32 replies to this topic

[ThomasM]

APM will soon offer a new 30 mm wide angle eyepiece. In contrast to other long focal length eyepieces (e.g. Nagler 31 mm) it is rather slim (~ 55 mm maximum diameter) and can be used for binoviewing:

 

https://www.cloudyni...e/#entry7825478

 

 

It is part of the Ultra-flat field series and the optical layout is available on the the APM web page:

 

 

http://www.apm-teles...yepiece FSD.pdf

 

According to this drawing and the description we can expect 70  degree AFOV, according to Marcus Ludes it is designed by an optical designer active here at CN.  The eyepieces looks very interesting it will be an excellent choice for binoviewing.

 

Now my question:

 

1. what true field of view can be expected? 

2. I don not understand the optical design, why is the field stop (diameter 30.4 mm)  indicated in the figure so small?

 

 

My understanding of super wide angle eyepiecs with a smyth lens  is the following:

 

The smyth lens as the first element acts as a Barlow, the  image will be magnified and as the result the  field stop diameter (the diameter of the aperture) is larger than  the effective field stop diameter which can be used to calcute the true field of view (TFOV).

So the effective field stop diameter will be smaller than the 30,4 mm given in the drawing, but that does not fit to a wide angle 30 mm  eyepieces. Then, the two optical elements following the field stop are rather small in diameter, that makes the eyepiece slim, but their diameter is even smaller than that  of the smyth lens. This seems to me very strange because the smyth lens magnifies the image. This is by the way the reason why the long focal length Naglers are so fat, the first lenses following the smyth lens must be large in diameter.

 

Can anybody help me out? May be the disigner of the eyepiece can explain?

 

 

with many thanks in advance

 

Thomas

 

edit: correct link to the APM web page

Edited by ThomasM, 16 April 2017 - 07:06 PM.

[lylver]

Hello, this may not be a classic Smyth negative lens, but a positive one, the external aperture seems to be 40 mm (negative Smyth are small), but we do not know with what type of glass they are made, we should expect additional information.
Eye side is specific, it reminds me Galoc design, perhaps an evolution, with a front focal reducer and two stages of flattening pair.

 

Edited by lylver, 16 April 2017 - 07:04 AM.

[ThomasM]

Hello, this may not be a classic Smyth negative lens, but a positive one, the external aperture seems to be 40 mm (negative Smyth are small), but we do not know with what type of glass they are made, we should expect additional information.
Eye side is specific, it reminds me Galoc design, perhaps an evolution, with a front focal reducer and two stages of flattening pair.

Yes I agree, if the first doublet would be a sort of Smyth lens with positive focal length (actually, I thought that by definition the focal length of a Smyth is negativ)  would be a good explanation. But this is very unusual, as you pointed out the Galoc design (is this the only one?)  has such first element. But if you look an the drawing, the first doublet does not look like a positve combination,  the first radius of curvature of the  positive front lens is much larger than that  of the back side of the negative lens. Or very unusual glass combinations have been used, the positve front lens with very high index of refraction while the negative with low.  Somewhat puzzling.

 In view of the large diameter of the front lens of 40 mm I would expect an effective field stop diameter of ~ 40 mm which might give 70 degree AFOV,  otherwise why making this lens so big?

 

Anyhow, very interesting, especially how this complex lens will perform in praxis.

 

Thomas

Edited by ThomasM, 16 April 2017 - 07:55 AM.

[lylver]

I said Galoc only for the specific shape of the eye doublet (thick positive meniscus, thin negative), without lens index & other properties this is hard to guess

http://atom.lylver.o...es/treediag.jpg

http://atom.lylver.o...ofEYEPIECES.pdf

 

It is a brand new optic formula

 

The others (except the 10mm that is new to me too) are made from a size scalable formula

http://www.apm-teles...yepiece FSD.pdf

Edited by lylver, 16 April 2017 - 08:34 AM.

[ThomasM]

I said Galoc only for the specific form of the eye doublet (thick positive meniscus, thin negative), without lens index & other properties this is hard to guess

http://atom.lylver.o...es/treediag.jpg

http://atom.lylver.o...ofEYEPIECES.pdf

 

It is a brand new optic formula

Ok, I misunderstood your comment regarding the Galoc eyepiece. I agree, the design is new, that's why I am asking questions. The actual optical diameter of the first positive lens is much larger than that of the exciting negative lens where the light exits.It seems that the two thick lenses are just collimating the light beam.

