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# Inexpensive 1.25" maximum TFOV EPs

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### #1 Jarrod

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Posted 03 April 2013 - 09:07 AM

I want to get an inexpensive 1.25" EP that maxes out the TFOV that can be offered by a 1.25" barrel.

Currently I'm looking at these two GSO Super Plossls at Agena, which Agena rates thusly:
32mm -> 52* AFOV
40mm -> 45* AFOV

Now, if I plug these into the TFOV = AFOV/Mag equation for my scope (1200mm f/5.9), I get:
32mm -> 1.39*
40mm -> 1.50*

However, I've read here that the 32mm and 40mm 1.25" Plossls do not offer a different TFOV, but rather you just get a smaller image in the 40mm EP. So that would argue to go for the 32mm EP. But the results from my equation say to get the 40mm EP. Can someone explain the discrepancy to me?

I've asked a similar question on the forum before, inside my other EP threads, but nobody ever answered. Basically, what is the physical limitation of TFOV as a function of barrel diameter and how do you *really* calculate it? The TFOV equation above has no terms that relate to the diameter of the EP barrel.

Many thanks!

### #2 dan_h

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Posted 03 April 2013 - 10:28 AM

Given that the barrel restricts the absolute maximum field, you can compare the actual FOV as the ratio of the focal lengths.

32/40=0.8, 52 degrees X 0.8 = 41.6 degrees for the 40mm eyepiece, not 45 degrees. That is one source of error and 41 degrees is probably closer to reality.

If you calculate the size of field for a 40mm FL and a 41 degree view, it comes to approximately 29.9mm, or just about equal the inside diameter of the eyepiece barrel.

Hope this helps,

dan

### #3 Thomas Karpf

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Posted 03 April 2013 - 10:32 AM

http://www.televue.c..._page.asp?id=79

It's based on the maximum field stop of the eyepiece.

For a 1.25" eyepiece, the maximum field stop is about 27mm. For a 2" eyepiece, the maximum field stop is about 46mm.

Simple trigonometry will give you the maximum field of view of a scope. It's based on a long, skinny triangle with long sides = the focal length of the scope and the short side = the field stop of the focuser.

maximum field of view with a 1.25" focuser is about 1.29 degrees (27mm / 1200mm * 180 / pi), and
maximum field of view with a 2" focuser is about 2.19 degrees (46mm / 1200mm * 180 / pi).

Frankly, I'd expect that the AFOV of the 40mm is probably more like 40* rather than 43*.

### #4 MingoT

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Posted 03 April 2013 - 10:34 AM

Hi,

The True Field of View is:
TFOV = 57.3 * Eyepiece_Field_Stop / Telescope_Focal_Length

The maximum Field Stop for a 1.25" eyepiece is 27mm, for a 1200mm telescope, this gives TFOV = 1.29 degrees. A given eyepiece could offer a little more TFOV than that, at the cost of some vignetting (darkening in the outer part of the image), how noticeable it is, depends on your sensitivity.

Unless you have very dark skies, you are quite young, and you want to maximize exit pupil, I would recommend the 32mm. It will show the same portion of the sky with more magnification and darker background.

Best,
Mingo

### #5 BDS316

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Posted 03 April 2013 - 11:04 AM

I would recommend the 32mm. It will show the same portion of the sky with more magnification and darker background.

Agree. I can't think of a single situation in which it would be better to use a 40mm Plossl over a 32mm Plossl.

Unless it's the 40mm Sterling Plossl, but that's a 2 inch eyepiece, of course

### #6 Mark9473

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Posted 03 April 2013 - 12:23 PM

Basically, what is the physical limitation of TFOV as a function of barrel diameter and how do you *really* calculate it? The TFOV equation above has no terms that relate to the diameter of the EP barrel.

Jarrod, the best calculation goes as follows:
FOV = ATAN (eyepiece field stop diameter / scope focal length).

