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Wide apparent field, or big exit pupil. Results..

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#1 Eddgie

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Posted 20 February 2013 - 11:24 AM

Ok, the results are in.

About 2 months ago, I started to become more concerned about the dimming associsted with binoviewer usage.

I noticed that in my C14, I used to use 13mm eyepecies in monoviwion quite frequently when seeing allowed, but when using binoviewers and a 13mm eyepiece pair, I just felt like the view was to dim to do my best observing. Instead, I found myself "Topping out" with a 15mm eyepeice pair even when seeing was exccellent. Higher than this and fiant detail just didn't seem to jump out the way it does at 266x using the 15s.

According to Edz's postings on the topic, when summation is factored in, binoviewers give an apparent brightness of about 70% of that you get when nonoviewing. This means that the 15mm Plossls were only giving a view that was about the same as my previsous result using 13mm Plossl (and even higher magnification).

Now I beleve that the ability to use two eyes on planets is indeed a huge advantage, and I seem to be seeing as much or more detail as I did using higher powers and monovision.

But this got me to thinking.. What is happening on the brighter extended objects in the sky. They look great, but honestly, I know that I have always been aware that they did nto look as bright using binocular vision.

And I missed some of that brightness.

The lowest power eyepiece pair I had with my Mark Vs was a pair of 24mm Hyperions. I noticed that when viewing clusters, the binoviewers offered with the 2468s offered a very immersive view and while I could tell it was a bit dimmer, it was not enough to be bothered by.

On extended targets though, the difference was more apparent. Showcase objects like large Nebula or Galaxies appeared dim enough that I started to feel that the 70% effective brightness imposed by the binoviewrs coupled with the very small exit pupil caused by the 2468s was hurting the view.

With all telescopes, the way to make something brighter is to use a bigger exit pupil. We tend to forget that in the days of 120mm Super-ultra-duber uber apparent field eyepices, but it is indeed a basic premise of telescope operation. The bigger the exit pupil (lower the power) the brighter an object will appear (though at a smaller image scale).

Anyway, I became interested in an alternative to give a brighter view than the 2468 Hyperions.

Now it is interesting to note that the 2468 Hyperions give almost the widest true field possible in the Mark Vs, and by going to longer focal lenght eyepices, I would be trading apparent field and image scale for a brighter image.

My initial effort was to maximize the exit pupil, and for this, I went to a pair fo 40mm Televue Wide Plossls. I reported my result here earlier, but for a recap, here it is... I liked them a lot. The view of Orion Nebula was wonderfully bright, and because of the low magnificaion, even on nights of less than good seeing, stars remained tiny pinpoints all the way across the field. In the C14, the lower magnifiction also eliminated the abberations at the edge of the field from the coma and field curvature of the scope so that the view accoss the rathar narrow 43 degree apparent field was wonderfully crisp and bright. I have to say that I was surprised, though I should not have been. The 41mm Panoptic was one of my most used eyepices in the C14 before getting binoviewers.

On the other hand, the view while bright, lacked the image scale (to be expected of course) but more importantly, the immersiveness that wide field eyepeices offer (and the Hyperions to me are similar to the T4 Naglers in the sense that they provide an "Immersive" quality that I don't find in some other 68 degree designs).

The next logical step was to "Split the difference" and go to a 32mm Plossl pair. This would hopefully be a middle ground... A bit more exit pupil than the 24s, restoring some of the brightness that results from the binoviewer, but retaining a little more apparent field, lost to the 40s.

I bought an new pair of very inexpensive 32mm Plossls (I don't think you have to spend a lot of money to get excellent eyepeice performacne these days) with a 52 degree apparent field.

The first thing I will admit though is that the eyepcies have excellent performance in general, they do not have the super-sharp field stop of the Televues. I have noticed this from other low cost Plossls. Not a big deal, but I like the razor sharp edge of field of the Televues.

Otherwise though, they are fine performers, and over a couple of nights over the last few weeks, I tried viewing multiple objects with all three pairs.

And here is what I found....

The 32s did indeed split the difference in performance. The slighty higher magnification over the Televues gave a bit of image scale improvement, but at the expense of magnifiying the field curvature and coma of the scope, and the bloating of seeing. The view was brighter though, and it was like observing in monovision using the 2468s.

I have to say, I like this compromise. It does indeed lack the immersiveness of the 24s, but still retains enough sharpness that for brighter clusters, I preferred the view over the 24s. Same size true field, but the lower magnificaiton rewared me with less bloated, sharper stars everywhere in the field.

The 40s though give such pinpoint stars (agian, not better eyepcies, just lower magnificaiton) that for bright clusters, I think they gave the best view.

The fainter the cluster members, and the denser the cluster, the more I preferred the 24 Hyperions. For example M37 is bright and jumps out in the 40s, but seems richer and more 3D in the 24s.

