Exit pupil & barlow query

[IB20]

Hoping a member can answer this for me as I can't seem to find a definitive answer anywhere.

What does placing a 2x Barlow before (rather than after) a prism star diagonal do to an eyepiece's exit pupil? I tried it yesterday with a 15mm eyepiece in an f10 scope and there were plenty of noticeable eye floaters. From what I've read it's assumed a 2x Barlow would increase to around 3x due to lengthening of optical path making the exit pupil in a 15mm eyepiece around 0.5mm. Yet I've used a 5mm in the same scope without the Barlow and don't see anything like the number of floaters? Am I missing something?

[vlaiv]

Barlow will have two distinct effects on eyepiece - regardless of positioning.

1. Will increase magnification - and hence reduce exit pupil as exit pupil is aperture divided by magnification

2. Will increase eye relief of eyepiece.

Difference of using barlow before and after diagonal is in barlow magnification. Barlow magnification depends on its focal length and distance between barlow element and focal plane.

Larger this distance - more magnification barlow gives.

If you place barlow before diagonal - you are adding another ~80mm of optical path between barlow and eyepiece - that means significant increase in magnification.

I highly doubt that it will only make x2 barlow act as x3 barlow. I think it will be closer to x4

We can do calculation on a example barlow to see what sort magnification change will diagonal make.

Say we have barlow like this:

Based on this image, I estimate barlow element to eyepiece distance to be around 50mm. Barlow formula is:

Magnification = 1 + distance / focal length

This is x2 barlow this means that it has focal length of about:  focal length = distance / (2 - 1) = 50mm

Here are optical paths of some diagonals:

We can safely say that using barlow in front of diagonal mirror will result in distance increase of about 80mm, so it will be 130mm (80+50) instead of 50mm.

Magnification = 1 + 130 / 50 = x3.6

As shown, magnification is probably closer to x4 then it is to x3 in that configuration. That is one reason you are seeing more floaters.

Second is extended eye relief.

With 5mm eyepiece - you have 0.5mm exit pupil - but it is concentrated in a single spot:

In above diagram - exit pupil is 0.5mm and as such it only covers small part of your pupil.

However, if exit pupil is significantly shifted - like when eye relief is pushed back, you'll get into this situation with normal eye positioning:

You still won't experience blackouts because exit pupil is still 0.5mm and even if you have 6 of them lined up (or rather not still merged into single) - it will be 3mm wide - so you'll be able to see without blackouts - but much larger area of your pupil will be illuminated.

This simply changes how light hits floaters inside your eye and can make them more visible.

People reported seeing floaters more with barlow lens and less in wider AFOV eyepieces.

 

 

[scotty38]

It could be that it's giving more the 3x in actual fact. Do the images look about the same barlow vs 5mm eyepiece?

Edit, or just read Vlaiv's response 🙂

Edited June 1, 2021 by scotty38

[IB20]

Thanks @vlaiv for your expertise and writing such a comprehensive response, that makes total sense. 🙂

[Louis D]

The shorter the Barlow, the larger the effect of placing it in front of the diagonal (that whole 1/FL deal).

If you can screw the optical element directly to the front of the diagonal, you can greatly reduce the magnifying effect.

[Don Pensack]

In a nutshell: the smaller the exit pupil, the more problem you'll have with floaters on the Moon and planets.

Floaters are not usually a problem on stars.