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Equipment Discussions >> Eyepieces

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bdcmd
sage
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Reged: 03/14/08

Loc: Glen Rose, Texas
Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Jarrod]
      #5777197 - 04/04/13 09:07 AM

Quote:



The scope came with a 2" 28mm 56* EP that gives a 1.3* TFOV in my scope, and it does do the job. This might sound silly to some, but the issue I have is that during alignment, I hate fumbling around with the 2"-1.25" adapter when swapping (frequently) between the 2" finder EP and the 1.25" illuminated reticle EP. . . . It should be much more convenient to just swap between two 1.25" EPs and leave the adapter in place. It's a small thing, but it hopefully should make for less hassle and get me observing sooner.




Maybe you should try a 2" illuminated reticle eyepiece to use as a finder AND alignment eyepiece. Stellarvue has their E7032R or E7026R, both about $150, so a little pricey, but very convenient. University Optics has their 32mm 2" illuminated reticle eyepiece also, seems much like the SV version, but only $130, and it looks like it comes with the illuminator, as well. Just a thought, if you are looking for the true minimum hassle way to go.


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Jarrod
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Re: Inexpensive 1.25" maximum TFOV EPs new [Re: bdcmd]
      #5777220 - 04/04/13 09:30 AM

Quote:


Maybe you should try a 2" illuminated reticle eyepiece to use as a finder AND alignment eyepiece.




Hmmm, those are interesting. My only concern would be the accuracy. At 1/3rd the magnification of my 12mm reticle, I think I might not be able to get the stars perfectly centered? I don't know. But it is a good idea...

Edited by Jarrod (04/04/13 09:31 AM)


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Jon Isaacs]
      #5777303 - 04/04/13 10:21 AM

Quote:

Quote:

Mfr's claims, not test results.
I just measured a 24 Baader Hyperion and got 27.0mm, so I don't know where that larger figure came from.
And I recall the 32mm Sirius Plossl had an iris in the barrel, so the field stop of that one is definitely less than 28mm, though I have no way to measure one now.
As I mentioned before, if the manufacturer or a calculator claims a 1.25" eyepiece is over 27mm of field stop, distrust it until you measure it. The Parks Gold Series 35 was 29mm, but then its field stop was above the 1.25" barrel, so was vignetted somewhat by the barrel itself.




Don:

Thanks for the clarification. I know that the 30mm and 35mm Ultima/Parks Gold/Antares Elite/Orion Orthoscopic eyepieces used that set back design with the larger field stop, I have never looked through one... one expect significant vignetting???

Jon



No, it was a reasonable compromise. I never noticed significant vignetting in the eyepiece, though calculations did show a fair amount of light loss at the field edge, because the scopes I was using the eyepiece in could easily illuminate the 29mm field.
In a scope where vignetting from the secondary mirror would result in more light loss at the edge of a 29mm field from the scope itself, then the natural vignetting in the eyepiece might be more noticeable.
But probably not in any scope designed to take 2" eyepieces. A 29mm field stop wouldn't be considered large for a 2" eyepiece.


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Tamiji Homma
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Reged: 02/24/07

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Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Starman1]
      #5777336 - 04/04/13 10:41 AM

Hi Don,

Quote:

I just measured a 24 Baader Hyperion and got 27.0mm, so I don't know where that larger figure came from.




What do you get field stop for Panoptic 24?

I measured:

26.9mm Televue Plossl 32
27mm Panoptic 24
28.5mm Hyperion 24
29mm Ultrascopic 35
31mm Hyperion 31 with 1.25" nosepiece
35mm Hyperion 31 2" mode with Baader Prism Star diagonal (clear aperture supposed to be 34mm)
37.8mm Hyperion 31 2" mode with 2" mirror diagonal

Hyperion 24 is definitely wider than Panoptic 24, no significant vignetting.
Ultrascopic 35 vignetting is acceptable level, not bother me.
Hyperion 31 with 1.25" nosepiece has noticeable vignetting but OK.

Tammy


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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Tamiji Homma]
      #5777417 - 04/04/13 11:07 AM

Quote:

Hi Don,

Quote:

I just measured a 24 Baader Hyperion and got 27.0mm, so I don't know where that larger figure came from.




What do you get field stop for Panoptic 24?

