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  #21  
Old 06-08-2006, 02:02 PM
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RB (Andrew)
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Have you guys noticed my signature.......

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  #22  
Old 06-08-2006, 03:26 PM
IanW
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Quote:
Originally Posted by 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

There are lenses IN an EP!
I refuse point blank to buy telescope parts from any camera shop.

Showing my age again, I'v always called eyepieces oculars.
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  #23  
Old 06-08-2006, 05:03 PM
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Argonavis (William)
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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.

end rant
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  #24  
Old 08-08-2006, 09:46 PM
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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
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  #25  
Old 09-08-2006, 12:35 AM
IanW
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Quote:
Originally Posted by 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.

end rant
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.
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  #26  
Old 09-08-2006, 03:14 AM
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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
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  #27  
Old 09-08-2006, 03:21 AM
IanW
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Quote:
Originally Posted by 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
ROFL - wanna buy a pair of used +20 dioptre coke bottles?
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  #28  
Old 09-08-2006, 04:38 AM
IanW
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Quote:
Originally Posted by 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
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.
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  #29  
Old 09-08-2006, 08:32 AM
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Quote:
Originally Posted by IanW
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.
Classic
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  #30  
Old 10-08-2006, 02:14 AM
IanW
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Quote:
Originally Posted by AstroJunk
Classic
So which telescope has a 20" finder telescope? No prize will be given
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  #31  
Old 10-08-2006, 08:23 AM
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Argonavis (William)
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Quote:
Originally Posted by 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.

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.
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  #32  
Old 10-08-2006, 08:28 AM
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Quote:
Originally Posted by Argonavis
...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.
AN

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  #33  
Old 10-08-2006, 04:03 PM
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ving (David)
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2 words:
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  #34  
Old 10-08-2006, 04:20 PM
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oh yeah and to make my point: potato!
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  #35  
Old 10-08-2006, 04:30 PM
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what a fun thread... well sort of

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?
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  #36  
Old 10-08-2006, 04:38 PM
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asimov (John)
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Quote:
Originally Posted by ving
oh yeah and to make my point: potato!
And heres mine. SPUD
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