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Abberation in Meade wide Field Eye Pieces

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#1 patindaytona

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Posted 10 October 2019 - 01:29 PM

I don't see it so much at night, but was just looking at the sun thru the 5.5mm and the 14mm Meade UWF EP.

It's very spherical and I see alot of distortion. Just curious what other's who own these eye pieces think about this.

I know you can twist them to get different eye relief's, but the best that I've found is when they are almost flush with the top.

Even then, you have to put your eye right up against it. They aren't bad at night though.



#2 junomike

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Posted 10 October 2019 - 02:45 PM

Most likely it's aberrations in your eye as the exit pupil in the daytime is far larger than at night (so daytime is more susceptible to seeing this)


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#3 SeattleScott

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Posted 10 October 2019 - 11:14 PM

Well technically the exit pupil is the same size day or night. But your pupil is dilated a lot more at night. But yes, distortion (including from one’s own eyes) is easier to see in the daytime.

Scott
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#4 Starman1

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Posted 11 October 2019 - 02:18 PM

I don't see it so much at night, but was just looking at the sun thru the 5.5mm and the 14mm Meade UWF EP.

It's very spherical and I see alot of distortion. Just curious what other's who own these eye pieces think about this.

I know you can twist them to get different eye relief's, but the best that I've found is when they are almost flush with the top.

Even then, you have to put your eye right up against it. They aren't bad at night though.

Any eyepiece wider than about 40° has distortion.

What differs is what kind of distortion, and the amount.

The wider the apparent field, the more distortion the eyepiece has, and distortion is distortion.

The most common form of visible distortion in an eyepiece created for astronomical use is rectilinear distortion.

If a straight line crosses the field, it goes from ")" to "|" to "(" as it crosses if the distortion is positive, called pincushion.

If a straight line crosses the field, it goes from "(" to "|" to ")" as it crosses if the distortion is negative, called barrel distortion.

Pincushion is the least visually objectionable and is usually the form you see in astronomical eyepieces.

This form of distortion can be suppressed, but if it is, another form of distortion becomes larger and more obvious: angular magnification distortion.

If a feature crosses the field and goes from A to A to A, that is considered rolling ball, or globe distortion.

If a feature crosses the field and goes from A to A to A, that is the opposite sign of the more common form of AMD.

Both are forms of AMD, or angular magnification distortion.

 

Daytime use eyepieces often have reduced RD so straight lines stay straight, but, as a result, billboard lettering will shrink as it nears the edge of the field.

Nighttime eyepieces often have reduced AMD so double star separations and craters and planets stay the same size as they near the edge.

 

Now, back to the eyepieces used on the Moon or sun:

With AMD, the disc nearer the edge will shrink in size, distorting the Moon/Sun into a slightly egg-shaped disc with the smaller end pointed out..

With RD in the most common form, the disc will stretch radially as it nears the edge, turning the round disc into a slightly oval shape.

 

As they say, distortion is distortion.

 

How do you avoid it?  Simple, use narrow field eyepieces.  Plössls make excellent sun and moon eyepieces.

Otherwise, don't look at the sun or moon right near the edge of the field.

 

Now, moving the eyeguards in or out does not change the eye relief of the eyepiece, it merely moves the pupil of your eye relative to the exit pupil of the eyepiece.

The distance from the top lens to the exit pupil is the eye relief of the eyepiece, which is fixed.

Whether day or night, you want the pupil of your eye positioned at the exit pupil.  It's harder in the daytime because your pupil is smaller, but it's the same process:

start back from the eyepiece and slowly move in toward the eyepiece.  At the point you can *just* see the entire field with peripheral vision, stop.  That is the exit pupil.

If you can't hold your head that far out, raise the eyeguard until holding your head there is instinctive.  If you go too close to the eyepiece, the iris of your eye will start

causing blackouts in the image.


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#5 patindaytona

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Posted 11 October 2019 - 02:50 PM

Thanks Starman. You always have great info.

"moving the eyeguards in or out does not change the eye relief of the eyepiece, it merely moves the pupil of your eye relative to the exit pupil of the eyepiece"

Eye Relief: distance from top lens to exit pupil.      How does an eye guard (meade's have the twist barrels) help with positioning your eye onto the exit pupil?

What you said "it merely moves the pupil of your eye relative to the exit pupil of the eyepiece"---so it's acting as a physical support? You are supposed to use it (eye guard) to physically rest your eye onto?

That's pretty much what i've been trying to do by adjusting it often to find the right place.

With the Meades' Im doing alot of experimenting to find the ideal amount and it makes a big difference. Finding by lowering it, from flush at top, to about 1/4" in is about the best. I have to press my eye socket right up against it to get in the whole field doing that. Is that ok? I'm not getting that field with ANY amount of twist on the barrel.

The same with the 24mm ES 82 degree. I have to press right onto the eye guard. It's the best way to see everything in the field.

Again "it merely moves the pupil of your eye.............." right?


Edited by patindaytona, 11 October 2019 - 03:07 PM.


