The Effects of Eye Relief using a Barlow Lens and Eyepiece

[leinad]

By request of scooper, erick and others.
This thread is intended to discuss the topic:

'The Effects of Eye relief using a Barlow Lens and Eyepiece'

I'd be interested in making this an informative topic if possible, so please feel free to provide links and words of experience. They'll be added to this thread topic with approved recommendation by members.
Please let me know if the topic is a little too general and a better title could be used.

So to start with here are a few quick primers.


What is Eye Relief?
-------------------
http://en.wikipedia.org/wiki/Eye_relief

What is the Exit Pupil?
---------------------
http://en.wikipedia.org/wiki/Exit_pupil

What is a Barlow Lens?
-----------------------
http://en.wikipedia.org/wiki/Barlow_lens
http://www.hypermaths.org/quadibloc/science/opt0501.htm



(Thanks Karlsson for Barlow lens links)

Gentlemen..start your posting.. :)

[Scoper]

Hi Leinad and many thanks for starting this thread.

It is just curiosity on my part but it appears to be an accepted fact that a barlow, apart from increasing the focal length of an optical train, will also increase the effective eye relief of any given eyepiece. I have two problems with this:

1. With the eyepieces I use, the eye relief remains unaltered when using them with a barlow.
2. On the internet I can find examples of those who attest the truth of an increase in eye relief and those who state that it is false but I can't find a web page which explains this phenomenon in optical terms though there has to be a web site out there that can shed light on this subject.

I understand the principle of placing a negative lens in an optical train to increase focal length complete with the arithmetic involved (I used to make my own barlows from old camera telenegative lenses in the bad old days) but I think i'm missing something here as I don't get the increase in eye relief with my plossls. The type and focal length of an eyepiece may be a large factor in whether a barlow increases the eye relief. And, of course, I may be doing something wrong when checking for eye relief.

Anyway,this should be an interesting thread, thanks once again Leinad and lets hope someone in the know can shed light on the subject.:)

Cheers
Malc

[Rodstar]

I have been doing some reading about the effects of Barlows in recent hours, and from what I have picked up, the effect of extending the eye relief is more exaggerated with the longer focal length eyepieces. So, for example, you might not notice a great difference with an eyepiece in the 5-15mm focal length range, but the effect may be quite marked (such as an increase by 5mm) in an EP in the 20-30mm focal length range.

Others with practical experience may wish to confirm or deny this.

BTW, one of the benefits of the Powermate is that apparently it does not increase the eye relief. So if this issue is a major problem for you, a Powermate may be worth the added investment.

[erick]

Rather than a problem, many would see it as an advantage. If they could increase the eye relief of short focal length plossls, say 9mm and below, they would be grateful. I bet many eyes have been frozen to 6mm plossls in cold conditions. Of course, the increase in apparent focal length and consequent increase in magnification may not help the visual observer much if the seeing won't sustain that magnification.

[leinad]

Majority of discussion I found was exactly how you have mentioned Rodstar.

As a beginner I found this quite useful:
http://www.televue.com/engine/page.asp?ID=107

Interesting to know if eye relief increase can be calculated with long focal length eyepieces and Barlow lens?

[Scoper]

Hi all,
Yes it appears that the increasing eye relief effect is more pronounced with long focal length eyepieces, eg, > 20 mm. This would explain why i don't measure a noticeable difference as the longest focal length eyepiece I use is 17mm.
The reason for the increase in eye relief is because the barlow, after intercepting the light-cone, narrows the cone, therefore increasing the focal ratio of the optical system as a whole. In doing so the exit pupil is pushed back but if that is the case then the eye relief of a given eyepiece must be variable. It follows that using the same eyepiece in telescopes of different focal ratios (leaving barlow lenses out of it for a while) must alter the eye relief also. Does anyone have any experience with this?

Cheers again
Malc

[Kokatha man]

Eh? - a barlow is a negative (concave) lens and it "expands the cone," not narrows.

[Scoper]

Ok. The convergence of the light cone is reduced; the angle of the cone is made less steep by the barlow; that is what i meant by it being narrower. With a 2x barlow the convergence is reduced by a factor of two therefore the focal ratio is doubled.

[Kokatha man]

Er, yes, Scoper; but no-one's questioning the above, that's the reality. Perhaps you could tie that into its' relationship to the question at hand?

Cheers, Darryl.:whistle:

[Scoper]

I thought I did that in my first post yesterday. Anyway, last night I was able to try out a couple of eyepieces and two barlows I rarely use: a generic shorty 2x barlow; a GSO 2" 2x barlow; a 25mm plossl and a 30mm superview.