And in the 32s, it was just as I expected, a compromise. Not quite as immersive with stars apparing alightly sharper than the 24s, but still a compelling view.

Large Nebula I have to say are quite differen. Orion really leaps out in the 40s but since it fills the entire field in all three pair, it is not so easy to notice how bright it is on the whole, but some of the larger, fainter structure was clearly more visible in the 40s. The 24s though provided a more immersive look at the complex pillowing and lane structure in the bright heart of the Nebula.

I tried may different targets, and in the end, this was what I concluded.

All three pair have their place. No one should underestimate the role that exit pupil plays in making an image brighter. The 40s were the absolute champion. Everything looked super bright and super sharp in the 40s. There is just on denying it, a big exit pupil is still your freind, and maybe more so in binoviewing where the binovewers themselves lower the brightness.

The 24s excell for small faint clusters because they give such an imersive and 3D feeling.

I found myself going back and forth between these two more than stopping up to the 32s.

But then.. I would put something in the eyepice that was just a bit to small and bright for the 40s, but a bit to large and bright for the 24s (I find the immersiveness to be less when the stars are bright and bloated by the 24s magnificaiton), and when I put in the 32s, I would admit that in that instance, it was the right pair.

I know it sounds silly, but I now have three pairs of eyepeices that give the same true field, but offer three ver differnt looks and feels, but that for some kinds of targets, one of these three pairs provides a better view.

I fact, while I found myself going back and forth between the 40s and 24s more often than not, I still liked the view well enough in the 32s that I am thinking of buying a pair of 32mm Telvue Plossls. I dislike the soft field stop of the inexpensive Plossls. I know that sounds crazy, spending a couple of hundred dollars on a pair of Televues, but for me, $200 is not so much money that I can't get what I like.

I hope that others find this post useful. If you are using a big prism bino, my advice would be to add a pair fo 40mm or 32mm Plossls if you don't already have some. You can buy them new these days for less than the cost of a tank of gas. (well, not me, becuse my car gets 65 MPG and I fill up every 6 weeks for $30, but you get the idea).

No, you won't sell your 24mm Panoptics. I am thinking of buying a pair to replace my Hyperions I (though I am on the fence... The Hyperions I don't think have as good an off axis performce, but again for dim clusters, this doesn't matter, and I think I prefer brighter clusters in the 32s). But with the price so low, you may find that many brighter objects are simply restored to mono-vision brightness levels just by moving to a slighty larger exit pupil.

Worth a try. I was pleased with the experiment and the result I now feel like all three pair have their role to play.

#2 t.r.

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Posted 20 February 2013 - 11:34 AM

You just stumbled upon why I have pairs of eyepieces from my 4.8 Naglers all the way to 40mm's plossls, in increments of only a couple millimeters for binoviewing, 15 pairs in all! It is NECESSARY for the reasons you have concluded! Powerswitches and compensators be damned! :lol: Now, take it one step further and get a pair of zooms! ;) Limitless exit pupils for just the right occasion. :p

#3 REC

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Posted 20 February 2013 - 11:41 AM

Very good report.....I'm getting out my BV's out tonight!

#4 mhilscher

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Posted 20 February 2013 - 12:39 PM

thank you, nice write-up

#5 Eddgie

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Posted 20 February 2013 - 01:20 PM

Goofy me.. At first I said the 40s were Wide fields, but of course that is not the case.. They are 40mm Plossls.

I regret the error.

#6 Ed D

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Posted 20 February 2013 - 08:26 PM

Excellent, detailed write up. Thanks.

Ed D

#7 tomcody

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Posted 21 February 2013 - 12:10 PM

Eddgie,
Have you had the opportunity to try Takahashi LE30's in your bino's?
If I remember correctly, Tammy stated that they were his most used bino eyepieces. I have wanted to try a pair myself, based on his comment.
Rex

#8 Eddgie

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Posted 21 February 2013 - 12:55 PM

No I haven't.

I doubt that I would though. First they are expensive, and second, for this kind of low power work (for a big aperture, 135x is king of low power) the quality of the the eyepeice is really not at all critical. I am sure the Taks are better, but in the C14, there is not enough magnification to really make this easily apparent.

So, the slighly smaller true field and exit pupil work opposite of my desire to get a brigher field.

But mostly it is price. I can find used TV 32s for reasonable prices.

Even then, I may just use what I have. The 32mm Plossls actually meet all of my requirments. The only thing I do not like about them is that the field stop is not crisp.

The 32mm Plossl I have is on of those designs that uses the end of the barrel for the field stop. These long focal lenght eyepieces that do this often suffer from this problem.. A soft kind of vignetted look at the edge of the field. It is not vignetting, but it kind of looks that way. It is simply the out of focus end of the barrel.

