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Equipment Discussions >> Binoviewers

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Eddgie
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Reged: 02/01/06

Eyepeices for widest true field?
      #5772819 - 04/02/13 12:31 PM

Ok, I was doing some aperture loss testing on my C5, and as a result of that, I had occasion to measure the widest possible true field that I could get out of my binoviewers.

Those that remember my quest for brighter deep sky remember that I had bought a pair of 32mm and 40mm Plossls in an attempt to have some choices with a bigger exit pupil.

As it turns out, I found myself using the 40mm Televue Plossls far more than the 32mm Generics. The view was as expected, much brighter because of the exit pupil, but the generics seemed to have a narrower true field and the soft field stop that I detest. I prefer a really sharp field stop.

Anyway, as a result of trying to diagnose some descripencies with my aperture reduction testting, I decided to do True field test comparing these eyepeices.

Honestly, I had expected a different outcome. Really. And I will explain why now.

In a moving mirror telescope, as the back focus increases, the focal lenght of the telescope increases both rasing the magnificaiton and lowering the true field size.

But this is not a great change, so should be hard to see.

Still, the Televue requires the absolute minimium amount of back focus of any eyepeice I own.

I was surprised that even though the Baader Hyperion and the Televue 40mm Plossl both have field stops very near the end of the barrel, the 24mm Hyperion takes more than a touch to move the focal plane far enough to the rear to reach focus. I would say maybe a full turn!

With the advantage of a shorter focal length, it would appear that the Televue had the edge.

But that is not how it worked out.

The winner (by a bit more than a whisker) was the Hyperion 24mm.

For a test, I used a distant shed at about 75 feet, and use features in the asbestos shingles on the side to compare true field.

And even though I had to move to increase the focal lenght of the scope to reach focus with the Hyperion, it still squeezed out the biggest true field. Not much. Not much at all. The width of two shingle butt gaps (one at either side of the field) as viewed from 75 feet.

So, small, but enough to add a couple of stars at the edge of the field in a tight cluster.

The surprise was the generic 32mm Plossl.

Because of the softness of the field stop, I had just assumed that this eyepeice used the edge of the barrel for a field stop. This caused me some problems in my apeture measurement because since I thought these eyepieces all gave about the same true field and because the the field stops were all (I thought) in about the same place, that the difference in focus position was minimal.

Boy was I wrong.

As it turns out, the generic 32mm Plossl uses a field stop well up inside the eyepiece barrel. It must be improperly placed though, because it is so soft. The view almost appears vignetted.

And I thought that the 40mm gave a slighlty wider true field, and seemed much brighter than the exit pupil differences alone would account for.

Well, here is what I found.

First, the generic 32mm Plossl took by far and away, the most back focus to reach focus. It seemed to take a couple of turns of the focuser knob which equate to maybe 20mm of focuser travel (and hence the apeture measurment problems).

(Hint.. This means that you may stand your very best chance of reaching focus without a GPC or barlow using the 40mm Plossl).

I measured the field stop, and it is almost exactly 27mm (as is the Televue Plossl vs 28mm for the Hyperion).

And yet the field in the generic 32mm was considerably smaller than the field in the 24mm Hyperion and the 40mm Plossl.

And the difference was not at all subtle! Where the difference in the 40/24 was the thickness of a shingle butt line on either side of the field, the difference in the 32mm was on the order of several nail head diameters! Now considering that a nail head diameter is appeared about 4 shingle-butt line widths apart, the difference of maybe 8 nail head diameters seemed extreme to me!

The true field was far narrower in the generic 32mm Plossl than I thought it would be.

I do not know if the difference can be attributed completly to the change in focal length.

The diffence in field stop location inside the barrel between the 32mm and 40mm is 21mm.

In an SCT, a 21mm change will change the focal lenght about 50mm. This would mean that in the C5, the focal lenght might go from about 1500mm with the 40mm to about 1550mm. This alone is not enought to account for the change.

Anyway, here is what I would summarize from this.

First, even though it will increase the focal length a bit, the 28mm field stop of the Hyperion (1mm larger than the field stop 40mm Telvue Plossl) still allows it to squeeze out ahead.

The 24mm Hyperion gave me the widest true field of these three pairs but only by a whisker over the 40mm.

More importantly, there is a lesson here. I felt like the 40mm Televue Plossls were indeed giving a better view than the generic 32s. The field was crisp and bright right to the feild stop, and under the sky, I flet like the field was a bit bigger using the 40s than the 32s.

And now the result is in.

If you want the widest possible true field in a 28mm eyepeice, the Hyperion appears to have a tiny edge.

And if you are using a Generic plossl in an SCT or MCT with moving mirrors (though I don't think this is more than a minor problem), you may be loosing some true field and more than a whisker. I am removing the generic 32s from my eyepice case.

And finally, the 40mm Televue may indeed be the best possible eyepecie for reaching focus in any telscope.

If you are only a tiny bit away from reaching focus with your current low power eyepeices and are forced to use a GPC, OCS, or barlow, the Televue 40 may give you just enough in-focus that you can now get there.

The 24mm Hyperion like the Televue 40, aslo requires perhaps less in-travel than many other designs.

Hope this is useful info for you. It took me a lot longer to write it up than to test it.

24mm Hyperion does indeed seem to offer the widest possibe true field in a 1.25" eyepiece, and much much wider than at lest the generic (sold under many brand name) 32mm Plossls that I bought.


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DaveJ
Carpal Tunnel
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Reged: 01/07/05

Loc: NE Ohio
Re: Eyepeices for widest true field? new [Re: Eddgie]
      #5772981 - 04/02/13 01:33 PM

Quote:

24mm Hyperion does indeed seem to offer the widest possibe true field in a 1.25" eyepiece, and much much wider than at lest the generic (sold under many brand name) 32mm Plossls that I bought.




