Barlow Eye relief

[Astro Imp]

I have been reading with interest the thread describing Barlows. Here:- http://stargazerslounge.com/topic/254245-barlow-how-does-yours-work/

If I have understood the theory the Barlow modifies the light cone thereby increasing the focal length of the OTA. How then does it affect the eye relief of the eyepiece. I know it tends to increase eye relief but isn't the eye relief a function of the eye piece. By my reasoning (obviously wrong) eye relief would then be a function of focal length and the eyepiece.

[John]

A barlow lens pushes the eye relief of an eyepiece outward by an amount that depends on a number of factors including the focal length of the barlow lens. All barlow lenses do this to some extent. TeleXtenders and Powermates don't.

With eyepieces that tend to have rather tight eye relief (eg: orthos and plossls with focal lengths shorter than 10mm or so) this can be an advantage for viewing comfort. With longer focal length eyepeices it can be a disadvantage because the observing point moves above the eyepiece top and the eye cup meaning that you need to hold your eye some way off the eyepiece to hold the view and that the eyecup won't do it's job in keeping stray light off the eye lens.

[Astro Imp]

Thanks John, I can understand that but was wondering why this happens as the eye relief (of an eye piece) is not related to a scope's focal ratio.

[John]

My understanding is that this is due to the barlow diverging the light beams coming from the obective lens / primary mirror, which in turn causes the exit pupil to be projected further out from it's original point. If I try to go deeper than this my head starts to hurt !

The effect has been measured at a 20% extension for long format barlow lenses (eg: the Tele Vue 2x barlow) and increasing to around 30% with short format barlows (eg: Celestron Ultima 2x).

[Stu]

Thanks John, I can understand that but was wondering why this happens as the eye relief (of an eye piece) is not related to a scope's focal ratio.

I'm with you there! If the eye relief of and eyepiece doesn't change with the focal ratio of a scope, why does it when a Barlow is used? Someone please explain!!

[Ruud]

First animation: Barlow. Note how the cones that leave the barlow diverge.

Second animation: Tele-extender. Note how the cones that leave it are parallel.

This is how the cones influence the eye relief. It's a bit like the lens formula.

Parallel cones are focussed more or less at the original eye relief position.

Diverging cones are focussed to a different eye relief position, behind the original eye relief position.

Actually, the telescopes objective also produces slightly diverging cones. This means that a tele-extender, which turns them into parallel cones, slightly shortens the eye relief.

Note: the diverging cones from a barlow can cause the outer rays to miss the lenses in the eyepiece. This is why a barlow can cause vignetting. Vignetting by barlows tends to happen in longer focal length eyepieces because these generally have a larger field stop which allows more strongly diverging cones through.

The parallel cones from a tele-extender will not cause vignetting

(Drawing and animations made with freeware Geogebra.)

[bingevader]

Great diagrams!

I was trying to think how to best explain it.

A picture tells a thousand words.

[Ruud]

By the way, a short focal length objective produces light cones that diverge stronger than a long focal length objective. This means that a short focal length objective causes an eyepiece to have more eye relief than a long focal length objective. In practice the difference won't be big though.

And one more thing. This is why 5x Barlows aren't made:

You'd get vignetting in just about any eyepiece

EDIT:  concerning post [6]. Vignetting by barlows in long focal length eyepieces is not caused by the field stop being big. It occurs because long focal length eyepieces the lenses are further behind the field stop, giving the diverging rays more of a chance to wander away too far from the optical axis and miss the lenses.

[Astro Imp]

Thanks guys, I'm not sure I've got my head round this yet but will spend some time digesting the information from Ruud.

[John]

Excellent posts Ruud

My head hurts a little less now !

I've experienced vignetting when using a shorty Ultima 2x barlow with eyepieces such as the 32mm plossl.

[Stu]

Great posts Ruud. Thanks!

[Dave In Vermont]

Cool, Ruud. I'll tell you a bad story - Later.....

Dave

[Ruud]

Okay Dave, I'll brace myself for that one.