Surplus shed: focal lengths are total chaos!

Hi,

I just found out that my Surplus Shed 26mm super plossl
(http://www.surplusshed.com/pages/item/l1714.html) is actually more
like 36mm !!!!

I need 36mm as much as I need 26mm, but with this trick, I was led to
believe that AFOV is 69, and real AFOV is more like 49! The view is
far from clear but I thought the 69 degrees AFOV more that compensate
it. Now I don't! I really don't like this kind of marketing and I
don't really believe that Surplus Shed didn't know about this FL
error!

I ordered some other lens sets, so this is what I think of them (I
have 114mm f/8 homemade dobsonian):

- 6.5mm wide angle ($32.5):
I don't use it very often because it's 10 lenses take away too much
light. Planetaries ussualy look much better with homemade 11.5mm
kelner. The view is far from clear. AFOV is OK.

- 7mm kelner (homemade from Surplus Shed lens set):
The lenses were badly schratched so I don't use this one at all.
Couldn't get replacement because these were sold out.

- 8mm kelner (homemade from Surplus Shed lens set):
This is new one and i didn't test it yet, but my friend Beri claims
that the view is not very clear.
Eyepieces assembled from first round of these lens sets were actually
13mm with far too much coma, so they are useles (we received
replacement from Shed). FL is around 8mm.

- 11.5mm kelner (homemade from Surplus Shed lens set):
Not so bad. Not perfectly clear, but I'm satisfied. FL is very close
to 11.5 mm.

- 19mm plossl: superbly sharp. AFOV only about 43, but I love it. FL
is very close to 19mm.
- 26mm superplossl: Not 26 but 36mm!!! Also, AFOV not 69 but 49!!!

Ante Perkovic
--
http://astro.fdst.hr

"Ante Perkovic" wrote in message
om...
Hi,

I just found out that my Surplus Shed 26mm super plossl
(http://www.surplusshed.com/pages/item/l1714.html) is actually more
like 36mm !!!!

I need 36mm as much as I need 26mm, but with this trick, I was led to
believe that AFOV is 69, and real AFOV is more like 49! The view is
far from clear but I thought the 69 degrees AFOV more that compensate
it. Now I don't! I really don't like this kind of marketing and I
don't really believe that Surplus Shed didn't know about this FL
error!

I wonder if they had some lenses that they thought were all alike, but
really weren't.

"Ante Perkovic" wrote in message
om...
Hi,

I just found out that my Surplus Shed 26mm super plossl
(http://www.surplusshed.com/pages/item/l1714.html) is actually more
like 36mm !!!!

I need 36mm as much as I need 26mm, but with this trick, I was led to
believe that AFOV is 69, and real AFOV is more like 49! The view is
far from clear but I thought the 69 degrees AFOV more that compensate
it. Now I don't! I really don't like this kind of marketing and I
don't really believe that Surplus Shed didn't know about this FL
error!

I wonder if they had some lenses that they thought were all alike, but
really weren't.

"Michael A. Covington" wrote:

"Ante Perkovic" wrote in message
om...

I just found out that my Surplus Shed 26mm super plossl
(http://www.surplusshed.com/pages/item/l1714.html) is actually more
like 36mm !!!!

I need 36mm as much as I need 26mm, but with this trick, I was led to
believe that AFOV is 69, and real AFOV is more like 49! The view is
far from clear but I thought the 69 degrees AFOV more that compensate
it. Now I don't! I really don't like this kind of marketing and I
don't really believe that Surplus Shed didn't know about this FL
error!

I wonder if they had some lenses that they thought were all alike, but
really weren't.

No, we ordered 7 sets.
Me and Beri (friend) measured them by measuring exit pupil. His scope is
200mm f/6 (FL=1200mm). The Exit pupil was very close to 6mm (we are 100%
sure it was 5.8mm) for all 7 eyepieces.

If EP = 6mm,

FL(eyepiece) = (1200mm/200mm) x EP = 36mm

The minimum FL of so-called "26mm" is 6 x 5.8mm = 34.8mm

Ante Perkovic

"Michael A. Covington" wrote:

"Ante Perkovic" wrote in message
om...

I just found out that my Surplus Shed 26mm super plossl
(http://www.surplusshed.com/pages/item/l1714.html) is actually more
like 36mm !!!!

I need 36mm as much as I need 26mm, but with this trick, I was led to
believe that AFOV is 69, and real AFOV is more like 49! The view is
far from clear but I thought the 69 degrees AFOV more that compensate
it. Now I don't! I really don't like this kind of marketing and I
don't really believe that Surplus Shed didn't know about this FL
error!

I wonder if they had some lenses that they thought were all alike, but
really weren't.

No, we ordered 7 sets.
Me and Beri (friend) measured them by measuring exit pupil. His scope is
200mm f/6 (FL=1200mm). The Exit pupil was very close to 6mm (we are 100%
sure it was 5.8mm) for all 7 eyepieces.

If EP = 6mm,

FL(eyepiece) = (1200mm/200mm) x EP = 36mm

The minimum FL of so-called "26mm" is 6 x 5.8mm = 34.8mm

Ante Perkovic