The Televue 40 uses (and the 32 from what I recall) has a very distinct field stop about 3 or 4 millimiters inside the barrel. It is only raised a tiny bit, but it make for a perfectly sharp edge of field.

I may also just post on the eyepeice forum for other inexpensvie 32mm eyepeices that have a distinct field stop. I don't want to spend the money even for a Televue if there is another alternative.

Or, I could just live with it. When the sky is dark, you really can't see it. Sadly, some of my sky is light pollluted and in these directions, I can see the soft edge.

I know, it is a little thing, but I am "The one" for my matrix and in my matrix, I can have a sharp field stop if I want one.. LOL.

But I just don't know if I am willing to spend $200 for it.

So will look for another inexpensive 32mm with a real field stop...

#9 tomcody

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Posted 21 February 2013 - 02:04 PM

I can understand your desire for a sharp defined field, it seems to add to my viewing comfort, perhaps it has something to do with the eye not trying to strain to focus at the very edge of the viewing field which is something I think we do subconsciously, perhaps a survival instinct?
Rex

#10 Eddgie

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Posted 21 February 2013 - 03:46 PM

I honestly don't know why I prefer it. Some people are not at all bothered by the sharpness of the field stop, and others really prefer it (and I of course am in the latter group).

I think that because it resembles vignetting and vignetting is a sign of optical degradation, it just makes me feel like I am using an improperly configured telescope.

I know what it is logically, and that it doesn't really degrade the view, but "emotionally," it bugs me.

Almost the only time I see it is with inexpensive Plossls, but you used to see it on Orthos many times as well.

Again, the inexpensive eyepieces were just a test, but they actually do work well enough that perhaps I will learn to look past it. If I am in parts of the sky that are dark, it really isn't even visible.

I am going to see what other choices I have in the lower priced Plossl range. If I can find another inexpensive pair that does have a true field stop, maybe I will just get them, but I do like the Televue 40s, and I know that the Televue 32 would make no compromise.

#11 thesubwaypusher

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Posted 22 February 2013 - 01:48 PM

That was very interesting, thanks Ed.

#12 aa6ww

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Posted 02 March 2013 - 06:51 PM

Ok, the results are in.


What binoviewer are you using? I'm finding out my denks are a huge compliment to my observing and have been slowly filling an entire arsenal of eyepieces for them. I currently have 40 Plossls, 32 Plossls, 24 pans, 19 pans and 16 naglers.
I cant even begin to describe what my 24 pans do in my 180mm F/6 with my binoviewer. My wife is getting tired of me always having to get her to come see how incredible the views are through them.
I'm about to try my refractors with a .63 focal reducer in them first, with the bino's in straight thru viewing, to see how they perform, to see if I can get an even wider field of view. I know it works beautifully with my TV-85 with one eyepiece for straight through viewing, using my 24 panoptic, and creates a huge wide field sharp view, similar to my 41 pan in my TV, but never tried it with the binoviewer. Im doing this in preparation of our comets which are coming next week since I now perfer 2 eye viewing over one, and with the focal reducer in place, I can get 2" 41 Pan optic views with two eyes this way, with way more contras. Straight through viewing would be preferred since the comets will ever get above 20 degs above the horizon. I should be able to reach focus using my 180mm F/6 since its a binoviewer cut scope and I need a long extension tube when using a diagonal.
Tonight is not looking very clear, but I'm planing on trying my 180mm F/6 and my 150mm F/5 this way tonight. Straight thru viewing with a .63 focal reducer, with the binoviewer.
I'm imaging my 1080mm 180mm refractor reduced to 680mm then using my 24 pans in my binoviewer for comet hunting, with my new 2" swan filter in place. It should be interesting!!

...Ralph

#13 Jon Isaacs

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Posted 03 March 2013 - 06:11 AM

Eddgie:

An interesting topic with interesting results. Fast scopes have other problems with binoviewers though nothing an extra set of truss poles and solve, but if you were working at F/5-F/6 rather than F/11 you could have your cake and eat it too, widefield eyepieces and large exit pupils.

How about a focal reducer for your C-14?

Jon

#14 Eddgie

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Posted 03 March 2013 - 11:10 AM

No, a focal reducer in the light path in the C14 will reduce the aperture to about 12". It is a terrible combination for binoviewing. The image is already dim enough.

In fact, my next scope will likely be a 12" or 14" Go-To Newtonian (can you call them Dobs anymore if they have Go-To and Tracking?).

It has been something I have been considering for several years, but just can't seem to pull the trigger on one.

Also can't decide between 12" and 14". Even though the 14" is faster (f/4.5 I think vs f/4.9 for the 12"), the 14" is still 150mm longer.

If I use a coma corrector, the field of the 12" grows proportionaly larger.

The true field of the C14 even with a 41mm Panoptic is nothing to write home about.

