Kowa 884 (1 Viewer)

[John goshawk]

What is the prism arrangement for this scope.
Can a straight through scope like this use a roof prism?

Thanks John

 

[John goshawk]

C'mon, where's the technical wizards?

Thanks John

 

[William Lewis]

Not a wizard unfortunately but your answer should be in here - Have scopes nowadays Porro or Roof prisms?

 

[henry link]

Straight scopes can use any of the prism arrangements that are used in binoculars, including roof prisms. The Kowa 884 uses a Porroprism.

 

[John goshawk]

Much appreciated, thanks

 

[Binastro]

Straight scopes can use no prisms, just relay lenses.

They are longer than prism scopes.

They can also use mirrors.

B.

 

[John goshawk]

Not a wizard unfortunately but your answer should be in here - Have scopes nowadays Porro or Roof prisms?

Thanks that's brilliant.

 

[John goshawk]

Is it true Kowa arrange their porro prisms horizontally (like porro Binoculars )where as Celestron and opticron vertically (no offset eyepiece)?

Thanks 👍

 

[William Lewis]

Porro prisms are arranged at an angle of 90 degrees from each other, in most binoculars this forms an inverted "V" shape.

You can see it in the photo below in the way the prism housings go diagonally up then down with there top edges forming a 90 degree angle. So I suppose none of the prisms will be vertical in either layout and the offset will be the same in both configurations, just one will be vertically offset in relation to the tripod foot and one will be horizontally offset to it.

 

[John goshawk]

Porro prisms are arranged at an angle of 90 degrees from each other, in most binoculars this forms an inverted "V" shape.

You can see it in the photo below in the way the prism housings go diagonally up then down with there top edges forming a 90 degree angle. View attachment 1545076So I suppose none of the prisms will be vertical in either layout and the offset will be the same in both configurations, just one will be vertically offset in relation to the tripod foot and one will be horizontally offset to it.

I just about grasp this. Would need a diagram to truly get it. Thanks for your help. John

 

[John goshawk]

This is what I mean by horizontal placement, as I image offset eyepiece spotting scopes, such as Kowa.

 

[John goshawk]

This is what I mean by vertical prism placement, in a celestron scope- no offset eyepiece.

 

[John goshawk]

Porro prisms are arranged at an angle of 90 degrees from each other, in most binoculars this forms an inverted "V" shape.

You can see it in the photo below in the way the prism housings go diagonally up then down with there top edges forming a 90 degree angle. View attachment 1545076So I suppose none of the prisms will be vertical in either layout and the offset will be the same in both configurations, just one will be vertically offset in relation to the tripod foot and one will be horizontally offset to it.

I can't see the inverted V in either diagram. Perhaps you could help me figure it out. Thanks John

 

[henry link]

This is the way a naked Porroprism looks as viewed from the eyepiece or the objective lens. The 90º angle is visible simply as the difference between the prism lying flat on its side and the one standing up vertically behind it. As long as the angle between the two prisms is a perfect 90º the image will be perfectly erected with no tilt no matter how it is rotated around the optical axis at the center of the circle seen at the top of the vertical prism.

Most scopes can be rotated around that axis by means of a locking collar located at about the middle of the scope tube without causing any disturbance to the correct image erection, so it really doesn't matter at all whether the eyepiece is above, to the side or below the objective lens.

 

[William Lewis]

If you want to see how this works you might find one of these for sale.

When the monocular as in photo 1 is flat, like the pictures in your diagrams show the image through the eye piece will be both inverted and flipped left to right. When the monocular is in use it is arranged as I have done in photo 2 and the image will be the right way up and the right way round - you can see the shape of the prisms like in Henry's photo by the outline of the monoculars casing.

Picture 1
picture 2

 

[John goshawk]

Fascinating.
But surely the celestron diagram I uploaded is wrong then? No right angles??

 

[henry link]

Yes it's wrong, but many two dimensional representations of Porro prisms are wrong in the same way, presumably because it's not so easy to represent the 90º twist between the two prisms in 3D space in a flat drawing.

William's first photo shows the prisms arranged like the Celestron diagram. In that arrangement the light will travel through the prisms just fine, but without being inverted, so the image at the eyepiece will look upside down just as if there were no prisms.

 

[John goshawk]

Good to talk to people with logical minds!
I swear dodgy diagrams by leading manufacturers have cause no end of confusion!
Very grateful for your time.
With this in mind, any idea why kowa often favours an offset eyepiece/bulge to one side in their scopes? Larger prisms?

 

[John goshawk]

Good to talk to people with logical minds!
I swear dodgy diagrams by leading manufacturers have cause no end of confusion!
Very grateful for your time.
With this in mind, any idea why kowa often favours an offset eyepiece/bulge to one side in their scopes? Larger prisms?

Any theories concerning my last comment?