You can get actual field stop sizes from Don's spreadsheet (or ask the supplier). Some of the widest:
Orion Sirius Plossl 40mm : 28.6 mm
Orion Sirius Plossl 32mm : 28.5 mm
TV Plossl 40mm : 27.3 mm
Baader Hyperion 24mm : 28.5 mm
TV Panoptic 24mm : 27.0 mm

The only way to go wider than that in a 1.25" barrel, that I know of, is by using a Baader Hyperion Aspheric eyepiece (31 or 36 mm) with the supplied 1.25" nosepiece. Field stop diameter is then 30 mm.

### #7 Jarrod

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Posted 03 April 2013 - 01:09 PM

Thanks all, that is very helpful.

Mark, I spent some time with the atan() function earlier trying to find the correct triangle. I was using the FL of the EP divided by the field stop diameter thinking I might get the AFOV but that doesn't appear to work. Thanks a lot for giving me the triangle that gives you the TFOV from the field stop diameter - that will come in handy!

Edit: I guess Tom gave the triangle too, but it didn't sink in because I didn't recognize the rad -> deg conversion

### #8 Thomas Karpf

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Posted 03 April 2013 - 01:42 PM

Edit: I guess Tom gave the triangle too, but it didn't sink in because I didn't recognize the rad -> deg conversion

Chuckle... Yeah, my formula included the 180/pi instead of 57.3, but half the time when I see an equation here that includes 57.3, somebody will ask 'what is 57.3'?

### #9 Mark9473

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Posted 03 April 2013 - 02:34 PM

I spent some time with the atan() function earlier trying to find the correct triangle. I was using the FL of the EP divided by the field stop diameter thinking I might get the AFOV but that doesn't appear to work.

Two problems that I see is that (1) the eyepiece FL may be off a bit (few % not uncommon), and (2) for a given FL and TFOV, the actual AFOV will depend on the geometric distortions in the eyepiece, so any AFOV calculation will be only an approximation.

That said, the formula that I would use for this approximation is:
AFOV = field stop size x 57.3 / EP focal length

### #10 David Knisely

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Posted 03 April 2013 - 04:12 PM

Basically, what is the physical limitation of TFOV as a function of barrel diameter and how do you *really* calculate it? The TFOV equation above has no terms that relate to the diameter of the EP barrel.

Jarrod, the best calculation goes as follows:
FOV = ATAN (eyepiece field stop diameter / scope focal length).

You can get actual field stop sizes from Don's spreadsheet (or ask the supplier). Some of the widest:
Orion Sirius Plossl 40mm : 28.6 mm
Orion Sirius Plossl 32mm : 28.5 mm
TV Plossl 40mm : 27.3 mm
Baader Hyperion 24mm : 28.5 mm
TV Panoptic 24mm : 27.0 mm

The only way to go wider than that in a 1.25" barrel, that I know of, is by using a Baader Hyperion Aspheric eyepiece (31 or 36 mm) with the supplied 1.25" nosepiece. Field stop diameter is then 30 mm.

Going much wider than around 28mm field stop with an 1.25" barrel eyepiece will result in some vignetting at the field edges. Indeed, I have tried the 36mm Hyperion Aspheric with its silly 1.25" adapter, and the vignetting in the outer field is horrid (31.5mm field stop with that adapter in-place). It was far better to just go with the eyepiece in its "native" 2" mode. Clear skies to you.

### #11 Jarrod

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Posted 03 April 2013 - 04:25 PM

I want this EP for one thing: initially locating alignment stars so I can center them up and swap in my 12mm illuminated reticle EP. It won't see much if any use beyond that. I'm looking for "cheap and easy" just like, ummm, fast food? I'll stop there

### #12 BDS316

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Posted 03 April 2013 - 04:37 PM

I want this EP for one thing: initially locating alignment stars so I can center them up and swap in my 12mm illuminated reticle EP. It won't see much if any use beyond that. I'm looking for "cheap and easy" just like, ummm, fast food? I'll stop there

You want an Asian 32mm Plossl, either Orion Sirius or GSO, whichever is less costly considering shipping

### #13 Starman1

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Posted 03 April 2013 - 04:59 PM

I want to get an inexpensive 1.25" EP that maxes out the TFOV that can be offered by a 1.25" barrel.

