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actually seeing the field stop in an EP?

Eyepieces
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#1 Jim Hermanson

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Posted 31 July 2021 - 10:15 PM

Dear Group:

I recently got the televue eyeguard extender for a Vixen 42 mm LWR eyepiece ... it fits and seems to work well, a much better eye position.

 

However I noticed that with the increased distance, looking into the eyepiece I no longer see the sharp edge which I assume corresponds to the field stop.  This was an eyepiece-only test against a bright background.

 

I performed the same experiment with my other eyepieces, which include a televue pan 24, TV radian 14 mm, and 11 mm Plössl, also Meade 55 Plössl and the Meade 40 superwide that the Vixen is replacing.  In nearly every case I can only see the sharp outline of the field stop when I press my eye fairly deep into the eyecups, deeper than I use observing.  Or only by moving the eye to one side can I glimpse the field stop. 

 

Never noticed this in the dark observing before, but the effect might explain why the actual FOVs that people report may be a bit narrower than the specification for that EP.

 

Comments, anyone ... has anyone noticed a similar effect?

 

thanks!
Jim H.



#2 SeattleScott

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Posted 31 July 2021 - 10:53 PM

The extender might not allow seeing the field stop. I have had no issues using the eyepiece without an extender. The AFOV has reportedly been measured at about 63.5 degrees, so pretty close to the advertised 65. I found the TFOV just barely larger than an Agena 38mm SWA with 45.7mm field stop. So I would say the 42lvw works as designed without the extender.

Scott

#3 Jon Isaacs

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Posted 01 August 2021 - 04:55 AM

Dear Group:

I recently got the televue eyeguard extender for a Vixen 42 mm LWR eyepiece ... it fits and seems to work well, a much better eye position.

 

However I noticed that with the increased distance, looking into the eyepiece I no longer see the sharp edge which I assume corresponds to the field stop.  This was an eyepiece-only test against a bright background.

 

I performed the same experiment with my other eyepieces, which include a televue pan 24, TV radian 14 mm, and 11 mm Plössl, also Meade 55 Plössl and the Meade 40 superwide that the Vixen is replacing.  In nearly every case I can only see the sharp outline of the field stop when I press my eye fairly deep into the eyecups, deeper than I use observing.  Or only by moving the eye to one side can I glimpse the field stop. 

 

Never noticed this in the dark observing before, but the effect might explain why the actual FOVs that people report may be a bit narrower than the specification for that EP.

 

Comments, anyone ... has anyone noticed a similar effect?

 

thanks!
Jim H.

 

To see the field stop, your eye has to be at the eyepoint, the point at which the rays converge to determine the eye relief.  If you are behind that, you won't see the edge of the field nor the field stop.

 

I typically fold the eyecups down when observing so I can see the field stop.  With some eyepieces, it's quite difficult to see the field stop but with the Vixen LVW, I would think there is sufficient eye relief to see the entire field and the field stop.  

 

Why are you using the eye guard extender?

 

Jon



#4 Echolight

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Posted 01 August 2021 - 09:33 AM

At normal eye position, I can see the field stop in all my eyepieces except the sole 100 degree eyepiece that I have, APM HDC 20mm Hyperwide.
With the Hyperwide I have to smush my eye in a little closer than I like to see the field stop. And it’s still a little soft.

 

With my LHD 14mm (80 degree), and 28 PWA (82 degree), I did use taller eye guards to facilitate easier eye placement. But I can still see the field stop. These field stops have a sharp appearance. 
 

With my Pentax XW40, which has about the largest available field stop in a 2 inch eyepiece at 46.5mm, I can see the sharp field stop with various eye placements, up until the point that when backing off too far the narrow eye guard begins to block the edge of the field of view. And this is the one problem I have yet to address, as I do like the feel of having my eye a little further back with this eyepiece.

The field stop when in view, and not blocked by the eye guard when I’m backed off a few mm, is always sharp.

I may try something like the eyeguard extender along with a wider conventional eyecup with this eyepiece some day, as I’d like to back off a couple or more millimeters and still be in contact with the eyecup as well as be able to see the field stop. Or I might just trim back the inside of the screw up eye guard and then fashion some kind of eyecup to fit inside it. Maybe a regular eyecup turned inside out.


