Author Topic: Eyepiece question  (Read 288 times)

firorectve

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Eyepiece question
« on: December 27, 2017, 09:34:49 PM »
Ok, I keep reading the discussions on eyepieces, but I have trouble figuring out the practical effect of things like the exit pupil and eye relief. However, I have found an eyepiece that, for me, works poorly. I have two Expanse eyepieces (9 mm and 20 mm). Hate using them as an example, but I'm thinking that if I describe what I don't like in an actual eyepiece maybe that would be helpful in figuring out what I should be looking for in an eyepiece.

When I use the Expanse eyepieces, without my eyeglasses, I find that they only work if I have my eye looking *perfectly straight* down the barrel. I do get the wide field effect (they are 66 deg AFOV) in that I see a wide field of view compared with my Plossls, and the view itself is good, but if I shift my head even a little either way I get a "blackout" effect (or is that vignetting?). For information, I was using the Nexstar 6SE scope, 1500 mm focal length. I calculate a 0.9-2.0 mm exit pupil for these, which I understand is a reasonable value. Is it an eye relief issue, or something else? And what should I look for in an eyepiece to *not* have this blackout effect?

The other issue I have with these eyepieces is that the outermost lens seems pretty close to the viewing end of the eyepiece (i.e. close to the eye), and the eyecup has little "height" over the top of the lens. I would prefer a "deeper" setting for the outermost lens, mainly for the practical reason it makes it harder to accidentally touch the glass, but also I seem to be able to get more absorbed in the view when looking deeper into the eyepiece. Is there anything to look for in the description of the eyepiece to assess this aspect?

For comparison, I have not had the blackout effect with my Meade SWA 5000 5.5 mm eyepiece. The outer lens is also set a bit deeper with that eyepiece. So I like that one better. I actually have also been pretty satisfied with my stock 10 mm and 25 mm Plossls that came with the Skywatcher dob as well, though the FOV is of course fairly narrow.

Thanks in advance for any information/suggestions.



Kapil Majmudar

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Re: Eyepiece question
« Reply #1 on: December 30, 2017, 11:52:26 PM »
Here is a thread on the subject: https://www.cloudyni...-what-they-are/

Mike Heck

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Re: Eyepiece question
« Reply #2 on: December 31, 2017, 03:10:24 AM »
Problems with blackouts is mostly payment for wide field...
NEOhio, could not it be that you described your day-time observing experience?
In night, when pupil of observer adapted to 6-8 mm wide the described problems are not so critical.

Yung Pryor

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Re: Eyepiece question
« Reply #3 on: December 31, 2017, 07:50:39 AM »
The technical name of the blackout is spherical aberration of the exit pupil.  Many wide field eyepieces will do this, but many don't or don't do it as badly. The best thing to do is read reviews for eyepieces and if a particular model is more prone to this, the reviews will usually say so.

The distance between your eye and the eye lens is called eye relief and this would be fixed to the eyepiece design, but from what you describe, I think you may want to look for a pair of winged eye guards.Winged Eye Guards can both help you keep your eye better positioned over the exit pupil, and will give you the effect you desire.

I love winged eye guards. You might too. They might help with your concerns.

This is a link to an example.  They come in many different sizes.

http://agenaastro.co...-id-33-5mm.html

notaslasof

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Re: Eyepiece question
« Reply #4 on: December 31, 2017, 08:25:02 PM »
Quote
The technical name of the blackout is spherical aberration of the exit pupil.

Spherical aberration is responsible only for specific sort of blackout named "Kidney bean vignetting".

piatimascomp

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Re: Eyepiece question
« Reply #5 on: January 04, 2018, 03:45:20 PM »
Quote
Ok, I keep reading the discussions on eyepieces, but I have trouble figuring out the practical effect of things like the exit pupil and eye relief. However, I have found an eyepiece that, for me, works poorly. I have two Expanse eyepieces (9 mm and 20 mm). Hate using them as an example, but I'm thinking that if I describe what I don't like in an actual eyepiece maybe that would be helpful in figuring out what I should be looking for in an eyepiece.

