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Building an eyepiece strategy on exit pupil and 'Ethos'


Well, not just exit pupil, but of the various numbers associated with eyepieces the exit pupil is a good way to set one limit. My new 'Ethos' arrived - the most expensive eyepiece I ever dreamed of purchasing - and that's what really got me to thinking about how to best use it in various scopes and developing a whole new strategy both for telescope use and mixing and matching eyepieces.

What I want in an eyepiece - and I suspect most of us want - is the widest field obtainable at the highest powers usable without sacrificing quality and that's where the Televue 'Ethos' comes in - it's a 13mm eyepiece with a 100-degree apparent field of view - unheard of in any eyepiece until it came out last year - and if the reviewers are correct, it maintains quality to the edges of this field and also provides great contrast. I can't wait to try it on the 15-inch Obsession and M51 is going to be my first target - just as soon as we get through the required week of rain and clouds ;-)

But I'm not complaining. The clouds have set me to reviewing my whole eyepiece situation and that has led me to evolving a sort of best strategy for using the various scopes I have, most of which were bought with public sessions in mind - but right now I'm in selfish mode. How can I use all these neat toys to my best advantage? Seriously - what I use frequently depends on mood, time available, my energy, seeing conditions at the moment, intended target and did I mention whim? Well - that's closely related to mood.

But as I started down the "best" eyepiece route I kept wandering off related side streets. Here's how it went.

First, having invested $620 in the 'Ethos,' how do I get the most out of it? Well, it will work on each of the scopes and here's what it delivers in terms of power and true field of view as approximated by the power divided into the apparent field of view.

Scope | power | true FOV
15-inch F4.5 | 130X | 46'
8-inch F10 | 154X | 39'
120mm F5 | 46X | 210'
100mm F9 | 69X | 127'
80mm F7.5 | 46X | 210'
66mm F6 } 31X | 314'

Next issue - how to double the power - and my solution is the 2X Televue 1.25-inch Barlow I already own. According to the instructions with the Ethos that will not vignette the fov - pleasant surprize there. I had assumed I needed to purchase a 2-inch "Big Barlow" for $200, or a PowerMate for $300. Not anxious to do either, so. . . again, according to the instructions with the 'Ethos' - this is the first eyepiece I ever owned that came with two pages of instructions - but according to these, this 2X Barlow will actually increase the power by a factor of 2.17X. So, my table of powers and fields now looks like this:

Scope | power | true FOV
15-inch F4.5 | 282X | 26'
8-inch F10 | 334X | 18'
120mm F5 | 100X | 1
100mm F9 | 150X | 40'
80mm F7.5 | 100X | 1
66mm F6 } 53X | 153'

Isn't this fun? But this is only the start. We haven't even touched the exit pupil business yet because it really isn't critical with the 13mm. The exit pupil there is well within bounds. It's with the wider field, low power eyepieces that I get concerned about it. What it boils down to is this. My old eyes can't handle any more than 5mm of exit pupil, I'm pretty sure, if they can handle that. This means for each scope you can calculate an extreme eyepiece size simply by working backwards from what delivers a 5mm exit pupil. Simply divide 5 into the objective diameter of the scope. So, for example, with the 15-inch scope you divide 5 into 375mm which gives you 75X - which is the lowest power I can use. What focal length eyepiece yields that power? Simply divide 75 into 1687, the focal length of the scope. The answer 22.5mm.

So the longest practical focal length in this case is a 22mm Nagler or if I want to stick with lower priced, existing eyepieces, a 21mm Hyperion. Should the 22mm Nagler be high on my list of potential purchases? They sell for $480. On the 15-inch that would yield 77X and a true field of 14'. Hmmm... is the extra fov worth the price? The trade offs here are the Nagler gives a 46' fov at 130X, vs 18' more at 77X. On this scope the prime targets are deep sky and the extra power is frequently important, so let's see what the comparative fields look like on a typical object, such as M35.


Aha! What's clear from this is the 22mm Nagler is worth it. Why? Because I not only like how the extra field frames M35, giving you a sense of context, but more importantly it brings NGC 2158 into the picture and I love being able to point out these two simultaneously to people because they are both open clusters of roughly the same size, but NGC 2158 is about five times further away. And what does this say about the Hyperion 21? Well, it's only a slight improvement on the Ethos in terms of field, but the Ethos will deliver almost the same field with significantly higher power, so the 21mm may end up on Astromart.

Now, having jumped to that conclusion, I would not dream of buying the 22mm Nagler without first trying the 21mm Hyperion and the Ethos together because what the Starry Nights simulation does not show you is the difference in power. The Ethos will deliver almost the same fov at 130X as the 21mm Hyperion gives at 80X. i think that's going to be a significant difference that will have to be weighed against the still larger fov - but at 77X - of the 22mm Nagler. And yes, the 22mm Nagler will include NGC 2158 when M35 is in the center of the field of view - but, the 13mm can capture both at once at higher power if you move M35 over to one edge. Bottom line - this is the sort of thing you need to play with - and, of course, i wouldn't make a decision on a single example. But I think it's a good illustration of the pros and cons and complexities of the eyepiece choices.

But to see where exit pupil really makes a difference we need to consider a second example - how this all plays out in the 8-inch SCT. Setting a maximum exit pupil of 5mm on this scope looks quite different. At 200mm a 5mm exit pupil results from a power of 40X. With a focal length of 2000 mm that sets an upper limit for the focal length of a low-power eyepiece of 50mm. Balancing this against apparent field of view of the various eyepiece designs - and price - I came up with a 35mm Panoptic as the best solution. My big assumption here is that Televue deserves its reputation for quality and that others can't match it. But I'm not entirely sure about this. I have an inexpensive ClearVue 30mm eyepiece with an 80 degree apparent field of view.