 

Thomas

[mrackerm]

Here is a copy of the comments I posted on the original thread regarding this eyepiece:

 

---

 

Let me attempt to explain the design particulars of this eyepiece, and, hopefully, in the process will answer some of the questions above.

I designed the eyepiece to the specifications requested by Markus Ludes.

The design comes from a family that some refer to as Nagler-type eyepieces and others simply refer to as wide-field eyepieces that include a Smyth lens.  The Smyth lens is the first two glass elements encountered as the light proceeds from the objective (or mirror) towards the initial focus, or image plane.  The Smyth lens group has a negative focal length.  Because this sits in a position before the telescope achieves its initial focus, and because it has a negative focal length, some of the optical concepts, such as image plane and field stop, become, more or less, virtual.  They do not exist at a clearly defined plane within the optical system.  Also, their approximate optical size is not precisely related to their physical size.

The subject eyepiece was designed for a 70 degree field of view.  Many wide-field eyepieces have a theoretical design for a given field of view, but in practice, lose much of their field due to vignetting from reduced size lens elements used by the fabricator to facilitate manufacture and assembly.  The 30mm, 70 degree eyepiece was designed for zero vignetting and should achieve very close to the full 70 degree field of view in practice (there are always slight adjustments internally for manufacture). 

Another factor that will change the size of a field stop (real or virtual) is distortion.  Most eyepieces of this type have barrel distortion.  This tends to make the field stop smaller in diameter than one would otherwise assume. 

For this eyepiece, the 40mm entrance diameter, as shown, will result in almost no vignetting and the full 70 degree field should be realized.  The internal field stop diameter is an approximate value which is based on the clear aperture diameter of the 3rd lens the light encounters.  The virtual field stop would actually be inside this lens slightly, so the mechanical diameter of the lens serves as the functional field stop.

I hope this helps.

I am rarely on Cloudy Nights so if someone posts a question, it is not certain that I will see it, but maybe there is some mechanism that will alert me.  If the observer community wants new products, by all means, ask Markus about them.  If he determines there is a market for such ideas, I am sure he will ask me or one of his other designer contacts to help him turn the idea into hardware.

Mark

[ThomasM]

Hi Mark,

 

thanks a lot for all the explanations, this is a copy of my question in Vendor section, please apologize for the dublication:

 

 

Ffirst I thougth I got, but now it is again unclear to me. Yes, I was expecting that it is a design with smyth lens, but that means the image increases in size. So I would expect that the diameter of the third lens should be rather big, at least I would expect it's diameter larger than that of the first lens. Then, if the third lens accepts a cone with 30.4 mm diameter and the image is enlarged by the smyth length,  I would expect an effective or virtual field stop diameter with significnat less than 30 mm. But with such small diameter a AFOV of 70 degree will be difficult. Were is my mistake? All in all, can you please provide a number of the effective field stop diameter which allows to calculate the true field of view for a given telescope?

 

with many thanks in advance

 

Thomas

 

p.s. I think it will be an excellent eyepiece for binoviewing because in contrast to other eyepieces it is very slim, I think many people will be very eager to use it

[Starman1]

Thomas,

See my post: https://www.cloudyni...gree/?p=7826462

about the relationship of apparent field and field stop.

If the designer says 30.4mm for field stop, you can use that for true field calculations until timing a star passage tells you different.

However, you cannot derive an apparent field from that unless you know the nature of the distortion in the eyepiece.

Hopefully, I explained it well enough in my post.

[ThomasM]

Thomas,

See my post: https://www.cloudyni...gree/?p=7826462

about the relationship of apparent field and field stop.

If the designer says 30.4mm for field stop, you can use that for true field calculations until timing a star passage tells you different.

However, you cannot derive an apparent field from that unless you know the nature of the distortion in the eyepiece.

Hopefully, I explained it well enough in my post.

