Galilean optics?

[pipnina]

I was not aware there was a way of making optics other than the keplerian type... But after reading about the vixen 2.1x42 bins I'm entrigued.

After reading a review where the reviewer said they were like "supervision" (as if the milky way looked brighter) and explained they didn't have a fixed exit pupil or something like that i'm now confused... And a little distressed that I can't use them to their full. (either the nearly 30 degree FOV with astigmatism or 12 degrees or less...)

So... are Galilean optics  way to actually make an image brighter than our eyes can normally do? Like compressing a 14mm exit pupil into a 7? Or am I misunderstanding?

Thanks,

   ~a confused pip

[ronin]

I assume that you are referring to the fact that Galileo used a positive objective lens and a negative lens for the eyepiece.

Kepler used a positive objective and positive eyepiece lens - which is the normal now.

The Galileo configuration gives a short optical path but the Kepler configuration produces better results. Think the opera glasses are actually Galileo designs, they are so short.

The Kepler set up means the length will be F_obj+F_ep for you to get collimated light out of the instrument. As a total guess is the Galilean design therefore F_obj-F_ep ???

Useful perhaps on something short but if the objective is 1000mm and the eyepiece 10mm you do not really gain much for a significant loss in performance as the objective focal length is the major contribution to the overall instrument length.

[pipnina]

I assume that you are referring to the fact that Galileo used a positive objective lens and a negative lens for the eyepiece.

Kepler used a positive objective and positive eyepiece lens - which is the normal now.

The Galileo configuration gives a short optical path but the Kepler configuration produces better results. Think the opera glasses are actually Galileo designs, they are so short.

The Kepler set up means the length will be F_obj+F_ep for you to get collimated light out of the instrument. As a total guess is the Galilean design therefore F_obj-F_ep ???

Useful perhaps on something short but if the objective is 1000mm and the eyepiece 10mm you do not really gain much for a significant loss in performance as the objective focal length is the major contribution to the overall instrument length.

Any idea why it was used in the vixen 2.1x42 bins? Can it be used to make images brighter than what we would normally see in a kepler configuration? All I've read so far about them is this one bit of utmost praise and a lot of places basically saying galilean configurations are outdated and terrible...

[DaveS]

The Galileo system also give right-way-up images without using prisms or additional lenses. Hence the use in opera glasses.

[Stu]

Pip, I don't know enough to answer your question but Matthew's review here contains a fair amount of detail if you haven't seen it already.

http://alpha-lyrae.co.uk/2014/10/25/vixen-sg-2-1x42-binoculars-review/

I don't think Gallilean optics are able to cheat the normal rules though, I think the perception of extra brightness is more an illusion of the very low mag.

[Alien 13]

I do wish someone made Galilean eyepieces I would love a scope with a magnification of around X5 or less.

Alan

[RichM63]

I do wish someone made Galilean eyepieces I would love a scope with a magnification of around X5 or less.

Alan

One of those "Lens to Scope" type of convertors would do that as they divide the focal length by 10. Thus your 50mm = 5x.

Of course if you want 1x, then an empty kitchen roll tube with no glass works fine.

Rich..

[ollypenrice]

This whole businesss of it being impossible to increase surface brightness may be true but it doesn't do a good job of describing what we really see.

I found the Vixens very enjoyable. I felt that they just gave me astoundingly good eyesight - when mine is really poor. I can normally see M31 as a faint streak but, in saying that, it's not an unambiguous faint streak, it's quite elusive and hard to disentangle from a nearby star. In the Vixens it becomes a clearly defined and totally unambiguous streak - and it doesn't seem particularly faint.

However, although they gave a wide FOV I found that I only wanted to use the central portion of the field. The rest brightened considerably. Not a prblem. They are not telescopes so you just move your head.

Olly

[pipnina]

This whole businesss of it being impossible to increase surface brightness may be true but it doesn't do a good job of describing what we really see.

I found the Vixens very enjoyable. I felt that they just gave me astoundingly good eyesight - when mine is really poor. I can normally see M31 as a faint streak but, in saying that, it's not an unambiguous faint streak, it's quite elusive and hard to disentangle from a nearby star. In the Vixens it becomes a clearly defined and totally unambiguous streak - and it doesn't seem particularly faint.

However, although they gave a wide FOV I found that I only wanted to use the central portion of the field. The rest brightened considerably. Not a prblem. They are not telescopes so you just move your head.

Olly

So would you say that the Vixens give the impression of increased brightness just like regular bins/scopes do?

In the alpha-lyrae review he said that you could hardly tell there was any magnification, would you vouch for that as well? But he also made no mention of brightening at the edge of the FOV, just a lot of distortion and loss of sharpness. What is the brightening like? A loss of contrast or perhaps more light entereing the side of your eye due to the massively oversised exit pupil?

Cheers,

    ~pip

[Ruud]

There is no exit pupil with opera glasses. Here's a rudimentary animation that shows how they work:

The wider the front lens, the wider the view.

Eye relief is zero if you want to see the whole view.

The incoming beam is wider than the exit beam, hence the light gain.

It's not an accurate illustration but it shows the principle.

Hope this helps.