I measured:

26.9mm Televue Plossl 32
27mm Panoptic 24
28.5mm Hyperion 24
29mm Ultrascopic 35
31mm Hyperion 31 with 1.25" nosepiece
35mm Hyperion 31 2" mode with Baader Prism Star diagonal (clear aperture supposed to be 34mm)
37.8mm Hyperion 31 2" mode with 2" mirror diagonal

Hyperion 24 is definitely wider than Panoptic 24, no significant vignetting.
Ultrascopic 35 vignetting is acceptable level, not bother me.
Hyperion 31 with 1.25" nosepiece has noticeable vignetting but OK.

Tammy



Tammy,
I think you meant Baader 24. The Panoptic figure came from TeleVue, and I trust it.
I took a 24 Hyperion out of the box, unscrewed the 1.25" bottom and calipered the opening in the bottom ring. It was 27mm. I didn't see a field stop in the 1.25" barrel, but this eyepiece requires a lot of in-travel, so I think the field stop may be the bottom lens retaining ring. It's possible that the field stop is lower than that, and in the barrel, and if so is larger than the opening in the lens retaining ring, but it would also mean there is no field stop per se in the eyepiece.

EDIT: I just compared the 24 Baader Hyperion and 24mm Panoptic in a TeleVue NP101 on land targets several miles away. The Baader DOES have a slightly wider field of view, but the edge of the field is a little unsharp and a little dimmer. This DOES imply the field stop for the Baader IS the inside diameter of the 1.25" barrel, and so is larger than the opening in the lens retaining ring. Interesting. It also explains why people who have used this eyepiece as a 2" eyepiece without the bottom barrel have commented about the "vague" edge of the field and serious vignetting. From what I see, the eyepiece, if used as a 2" eyepiece, should have the 1.25" bottom barrel left attached.

Edited by Starman1 (04/04/13 11:22 AM)


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Jarrod]
      #5777494 - 04/04/13 11:46 AM

As a general rule of thumb, in 1.25" format, the largest TFOV eyepieces will be: 32mm to 35mm at ~50-deg AFOV; 24mm to 25mm at ~68-deg AFOV and 16mm to 18mm at ~82-deg AFOV. Each of these will produce fairly similar TFOVs. The wider AFOV designs will just do so at a higher magnification, larger image scale and smaller exit pupil (darker background, better control of any astigmatism in your eye).

It's not exact, but it does provide a practically useful equivalency model.

Regards,

Jim


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Tamiji Homma
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Reged: 02/24/07

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Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Starman1]
      #5777504 - 04/04/13 11:52 AM

Quote:

I think you meant Baader 24. The Panoptic figure came from TeleVue, and I trust it.




Hi Don,

I meant Panoptic 24.

I noticed that there is some discrepancy in Televue eyepiece field stop number.

For example,

27.4 mm Nagler 20T5
27mm Panoptic 24
27mm Plossl 32

If you measure TFOV, Panoptic 24 gives the widest among 3 eyepieces.
The Nagler 20T5 gives the narrowest TFOV among them.

Nagler 20T5 field stop is more like 26.6mm instead of 27.4mm.

That's why I asked what you get for Panoptic 24 if you found Baader Hyperion 24 field stop was 27mm. Because I see wider TFOV with Baader Hyperion 24 than Panoptic 24.

Tammy


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Tamiji Homma]
      #5777734 - 04/04/13 01:26 PM

Thanks for making me take a look at the 24 Hyperion.
TeleVue says:
32 Plossl--27.0 field stop
24 Panoptic--27.0 field stop
20 Nagler T5--27.4 field stop.

But remember, the field stop size is only part of the equation for determining true field.

For example, hypothesize an eyepiece with severe angular magnification distortion, in which the magnification at the edge was 50% as great as in the center.
Timing the passage of a star, you would get a large true field. Yet, the apparent field and field stop would both be smaller than the figures you would derive from the true field size.

Hypothesize an eyepiece in which the edge of the field had twice the magnification of the center. A timing of the passage of a star would yield a smaller true field than that derived from the apparent field or the field stop.

And remember than RD and AMD both distort the edge of the field.

So if the true fields differ for the 3 eyepieces, and the Nagler is not larger than the others in terms of true field, the difference is due to differences in distortion at the edges of the field. The Nagler may have a different amount of distortion than the others.

A ZERO distortion eyepiece (impossible in eyepieces this wide) of 20mm and 27.4mm field stop would have a 68.8 degree field. And a 24mm eyepiece with a 27.0mm field stop would have a 56.9 degree field. And a 32mm with a 27.0mm field stop would have an apparent field of 45.7 degrees. The differences are due to distortion.

