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Effect of Barlow mag. on eye relief

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#1 Jeronimo Cruz

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Posted 06 January 2012 - 02:26 PM

Hi all,

I'm aware that Barlow lenses can increase the eye relief of eyepieces. My question to you is this:Do Barlows with a stronger magnification factor add more or less eye relief to eyepieces than weaker Barlows do? For example, will a 5x Barlow increase the eye releif more so than a 2x Barlow? Or is the other way around?

Thanks for your responses.

#2 Ed D

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Posted 06 January 2012 - 08:34 PM

I have always read that one could use a longer focal length eyepiece and barlow to equal the higher magnification of a shorter focal length eyepiece, yet retain the eye relief of the longer one. An example would be using a 15mm Plossl with a 2x barlow, which would give equivalent magnification to a 7.5mm Plossl but with the eye relief of the 15mm. This has also been my experience with barlows.

Ed D

#3 faackanders2

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Posted 06 January 2012 - 09:07 PM

To maintain eye relief of the original eyepiece, you would need to have a televue Powermate (inverse double doublet).

#4 sixela

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Posted 10 January 2012 - 06:03 AM

How much the eye relief changes is more a function of the focal length of the barlow (shorter barlows yield more effect) and of the eyepiece (longer eyepieces have more change in the eye relief) than the magnification factor.

#5 howard929

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Posted 10 January 2012 - 07:48 AM

Hi all,

I'm aware that Barlow lenses can increase the eye relief of eyepieces. My question to you is this:Do Barlows with a stronger magnification factor add more or less eye relief to eyepieces than weaker Barlows do? For example, will a 5x Barlow increase the eye releif more so than a 2x Barlow? Or is the other way around?

Thanks for your responses.


I'm confused. I was under the impression that a barlow doesn't effect or change the ER of a EP at all.

Howard

#6 BDS316

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Posted 10 January 2012 - 08:07 AM

Hi all,

I'm aware that Barlow lenses can increase the eye relief of eyepieces. My question to you is this:Do Barlows with a stronger magnification factor add more or less eye relief to eyepieces than weaker Barlows do? For example, will a 5x Barlow increase the eye releif more so than a 2x Barlow? Or is the other way around?

Thanks for your responses.


I'm confused. I was under the impression that a barlow doesn't effect or change the ER of a EP at all.

Howard



Most types of Barlows will increase the eye relief of an eyepiece. The exceptions to this are telecentric devices like Powermates which do not change the eye relief at all.

Shorty Barlows increase the eye relief moreso than standard barlows of the same magnification factor.

Longer focal length eyepieces wil experience more of an eye relief increase with a given Barlow than shorter focal length eyepieces. Unfortunately this is the exact opposite of what we would want.

For example if you use a 32mm plossl with a shorty barlow the eye relief would increase so much that It would be tough to hold the exit pupil and see anything, my friend.

#7 howard929

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Posted 10 January 2012 - 08:31 AM

Bryan,

Thank you for explaining that.

Howard

#8 BDS316

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Posted 10 January 2012 - 10:18 AM

Bryan,

Thank you for explaining that.

Howard


No prob. In addition to powermates, Seibert I believe makes telecentric devices (some would say these are not actually barlows, some would) that would not change eye relief.

#9 kevint1

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Posted 10 January 2012 - 10:36 AM

Does anyone know if there is a formula for calculating the amount of change in eye relief that a Barlow will give an eyepiece? I am using a TV 2x Barlow and do notice an increase in eye relief with longer focal length eyepieces.

#10 MitchAlsup

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Posted 10 January 2012 - 07:21 PM

My question to you is this:Do Barlows with a stronger magnification factor add more or less eye relief to eyepieces than weaker Barlows do?


If you place a 2X Barlow lens at the prescribed distance it operates as a 2X Barlow. If you place the lens at 1.5X its prescribed distance it operates as a 3X Barlow.

The lens creates the same divergence of the converging light cones in both cases, its just that the image scale is bigger when the Barlow lens is farther into the light path.

Thus, it is NOT the power of the Barlow that causes the push of the exit pupil outward, but the amount of divergence of the converging light cones.

The amount of divergence can be directly calculated from the FL of the Barlow lens.

Unfortunately, no Barlow lens manufacture states the FL directly.

Yes shortie Barlows create greated divergence and push the exit pupil more than normal length Barlows.

#11 walrus

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Posted 22 January 2014 - 04:32 PM

I found a great little online UK program called 'Telescope Calculator' that tries to estimate the eye relief for a given Barlow used. I don't think it really accounts for the difference between a shorty or a normal barlow.

I spent some time trying to figure what formula they might be using to generate the estimate but believe I finally figured it out.

'Example for estimated eye relief using a x2 barlow
'Meade 1.25 Super Plossl
'12.4mm Focal Length
'7mm Eye Relief
'x2 Barlow

'
' ER Estimate = ((Eye Relief / Ocular F/L) + (Balow Value /4)) * Ocular F/L )
' ER Estimate = ((7mm / 12.4mm) + (x2 barlow /4)) * 12.4mm )
' ER Estimate = ((7 / 12.4) + (2 / 4)) * 12.4 )
' ER Estimate = ((0.5645161290322581) + (.5)) * 12.4 )
' ER Estimate = ((1.064516129032258) * 12.4 )
' ER Estimate = (13.2)

:jump:

#12 dyslexic nam

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Posted 22 January 2014 - 04:44 PM

Bryan,

Thank you for explaining that.