Don,

 

thanks a lot, this all sounds very reasonable. Distortion has certainly an effect on the aparent field of view. But this is not my  key problem:  The field stop with it's 30.4 mm indicated in the figure (see the figure above taken from the APM web page) is a true mechanical measure. Taking into account the magnification of the smyth lens ( for a Nagler 13 mm eyepiece the effective field stop diameter ist 17,2 mm - taken for TV Web page- the mechanical aperture 22.5 mm, I measured it, so 1.3x magnification) the effective or virtual field stop diameter is much smaller than 30.4 mm, if we assume the same magnification of 1.3x it is only 23.4 mm. Even with constant angular magnification, i.d. large pincushion distorsion this will result in 45 degree AFOV, way of the specified 70 degree. Presumably, the magnification is somewhat smaller, but with a negative smyth length I can not see how to get 70 degree. Just taking the 30.4 mm field stop diameter (no magnification), assuming constant angular magnification I get only 58.6 degree for a 30 mm eyepiece, not 70 degree.

 

May be this explains why I am puzzled

 

Thomas

Edited by ThomasM, 16 April 2017 - 05:43 PM.

[Starman1]

Not necessarily.

You don't know what happens in the lenses.

If the Smyth lens works normally, it expands the field size to hit the next lens larger than the Smyth lens.

Then, it may shrink or expand as it goes through the lenses, until it passes through the eye lens and narrows to the exit pupil.

 

The exit pupil (determined by the telescope) can be ignored, but the eye relief is a design element.

The combination of eye lens diameter and eye relief determines the maximum possible apparent field with any given diameter of field stop.

If the eyepiece had an eye lens of X diameter, but the eye relief was designed to be 20mm, it could have less apparent field than if the 

eye relief were 10mm.

 

One way to think of it is that with a constant diameter of field stop, the closer to it you are, the wider it appears to be.

Have you ever noticed that the wider the apparent field, the closer the image seems to be?  That's a psychological phenomenon.

Look through a 40mm Plössl some day--the field seems to be a long way from your eye down a long tube.

In a 100° eyepiece it seems so close part of it is tucked under the edges of the eyepiece and you have to look through at an angle to see it, like looking through a porthole.

 

Now, in practice, the field stop diameter size does determine the maximum apparent field if lens curves are kept reasonable and good control of chromatic aberration is required.

Still, there are multiple solutions for the same size field stop diameter and focal length for eye relief and apparent field.  Always.

A 32mm Plössl and 24mm 68° eyepiece may have exactly the same field stop diameter, yet have widely different apparent fields, eye reliefs, eye lens diameters, etc.

 

If you know the exact eye lens diameter and eye relief, you can calculate the maximum apparent field.  That will tell you only a bit about the field stop unless you know other characteristics of the eyepiece.

[Starman1]

If the eye lens is 29mm in diameter (can't tell from the drawing), and eye relief is 22mm from the lens and the concavity of the eye lens is 2mm,

the apparent field could be 72°.

That follows from trig.

The apparent field could be smaller than that if the eye lens isn't entirely used, but it couldn't be larger than 72° with a 29mm lens.

 

What is the field stop?  They say 30.4mm.

Now maybe that backs into a 65° apparent field with simple math, like AF = TF x M

And that could be, and it might possibly show a discrepancy with the maximum apparent field figured from the eye lens diameter.

True field, however, I presume, is a 30.4mm field, and that'll give you the size of the true field in any telescope.

 

When these get into circulation, we'll see some people testing them using the flashlight test and then we'll know more accurate apparent fields.

At this time, I see no reason to doubt the manufacturer on eye relief or field stop size.

Once we know actual field stop, apparent field, lens diameters and eye relief, we can also figure out the edge of field distortion, but it likely won't be important.

[mrackerm]

Thomas:

 

This eyepiece has an entrance pupil that is over 1 meter distant from the first lens - this was done so that it is expecting a near telecentric light bundle from the objective.  This allows the the eyepiece to accept light rays over a wide range of ray angles on what would have been the image surface of the objective (different styles of telescopes produce different ray angles for the chief ray as a function of field position).  So, in English, the eyepiece was designed to be as flexible as possible regarding how the light gets to it.  As a result, the entrance pupil diameter won't do you a lot of good - it does not have any physical meaning.