The differences in apparent field are:
82deg.20mm--16.1%
68deg.24mm--16.3%
50deg.32mm--8.6%
And all 3 have pincushion distortion.

Since distortion at field edge grows along with apparent field, I see some vindication for the comments from some here on CN that the 24 Panoptic has more rectilinear distortion than it should have to correct angular magnification distortion.

Judging from the figures, I would guess the 24 Panoptic to have the largest true field of the 3, but not because of its field stop size.

In order to be able to pre-calculate the true field seen by an eyepiece, you would have to know the amount of distortion at the edge of the field.
You could derive an "effective" field stop size for the eyepiece based on that, but it might not be the same as the actual field stop size.
When you use true field to derive a field stop size, you too are deriving an "effective field stop" size. In my experience, those derived field stop figures make figuring out the true field in a second instrument more accurate than measuring the actual field stop in the eyepiece.


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David Knisely
Postmaster
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Starman1]
      #5777819 - 04/04/13 02:04 PM

My 24mm Panoptic has a measured field stop of 27.00mm +/- .02mm (the accuracy of the caliper). In my 10 inch telescope with its measured focal length of 1410mm +/- 0.5mm, it yields a true field of view of 1.113 degrees via multiple timings using the star-drift method. The field stop equation yields a value of 1.097 degrees which is only 1.4% off the actual value. I also measured the eyepiece's apparent field at 68.0 degrees as well, so at least for this eyepiece, Tele Vue's values here jive with actual physical measurement. Clear skies to you.

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bdcmd
sage
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Reged: 03/14/08

Loc: Glen Rose, Texas
Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Jarrod]
      #5777918 - 04/04/13 03:10 PM

Quote:

Quote:


Maybe you should try a 2" illuminated reticle eyepiece to use as a finder AND alignment eyepiece.




Hmmm, those are interesting. My only concern would be the accuracy. At 1/3rd the magnification of my 12mm reticle, I think I might not be able to get the stars perfectly centered? I don't know. But it is a good idea...




Stick a 2" 2x barlow on it if you need more magnification. Agena or Astro-Tech both have suitable inexpensive barlows.


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dyslexic nam
Carpal Tunnel
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Reged: 01/28/08

Loc: PEI, Canada
1.25" maximum TFOV EPs - lightweight option? new [Re: bdcmd]
      #6493962 - 04/28/14 01:57 PM

I am resurrecting this thread instead of starting a new thread to ask a similar question.

I am looking for a fairly light weight 1.25" ep that approaches max TFOV. I will be using it in an Astroscan (in conjunction w/ a 1.25 Paracorr) and an 80mm Antares finder. Both would potentially have/introduce balance issues the Astroscan due to its mounting mechanism, and the Antares because it is adding weight at the end of my dob so the lighter the ep, the better.

It looks like there are a few clusters of eps that approach max TFOV using similar focal length ranges and similar AFOVs. As Jim noted above, the general groupings are:
32mm to 35mm = 50 deg AFOV
24mm to 25mm = 68 deg AFOV
16mm to 18mm = 82 deg AFOV

What I am wondering is if there are relatively lightweight options in the wider categories? I know a 32mm plossl is feasible in terms of scope balance, but I would prefer a wider view if possible. Plus, the fast focal ratios (Antares finder is f4.4, Astroscan is f4 though slower with Paracorr if balance isnt too bad) suggest the need for a shorter fl ep to avoid exit pupil issues. Anything above around 24mm would waste light, and ideally it would be in the 19-20mm range. Unfortunately (and unsurprisingly), the general trend seems to be more weight as the focal length decreases and AFOV increases. I know much of this weight is driven simply by the physics of providing the wide view, but I am hoping that someone has found some workarounds.

If anyone knows of a wide, lightweight option in 1.25 (ideally between 18mm and 24mm), suggestions would be appreciated.


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Shneor
scholastic sledgehammer
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Re: 1.25" maximum TFOV EPs - lightweight option? new [Re: dyslexic nam]
      #6494032 - 04/28/14 02:46 PM

The widest field 1.25" eyepiece I have ever owned was a 30mm Rini, with a field stop measured at 29mm. The claim was a 60 AFOV. It was a pretty nice eyepiece for having single-coated lenses. I used a pair in a binoviewer for awhile.

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JustaBoy
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Re: 1.25" maximum TFOV EPs - lightweight option? new [Re: Shneor]
      #6494061 - 04/28/14 02:58 PM

The Very Best answer for those requirements (over-looking the weight) is the Vixen 22mm LVW - Bar None!

