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Meade 40mm Plossl ... not a fan.

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#1 OldCarolina

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Posted 01 December 2023 - 07:50 PM

All I own is Meade Plossls for my ETX90.

 

I have been digging around here and trying to take an elementary deeper dive into understanding FOV and magnification for eyepieces.

 

From what I think I understand …

 

The AFOV for the Meade Plossl design is advertised as 52*.

 

If I use the general relationship AFOV=FS/FL radians (that need converting to degrees) where FS is field stop for 1.25 barrel (27.4mm is what I’m using) and FL (focal length of the eyepiece in mm) … then it would seem that the AFOV for all of my eyepieces (up to the 26mm that came with my scope) is greater than 52* and so the AFOV of these is limited to 52* introduced by the Meade Plossl design.

 

On the other hand … my 40mm Plossl (excuse me …”Super” Plossl) calculates to around 39* (Meade advertises 44*).  So the actual barrel diameter and field stop, not the Plossl design is now the limiting factor to the much narrower AFOV.

 

 

This with the greater eye relief of the 40mm (eye relief increases with focal length, right?) makes it hard for me to use this 40mm unit.  Like looking through a straw.  It sucks.  In my opinion, of course.

 

Furthermore … I’m getting 49* AFOV for the 32mm Plossl which approaches the max AFOV (in seeking lowest magnification) for the Plossl eyepiece design.

 

And using the relationship TFOV=AFOV/Magnification … I’ll use the 52* and 44* AFOV advertised by Meade for the 32mm and the 40mm … getting around 1.3 TFOV for both.

 

So the same TFOV for both the 32mm and 40mm just with higher magnification and more desirable (to me) 52* AFOV for the 32mm Plossl.

 

I’m trying to understand mathematically why I don't like this 40mm.

 

I don't like it no matter the math and will probably get rid of it and try a 32mm for lowest magnification.

 

But ... am I generally on the right track here in terms of the analysis?

 

Thanks for reading.


Edited by OldCarolina, 01 December 2023 - 07:58 PM.


#2 deSitter

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Posted 01 December 2023 - 08:27 PM

The 40mm Plossl is limited by the 1.25" barrel diameter. The apparent field of view is thus about 45 degrees. It is actually one of my favorite eyepieces, because it gives a low power view in long focus (high f/ratio) classic refractors. Absolutely razor sharp and colorless field stop and beautiful clarity across the necessarily limited field. In a 2" barrel diameter you can have a bigger 70 degree field of view.

 

Remember the field of view ultimately depends on the primary image made by the objective. You have to put a barrel around that.

 

-drl


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#3 donniesoprano

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Posted 01 December 2023 - 08:33 PM

Most people don’t have a need for a 1.25” 40mm plossl. It’s a uniquely useful eyepiece for specific circumstances.  It’s not a bad eyepiece, it’s just a bad fit for you, here.

 

Stick with the 32mm in 1.25” format.

 

ds


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#4 Dave Mitsky

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Posted 01 December 2023 - 09:00 PM

This situation is the same for any 40mm Plössl, i.e., long eye relief and an apparent field of view of approximately 43 degrees. 

The maximum field stop for a 1.25" eyepiece is about 27mm, no matter what the design.  


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#5 sunrag

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Posted 01 December 2023 - 09:09 PM

I have a Televue 40mm Plossl in 1.25” barrel and the Meade Super Plossl 40mm. I prefer the Meade, it is easy to view through and seems more forgiving of my eye position (on axis or not). The TV 40 is terrible unless i am viewing exactly on axis. I only keep i because it has the correct size flange to attach to my Night Vision Device.


Edited by sunrag, 01 December 2023 - 09:10 PM.

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#6 vtornado

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Posted 01 December 2023 - 09:35 PM

Both the 32mm plossl and 40mm plossl have roughly the same true field of view (TFOV). 

 

What the 40mm brings to the table is a larger exit pupil for scopes with a high f ratio such as an SCT, Mak or long focus refractor.

 

You might use this eyepiece if you were hunting for extended targets at a dark site.  In an urban site with high light pollution, the 32mm will present a darker sky which makes stars have more "pop".  It may also help when using a nebula filter.

 

The Meade Super plossl  long focal length eyepieces have deeply recessed eye lenses which helps for eye placement.   Not good for glasses wearers however


Edited by vtornado, 01 December 2023 - 09:37 PM.

