for the secondary. I think that because the newtonian design was the first one to use the terms, most newtonian users use the term diagonal, although I use secondary.

Geoff: isnt that 90 degree mirror actually a 45 degree mirror? it's angle is at 45 degrees exactly the same as a secondary in a newtonain. Look at a periscope, same system.

boy if that didn't confuse me nothing will

asimov (John) · #11 29-07-2006, 10:42 AM

Quote:

Originally Posted by Starkler

So if the 45 degree mirror in a newtonian is called a diagonal, what would you call that 90 degree mirror adapter that refractor and sct users put their eyepieces in ?

Your referring to the star diagonal, or at least thats what I've been calling it all these years.

http://observers.org/beginner/glossary.html << Might make things clearer.

Orion · #12 30-07-2006, 08:45 AM

In a folded Newtonian the secondary is not the diagonal the tertiary mirror is. (my bit to add to the confusion)

IanW · #13 05-08-2006, 04:35 PM

Quote:

Originally Posted by leon

Hi All

Some weeks ago i posted a thread, where i spoke about the secondary mirror and called it a Diagonal, and was corrected, and some didn't know what i was talking about, ( you don't have a diagonal in a telescope) they muttered...

Well we did, (that is us older folk on this site)

When we, us older people built, or purchased telescopes, it was always called a diagonal, and not a secondary as used today buy all you young blokes.
The secondary was actually the main mirror, and the diagonal was the other one at the other end of the tube.

Leon

Being from the 'old school' myself i.e: the dinosaurs who use the correct terminology, rather than the fad as popularised by the semi literate magazine writers

I for one still call it a diagonal, though I will use secondary if I'm posting on yank boards. I always refer to the diagonal placed after a focuser as a star diagonal or 'prism' depending on my mood.

OneOfOne (Trevor) · #14 07-08-2006, 08:27 AM

I call the "main mirror" the primary because it is the first mirror the light hits. The next mirror it hits is the "secondary" for the same reason, however, I don't have a problem if someone calls it the "diagonal" (providing they are talking about a Newtonian, or similar design) although I prefer to use secondary. If you have enough "in focus" you could then put in a "diagonal" (or "star diagonal") to bend the light path around, although technically, I guess it should be called the "tertiary" in order to be consistent. In the case of an SCT, I would also call this a "diagonal" as it achieves the same effect as in a Newtonian.

I think using the primary and secondary terms makes it obvious what you are talking about. So when you say "secondary" you mean the second mirror the light hits, regardless of Newtonian, SCT, Mak, or whatever type of design you are talking about. If you were talking about collimating a "reflecting" type of scope, you would say that, for example, the secondary must be in the centre of the primary, which is true, AFAIK, for all types (that have a secondary). If you were aligning the "diagonal" it could be assumed you are talking about a Newtonian type of design.