That is all that I will say about this, as repeating it is even beginning to tire me.

I'm very sorry, but it's the Only Answer that I have...

:-)


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dyslexic nam
Carpal Tunnel
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Reged: 01/28/08

Loc: PEI, Canada
Re: 1.25" maximum TFOV EPs - lightweight option? new [Re: JustaBoy]
      #6494065 - 04/28/14 03:00 PM

Quote:

The Very Best answer for those requirements (over-looking the weight) is the Vixen 22mm LVW - Bar None!

That is all that I will say about this, as repeating it is even beginning to tire me.

I'm very sorry, but it's the Only Answer that I have...

:-)




Unfortunately, a 22 LVW is too heavy to meet my needs (you may be able to overlook it as a consideration, but I can't under the circumstances).


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JustaBoy
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Re: 1.25" maximum TFOV EPs - lightweight option? new [Re: dyslexic nam]
      #6494078 - 04/28/14 03:08 PM

You could perhaps "get by" with a 19mm Pan, but it would be No Where Near as Good. - Might try a 20mm or 21mm TV Plossl.

If I were in this same boat I would lie awake at night, thinking about how to modify these scopes to where the balance would no longer be an issue.

Anyway - I said that I wasn't going to say anything more and I did - Sorry.



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dyslexic nam
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Reged: 01/28/08

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Re: 1.25" maximum TFOV EPs - lightweight option? new [Re: JustaBoy]
      #6494089 - 04/28/14 03:13 PM

Interestingly, a 19mm Pan or 20mm ES68 were a couple I was thinking of. Around 1/2 pound, 5mm(ish) exit pupil, and still a good sized TFOV. If the Meade 18mm UWA wasn't so heavy, it would be a frontrunner as well.

As for addressing the balance issues, I could do it with counterweights for the Antares Finder, but the Astroscan would be more challenging, and it is the main reason I am thinking of this.


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John Huntley
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Re: 1.25" maximum TFOV EPs - lightweight option? new [Re: dyslexic nam]
      #6494102 - 04/28/14 03:17 PM

The out of production Kokusai Kohki Widescan III 20mm claimed 84 degrees AFoV. It's reasonably light too - about 185 grams I think. Trouble is that the edge correction starts to crumble at around F/8.

I think the challenge is that to do wide field tricks with the light cone of a fast scope without messing the outer half of the field of view up takes skillful design, quality build and a fair amount of carefully ground and polished glass. These factors add up to both cost and weight not to mention bulk in some cases.

Edited by John Huntley (04/28/14 03:17 PM)


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dyslexic nam
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Re: 1.25" maximum TFOV EPs - lightweight option? new [Re: John Huntley]
      #6494111 - 04/28/14 03:21 PM

That sounds interesting, but if it gets sketchy at f8, I can only think what it would look like at f4-4.4. I wonder how much the Paracorr would clean things up?

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penguinx64
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Re: 1.25" maximum TFOV EPs - lightweight option? new [Re: dyslexic nam]
      #6494456 - 04/28/14 06:28 PM

I think a Ganymede 26mm Erfle is a good low power wide angle eyepiece with a 56 degree AFOV. I picked up one of these on the Cloudy Nights Classifeds for $13. Can't beat it at that price. Not too heavy if you decide to barlow it.

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EuropaWill
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Reged: 02/12/09

Re: Inexpensive 1.25" maximum TFOV EPs new [Re: Jarrod]
      #6494715 - 04/28/14 08:56 PM

In the spirit of your original question: Inexpensive and largest TFOV in 1.25"... That would be the GSO 32mm and 40mm Plossls.

Since there is a plethora of theory being presented in this thread, i'll take a different route and let you know that I own both GSO's. Neither is barrel limited so they have an installed field stop. I have not measured it. Despite the manufacturer's claims the GSO 32mm is probably at around 50 degrees AFOV while the GSO 40mm is at around 43 degrees AFOV.

For the record: The 40mm has in fact a slightly larger TFOV than the 32mm and I have confirmed this on multiple occassions including just a few minutes ago during an indoor test. Although the 40mm's ~43 degree AFOV seems much smaller on paper, due to the longer eye relief it feels larger at around 44-45 degrees. Not a big "seat of the pants" difference, but perceived nonetheless and worth mentioning.

The 40mm is brighter than the 32 due to the larger exit pupil. The 40mm is easier to use with glasses and doesn't require you get up and personal with the eyecup to see the entire FOV like the 32mm does. The 32mm is more immersive feeling due to the larger AFOV.


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