Oh this is interesting! The 30mm ClearVue (which I think I paid $80 for and is quite nice) gives me the same fov for all practical purposes - and at a higher power (67X vs. 57X) - than the 35mm Panoptic which sells for $380! Of course, I could get a significantly larger field - but at still lower power (49X)- by using a 41mm Panoptic. Of course the 41mm Panoptic is almost in the same cost category ($510) as the Ethos! Hey, I think I'm saving myself some money here! (Wonder if there is an inexpensive - but good - 22mm eyepiece with an 80-degree afov? )

OK - so where we stand now is that the Ethos with 2X Barlow makes a great eyepiece for the 15-inch and 8-inch. That it may be worth it to get the 22mm Nagler, particularly for the 15-inch, but that the 30mm ClearVue is a fine low-power/wide field choice for the 8-inch SCT. Why not use the 30mm ClearVue on the 15-inch? I do sometimes when desperate - but this runs right up against the exit pupil problem. This combination yields an exit pupil close to 7mm and that is wasted on my eye. (Should work for a healthier, much younger person, however.)

It was at this point I began to wonder that if I was heading in the wrong direction entirely by looking at more eyepieces as the solution to nice, low-power view. I got hooked on this idea of wide fields of view decades ago, but now that they are within my reach (financially) and there are several excellent new technical solutions as well - well, I'm not at all sure the super wide fields - at low power - are nearly as desirable as I once thought. What's more, they're easily within reach by piggy-backing small refractors onto the larger scopes. So. . .

Part of the new strategy is to put the 66mm ED (AstroTech) on the LX90 Meade. This works as long as I don't put large eyepieces in both scopes at once - do that and it's hard to balance. But the 66mm would make a wonderful, richest field and finder combination on the LX90. Using it strictly as a finder, a 40mm Plossl would give me a 5-degree fov. With the 12.8 mm lighted reticle it still gives 31X and a 136' fov. And if I want a super view of something like the Pleiades or the Double Cluster in Perseus, why then I'll put the 13mm 'Ethos' in it - that yields 31X and a 314' fov! Think of using that on the Double Cluster, then zooming in by switching the 'Ethos' to the 8-inch and getting 154X and a 39' fov. Makes me drool! Go away clouds!

What about the 15-inch? I'm sure the same combo would work fine. But I may be able to buck up the piggy back scope to the 80 ED. I'll check on the Obsession online forum to see if someone has tried that - not sure the 15-inch can handle it, though I'm sure it's no problem for the ServoCat. If the 80 ED doesn't work there I think it has been driven to AstroMart by the less expensive 120mm ST achromatic. The reason is the greater light grasp of the 120mm - and I still have the 100mm ED as a great solution for planets and double stars.

In fact this last pair - the 120mm and the 100mm should make perfect mates on the Desert Sky Astro Products dual mount I've ordered. On one side you have the 120mm yielding a 4 fov at 20X with the 30mm ClearVue, and on the other you can have a 9mm TMB Planetary giving 100X in a 36' fov. The 120mm is great for richest field work, but doesn't do well on the planets and moon. But that's just where's the 100mm excels, and both could do a fine job on most doubles.

Hmmm. . . now what's the best way to get high power out of the 100mm ED? Well, since it is on a "push-to" mount all the time - not unlike a Dobsonian - then the wider the fov, the better. That says use the 'Ethos.' At 13mm you get 69X in a 127' fov. Put in the 2X Barlow and you have 150X and a 40' fov. To give you an idea what that means, a 6mm TMB would give 150X as well - but a fov of 24' . What's that mean when you're viewing Saturn, for example? Well with the 13mm Barlowed to 150X it will take Saturn about 2.5 minutes - real time - to cross the fov. It would be less than a minute and half for it to pass across the 6mm TMB fov. Bottom line - you will get a much better look before you have to move - and thus jiggle - the scope if you're using the 'Ethos' with a Barlow.

Can you go higher? How about a 3X Barlow? Televue makes one and these don't break the bank - just $110. So - the 3X with the 'Ethos' actually yields 3.25X, reducing the 13mm 'Ethos' to the equivalent of a 4mm. That's 225X and a 27' fov. Not too shabby. But you would need a really good night to benefit from it. On most nights the highest power you could use in the 100mm is 200X, (50X per inch). So would 3X Barlow be worth the $110 investment? maybe. In the 120mm it would give just 150X, well within it's maximum and useful on double stars and such, More importantly, on the 15-inch it would yield 422X and that would be worth trying on an exceptional night. But that's the key. To get the most out of the 3X Barlow I would need an exceptional night.

Oh - and Televue has just announced an 8mm 'Ethos." Is that in my future? I doubt it. What might be in my future is another 13mm Ethos. I mean think of using them both in the Denkmeir binoviewer with the Power Switch that gives me three different powers? Wow! Let's see . . .

(Note to newbies - it wasn't always this way. I've been observing half a century and a good deal of that time my largest scope was a 6-inch Criterion Dynascope and I had three eyepieces - two achromatic Ramsdens and one pricey ($12, I think) orthoscopic. They had apparent fields of roughly 40 degrees , little eye relief, and gave the general feeling that you were looking into a straw. But I did eventually get one fancy, low-power, wide-field eyepiece - a 40mm Kellner.)


What you didn't mention is the increased contrast you get with higher
magnification while maintaining the equivalent fov. This is perhaps the
biggest factor of all.


Posted by Greg Stone at May 3, 2008 09:50 AM Comments? Please email me: gstone@umassd.edu

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