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#5 Starman1

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Posted 01 August 2021 - 10:12 AM

Dear Group:

I recently got the televue eyeguard extender for a Vixen 42 mm LWR eyepiece ... it fits and seems to work well, a much better eye position.

 

However I noticed that with the increased distance, looking into the eyepiece I no longer see the sharp edge which I assume corresponds to the field stop.  This was an eyepiece-only test against a bright background.

 

I performed the same experiment with my other eyepieces, which include a televue pan 24, TV radian 14 mm, and 11 mm Plössl, also Meade 55 Plössl and the Meade 40 superwide that the Vixen is replacing.  In nearly every case I can only see the sharp outline of the field stop when I press my eye fairly deep into the eyecups, deeper than I use observing.  Or only by moving the eye to one side can I glimpse the field stop. 

 

Never noticed this in the dark observing before, but the effect might explain why the actual FOVs that people report may be a bit narrower than the specification for that EP.

 

Comments, anyone ... has anyone noticed a similar effect?

 

thanks!
Jim H.

When you focus in a telescope, you are focusing at a distance, and since the field stop is at or near the focal plane, you see it in focus.

When you look through an eyepiece at the sky, there is no way to know what focal point your eye takes, but if you look directly at the field stop, it is likely to be seen with a close-up focus.

If you are young, it may still be in focus.  If you're older, it likely won't be.

That does not mean you won't see it in focus in the scope.  

 

Also, your daylight pupil will be quite small.  To field the image from the eyepiece, you will have to avert your gaze to see the edge of the field.

At night, with a larger pupil, the field stop will be more inside your field of vision because the entire exit pupil will be.

 

To check for the field stop focus, always use it in a telescope at night.

 

A couple things you can check for looking through an eyepiece at the sky:

--lateral field vignetting (the edge is darker than the center)  Note: most long focal length eyepieces will have some.

--lateral field color/tinting, like the "ring of fire" in the outer 10% of the field in the 31 Nagler and 30mm ES.

--dirt particles at/near the focal plane

--reflective surfaces inside the eyepiece (spacers, lens edges, etc) that could be a problem at night.  If you see the threads at the bottom of the eyepiece, blacken them with flat black paint.

Otherwise, bright stars just outside the field at night may cause a glare artifact inside the FOV.


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#6 SeattleScott

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Posted 01 August 2021 - 11:32 AM

Ultimately eyepieces with big eye lenses make it a little harder to know right where to place your eye. No one has eye placement problems with a 6mm Plossl. But a 42mm is gonna have a big eye lens. The LVW doesn’t even have a very big eye lens for a 40mm class wide angle. Using an eyeguard extension can help with eye placement but if you back off too far you don’t get the whole FOV.

Initially I had a little trouble with the 42lvw but after a couple sessions I got used to it. I suggest you try this rather than limit your FOV and not get the full benefits of this wide field eyepiece.

Scott
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#7 Jim Hermanson

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Posted 02 August 2021 - 11:53 PM

To Jon:

 

the eyeguard extender is to help position the eye for best viewing ... with the Vixen 42 I encounter "kidney beaning" when I push against the stock eyeguard.  The Televue extender should preclude that ... but it might be a bit "too long" so that I lose the field stop, as we've been discussing.   I might experiment with a few other eyecups as well.


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#8 Miranda2525

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Posted 04 August 2021 - 12:56 AM

To Jon:

 

the eyeguard extender is to help position the eye for best viewing ... with the Vixen 42 I encounter "kidney beaning" when I push against the stock eyeguard.  The Televue extender should preclude that ... but it might be a bit "too long" so that I lose the field stop, as we've been discussing.   I might experiment with a few other eyecups as well.

You've just got to learn to "hover" at the right point. Some eyepieces are not made right. The company assumes that by just adding a rubber eyecup that your eye will be at the right position to see the field stop, and this isn't always the case.

 

Or as you said, experiment with other types of eye cups.


Edited by Miranda2525, 04 August 2021 - 12:57 AM.


#9 darkandstormynight

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Posted 04 August 2021 - 12:39 PM

Yes, experimentation may be necessary.  Even without glasses the eyepiece/facial anatomy match can vary between individuals, so an eyepiece that’s right for one may be wrong for another.


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