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#7 TOMDEY

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Posted 01 December 2023 - 10:03 PM

With your F/13.8 telescope, the 40mm will provide a 56% brighter and 20% smaller image than the 32mm. That could be preferable on targets like the Andromeda and Triangulum galaxies and the bigger nebulae, under good dark rural skies. So they each have their place.    Tom


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#8 Tony Flanders

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Posted 02 December 2023 - 05:57 AM

Your analysis is spot-on. Most people dislike 40-mm Plossls in 1.25-inch barrels for exactly the reasons that you describe.

 

My own opinion is that the focal length where the Plossl design works best of all is 25 mm, which as far as I'm concerned yields perfect eye relief -- neither too long nor too short. Quite a number of people find that even 32-mm Plossl have longer eye relief than they like.

 

In general, the solution to excessive eye relief is a well-designed eye cup that folds out from the eyepiece and guides your eye to the perfect position. Many eyepieces come with those off the shelf, but you can always make your own with some thin cardboard (as in many food boxes) or thick paper plus some sticky tape.

 

It is true, as some people have said here, that despite the limited field of view, there are some cases where the larger exit pupil of a 40-mm Plossl might be desirable. But frankly, such cases are not very common. Truth be told, if you want to observe the Andromeda Galaxy in all its glory, you need a different telescope, not a different eyepiece.


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#9 Jon Isaacs

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Posted 02 December 2023 - 07:54 AM

My two cents:

 

The ETX-90 has a focal ratio of F/13.8   

 

A 40mm Plossl is an eyepiece that is useful in certain situations but most of the time, it will go unused.  As others have said, the 40mm will be 56% brighter than the 32mm, there are times when the brighter view is a help in seeing faint objects.  

 

This not the forte of the ETX-90 but the 40mm can make for the best under certain circumstances. 

 

Jon


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#10 OldCarolina

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Posted 02 December 2023 - 09:55 AM

Thanks for the replies.  There seems to lot out there on the 40mm Plossl.

 

I should have changed the title of my post to ... "The math behind why I loath the 40mm ..."

 

I was more looking for feedback as to if my eyepiece analysis (the equations/relationships, terminology, etc ..) was generally on the right track.

 

Just as an academic exercise more than as to whether or not it would explain anything,

 

Because sometimes you either like things or you don't! 

 

T Flanders seemed to suggest I was on the right track. 

 

Thanks again.


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#11 deSitter

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Posted 03 December 2023 - 03:30 AM

Here is a formula for max field of view vs. barrel size.

 

Let barrel size be B. Let focal length be F. Let max field of be A. Then

 

A = 2 arctan ( B / 2F )

 

Of course, B and F must be in the same units.

 

Example. B = 32mm (= 1.25") and F = 40mm.

 

A = 2 arctan 0.4 = 44 degrees.

 

Example. B = 52mm (= 2") and F = 40mm.

.

A = 2 arctan 0.65 = 66 degrees.

 

Example. B=24.5mm (= 0.965") and F = 40mm.

 

A = 2 arctan 0.31.. = 34 degrees.

 

(Such a soda straw eyepiece actually exists! Meade MA40mm essentially a Kellner for classic refractors.)

 

 

Note that the scope is not involved at all.

 

This only strictly works for eyepieces with an external field stop. A wide field design like the Nagler has an internal field stop. Such eyepieces have a sort of built-in Barlow diverging element before the field stop (so your telescope is in effect always using a barlow). You would need to find the effective focal length of the remaining elements behind the internal field stop after removing those in front of it. That in fact may be how the field of view and effective focal length of the eyepiece are determined. Because the field stop is internal, it need not be the same size as the eyepiece barrel.

 

-drl


Edited by deSitter, 03 December 2023 - 05:13 AM.

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#12 Jon Isaacs

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Posted 03 December 2023 - 05:14 AM

Here is a formula for max field of view vs. barrel size.

 

Let barrel size be B. Let focal length be F. Let max field of be A. Then

 

A = 2 arctan ( B / 2F )

 

Of course, B and F must be in the same units.

 

Example. B = 32mm (= 1.25") and F = 40mm.

 

A = 2 arctan 0.4 = 44 degrees.

 

Example. B = 52mm (= 2") and F = 40mm.

.

A = 2 arctan 0.65 = 66 degrees.

 

Note that the scope is not involved at all.