How about the 24mm Panoptics? From everything I've read over the years in the binoviewer forum, the 24Pans are reputed to have the widest true field available in a 1.25" eyepiece.


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Tamiji Homma
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Reged: 02/24/07

Loc: California, USA
Re: Eyepeices for widest true field? new [Re: DaveJ]
      #5773058 - 04/02/13 02:18 PM

You can get a little wider with Ultrascopic 35 (field stop 29mm), Hyperion 31 with 1.25" nosepiece (field stop 31mm) given you have vignetting.

Tammy


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Eddgie
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Reged: 02/01/06

Re: Eyepeices for widest true field? new [Re: DaveJ]
      #5773180 - 04/02/13 03:41 PM

One would think that maybe Televue's claim is now dated.

Since the field stop of the eyepiece determines the true field size (assuming that there is no other component before it in the light path that acts as a field stop) then the eyepiece with the largest field stop would have the widest true field.

The Televue site lists the field stop of the 24mm Pan as 27mm.

The field scope of the 24mm Hyperion is 28mm. that is a 2% increase in field size. Not much, but bigger is bigger.

As for the true field, that is somewhat limited by the size of the aperture in the bino in the sense that if the rear aperture of the bino is smaller than the field stop of the eyepiece, it may in fact reduce the size of the true field.

On eyepieces where the field stop of the eyepiece is very far down in the barrel, the rear aperture of the bino may in fact act as the real field stop of the system. Because it is not in focus, it looks like vignetting, but in fact, it is just the edge of the more distant unfocused field stop.

The aperture of the Mark V (as shipped) is 28mm, so using an eyepiece with a larger field stop may not really be giving a wider field than the 24mm Hyperion.

Hopefully someone can test it.

I believe that someone also had their Mark Vs modified to be able to take a 30mm field stop, so it may be possible that the 31mm Aspheric could go wider, but only if the bino aperture could be made bigger.


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MrGrytt
scholastic sledgehammer


Reged: 07/28/05

Loc: Upstate Cuomostan
Re: Eyepeices for widest true field? new [Re: DaveJ]
      #5774400 - 04/02/13 11:11 PM

Quote:


How about the 24mm Panoptics? From everything I've read over the years in the binoviewer forum, the 24Pans are reputed to have the widest true field available in a 1.25" eyepiece.




Right on.

Harvey


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rodnocjolly
member


Reged: 10/13/12

Re: Eyepeices for widest true field? new [Re: Tamiji Homma]
      #5778237 - 04/04/13 06:02 PM

Quote:

You can get a little wider with Ultrascopic 35 (field stop 29mm), Hyperion 31 with 1.25" nosepiece (field stop 31mm) given you have vignetting.

Tammy




+1 I really like them, much better than the Pan 19mm


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Eddgie
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Reged: 02/01/06

Re: Eyepeices for widest true field? new [Re: rodnocjolly]
      #5783438 - 04/07/13 09:55 AM

Once again, the limit (unless the specs are wrong) is 28mm imposed by the rear apeture of the binoviewer.

You can put in an eyepeice with a wider field stop, but you still get limited to a field with the same as a 28mm field stop.

This has already been tested and reported. Once you go past 28mm, there is no more improvement in true field size.

It does not look like "Vignetting." In these eyepeices, the rear aperture is so close to the field stop of the eyepeice that it is almost in focus.

As a result, it still appears pretty sharp, but you get cut off at 28mm.


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Tamiji Homma
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Reged: 02/24/07

Loc: California, USA
Re: Eyepeices for widest true field? new [Re: Eddgie]
      #5783565 - 04/07/13 11:11 AM

Quote:

This has already been tested and reported. Once you go past 28mm, there is no more improvement in true field size.




Hi Ed,

I have tested TFOV with Panoptic 24, Ultrascopic 35, and Hyperion 31/1.25" nosepiece on MarkV without GPC.

Ultrascopic 35 gives you about 6.6% wider TFOV than Panoptic 24,
Hyperion 31/1.25" nosepiece gives you 14% wider TFOV than Panoptic 24.

Last 4% of Hyperion 31/1.25" nosepiece view has a lot of vignetting but Ultrascopic 35 is acceptable, I am happy with Ultrascopic 35 on MarkV.

Tammy


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DRodrigues
super member


Reged: 08/08/11

Re: Eyepeices for widest true field? new [Re: Tamiji Homma]
      #5784432 - 04/07/13 06:02 PM

On my day-light bino combo http://www.pt-ducks.com/cr-telescopes.htm#CR-binoviewing I found that the largest TFOV also was obtained with the Hyperion 24 (don't use them since the eye-relief isn't good enough for a comfortable observation with eye-glasses...), the Vixen NLV 40 and the Meade 5000 26/60.
The TV Pano 24 has similar TFOV to the TS Planetary HR 25 (as also the new Meade 25/60) and the TS SuperView 32. This induced me to feel that the NLV40 almost had similar AFOV to the TS SV 32...
The NLV40 has the best image quality but as I need higher magnifications I use the Meade 5000 26/60 since with my combo shows the flattest field at minimum magnifications, almost as good as the NLV!


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Eddgie
Postmaster
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Reged: 02/01/06

Re: Eyepeices for widest true field? new [Re: Tamiji Homma]
      #5785801 - 04/08/13 11:57 AM

Well, I don't doubt that these other eyepeices give a wider true field than the 24mm Pan.

Even the 24mm Hyperion has a wider field stop.

That is why I said above that Telvue's marketing may be outdated. Clearly there are wider true fields avilable from other eyepices today.

And I should have been clear in my post title.

The results were of course only valid for the three eyepeices I tested.

Of those three eyepeices, I got the widest true field from the 24mm Hyperion.


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