How Does A Barlow Increase Eye Relief?

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Refractordude
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How Does A Barlow Increase Eye Relief?

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Post by Refractordude »

Example

If I barlow a 10mm eyepiece with an eye relief of 8mm, how much more eye relief will I get?

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Telescopes: Celestron 150mm f/8 Refractor, Meade LX70 120mm f/8 Refractor, Vixen 70mm f/12.9 Refractor, Tasco 49N 50mm Red Refractor
Binoculars: Zhumell 20x80 Giant, Oberwerk LW 11x70, Levenhuk Sherman 7x50
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notFritzArgelander
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Re: How Does A Barlow Increase Eye Relief?

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Post by notFritzArgelander »

This is NOT an easy question to answer since it depends of the designs of the Barlow lens and the eyepiece involved.

Only a design like the TeleVue Powermates will preserve the placement of the exit pupil and therefore the eye relief is my understanding. So if you're using an eyepiece that has a native 8mm eye relief that should be what you get (approximately) with all of the Powermates.

With a classic Barlow you get a little more eye relief maybe 9-11mm. Maybe more. It depends on details.

I've not desired to go into the weeds on this. From a practical POV with a Powermate I put my eye where I usually put it. With a classic Barlow I know I'll get more eye relief. But as a practical matter for an 8mm eye relief ocular I'd just start at 8mm and back off.

On CN there was a post on this topic with formulas specific to various Barlows. FWIW it's here:
https://www.cloudynights.com/topic/4857 ... ye-relief/

I'm not going to bother since I regard this as lily gilding. I prefer to assume the eye relief is roughly the same and go from there.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
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Gfamily
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Re: How Does A Barlow Increase Eye Relief?

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Post by Gfamily »

I'm reading the text in the attachment as saying that you 'increase' the eye relief because...
A 20mm eyepiece will have a longer eye relief than a 10mm eyepiece (typically at least)
A 20mm eyepiece used with a barlow will have a longer eye relief than a 10mm eyepiece without one (ditto)
so using a 20mm eyepiece with a barlow will give the same magnification as a 10mm, but with more eye relief (usual caveats apply!)

I was not aware that barlowing an eyepiece would change its eye relief - so thanks nFA, for the info.
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Lady Fraktor
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Re: How Does A Barlow Increase Eye Relief?

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Post by Lady Fraktor »

I have found that using a barlow if it does increase eye relief it is a slight amount as nFA has posted above.

Some people find it an advantage to use a barlow with eyepieces under around 10mm as the eye relief starts getting shorter so using a barlowed 20mm as a 10mm with the 20mm eye relief.
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Refractordude
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Re: How Does A Barlow Increase Eye Relief?

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Post by Refractordude »

Lady Fraktor wrote:
Thu Dec 12, 2019 5:39 pm
I have found that using a barlow if it does increase eye relief it is a slight amount as nFA has posted above.

Some people find it an advantage to use a barlow with eyepieces under around 10mm as the eye relief starts getting shorter so using a barlowed 20mm as a 10mm with the 20mm eye relief.
Yep, my 10mm is a barlowed 20mm.
Telescopes: Celestron 150mm f/8 Refractor, Meade LX70 120mm f/8 Refractor, Vixen 70mm f/12.9 Refractor, Tasco 49N 50mm Red Refractor
Binoculars: Zhumell 20x80 Giant, Oberwerk LW 11x70, Levenhuk Sherman 7x50
Mounts: Orion SkyView Pro Equatorial, Orion Versago II Altazimuth, Farpoint Universal Parrallogram Mount, Orion Tritech II Fluid Head Tripod
Finders: GSO 8x50 Raci, Svbony Red Dot
Diagonals: GSO Dielectric 2", GSO Dielectric 1.25"
Eyepieces: GSO Superview 30mm, Astromania-Olivon 22mm, GSO Plossls 32mm/25mm/20mm, GSO 20mm Superview, Svbony 20mm/15mm/9mm/6mm, Svbony Aspheric 23mm/10mm/4mm, Agena Super Wide Angle 15mm, Few No Name Brand Ebay Plossls
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