N-8A gunsight

[Kingscoy]

Hello,

Does anyone perhaps have something on this gunsight? Manual or something like that.
I browsed the forum but couldn't find anything (or I overlooked) and the web is also limited in its info.

Help is aprreciated!

Cheers,
Sander

 

[Snautzer01]

used on the b25 and b17

ŽË@"š@Æ@€@Ší

 

[Kingscoy]

used on the b25 and b17

ŽË@"š@Æ@€@Ší

Hi,

Thanks for the link. I did came across it too browsing the internet. Very basic info but useful.
I still hope someone has (some sort of) a manual for it.

Cheers,
Sander

 

[cptsmith]

Does the Gunners Information File have any data on it?

 

[Kingscoy]

I'm not familiar with that file/ manual. Is it a document?

regards,
Sander

 

[akosgy]

Hello,

Does anyone perhaps have something on this gunsight? Manual or something like that.
I browsed the forum but couldn't find anything (or I overlooked) and the web is also limited in its info.

Help is aprreciated!

Cheers,
Sander

Hi there! Hope my answer does not come to late.

The N-8A gunsight is well described in an article by Robert Hurst in Axis History Forum (Axis History Forum • View topic - American Fixed and Free-Mounted Gunsights used by the RAF) (actually taken from a book by R. Wallace Clarke).

Optically this is an unusual solution (at least for WWII gunsights, the principle is more used nowadays). To have a "good" projection of the reticle ("graticule" - centre dot + circle(s), + mil marks in US sights), you need "good" (well corrected) optics.

If you look at the projection optics of WWII British or German gunsights, you can notice that complicatred 3 element optics are usually used. A way to go around this, is the use of so called Mangin mirror, invented originally for celestial telescopes (see Wikipedia article on this). This is a combined lens/mirror optics, where one surface radius of the lens matches that of the spherical mirror. The lens is made so (i. e. other radius and index of refraction are calculated so) that its spherical abberation exactly compensates that of the spherical mirror. Thus there is no sperical abberation (i.e. the optical element is abberation free fore a "point source". (A filament of the bulb, used in reflecion gunsights is of course not a point source, nevertheless the aberrations are diminished.) This optical solution is much thinner than let's say a 3 element lens. The complete optics then work the following way (see figure on N-8A in the named article): the graticicule is illuminated from below as usual, but the light then passes through the the semi transparent rreflector glass (some light beeing lost by directing it foreward by its front surface). Light the falls on the mangin lens, from where it is reflected down, and collimated (making the rays more or less parallel, as opposed to "point source") and reflected back from the rear side of the semi transparen reflector, into the eye of the pilot/gunner. In all other respect, this sight works the same way and on the same principle as ususal, objective lens based reflector sights.

Hope I could clear up the subject somewhat.

 

[Kingscoy]

Hi,
Thanks for that link and info. Very useful indeed!!

Cheers,
Sander