My comments are below, in bold type.

When I use the Expanse eyepieces, without my eyeglasses, I find that they only work if I have my eye looking *perfectly straight* down the barrel. I do get the wide field effect (they are 66 deg AFOV) in that I see a wide field of view compared with my Plossls, and the view itself is good, but if I shift my head even a little either way I get a "blackout" effect (or is that vignetting?). For information, I was using the Nexstar 6SE scope, 1500 mm focal length. I calculate a 0.9-2.0 mm exit pupil for these, which I understand is a reasonable value. Is it an eye relief issue, or something else? And what should I look for in an eyepiece to *not* have this blackout effect?

If the eyepiece has too long an eye relief, it is easily possible to drift too close and start getting blackouts. An eyepiece with a bit less eye relief might be preferred, especially if you use the eyepieces without glasses. Both of those eyepieces have fairly long eye reliefs, so this could be your problem. Also, when using an eyepiece with that wide a field, one does not hold one's head perfectly still and then just avert the eye to look at the edge, since this could easily move the eye away from the exit pupil. Better to roll the head a bit, holding the eye in place, so the pupil of your eye is not moved away from the exit pupil. So eyepieces with a bit less eye relief and perhaps a bit of practice in using them might pay off.

The other issue I have with these eyepieces is that the outermost lens seems pretty close to the viewing end of the eyepiece (i.e. close to the eye), and the eyecup has little "height" over the top of the lens. I would prefer a "deeper" setting for the outermost lens, mainly for the practical reason it makes it harder to accidentally touch the glass, but also I seem to be able to get more absorbed in the view when looking deeper into the eyepiece. Is there anything to look for in the description of the eyepiece to assess this aspect?

In order to find this, you will need to look at eyepieces with longer eye reliefs but with eyecups that adjust upwards sufficiently to hold your eye steadily at the right distance from the lens. If you did this with a short eye relief eyepiece, you'd never see the entire field of view

For comparison, I have not had the blackout effect with my Meade SWA 5000 5.5 mm eyepiece. The outer lens is also set a bit deeper with that eyepiece. So I like that one better. I actually have also been pretty satisfied with my stock 10 mm and 25 mm Plossls that came with the Skywatcher dob as well, though the FOV is of course fairly narrow.

And all those eyepieces have less eye relief, which says your blackouts problem is from drifting too close. In the case of the Expanse eyepieces, then, don't even think about using them with the eyecups folded down

Thanks in advance for any information/suggestions.


Sam Noble

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Re: Eyepiece question
« Reply #6 on: January 10, 2018, 11:33:27 PM »
Just fiddled with the Expanse 20mm and 9mm (the 9mm has internal barlow).The 20mm is a 1,1,2 Modified Konig, almost the old Konig-II patent.
The 9mm is a 1,2 at the top...

They both have a very severe eye lens at the top, the 20mm more than the 9mm,
and a concave top. Sensitive eye placement is inevitable, though understanding
where your eye should be helps a lot.

Weak lenses remove the sensitivity but always cause chromatic lare at the edges.
However, a ring/sleeve with a top 0.400 inches above the eyepiece top flange
 locates the eye and practically eliminates all blackouts. That's without glasses.
 With glasses, you only need a very low extra length.
This is for the 20mm...

The 9mm seems to be more sensitive, but a semi-solid or solid collar should
to make eye placement much easier...

riaherrvodo

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Re: Eyepiece question
« Reply #7 on: January 11, 2018, 05:09:59 AM »
Not worth buying IMO...

tersrhythopes

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Re: Eyepiece question
« Reply #8 on: January 11, 2018, 07:21:27 AM »
Quite happy with the 20mm, with eye spacer...