Currently I'm looking at these two GSO Super Plossls at Agena, which Agena rates thusly:
32mm -> 52* AFOV
40mm -> 45* AFOV

Now, if I plug these into the TFOV = AFOV/Mag equation for my scope (1200mm f/5.9), I get:
32mm -> 1.39*
40mm -> 1.50*

However, I've read here that the 32mm and 40mm 1.25" Plossls do not offer a different TFOV, but rather you just get a smaller image in the 40mm EP. So that would argue to go for the 32mm EP. But the results from my equation say to get the 40mm EP. Can someone explain the discrepancy to me?

I've asked a similar question on the forum before, inside my other EP threads, but nobody ever answered. Basically, what is the physical limitation of TFOV as a function of barrel diameter and how do you *really* calculate it? The TFOV equation above has no terms that relate to the diameter of the EP barrel.

Many thanks!

First, without severe angular magnification distortion, a 32mm 1.25" eyepiece maxes out at 50 degrees and a 40mm maxes out at 43 degrees. However, that is if the field stop is held to 27-27.4mm.
If the field stop is slightly larger, then the field stop must be positioned above the 1.25" barrel. Though uncommon, this has been done in the 35mm Parks Gold Series, the 24mm Konig, and a few others through the years. Because of the high position of the field stop, these eyepieces require a lot more inward focuser travel than others. Plus, the edge of the field of view is vignetted, though not terribly so. The field stop of the Parks, for example, is 29mm.

If you are figuring out the field of view, it's reasonable to presume the field stop maxes out at 27.0-27.4mm in a 1.25" eyepiece.
Using the formula:
TF=(EPFS/TFL) * 57.3
where TF = true field, EPFS = eyepiece fieldstop, and TFL is telescope focal length.
Since your telescope has a 1200mm focal length, the largest possible true field in 1.25" is (27.4/1200) * 57.3 = 1.31 degrees.
The TeleVue 40mm Plossl has a 27.3mm field stop (1.30 degrees) and the 32mm Plossl has a 26.9mm field stop (1.28 degrees).
If you just figure than any 1.25" Plossl will deliver a max of 1.3 degrees with 32-40mm, you'll be very close. Note that many 24mm Widefields will also yield the same field size (24mm ES 68 degree, Baader Hyperion 24, TeleVue Panoptic 24, etc.)

My Eyepiece Buyer's Guide spreadsheet calculates field stops for eyepieces where the MFR doesn't give data, but the presumption is 4% distortion at the edge of the field. With more distortion, the true field can be squeezed into a smaller width, so distrust any figures you see above 27.4mm.

The 40mm focal length is primarily for users of long focal ratio telescopes in order to get a decently large exit pupil. At almost f/6, your scope gives a nice big exit pupil with a 32mm, so no reason to restrict the apparent field and go to a 40mm. Frankly a 24mm Widefield, with the same field size and a magnification of 50X might be an eyepiece you use for observing rather than just finding.

### #14 Jarrod

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Posted 03 April 2013 - 09:26 PM

a 24mm Widefield, with the same field size and a magnification of 50X might be an eyepiece you use for observing rather than just finding.

I have the ES68 24mm on backorder but ended up with the Meade 5000 24mm/82*. I should cancel the 24mm/68* since it probably will not get used much now and I feel would be overkill as a locator EP. Thanks for the help.

### #15 buddyjesus

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Posted 03 April 2013 - 09:39 PM

good eyepiece from what I gather. gl with it.

