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Surplus Shed 38mm Erfle eyepiece focal length error?



 
 
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  #1  
Old November 7th 03, 01:04 AM
Jon Isaacs
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Default Surplus Shed 38mm Erfle eyepiece focal length error?

PS I also measured my 6mm and 9mm Synta UltraWide EPs with and without
their built-in Barlow (which can be easily unscrewed from the eyepiece
body). The 6mm became a 15mm and the 9mm became a 16mm.


Have you ever tried swapping the "built-in Barlows" between the 6 and the 9? I
wondered if the only difference was the "built in barlow" but after swapping
them, it is clear the entire eyepiece is different.

Also, I believe these are not barlows but maybe Smyth (?) lenses as the view
without them is quite distorted.

jon
  #2  
Old November 7th 03, 01:58 AM
Joseph O'Neil
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Default Surplus Shed 38mm Erfle eyepiece focal length error?

On Thu, 6 Nov 2003 23:14:45 -0000, Jim Easterbrook
wrote:

My measured focal lengths were generally quite close to the stated
values except for my 38mm Surplus Shed Erfle, which measured 31mm. Has
anyone else with one of these eyepieces had a go at measuring it?

-snip-

Not at all uncommon with many, many eyepeices. Never tried
that one personally myself, but ai remember a chat I had with Al
Nagler on the hill at Stellafane many moons ago, whent he first
"clones" (wink, wink, nudge, nudge) of his Naglers came out.

He told me, without naming any names, measure any of his
eyepieces,a nd you will see his measurements are exact and true.

Then he told me to accurately measure other brands, exactly,
and see if the reported focal lenght is accurate.

You see, if you are out even a wee bit, your claims for apparent feild
of view are out too.

So do not be too surpized, but it is worthy point to note.

anytime
joe



http://www.oneilphoto.on.ca




  #3  
Old November 7th 03, 08:26 AM
Beri
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Default Surplus Shed 38mm Erfle eyepiece focal length error?

Jim Easterbrook wrote

My measured focal lengths were generally quite close to the stated
values except for my 38mm Surplus Shed Erfle, which measured 31mm. Has
anyone else with one of these eyepieces had a go at measuring it?


I have a "32 mm plossl by Paul Rini" in a 2" barrel as labeled by Surplushed....
Bought that about a year and a half ago...

When dissasembled... in fact it is a 3 element RKE... :-/
And it yields a 8 mm exit pupil in my 8"F6 dob...

Now... how can that be 32 mm?



--
------------------------------------------------------------------------
The two most common things in the Universe are hydrogen and stupidity.

http://www.pondi.hr/~beri (personal pages)
http://www.astronomydaily.com (moderator)
43.4970 N, 16.2220 E
  #4  
Old November 7th 03, 09:39 AM
Ante Perkovic
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Default Surplus Shed 38mm Erfle eyepiece focal length error?

Jim Easterbrook wrote:

Today I had my first go at measuring the focal length of my eyepieces,
using Don Taylor's magnification of a grating technique. I don't possess
a proper Ronchi grating, but made my own 100 lines/inch grating with a
laser printer and some OHP film.

My measured focal lengths were generally quite close to the stated
values except for my 38mm Surplus Shed Erfle, which measured 31mm. Has
anyone else with one of these eyepieces had a go at measuring it?


Yes,

My so-called 8mm is actually 13mm. I also (together with Beri, whose post
you can find in this thread) assembled or bought some eyepieces, notably:
- 6.5mm (bought in one piece, 10 elements!): very good AFOV, 10 elements
steals at least 1 magnitude
- 7mm Kelner: there were scratched, probably last of its kind in Shed.
Since I don't have shorter ERs, I sometimes use this one for the moon.
- 8mm Kelner: FL is more like 13mm, with *big* coma ta the edges. But, new
ones are on the way (for free!!).
- 11.5mm Kelner: Very good Kelner
- 19mm plossl: superbly sharp. AFOV only about 43, but I love it, it's
great complement to 26mm
- 26mm superplossl: AFOV is 69 degrees!!! Not as clear as 19mm, but who
cares if You can see all of M45 or Coathanger at 35x!!!