In the shorty barlow the 25mm plossl did indeed exhibit increased eye relief, it was awkward to use having to hold my head away from the eyecup, however, I also noted that the field stop of the 25mm was not visible and the field had narrowed due to vignetting by the barlow. In the GSO barlow the increased eye relief effect was greatly reduced, I could still hold my eye on the eyecup but only just, there was also a small amount of vignetting in this barlow. The 30mm superview retained its eye relief in the GSO barlow as far as I could tell with no noticeable difference with or without the barlow; also there was no vignetting in the superview.
I did this test at 2 AM after working an evening shift and I admit that tiredness and hasty set-up mean that this is a tentative result. I hope to test and measure the results again when time permits.
Cheers
Malc

[Kokatha man]

"Ok. The convergence of the light cone is reduced; the angle of the cone is made less steep by the barlow; that is what i meant by it being narrower. "

I wasn't meaning to be rude Malcolm, it's just that neither of those 2 quotes above really make sense (not saying you don't know what you mean; just that it isn't correct to call a beam that diverges (expands) as "narrower" whether you say "the convergence of the light cone is reduced" or "the angle of the cone is made less steep...")

I've found myself (without really taking the time to check various eps like you've just done) that eye relief is enhanced with the use of a barlow: which is why I made my original comment: but I wouldn't disagree with your trials that found that the longer f/l eps showed this more markedly - I haven't tried what you've just done, and I accept your results.

I'd just like to hear/see some optical science/illustrations that explain the situation; to be able to get my head around it all from a logic aspect, that's all.

Best regards, Darryl.

[tbentley]

Here's my idea on how it works but I'm relying on first year engineering physics which I learnt over 13 years ago so I'm happy to be corrected on any of this.
The first diagram shows the basic effects of a barlow on the light path in a telescope. I've used a refractor for simplicity but a reflector is identical after the objective anyway of course.
The second diagram represents the light path of the outer edge of the image both with a barlow and without. My understanding is that exit pupil and eye relief are directly proportional, therefore larger exit pupil = more eye relief. If this isn't right then my whole theory falls down anyway and the diagrams are useless.
I believe that without a barlow the image is focused with it's outer edge well inside the limits of the eyepiece lens. The additional divergence created by the addition of a barlow lens pushes the image edge closer to the edge of the eyepiece lens. If the edge of the image is already at the limits of the lens without a barlow then the addition of one will make no difference to the eye relief.
Hopefully someone can confirm or correct this as I've tried to work it out from a very basic level of understanding of optics and could therefore be way off.
Travis

[syzygy]

Sorry to say, but it isn't right. :( The 2 things are unrelated.

Exit pupil is a function of the telescopes aperture and the magnification used:
aperture in mm / magnification = exit pupil in mm. eg. with a 100mm F10 telescope, a 20mm eyepiece will give 50x and a 2mm exit pupil.

Eye relief is a function of the eyepiece design and it's focal length. I do not believe it is affected by a barlow (which simply changes the telescope's effective f-ratio), but I don't use a barlow myself, so I can't test it.

[Scoper]

Here is a link which describes the funtion of a barlow lens complete with a ray diagram and a formula. There is nothing in this link to suggest that eye relief is modified by the barlow. Usng the formula it can be seen that the cone of light from the objective is modified by the divergence of the barlow and this modification is very sensitive to small changes of the distance of the barlow inside the focal plane. The only effect should be an increase in focal ratio which should not affect eye relief as far as I'm aware.

http://www.astunit.com/tutorials/barlow.htm

[tbentley]

Thanks for that Syzygy. You've made me realise that my knowledge of what eye relief is was sorely lacking. Consequently I've done a little more research and tried to apply this to a barlow lense. What I've come up with is the attached ray trace diagram.
The top diagram shows how eye relief works with a standard objective->eyepiece scenario. The bottom one I've inserted a barlow and shown how the light rays are affected by this. Assuming that I've got it roughly right you can see that the eye relief is affected (lengthened) but how much would be dependent of focal length of the lenses of course.
How does this sound? As I say I have very little knowledge of optics (a bit more now than when I started though) so I'd be happy to be corrected. Anything that brings us closer to the truth is good.
Trav

[Kokatha man]

"Many eyepieces have an eye relief (distance of exit pupil from eye lens) that is directly related to its focal length. For example, the eye relief of a Plössl is 0.73 × its focal length. Thus, with these eyepieces, for a given magnification there will be greater eye relief with a barlow than without."

The above is taken from Scoper's link - I'm not picking on you Scoper (glad you're part of this thread actually) but it does say that.

Which is to say that this thread is absolutely contradictory: or at least darn confusing!!??!!

No offence to anyone here - but jist wot is we sayin???

Regards, Darryl.

[Scoper]

Thanks Darryl, yes you are quite correct, the link does say that. Quite confusing in a way because no reason is given in the ray trace or the formula for this, unless I've missed something. My tentative experiment the other night suggested a shift in eye relief but not in a consistant way. It realy needs to be measured, I have done that before but it's a bit tricky if you want to be exact. I wish someone could find an optical explanation for the shift in eye relief.

[tbentley]

That staement confused me when put in the context of the ray trace of the barlow. The diagram shows an increase in the focal length of the objective and doesn't affect the focal length of the eyepiece so therefore(?) the eye relief shouldn't be affected if the above is true.
Any comments on my second attempt at an eye relief ray trace diagram Scoper? If it's right then it seems to explain the phenomenon but I can't find confirmation anywhere. Is there an optician in the house?
Trav