### #16 Jon Isaacs

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Posted 03 April 2013 - 11:10 PM

Orion Sirius Plossl 40mm : 28.6 mm
Orion Sirius Plossl 32mm : 28.5 mm
TV Plossl 40mm : 27.3 mm
Baader Hyperion 24mm : 28.5 mm
TV Panoptic 24mm : 27.0 mm

Are these all measured values? Given that OD of a 28mm filter thread measures about 28.2mm, this implies that a filter would not fit the barrel of the 24mm Hyperion and the Sirius Plossls.

Jon

### #17 Starman1

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Posted 04 April 2013 - 01:15 AM

Orion Sirius Plossl 40mm : 28.6 mm
Orion Sirius Plossl 32mm : 28.5 mm
TV Plossl 40mm : 27.3 mm
Baader Hyperion 24mm : 28.5 mm
TV Panoptic 24mm : 27.0 mm

Are these all measured values? Given that OD of a 28mm filter thread measures about 28.2mm, this implies that a filter would not fit the barrel of the 24mm Hyperion and the Sirius Plossls.

Jon

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.

### #18 Jon Isaacs

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Posted 04 April 2013 - 06:45 AM

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???

Otherwise, it would seem anything over about 27mm is suspect and anything over 28mm is definitely so. I have the 32mm and 40mm Celestron orange and black Taiwanese Plossls from the mid 90's, both have 27.5 mm field stops as measured with a caliper.

In Jarrod's scope with the 1200mm focal length, both would provide a TFoV of 1.31 degrees. With a 2 inch eyepieces with the maximum 46 mm field stop, a 2.2 degree TFoV is possible. One 2 inch eyepiece can make a great finder/widefield eyepiece, great ones will break the bank but reasonable ones, both performance wise and cost wise, are available.

Jon

### #19 Jarrod

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Posted 04 April 2013 - 07:10 AM

With a 2 inch eyepieces with the maximum 46 mm field stop, a 2.2 degree TFoV is possible. One 2 inch eyepiece can make a great finder/widefield eyepiece, great ones will break the bank but reasonable ones, both performance wise and cost wise, are available.

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. And I certainly wouldn't enjoy swapping my HUGE Meade UWA EPs in and out while aligning. 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.

### #20 Jon Isaacs

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Posted 04 April 2013 - 07:23 AM

FOV = ATAN (eyepiece field stop diameter / scope focal length).

A few thoughts:

If one uses the arc-tangent, I believe the formulation is something like:

TFoV = 2 x Atan (field stop/(2 x focal length scope))

To use the ATan, a right triangle is necessary, that is where the 2 comes from... divide the Isosceles triangle down the middle to make a right triangle, use the Atan and then double the angle.

But for any possible field stop and focal length of the telescope, the angles are so small that there is no practical difference between the arctangent and the angle itself. With the widest field combination I have, a 42mm field stop in a 400mm focal length telescope, the difference between using the TFoV = 2 x Atan(FS/2xFL) and TFoV = FS/FL radians is less than a tenth of a percent. With the combinations being discussed here, the difference is less than 0.005%.

As far as the source of the 57.3.. I always write the equation:

TFoV = 57.3 degrees/radian x (FSeyepiece/FLscope) or

TFoV = 57.3 deg/rad x FS/FLscope

The difference between using 57.3 degrees/radian and 180/pi is 0.0074%... One cannot determine the focal length of the scope or the field stop accurately enough to make this tiny difference matter.

Jon

### #21 bdcmd

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Posted 04 April 2013 - 08:07 AM

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.

### #22 Jarrod

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Posted 04 April 2013 - 08:30 AM

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...

### #23 Starman1

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Posted 04 April 2013 - 09:21 AM

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.

### #24 Tamiji Homma

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Posted 04 April 2013 - 09:41 AM

Hi Don,

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

### #25 Starman1

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Posted 04 April 2013 - 10:07 AM

Hi Don,

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.

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