Dumbing down astronomy

[IanW]

After reading Leon's post on the use of the term diagonal or secondary and being from the old school of amateur astronomy and a declared pedantic when it comes to telescope nomenclature I figured it was time to have a bit of a rant about my all time pet peeves, namely the terms prime focus and plossl.

Rant mode:ON :tasdevil:

First off the rank is the term "Prime Focus". This term has been Bast**rdised beyond belief in the past 25 years or so by the amateur community, most notably by US magazines and authors writing to the amateur end of the market. Thankfully the professional astronomy world still uses the correct terminology.

For those of you who don't know, "Prime Focus" is short for Primary Focus and refers to the focus point of the first mirror or lens in an optical system, thus for example in a Newtonian Primary focus occurs at the location of the diagonal (a.k.a secondary mirror, flat). The focal point where your eyepiece is located is properly the Newtonian Focus. In a SCT it's the Schmid-Cassegrainian focus, etc. Really "Prime focus" can only be applied to refractors, tilt mirror reflectors and the visual cage on telescopes like the AAT.

Secondly the plossl eyepiece. For reasons that I'm too cynical to list a very specific optical design that uses 4 lens elements has mysteriously grown extra elements, no longer makes it a Plossl. Nor is it a "super Plossl" or any similar name, it's a whole new design folks and should be named as such.

I wish, how I wish that various magazine editors, authors and columnists would be a bit more accurate and pedantic in their terminology, and spend the time to educate the newer, and not so new members of our fraternity in to using the correct terminology, rather than taking the easy way out and just dumbing down the terms.

Rant mode:OFF ;)

[[1ponders]]

:lol: I can appreciate your "prime focus" rant. ;) Don't know about eyepieces though, I've never found where to put them in the camera :shrug:;)

[sheeny]

Thanks Ian!

I didn't realise I was mis-using the term prime focus. I assumed it was the primary focus of the scope, not the focus of the primary lens.

:thumbsup:

Al.

[Starkler]

This doesnt sound right to me. Light at the the position of the secondary/diagonal isnt at focus. That mirror merely reflects the pre-focused light cone, and doesnt modify the focal length in any way.

[acropolite]

Ian, you have too much time on your hands.... :lol2: We define a motor as a device that propels a car or vehicle, the poms call a car a motor, a rocket motor is nothing like a car motor, motor skills have nothing to do with internal combustion and our muscles don't have pistons. Gay used to be happy.... Where do we stop. The english language evolves, why not just evolve with it....:P

[syzygy]

Well the "prime focus" of a Newtonian would, by your definition, be at some distance beyond the diagonal. Anyway the term "prime focus photography" has been around for a very long time and applied to camera setups at the Newtonian focus.

[jjjnettie]

Ian,
Here is an excerpt from the book Advanced Skywatching co-authored by David H Levy. It basically describes the type of EP's available.
Would you agree with it's description of a Plossl eyepiece?
Or have I missed the point of your complaint completely?:)

"Some manufacturers have exclusive rights to certain designs-such as Edmund Scientifics RKE models, Meades Modified Achromats and TeleVues Nagler and Panoptic series. But the most common designs are sold by nearly every dealer. Kellner, RKE and Modified Achromat eyepieces use three lens elements for good image quality at low cost.
A step up in quality and price are the four-element Orthoscopic and Plossl models.....they represent the best buys in todays marketplace.
In the premium price bracket are eyepieces known by generic names such as Erfles, and brand names such as Wide Fields, Ultra Wide Angles, Panoptics and Naglers. These five- to eight-element models.....
Compared to a standard Plossl these models all show a much wider field, providing a wonderful Picture-Window view of the universe."

[astroron]

Good article jjj.
By the Way No Coffee tonight?;) :thumbsup:

[jjjnettie]

I'll bring some up for you to try at the next new moon Ron.

[ballaratdragons]

and how many times do we hear Eyepieces being called "Lenses"? Too many times, and yet that's what the local Camera shop called them when I went to price Meade EP's :screwy:

There are lenses IN an EP!

[jjjnettie]

Ken, I agree with you about camera stores.
Geez, the last one I went into, they thought I was an idiot buying binoculars for astronomy. You can't use them to look up at night, you wouldn't see anything, here, buy this 60mm refractor instead.

[ballaratdragons]

:rofl:

[IanW]

Phil Phil Phil, While english does indeed evolve, scientific and physics terms have very fixed meanings and are generally absolute in their meaning.

The English slang expression "Motor" for a car is derived from shortening the term "Motorised Carriage", which was a very accurate description of the early motor carriage. A motor however is defined as a device that converts any form of energy in to mechanical energy, hence it is applied to both mechanical devices that produce motion, and also to human muscles.

As for Gay, the word was hijacked quite late on, from memory in the late 50s or 60s and was used due to the colourful nature of the lifestyle lead by many of that sexual persuasion.

jjnettie

There's some innaccuracies in that quote you posted. Firstly an Erfle is not a generic lens design per se, it's a very specific design in it's proper form, being a 5 element design with 2-1-2 element layout. The Erfle is named after Heinrich Valintin Erfle, who worked for Steinheil & Soehne before taking up a post at Carl Zeiss. The true Erfle has an apparent FOV of around 55° with some modern designs running out to 70° FOV.