#6 Starman1

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Posted 11 October 2019 - 04:08 PM

You don't necessarily want the eyeguard to touch your face, though some observers like to "bury" their eyes into the cup,

but, at some distance from the eye, your instinct for eye preservation forces you to back up a bit for comfort.

I use Delite eyepieces on my 4", and I can raise the eyeguards all the way and still use the eyepieces, but I don't like the pressure

against my eye socket, so I lower them a tad and I still have no tendency to drift closer, yet I am comfortably away from the eyeguard.

 

An eyepiece with less eye relief will require a lower eyeguard position, and an eyepiece with a recessed top lens or a very short eye relief

may be most comfortable to use with the rubber eyeguard down all the way.


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#7 SeattleScott

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Posted 11 October 2019 - 04:26 PM

Generally if you don’t wear glasses you raise the eye guard up to the lowest point where you can see the entire FOV with your eye next to or against the eye This way it helps guide your eye and block stray light.

I have the ES 24/82 and a Meade 8.8/82. Definitely not eyeglass friendly. That is a trade off. With my LVWS I can wear glasses and see the entire FOV. But they are only 65 AFOV. If you want 80+ you are going to have to get cozy with that lens, or get a giant lens, like the ES 92’s. Don’t those look beautiful? I just got back to the gym today after a hiatus. No point in buying an eyepiece I cannot lift.

Scott
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#8 patindaytona

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Posted 11 October 2019 - 05:25 PM

You don't necessarily want the eyeguard to touch your face, though some observers like to "bury" their eyes into the cup,

but, at some distance from the eye, your instinct for eye preservation forces you to back up a bit for comfort.

I use Delite eyepieces on my 4", and I can raise the eyeguards all the way and still use the eyepieces, but I don't like the pressure

against my eye socket, so I lower them a tad and I still have no tendency to drift closer, yet I am comfortably away from the eyeguard.

 

An eyepiece with less eye relief will require a lower eyeguard position, and an eyepiece with a recessed top lens or a very short eye relief

may be most comfortable to use with the rubber eyeguard down all the way.

Ok, it's a matter of preference then.

I'm looking at specs I made of my eye pieces. Is the exit pupil indicating the amount of eye relief?  As magnification increases, the exit pupil becomes shorter.

If that's what you are refering too, then with my say 5.5.mm (1.70mm exit pupil), i would want to lower the eye guard.

My 25mm ES 82 degree (all my eye pieces are 82 by the way), i have a 5.1 exit pupil, so i would be wanting to back away more?

............I'm finding with all of these eye pieces i have to really fit them right into my eye socket to encompass the wide field



#9 patindaytona

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Posted 11 October 2019 - 05:30 PM

Generally if you don’t wear glasses you raise the eye guard up to the lowest point where you can see the entire FOV with your eye next to or against the eye This way it helps guide your eye and block stray light.

I have the ES 24/82 and a Meade 8.8/82. Definitely not eyeglass friendly. That is a trade off. With my LVWS I can wear glasses and see the entire FOV. But they are only 65 AFOV. If you want 80+ you are going to have to get cozy with that lens, or get a giant lens, like the ES 92’s. Don’t those look beautiful? I just got back to the gym today after a hiatus. No point in buying an eyepiece I cannot lift.

Scott

Yes, i do have to get cozy and try and get "inside" those 82 field eye pieces. It's the only way i can see doing it, but I'll experiment later and see if i can find any other relief distance.

I don't wear glasses, but I'm sure I'll end up showing someone who does. I don't know how bad that is if they look thru it.



#10 Starman1

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Posted 11 October 2019 - 07:28 PM

Ok, it's a matter of preference then.

I'm looking at specs I made of my eye pieces. Is the exit pupil indicating the amount of eye relief?  As magnification increases, the exit pupil becomes shorter.

If that's what you are refering too, then with my say 5.5.mm (1.70mm exit pupil), i would want to lower the eye guard.

My 25mm ES 82 degree (all my eye pieces are 82 by the way), i have a 5.1 exit pupil, so i would be wanting to back away more?

............I'm finding with all of these eye pieces i have to really fit them right into my eye socket to encompass the wide field

Focal length, and exit pupil, isn't related to eye relief necessarily.

If a line is "scaled", each focal length shorter is proportionately smaller in weight, size, eye relief.

Example: TeleVue Panoptics.

 

But most lines of eyepieces these days are not scaled.

Either they have constant eye relief over the range (and different internals on different focal lengths), examle TeleVue Delites or Delos or Ethos, or

perhaps are partially scaled over sets of 3 in a range of 9 eyepieces (where you have 3 sets of three, scaled), or there is

no rhyme or reason and the eye relief is just related to whatever the internal design is, and varies all over the place across a series, example: ES 62° series.

The 82° ES series is more or less constant eye relief which means the internals differ from focal length to focal length.

And, unfortunately, they recess the eye lenses a bit, so eye reliefs feel shorter than they are.


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