Had a well packaged Go-To 12" or 14" Newt been around when I bought the C14, I would have preferred to own one, but I was not at all enamored with the prospect of cobbling one together.

Also, I do have the EdgeHD 8" that I enjoy binoviewing with when I want a bigger true field. Most stuff fits into the field of the EdgeHD 8".

In fact I had the EdgeHD 8" out last night. Excellent performance with the 24mm Hyperions.

Many larger clusters that filled the field of the C14 were beautifully framed in the EdgeHD 8".


But I have been saving for a new Go-To reflector. Most likely the 12".

I don't want to sell the C14 before I have had a chance to get the reflector and have the mirrors re-figured, so I have to save a bit of money first.

#15 Jon Isaacs

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Posted 03 March 2013 - 06:57 PM

Eddgie:

My rough calculations suggest that even at an effective aperture of 12 inches, with a F/6.3 focal reducer, you would be operating at F/7.3. With a 24mm eyepiece the exit pupil would be 3.3 mm, same brightness as a 35mm but with a much wider field of view.

Jon

#16 Eddgie

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Posted 03 March 2013 - 10:59 PM

I really don't want to use my scope as a 12" scope.

I did that with the Denkmeier Supersystem Low Power arm, and the view was so dim I thought I was looking though a C9.

I have no real desire to operate the scope at a 30% light loss penalty.

#17 Jon Isaacs

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Posted 04 March 2013 - 05:14 AM

I really don't want to use my scope as a 12" scope.

I did that with the Denkmeier Supersystem Low Power arm, and the view was so dim I thought I was looking though a C9.

I have no real desire to operate the scope at a 30% light loss penalty.


You are losing light but the image is brighter. If you were operating at the same magnification with your 8 inch SCT the image would be a whole lot dimmer...

I am not sure how much experience you have viewing through Newtonians but my gut feeling is that the important differences between the Newtonian/Dob experience and the SCT experience are not at the eyepiece, that is how it is for me anyway.

Jon

#18 Eddgie

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Posted 04 March 2013 - 10:03 AM

Jon, I am not sure that we are on the same page here.

What I am talking about is the physical reduction in aperture that occurs when you use a focal reducer and a binoviewer in an SCT.

The mirror has to be moved so far forward to reach focus that the light cone gets cut of.

The scope itself gets turned into a physically smaller apterure.

It isn't that the binoviwer dimming alone is causing the appearance of a dimmer image. It as very real and very large physical reduction in the aperture of the telescope itself.

I do not have this problem when using the binoveiwers straight through. The scope opertaes at the full aperture.

But a focal reducer with binoviewers requries so much back focus the real aperture reduction is (to me any) extremly dibilitating.

Whith the Binoviewer and focal reducer, I felt like I was using a C10. The view was horribly dim.

The contrast also nose dived because of the larger central obstruction (by percentage) that resulted from the reduced aperture.

I have people tell me all the time that they can't see the difference when they use focal reducers and binoviewers, but to me the effects were glaring. It was soooooo easy for me to see the the apertue was being reduced that it wasn't funny. The results (in my own book) were a total disaster.

If others are OK using their C14 as a 50 lb C11, that is ok with me, but I personally found the damage to be so bad that I would never do it for any reason at all. The extra true field would not be worth it if the added field was dimmer than what I could get in a C8 giving the same true field.

#19 Jon Isaacs

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Posted 04 March 2013 - 11:32 AM

Jon, I am not sure that we are on the same page here.

What I am talking about is the physical reduction in aperture that occurs when you use a focal reducer and a binoviewer in an SCT.

The mirror has to be moved so far forward to reach focus that the light cone gets cut of.

The scope itself gets turned into a physically smaller apterure.


I think we are on the same page. My point is that a 14 inch F/6.3 that has an effective aperture of 12 inches is still a 12 inch F/7.3 (or so) and so with the 24mm Widefields, will provide brighter views with a wider true field of view than a 14 inch F/11 with the same eyepieces.

Jon

#20 Eddgie

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Posted 04 March 2013 - 05:05 PM

Ah, I see.. Forgive me for not understanding... I see your point.

But honestly, I am pretty content with monovision. I use the EdgeHD 8" when I need a wider field.

The EdgeHD 8" is like using my 6" APO. It is just so amazingly sharp across the field.

Anyway, I see your point now.

Maybe I will try it again some day, but I am not really wild about the idea.

Also, my viewing tends to run towards mostly stellar targets. Too much light pollution for all but the brightest Nebula.

#21 Jon Isaacs

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Posted 05 March 2013 - 06:28 AM

But honestly, I am pretty content with monovision.


Me too.. :)

I am big on widefields of view, 4 degrees, even 6 degrees, but I regularly observe where the skies are dark and clear and Milky Way is bright and shining.

Jon






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