For what it's worth, the so called "Five element Plossl" is not a plossl at all, as it was designed by Herr Erfle and used the "Erfle" as the basis of it's design, rather than the Plossl which is a much earlier design by George Simon Plossl (d. 1868). So called Super Plossls (Meade etc) are just using the Plossl name for marketing purposes, same with the name Erfle with respect to most so called Erfle designs. Hence the confusion amongst many amateur astronomers.

syzygy You are quite corrrect, the actual point of focus is in front of the diagonal/secondary mirror, as it is 'behind' the tertiary mirror in a Coudé focus system. (When viewed from the front of the telescope tube)

Using "prime focus" to define Newtonian, Coudé, Cassegranina focus etc did NOT start to happen until the mid to late 70s and was a specifically American twisting of true terminology. As far as I can recall it was Meade and Celestron that pioneered this travesty in their advertising and the gutless editors at S&T and Astronomy Now magazines aquiessed rather than do the correct thing and refuse to run advertisements with such poor copy.

Ian pot stirring on a cloudy night

[acropolite]

Ian, the words prime and focus, were stolen and twisted by the astronomy community from the original latin derivations which were nothing to do with Astronomy or telescopes so how they can lay claim to exclusive use eludes me. Another example, the term hole in "black hole" is totally incorrect, the object, if it exists, is not a hole, nor does it have any of the properties of a hole, so astronomers, even professional ones are also guilty of twisting the meanings of words to suit their ends. Now let's examine your Sig I note you have in your sig a 12" F5 Dob, rather odd because as everyone knows the term dob relates to the mount and not the optical assembly; one presumes you have a Newtonian on a Dobsonian base. Really....I would have expected better from one so pedantic.:poke: :whistle:

[Gargoyle_Steve]

re: your sig line again Ian
Is it not also incorrect to speak of a telescopes F ratio, should it not correctly be "f" ratio?
;)

Speaking scientifically, in a similar vein there is a very big difference between v (velocity) and V (voltage), and numerous other examples.

:thumbsup:
Cheers guys!


Rule #1 : Don't sweat the small stuff.

Rule #2 : EVERYTHING is small stuff.

[IanW]

To be really specific it's properly f. when refering to a telescopes focal ratio ;) and note I have updated my signature. :eyepop:

[Gargoyle_Steve]

Quick work Ian ..... and rest assurred I went and had a good look at my sig line and corrected a couple of things before I posted my message too, because previously I had owned a (quote)10" dob(unquote) too.

Glad you accepted my jest in the fashion it was meant - good natured
sheet stirring!
:eyepop:
Cheers mate!

[IanW]

Acropolite,

When I did my sig I wasn't aware that I could use the normal V.Bulletin control codes in it, so I used the standard capital F used in many photographic forums to denote Focal Ratio.

Excellent point re focus, which derives from the latin word fo "hearth" (fireplace). Now I'm no expert in Latin though I did study it many years ago, however , the lingua franca of science was Latin and in many cases the likes of Sir Isaac Newton, Francis Bacon and so on derived new meanings based on concepts as much as the meaning of the word, thus while a hearth may at first have no apparent connection to the focal point of an optical system, both are the centre of attraction and focus of the energy of light. Feel free to toss in a more accurate method of explaining it.

Black holes and professional astronomers, oh what fun! Professional astromers are, like many other scientists and researchers faced with an intriguing problem of having to explain in lay terms what is often highly bizzare and difficult to decribe in nature so they use metaphorical descriptions. The term was coined by Jophn Archibald Wheeler to describe what was previously known as "frozen stars", again a term used to try to convey what is in reality a highly complex piece of mathematical and cosmological theory. Thus we have all sorts of interesting analogies (pizza shaped galaxy groupings, sausage strings blah blah blah) in astronomy as often mere words cannot convey the theory in a reasonably easy to understand way.

[IanW]

No worries Steve, If I'd been thinking I'd have sorted out my sig first :shrug: ..

Now if only the cloud would lift here for one night.

[netwolf]

Communication requires us to establish a protocol, and sometimes we must establish the difrences in the use of words to communicate succesfully. To see example of this try to follow any discussion threads about Collimation howto or similar.

The focal point of the the Primary mirror in a Newtonian is "displaced/reflected/redirected" by the secondary into the Focuser assembly. The focal point is not the secondary its somwhere behind it.

Regards

[RB]

Have you guys noticed my signature.......:whistle:

:rofl:

[IanW]

I refuse point blank to buy telescope parts from any camera shop.

Showing my age again, I'v always called eyepieces oculars.