I think I got to the bottom of the persistent blackouts on the 9mm.
The top section was simply built wrong, or with the wrong values.
I dropped the spacing of the top 2 elements to touching.
Blackouts gone,
Some QC issue, maybe....too strong an eye lens.
People do swear there are no blackouts...others swear there are.
MAybe there are glitches sometimes.

Ryan Miller

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Re: Eyepiece question
« Reply #9 on: January 12, 2018, 01:05:52 AM »
Quote
Not worth buying IMO...


What would be an eye-comfortable alternative, assuming it's under $100 ?
I was wondering about the Agen Astro Dual-ED StarGuider...

tingdermeli

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Re: Eyepiece question
« Reply #10 on: January 17, 2018, 01:12:15 AM »
Quote
Quote

Not worth buying IMO...


What would be an eye-comfortable alternative, assuming it's under $100 ?
I was wondering about the Agen Astro Dual-ED StarGuider...
I use the Astro Tech version of the same eyepiece. Very nice with a twist up eye guard.

Jeremy Kelley

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Re: Eyepiece question
« Reply #11 on: January 20, 2018, 02:40:40 PM »
I see...you mean the same thing is available for less...makes sense.
It's likely to have the same issues in the OP, though.

I have lower-afov versions of the 1,1,2-Konig (60 degrees) with almost
no blackout, but I don't know if anyone makes that.

When I say "no blackouts, I mean that you can move the eye side to side,
 and you see smaller pieces of the field, but it does not 'wink out' like there is a black shutter
 closing your view off faster than your eye moves.

Plossls have low blackout but just the 50 degrees afov.

larterpchaka

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Re: Eyepiece question
« Reply #12 on: January 20, 2018, 05:58:31 PM »
Quote
When I say "no blackouts, I mean that you can move the eye side to side,
 and you see smaller pieces of the field, but it does not 'wink out' like there is a black shutter
 closing your view off faster than your eye moves.

Plossls have low blackout but just the 50 degrees afov.

Wouldn't that depend on the exit pupil?

For example, if your pupil is 5mm and the exit pupil is 4mm, you could move your eye a lot to the side before the exit pupil is completely eclipsed.
But if the exit pupil is 1mm, you can move your head a bit to the side and still have the entire exit pupil enter the pupil of your eye until it is suddenly, completely, eclipsed.
In the second example, moving your head slowly enough to the side that the field slowly gets eclipsed would be nearly impossible.
In the first example, it would be fairly easy to move your head to the side slowly enough.

Otherwise, you are describing a characteristic that makes no sense if you are talking about 2 eyepieces that yield the same exit pupil.

Spherical aberration of the exit pupil can make positioning the head difficult. I keep running into eyepieces that have been recently designed that ignored
that parameter of design. They can have large or small exit pupils and still have the same issue. It doesn't make the exit pupil suddenly disappear, however.

wagishohots

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Re: Eyepiece question
« Reply #13 on: January 26, 2018, 02:18:02 AM »
It can be an aggravating factor,
but I almost always see it in eyepieces of lower power and very wide
eye lens.  The curvature on the Expanses is rather spectacular so
it's no surprise it's sensitive.

I tend to associate it with an EP that has been pushed really hard for
field width without a lower number of elements than the afov warrants....
Even a 6-element can have blackouts if you push it too hard...then the
eyeball is a in a tight box to make it work.

coepupinsynch

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Re: Eyepiece question
« Reply #14 on: January 30, 2018, 07:05:45 AM »
Quote
It can be an aggravating factor,
but I almost always see it in eyepieces of lower power and very wide
eye lens.  The curvature on the Expanses is rather spectacular so
it's no surprise it's sensitive.

I tend to associate it with an EP that has been pushed really hard for
field width without a lower number of elements than the afov warrants....
Even a 6-element can have blackouts if you push it too hard...then the
eyeball is a in a tight box to make it work.

What do you mean? Exit pupil is exit pupil. A 20mm 9-element design has the exact same exit pupil size as a 20mm 3-element eyepiece.
What newly-defined characteristic of the exit pupil are you describing if it's not SAEP?