Howard


No prob. In addition to powermates, Seibert I believe makes telecentric devices (some would say these are not actually barlows, some would) that would not change eye relief.


As does Meade, with the "Tele-Extenders". I have the 2x, and it works great and reserves the native ER of the original ep.

#13 BillP

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Posted 22 January 2014 - 05:04 PM

Does anyone know if there is a formula for calculating the amount of change in eye relief that a Barlow will give an eyepiece? I am using a TV 2x Barlow and do notice an increase in eye relief with longer focal length eyepieces.


In my tests, the TV 2x increased ER of Plossls by approx 20%. So not enough to notice all that much. E.g., if a 12.5mm Plossl has an ER of 8.2mm, then the TV 2x will make its ER approximately 9.8mm. So not a big increase.

Shorty Barlows I found increase ER more. So the Klee 2.8x increases the ER by about 40%. The almost same mag factor 2.7x APM ED Barlow, which is not a shorty, only increases ER my 20%, like the TV 2x.

Rusty - FWIW, my tests above do not match up with the calculator you found unfortunately. If I tried to fit a formula to my 3 data points from my measures, something like this works ok: -->

New ER = Old ER * ( 1 - (Barlow Mag / Barlow FL) * 5)

Most Barlow makers do not give the FL of the Barlow. It is always a negative number. For the APM 2.7x ED the FL = -63mm, I estimate the TV 2x is -42mm. The Klee 2.8x I estimate at -35mm based on measures.

#14 Starman81

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Posted 22 January 2014 - 05:08 PM

Most types of Barlows will increase the eye relief of an eyepiece. The exceptions to this are telecentric devices like Powermates which do not change the eye relief at all.


I am quite sensitive to changes in eye relief and IME I would say that the TV 2.5x Powermate slightly decreases eye relief! I know that may be a controversial statement but that is what I have experienced time and time again. This was experienced when used with Pentax XW eyepieces.

#15 SteveG

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Posted 22 January 2014 - 06:32 PM

Bryan,

Thank you for explaining that.

Howard


No prob. In addition to powermates, Seibert I believe makes telecentric devices (some would say these are not actually barlows, some would) that would not change eye relief.


As does Meade, with the "Tele-Extenders". I have the 2x, and it works great and reserves the native ER of the original ep.


Meade Tele Extender is not a barlow, it is an empty tube used for imaging:
http://www.amazon.co...er/dp/B00023...

Meade TeleXtender is a telecentric barlow, with a very confusing name:
http://telescopes.ne...elextender.html

#16 walrus

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Posted 22 January 2014 - 07:56 PM

Thank you for that info. There is nothing like experience by testing. I must say I feel much better about the numbers you have experienced than the values the 'Telescope Calculator' was coming up with on longer focal lenght eyepieces.

#17 Vondragonnoggin

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Posted 23 January 2014 - 08:09 AM

Bryan,

Thank you for explaining that.

Howard


No prob. In addition to powermates, Seibert I believe makes telecentric devices (some would say these are not actually barlows, some would) that would not change eye relief.


As does Meade, with the "Tele-Extenders". I have the 2x, and it works great and reserves the native ER of the original ep.


Meade Tele Extender is not a barlow, it is an empty tube used for imaging:
http://www.amazon.co...er/dp/B00023...

Meade TeleXtender is a telecentric barlow, with a very confusing name:
http://telescopes.ne...elextender.html


Explore Scientific carries this line now and they are called focal extenders, but they are Telecentric Barlows. Just picked up a 5x last month. I have the Meade 2x, 2" TeleXtender also. Had for four years now and used frequently.

#18 Starman1

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Posted 23 January 2014 - 03:11 PM


Most types of Barlows will increase the eye relief of an eyepiece. The exceptions to this are telecentric devices like Powermates which do not change the eye relief at all.


I am quite sensitive to changes in eye relief and IME I would say that the TV 2.5x Powermate slightly decreases eye relief! I know that may be a controversial statement but that is what I have experienced time and time again. This was experienced when used with Pentax XW eyepieces.

The TeleVue PowerMate is parfocal with the eyepiece (the focus position of the focuser doesn't change when you insert the PowerMate), but only IF the focal plane of the eyepiece is coincident with the shoulder in the eyepiece barrel. If the focal plane in the eyepiece is above that or below that, it will not be parfocal.
The ES focal extenders all require additional in-travel of the focuser, so they did not design in a parfocal characteristic.
Yet, both are telecentric designs. Magnification does not change with distance from the lens (OK, doesn't change significantly), and eye relief should not change in the eyepiece used in one.

If you found a slight decrease in eye relief in a particular eyepiece, odds are the eyepiece had a negative lens assembly in its bottom that depended on a particular angle of light entering to create a particular eye relief. By altering the angle of light entering, it changed the internal way light was handled in the eyepiece. I would also bet the PowerMate didn't yield 2.5X with that eyepiece, either.

#19 SteveG

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Posted 23 January 2014 - 03:12 PM

Yes - I have the ES 3x focal extender. I believe these are the same as the Meade TeleXtenders.


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