 

The best way to calculate actual field of view is to use the diameter of the first lens - it is very close to the diameter of the focal surface for the largest and fastest objective the eyepiece can support.  There is a virtual image surface 25mm into the eyepiece from the first lens and its diameter is about 2mm smaller than the first lens.  If your telescope produces a smaller diameter image circle than 40mm, then the telescope will limit the actual field of view of the system.  If the telescope produces an image circle larger than 40mm diameter, then the eyepiece will limit the field of view.  

 

I always look at the telescope to determine the actual field of view.  Since the entrance of the eyepiece is 40mm diameter, if we use this as the approximate diameter of the telescope image circle, then the actual field of view will be

 

AFOV = 2 * atan(20/focal length)

 

I am not sure any of this helps you.  Eyepieces are actually imaging systems if you use them in reverse.  The problem with designs that include the Smyth lens is that the image is formed internal to the eyepiece and the diameter and shape of the focal surface (not necessarily a plane) can really only be determined from ray tracing, or extremely laborious calculations.

 

Mark

[lylver]

 

There is a virtual image surface 25mm into the eyepiece from the first lens and its diameter is about 2mm smaller than the first lens.

Ok ... impossible to know with the optomechanical design. New generation EP with the focus inside are harder to understand.

Enough informations to me, my future scope got a much smaller circle at this distance from focus, even hard bending incoming rays ^^

 

Just a general thinking : small lenses may be harder to polish and to center well in a housing.

 

Anyway, we the "old" people like flat field eyepieces, we are still able to see sharp details but presbytie impair our focus capability.

Edited by lylver, 16 April 2017 - 07:00 PM.

[ThomasM]

Thomas:

 

This eyepiece has an entrance pupil that is over 1 meter distant from the first lens - this was done so that it is expecting a near telecentric light bundle from the objective.  This allows the the eyepiece to accept light rays over a wide range of ray angles on what would have been the image surface of the objective (different styles of telescopes produce different ray angles for the chief ray as a function of field position).  So, in English, the eyepiece was designed to be as flexible as possible regarding how the light gets to it.  As a result, the entrance pupil diameter won't do you a lot of good - it does not have any physical meaning.

 

The best way to calculate actual field of view is to use the diameter of the first lens - it is very close to the diameter of the focal surface for the largest and fastest objective the eyepiece can support.  There is a virtual image surface 25mm into the eyepiece from the first lens and its diameter is about 2mm smaller than the first lens.  If your telescope produces a smaller diameter image circle than 40mm, then the telescope will limit the actual field of view of the system.  If the telescope produces an image circle larger than 40mm diameter, then the eyepiece will limit the field of view.  

 

I always look at the telescope to determine the actual field of view.  Since the entrance of the eyepiece is 40mm diameter, if we use this as the approximate diameter of the telescope image circle, then the actual field of view will be

 

AFOV = 2 * atan(20/focal length)

 

I am not sure any of this helps you.  Eyepieces are actually imaging systems if you use them in reverse.  The problem with designs that include the Smyth lens is that the image is formed internal to the eyepiece and the diameter and shape of the focal surface (not necessarily a plane) can really only be determined from ray tracing, or extremely laborious calculations.

 

Mark

Mark,

 

once again, thank you very much for your explanations, for your patience and taking your time reading my questions. I have to admit, that I still do not understand the design of the 30 mm UF eyepiece.

In essence, I do not understand how the full 40 mm wide light beam can propagate through the eyepiece.  Usually, the diameter of the smyth lens is smaller than the field stop and that of the next optical element (e.g. in the other eyepieces of the Ultra Flat series, Naglers). This natural because the smyth lens magnifies the diameter of the ligth beam as illustrated here:

 

 

 

http://www.brayebroo...low lenses.html

page 129, the Nagler design

 

or here:

 

http://cs.astronomy....65.Nagler04.jpg

 

 

  In contrast the large 40 mm diameter smyth lens of the UF 30 mm is followed by significantly  smaller elements.