 

This only strictly works for eyepieces with an external field stop. A wide field design like the Nagler has an internal field stop. Such eyepieces have a sort of built-in Barlow diverging element before the field stop (so your telescope is in effect always using a barlow). You would need to find the effective focal length of the remaining elements behind the internal field stop after removing those in front of it. That in fact may be how the field of view and effective focal length of the eyepiece are determined. Because the field stop is internal, it need not be the same size as the eyepiece barrel.

 

-drl

 

A couple of issues:

 

- One needs to use the inner diameter of the barrel rather than the outer diameter.  The maximum field stop diameter for 1.25 inch eyepieces is about 27.5 mm, for 2 inch, about 46.5.  

 

- The analytical relationship between the field stop diameter, the eyepiece focal length and the apparent field of view is not exact and depends on the individual eyepiece.  Both the field stop and the AFoV can be measured and the focal length is specified.  Measuring the AFoV using Beam Projection:

 

AFOV Measuring .jpg
 

In comparing these three variables, the relationship that seems to best fit the actual data is

 

AFoV = 180 deg/ Pi x Field Stop / focal length.  

 

I believe this is because magnification is angular.  

 

Using this relationship, the maximum distortion-less AFOV for a 32mm Plossl is 

AFoV = 57.3 deg/rad x 27.5 mm /32mm = 49.2 degrees, for the 40mm, 39.2 degrees. 

 

Jon


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#13 deSitter

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Posted 03 December 2023 - 07:23 AM

A couple of issues:

 

- One needs to use the inner diameter of the barrel rather than the outer diameter.  The maximum field stop diameter for 1.25 inch eyepieces is about 27.5 mm, for 2 inch, about 46.5.  

 

- The analytical relationship between the field stop diameter, the eyepiece focal length and the apparent field of view is not exact and depends on the individual eyepiece.  Both the field stop and the AFoV can be measured and the focal length is specified.  Measuring the AFoV using Beam Projection:

 

 
 

In comparing these three variables, the relationship that seems to best fit the actual data is

 

AFoV = 180 deg/ Pi x Field Stop / focal length.  

 

I believe this is because magnification is angular.  

 

Using this relationship, the maximum distortion-less AFOV for a 32mm Plossl is 

AFoV = 57.3 deg/rad x 27.5 mm /32mm = 49.2 degrees, for the 40mm, 39.2 degrees. 

 

Jon

Yes the formula is theoretical and in practice there are certainly adjustments expected. And yes about the inner vs. outer diameter of the barrel, that's just a small delta. A good eyepiece (e.g. the Meade mentioned in the OP's post) will have a field stop that is even a bit smaller than the barrel inside diameter. Some cheap eyepieces just use the barrel itself as a stop.

 

Classic wide fields - Erfles - have external field stops which explains why you don't see long ones in 1.25" format.

 

Your projection method should be possible without using the scope. Say, place a disc of translucent plastic (e.g. from a dissected dead LCD screen) over the field stop and then use strong illumination on it from the front.

 

-drl


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#14 OldCarolina

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Posted 03 December 2023 - 11:21 AM

OK I can see the 2D theoretical triangle that gives AFOV=2InverseTan(.5B/F) which gives me 43.6* for the 40mm Plossl.

 

I'm sure Meade likes to use this measurement more for its marketing propaganda than the 39* calculated with the AVOF=FS/FL.

 

Seems like there are too many variable to really nail it down with formulas and such which is the big takeaway.

 

And I'm gonna hang on to the 40mm and give it another chance sometime.

 

Thanks for all the help.


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#15 Jon Isaacs

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Posted 03 December 2023 - 11:50 AM

Yes the formula is theoretical and in practice there are certainly adjustments expected. And yes about the inner vs. outer diameter of the barrel, that's just a small delta. A good eyepiece (e.g. the Meade mentioned in the OP's post) will have a field stop that is even a bit smaller than the barrel inside diameter. Some cheap eyepieces just use the barrel itself as a stop.

 

Classic wide fields - Erfles - have external field stops which explains why you don't see long ones in 1.25" format.

 

Your projection method should be possible without using the scope. Say, place a disc of translucent plastic (e.g. from a dissected dead LCD screen) over the field stop and then use strong illumination on it from the front.

 

-drl

 

The validity of the arc-tan formulation is questionable. Certainly the purely angular formulation is more consistent with the real world.  

 

People normally use the projection method without the telescope but I do it for couple of reasons:  The telescope on the tripod provides an easy way to mount the eyepiece. It also provides a semi-collimated beam.