[Argonavis]

That was a bit of a rant...well

Prime Focus is generally used in the amateur literature to denote the placement of the camera body at the focuser of a Newtonian. There are no lenses between the mirrors (primary and secondary) before the film plane in this arrangement. Professionals generally have bigger scopes than amateurs so they can mount the camera (in the days of film) inside the scope where the secondary mirror would normally go. Amateurs are generally reduced to attaching the SLR to the focuser. To me both are Prime (ie first) Focus. As I recall Steve Lee had a similar rant, and I think you are both wrong. DSLR's are generally used in the same position but with thier camera lens, in what used to be called the afocal method. Essentially, words mean what they are popularily accepted to mean.

My understanding of a Plossl eyepiece is one with 2 sets of lens elements, generally of 2 each. I understand that TeleVue actually has a patent on thier version of the Plossl design. I can't see why other designers can't introduce 5 or 6 lens elements in 2 groups, as a modified Plossl. This would give some design freedom, and the basic characteristics of the Plossl (ie 50 degree AFOV) is preserved. It is a marketing challenge to sell a new design of eyepeice is it is only a modification of an existing design and you call it something else. People will say that is it only a Plossl. To the end user, if they buy a Plossl eyepeice, they don't take it apart to check the number of lens elements or groups of elements, most of us only care that it works are advertised. So get a grip.

The popular usage as defining authority is most illustrated in the term "dobsonian telescope" for just about every alt-az mounted newtonian reflector, even though the original dobsonians were plate glass mirrors with sonotube OTA's and big wooden mountings. If you used pyrex mirrors in a metal tube with a truss is it still a dobsonian? Popular usage would suggest yes.

Why? Do you usually call your telescope by its mounting type or optical design? Do I call my fork mounted SCT a "fork"? My GEM a "GEM"?

However the multitudes have spoken, so I am forced to call my dobsonians "dobsonians", not alt-az newtonians. :mad2:

end rant

[AstroJunk]

Yippeee, a semantics rant, can I join in!!!

Methodology rather than Method
Program rather than Programme
inquiry instead of enquiry
Aluminium rather than Aluminum (one of many that the Americans actually got right)
Refractor instead of Finder Scope:lol:

[IanW]

Ahh the sainted Steve Lee, there's a name that brings back memories. Anyway you are 100% incorrect and here's why.

To keep it simple I'll just use a mirror as an example as this applies to any curved mirror surface, not just the parabaloid of a Newtonian.

The focus point is fixed in relation to the surface of the mirror. it is a physcial location about which the surface of the mirror can be plotted both mathematically and physically. Diverting the light (image) cone via a mirror does not alter the focus point of the mirror, it merely diverts the light (image) cone. It's a simple concept yet one that many amateurs these days never come to grips with as so few of us actually make mirrors or lenses these days. It's elementary optical theory as well.

[Harpspitfire]

i can appreciate the correct definition of prime focus terminolgy- however in my case Prime/ newtonian/ &SCT focus are all entirely incorrect-- in any optical system, my standardized terminology is UNfocused- LOL

[IanW]

ROFL - wanna buy a pair of used +20 dioptre coke bottles?

[IanW]

Ahh the US vs English spelling issue. Not going to buy in to this one as the history of why the Americans use out of date spelling is long, complex and most would find it dead boring.

However I can't resist a shot at two you've listed. Aluminium vs Aluminum and Refractor instead of Finder Scope.

When Humphrey Davy discovered Aluminium in 1808 he originally named it Aluminum, however in 1812 he changed the spelling (Discoverer's perogative) to Aluminium as this spelling properly followed the Latin root of the mineral Alumina. Hence both spellings are originally British! The Americans used Aluminium as their spelling up until early in the c.20 when they changed it to Aluminum, a spelling varient that is only found in the USA and Canada.

Finder Scope is incorrect. ;) The correct (über pedantic version) term is finder telescope. Indeed not all finder telescopes are refractors, I can think of at least one telescope that uses a 20" reflecting telescope as it's Finder Telescope.

[AstroJunk]

Classic:lol: :thumbsup: :lol:

[IanW]

So which telescope has a 20" finder telescope? No prize will be given ;)

[Argonavis]

I am not sure where you have gone with this. If I place an imaging device at the focal plane, why would I not call this prime focus astrophotography/astroimaging, whether or not the light cone from the primary had been diverted by a secondary mirror placed in the light path?

Prime focus = first focus = the focal plane unadulterated by a negative or positive projection lens = placing your camera at the focal plane without a (camera) lens or ocular in the lightpath. A secondary mirror merely diverts the light cone, doesn't alter the point of focus.

simple really

remindes me of the day I spent in the Federal court observing a flock of barristers arguing over the meaning of a single word.

[RB]

:rofl: AN

:thumbsup:

[ving]

2 words: :rolleyes:

[ving]

oh yeah and to make my point: potato!

[janoskiss]

what a fun thread... well sort of :P

Does "prime focus" allow for the bending of light due to curvature of space time when using the telescope in high gravity environments, e.g., near a black hole, or is "prime focus" strictly defined to be the zero-G focal plane of the mirror? :confuse3: :nerd:

[asimov]

And heres mine. SPUD