The diameter of the field stop which is more or less identical with the diameter of the second optial element is only 30.4 mm, this restricts the diameter of the beam in the virutual image plane. With my naive undertanding I would expect that with such a field stop diameter  the eyepiece can only provide 54 degree AFOV (if I take your equation and replace the 20 by 30.4 /2). As the smyth lens enlarges the image, the field stop is an internal aperture,  I would expect that the AFOV is even smaller than 54 degree,  much less than 70 degree.

So I can only speculate that the first optical element is very different from a standard smyth lens, it is very thick and the distance to the next element is very short. Is that the explanation?

 

Best regards

 

 

Thomas

Edited by ThomasM, 17 April 2017 - 09:33 AM.

[lylver]

I checked this way. I think Mark meant using virtual field stop (40).

AFOV = 2 * tan^-1 ( half_field_stop / f ).

It makes 66.7°, a compressed field of ~ 70° due to barrel distorsion

[Starman1]

https://www.cloudyni...gree/?p=7828031

[lylver]

Yep, you are right we don't have the info. I found a nice picture too about complex EP.(down)

You can place field stop at many places where it should help positionning the eye.

Down it is between Smyth & Flattening part / Core eye part.

This one had room for the FS. Moved where the displaced scope focus plane is.

a) Red dot : the original focal plane

b) Razor cut aperture stop at the virtual image edge position (doesn't mean the image is flat )

You can put the FS elsewhere, I hadn't thought it could be placed on the original focus plane and remain sharp for the eye

Edited by lylver, 17 April 2017 - 05:49 PM.

[BillP]

The subject eyepiece was designed for a 70 degree field of view.  Many wide-field eyepieces have a theoretical design for a given field of view, but in practice, lose much of their field due to vignetting from reduced size lens elements used by the fabricator to facilitate manufacture and assembly.  The 30mm, 70 degree eyepiece was designed for zero vignetting and should achieve very close to the full 70 degree field of view in practice (there are always slight adjustments internally for manufacture).

 

Something I do not see often...so nice to see it here...and why people should always take "design" performance specs, whether for an eyepiece or an objective, with a grain of salt.  Design performance in probably most circumstances does not equal production performance.

[mrackerm]

Thomas:

 

Sorry - I don't check CN often.

 

The Smyth lens for this eyepiece is somewhat unusual.  The first lens (positive lens) uses a very high index glass.

The second lens (negative lens) uses a low index glass.  This causes the pupil and the diameter of the third lens

to be smaller than one would expect.

 

Mark

[ThomasM]

Thomas:

 

Sorry - I don't check CN often.

 

The Smyth lens for this eyepiece is somewhat unusual.  The first lens (positive lens) uses a very high index glass.

The second lens (negative lens) uses a low index glass.  This causes the pupil and the diameter of the third lens

to be smaller than one would expect.

 

Mark

Mark,

 

thanks a lot for the explanation, this is a very interesting design  and it explains  how you can achieve a wide AFOV with a rather slim eyepiece.

 

Thomas

[nicknacknock]

And here it is...

[lylver]

Positive smyth :an uncommon choice that implies here smaller and thinner lenses. Light transmission is kept under control.

More than 95% did I guess right ?

Edited by lylver, 29 June 2017 - 02:55 PM.

[ThomasM]

Positive smyth :an uncommon choice that implies here smaller and thinner lenses. Light transmission is kept under control.

More than 95% did I guess right ?

I would not interpret Mark's comment such that it is a positive smyth lens, he is only saying that the intermediate image ( pupil) is smaller than one would expect because of the combination of glass and ( that's my interpretation) that both lenses are rather thick.

 

Thomas

[Starman1]

The bottom lens still looks slightly concave.  The upper curve appears deeper than the lower curve.

It's only slightly negative, though, so not a lot of expansion of the image scale takes place.  That allows the following lenses to be smaller in diameter and the eyepiece lighter.

560g is fairly light for a 9 element 30mm eyepiece.

[lylver]

The glass/glass inner surface has stronger curve, so the index ratio he gaves should make the difference.

... Had been surprised before with correcting lenses.

It may be only a local (to the thick front glasses) light cone resizer (act as focal point displacer and magnifier/reducer)

Mark may remain silent to preserve the design, I already said to much.

Edited by lylver, 29 June 2017 - 04:53 PM.