Regarding focal lengths, I'm also little suspicious about that, but have no
idea how to measure it.

Altogether, I still think SurplusShed stuff is bestbuy for ATM.

Tonci
--
http://astro.fdst.hr

  #5  
Old November 7th 03, 02:34 PM
guid0
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Default Surplus Shed 38mm Erfle eyepiece focal length error?

On Thu, 6 Nov 2003 23:14:45 -0000, Jim Easterbrook
stepped up to the plate and batted:

Today I had my first go at measuring the focal length of my eyepieces,
using Don Taylor's magnification of a grating technique. I don't possess
a proper Ronchi grating, but made my own 100 lines/inch grating with a
laser printer and some OHP film.

My measured focal lengths were generally quite close to the stated
values except for my 38mm Surplus Shed Erfle, which measured 31mm. Has
anyone else with one of these eyepieces had a go at measuring it?

PS I also measured my 6mm and 9mm Synta UltraWide EPs with and without
their built-in Barlow (which can be easily unscrewed from the eyepiece
body). The 6mm became a 15mm and the 9mm became a 16mm.


I also have that eyepiece but I've never measured the focal length
(wouldn't know where to begin!)

I'm not so sure about the claim of 60* AFOV for this piece. Seems a
bit tighter to me. Did you measure this aspect of the EP by any
chance?

G../0
  #6  
Old November 7th 03, 06:38 PM
Jim Easterbrook
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Default Surplus Shed 38mm Erfle eyepiece focal length error?

In article ,
says...

I also have that eyepiece but I've never measured the focal length
(wouldn't know where to begin!)


The method is described he
http://groups.google.co.uk/groups?th=16bb70d0cac07219

I'm not so sure about the claim of 60* AFOV for this piece. Seems a
bit tighter to me. Did you measure this aspect of the EP by any
chance?


Not yet. I need a nice long evening of clear sky and no other tasks. I
plan to do it "soon" and will put all my results on my website. I also
want to refine my focal length measuring apparatus.
--
Jim Easterbrook http://www.easter.mersinet.co.uk/
N51.36 E0.25
  #8  
Old November 8th 03, 01:54 PM
Jim Easterbrook
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Default Surplus Shed 38mm Erfle eyepiece focal length error?

In article , I wrote:

Isn't that an incentive to quote a longer focal length though?


I should have written *shorter* focal length, i.e. to give a larger
AFOV.
--
Jim Easterbrook http://www.easter.mersinet.co.uk/
N51.36 E0.25
  #9  
Old November 8th 03, 05:10 PM
Jon Isaacs
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Default Surplus Shed 38mm Erfle eyepiece focal length error?


Isn't that an incentive to quote a longer focal length though? E.g. you
measure a true field of view that suggests your EP is either a 32mm 60
degree or a 38mm 50 degree. I know which I'd rather buy.


I think you probably meant to say: "Isn't that an incentive to quote a shorter
focal length...."

jon


  #10  
Old November 8th 03, 08:05 PM
guid0
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Default Surplus Shed 38mm Erfle eyepiece focal length error?

On Fri, 7 Nov 2003 17:38:48 -0000, Jim Easterbrook
wrote:

In article ,
says...

I also have that eyepiece but I've never measured the focal length
(wouldn't know where to begin!)


The method is described he
http://groups.google.co.uk/groups?th=16bb70d0cac07219


Thanks for the info Jim. I'll give this experiment a whirl.

I'm not so sure about the claim of 60* AFOV for this piece. Seems a
bit tighter to me. Did you measure this aspect of the EP by any
chance?


Not yet. I need a nice long evening of clear sky and no other tasks. I
plan to do it "soon" and will put all my results on my website. I also
want to refine my focal length measuring apparatus.


Same thing here. I got this EP recently and haven't had a chance to do
a star drift test. I'll be curious to see if your results match mine.


Clear skies

G../0
seleblan at u ottawa * cannabis
(to reply remove "nnabis")
 




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