You are not logged in. [Login] Entrance · Main Index · Search · New user · Who's Online FAQ · Calendar

Equipment Discussions >> Eyepieces

Pages: 1 | 2 | (show all)
Jarrod
scholastic sledgehammer

Reged: 01/20/13

Loc: SE USA
Inexpensive 1.25" maximum TFOV EPs
#5775127 - 04/03/13 10:07 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!

Edited by Jarrod (04/03/13 10:09 AM)

 Post Extras:
dan_h
Carpal Tunnel

Reged: 12/10/07

Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jarrod]
#5775334 - 04/03/13 11:28 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

 Post Extras:
Thomas Karpf
Pooh-Bah

Reged: 02/09/09

Loc: Newington, CT
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jarrod]
#5775345 - 04/03/13 11:32 AM

http://www.televue.com/engine/TV3b_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*.

 Post Extras:
MingoT
member

Reged: 09/12/10

Loc: Zaragoza, Spain
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jarrod]
#5775352 - 04/03/13 11:34 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

 Post Extras:
BDS316
Carpal Tunnel

Reged: 09/16/09

Loc: Sol 3
Re: Inexpensive 1.25" maximum TFOV EPs [Re: MingoT]
#5775414 - 04/03/13 12:04 PM

Quote:

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

 Post Extras:
Mark9473
Postmaster

Reged: 07/21/05

Loc: 51°N 4°E
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jarrod]
#5775561 - 04/03/13 01:23 PM

Quote:

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.

 Post Extras:
Jarrod
scholastic sledgehammer

Reged: 01/20/13

Loc: SE USA
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Mark9473]
#5775680 - 04/03/13 02:09 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

Edited by Jarrod (04/03/13 02:33 PM)

 Post Extras:
Thomas Karpf
Pooh-Bah

Reged: 02/09/09

Loc: Newington, CT
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jarrod]
#5775729 - 04/03/13 02:42 PM

Quote:

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'?

 Post Extras:
Mark9473
Postmaster

Reged: 07/21/05

Loc: 51°N 4°E
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jarrod]
#5775822 - 04/03/13 03:34 PM

Quote:

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

 Post Extras:
David Knisely
Postmaster

Reged: 04/19/04

Re: Inexpensive 1.25" maximum TFOV EPs [Re: Mark9473]
#5776004 - 04/03/13 05:12 PM

Quote:

Quote:

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.

 Post Extras:
Jarrod
scholastic sledgehammer

Reged: 01/20/13

Loc: SE USA
Re: Inexpensive 1.25" maximum TFOV EPs [Re: David Knisely]
#5776022 - 04/03/13 05:25 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

Edited by Jarrod (04/03/13 05:28 PM)

 Post Extras:
BDS316
Carpal Tunnel

Reged: 09/16/09

Loc: Sol 3
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jarrod]
#5776033 - 04/03/13 05:37 PM

Quote:

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

 Post Extras:
Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jarrod]
#5776084 - 04/03/13 05:59 PM

Quote:

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.

Edited by Starman1 (04/03/13 06:02 PM)

 Post Extras:
Jarrod
scholastic sledgehammer

Reged: 01/20/13

Loc: SE USA
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Starman1]
#5776652 - 04/03/13 10:26 PM

Quote:

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.

 Post Extras:
buddyjesus
Carpal Tunnel

Reged: 07/07/10

Loc: Davison, Michigan
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jarrod]
#5776673 - 04/03/13 10:39 PM

good eyepiece from what I gather. gl with it.

 Post Extras:
Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Mark9473]
#5776839 - 04/04/13 12:10 AM

Quote:

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

 Post Extras:
Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jon Isaacs]
#5776951 - 04/04/13 02:15 AM

Quote:

Quote:

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.

 Post Extras:
Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Starman1]
#5777132 - 04/04/13 07:45 AM

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???

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

 Post Extras:
Jarrod
scholastic sledgehammer

Reged: 01/20/13

Loc: SE USA
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Jon Isaacs]
#5777149 - 04/04/13 08:10 AM

Quote:

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.

 Post Extras:
Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Inexpensive 1.25" maximum TFOV EPs [Re: Mark9473]
#5777156 - 04/04/13 08:23 AM

Quote:

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

 Post Extras:
Pages: 1 | 2 | (show all)

Extra information
45 registered and 40 anonymous users are browsing this forum.

Moderator:  ausastronomer, Scott in NC, iceblaze

Forum Permissions
You cannot start new topics