 

Using the inner diameter of the barrel as the maximum possible field stop diameter is an important factor and in this case, it is more significant than whether or not one uses an angular formulation or the arc-tangent formulation. With narrower field eyepieces, they converge.  Where it becomes important is with wide field eyepieces.  I measured the effective field stop diameter of the 28mm UWA at 40.8 mm via drift timing and the AFoV at 84.2 degree.  The aTan formulation says the AFoV would be 72.2 degrees, the angular magnification approximation says 83.5 degrees.  

 

See below:

 

OK I can see the 2D theoretical triangle that gives AFOV=2InverseTan(.5B/F) which gives me 43.6* for the 40mm Plossl.

 

I'm sure Meade likes to use this measurement more for its marketing propaganda than the 39* calculated with the AVOF=FS/FL.

 

Seems like there are too many variable to really nail it down with formulas and such which is the big takeaway.

 

And I'm gonna hang on to the 40mm and give it another chance sometime.

 

Thanks for all the help.

 

If one uses 27.5 mm as the field stop diameter, then the AFoV = 37.9 degrees using 2 x Atan(B/2F) whereas it is equal to 39.4 degrees using the AFoV= 57.3 deg/rad x B/F.  

 

Field stops of Plossls, Erfles and such can be measured directly. The inner diameter of the barrel is approximately 28.0 mm but the largest field stops I have measured have been 27.5mm. Those were on 32mm and 40mm Celestron Halloween Plossls.  That is tiny step.  

 

If I were to measure the AFoV, my experience suggests it would probably be around 40 degrees but it could be somewhat more.  

 

Jon


Edited by Jon Isaacs, 03 December 2023 - 11:52 AM.


#16 deSitter

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Posted 03 December 2023 - 12:59 PM

The validity of the arc-tan formulation is questionable. Certainly the purely angular formulation is more consistent with the real world.  

 

People normally use the projection method without the telescope but I do it for couple of reasons:  The telescope on the tripod provides an easy way to mount the eyepiece. It also provides a semi-collimated beam.

 

Using the inner diameter of the barrel as the maximum possible field stop diameter is an important factor and in this case, it is more significant than whether or not one uses an angular formulation or the arc-tangent formulation. With narrower field eyepieces, they converge.  Where it becomes important is with wide field eyepieces.  I measured the effective field stop diameter of the 28mm UWA at 40.8 mm via drift timing and the AFoV at 84.2 degree.  The aTan formulation says the AFoV would be 72.2 degrees, the angular magnification approximation says 83.5 degrees.  

 

See below:

 

 

If one uses 27.5 mm as the field stop diameter, then the AFoV = 37.9 degrees using 2 x Atan(B/2F) whereas it is equal to 39.4 degrees using the AFoV= 57.3 deg/rad x B/F.  

 

Field stops of Plossls, Erfles and such can be measured directly. The inner diameter of the barrel is approximately 28.0 mm but the largest field stops I have measured have been 27.5mm. Those were on 32mm and 40mm Celestron Halloween Plossls.  That is tiny step.  

 

If I were to measure the AFoV, my experience suggests it would probably be around 40 degrees but it could be somewhat more.  

 

Jon

The formula is just simple ray optics. You replace the eyepiece by a single lens at the principal plane of the elements. The angle subtended by the field stop from this point is the apparent field of view. There is nothing handwaving about it. The limiting case is when the focal length -> 0. The arctan then -> 90 and double it to 180, that is, the eye relief is 0 and the exit pupil is a single point in the middle of the principal plane, which is coincident with the field stop.

 

-drl


Edited by deSitter, 03 December 2023 - 12:59 PM.


#17 Jon Isaacs

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Posted 04 December 2023 - 02:05 PM

The formula is just simple ray optics. You replace the eyepiece by a single lens at the principal plane of the elements. The angle subtended by the field stop from this point is the apparent field of view. There is nothing handwaving about it. The limiting case is when the focal length -> 0. The arctan then -> 90 and double it to 180, that is, the eye relief is 0 and the exit pupil is a single point in the middle of the principal plane, which is coincident with the field stop.

 

-drl

 

An eyepiece is not a simple lens. The relationship between AFoV, Field Stop and Focal length is actually a two dimensional integral that depends on the particular optical design of the eyepiece.

 

If one assumes a fixed focal length, then the angular approximation seems to provide a more accurate approximation of the actual relationship between measured field stop diameter and measured AFoV.

 

Jon


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#18 CHASLX200

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Posted 04 December 2023 - 07:39 PM

I never used a 1.25" 40mm that much.  I like the TV 32mm plossl much better.




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