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Equipment Discussions >> Eyepieces

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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Starman1]
      #5856848 - 05/13/13 04:36 AM Attachment (16 downloads)

Don,

Much of the unwanted light from the vicinity of the primary does not lie outside of the field stop. Here is another diagram to illustrate this more clearly. In the short-focus telescope shown, you can see some of the many rays of light (from various sources in the image) which come from the primary. These rays completely span the field-stop, go through the eyepiece and become part of the final image - just as you might expect.

If we move on to the long focus version of the same telescope (with the same eyepiece) you can see that those exact same paths of light come from unwanted, reflected light coming from the shiny interior of the extended tube.

Since the eyepiece is not a dynamic device it cannot behave differently in these two scenarios.

You are familiar with the issue of "unused light" in overly-large exit-pupils at low magnification I believe? Well, these unwanted reflections - lying outside of the circumference of the primary - form part of that "unused light" at low magnifications, resulting in better contrast at low powers compared to when you are pushing the magnification.

In all of this, it's important to remember that the exit-pupil-thick beam of light entering the eye from any specific angle does not contain a representation of the entire stellar image. It is a spread-out representation of a single point; an exit-pupil-thick beam of light either bright, or dark - or whatever is needed to recreate that spot that it will be focused back into on the retina by the human eye-lens.

Instead of countering with perfectly correct arguments against things I've never said, it would perhaps be more instructive to CN readers if you state what it is in the diagram that you have issues with.

Clear skies to you.


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Pushing the magnification new [Re: ibase]
      #5856902 - 05/13/13 07:16 AM

Hernando,

Quote:

Magnification can be pushed further on the WO66 without image breakdown, the 80mm ShortTube was outclassed. Probably better optics, better baffling, better Petzval design, either or all have contributed to the better views on the WO66 when pushing the magnification.

So someone who owns the ST80 will say that magnification can only be pushed to x factor, while the WO66 owner will say a different higher figure. One reason maybe why everybody have divergent views on max. magnification.




Not surprising, since the ST80 is an f/5 achromat. It is a nice rich-field wide-field for dark sites, especially since I've upgraded the focuser to a 2" Crayford. But an ST80 is not exactly the best scope for planet/lunar viewing. The image falls apart rapidly with higher magnification due to CA. My A70LF - a 70mm f/12.9 refractor - is much better for observing planets and the Moon, despite the smaller aperture. The CA ratio for the ST80 is 1.59, that for the A70LF is 4.69.

It's important to get the best - or at least a better - tool for the job. That goes for telescopes as well as eyepieces.

Equivalent Chromatic Aberration of Achromatic Refractors

Mike


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Pushing the magnification new [Re: Geo31]
      #5857013 - 05/13/13 08:57 AM

Quote:

I'm curious...

If stray light that won't reach the image are blocked by the field stop, why does it matter than elements have their edges blackened?

It would seem to me that if imaging light is all that reaches the eye, then blackening the edges would be a red herring?

What am I missing?




I don't think you are missing anything. If lens are properly sized and proper stops are in place, there should not be any possibility of light striking the edges of the lenses.

However this is not a popular point of view. It seems that there are a number of magical properties that have been given to edge blackening.

Yes, there can be stray light as a result of defects in the surface polishes or even in the glass itself but the amount of light that is scattered and actually hits the edges of any lens has to be incredibily tiny. The amount that gets reflected back into the image is much smaller again.

dan


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ibase
Vendor Affiliate
*****

Reged: 03/20/08

Loc: Manila, Philippines 121*E 14*N
Re: Pushing the magnification new [Re: Sarkikos]
      #5857067 - 05/13/13 09:33 AM

Quote:

Hernando,

Quote:

Magnification can be pushed further on the WO66 without image breakdown, the 80mm ShortTube was outclassed. Probably better optics, better baffling, better Petzval design, either or all have contributed to the better views on the WO66 when pushing the magnification.

So someone who owns the ST80 will say that magnification can only be pushed to x factor, while the WO66 owner will say a different higher figure. One reason maybe why everybody have divergent views on max. magnification.




Not surprising, since the ST80 is an f/5 achromat. It is a nice rich-field wide-field for dark sites, especially since I've upgraded the focuser to a 2" Crayford. But an ST80 is not exactly the best scope for planet/lunar viewing. The image falls apart rapidly with higher magnification due to CA. My A70LF - a 70mm f/12.9 refractor - is much better for observing planets and the Moon, despite the smaller aperture. The CA ratio for the ST80 is 1.59, that for the A70LF is 4.69.

It's important to get the best - or at least a better - tool for the job. That goes for telescopes as well as eyepieces.

Equivalent Chromatic Aberration of Achromatic Refractors

Mike




Mike, well said, different eyepieces/telescopes for different jobs.

Really like the ST80, as you said, for the rich wide starfields, although the WO66 has become a sort of rapid deployment scope for lunar/planets, the Delos 6mm is current favorite on it - really enjoyed using this scope when Jupiter was at opposition and missing a stripe, seeing the GRS distinctly and following moon transits. For closer looks though, will bring out the WO 102ED or 6" Antares refractors.

PS. Nice CA chart, thanks!

Best,


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Pushing the magnification new [Re: great_bear]
      #5857208 - 05/13/13 10:59 AM Attachment (5 downloads)

Quote:



If we move on to the long focus version of the same telescope (with the same eyepiece) you can see that those exact same paths of light come from unwanted, reflected light coming from the shiny interior of the extended tube.





<< shiny interior of the extended tube >> ?

Seems the appropriate thing to do is eliminate the problem at the source. Get rid of the shiny interior and add a baffle or two. And don't forget to add an appropriate glare/dew shield to block the off axis light from entering the lens. Problem solved.


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: dan_h]
      #5857225 - 05/13/13 11:08 AM

- yes for sure - no doubt about that


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Pushing the magnification new [Re: great_bear]
      #5857299 - 05/13/13 11:47 AM

And note that the unwanted light from outside the field of view gets into the field of view by reflection. I was never arguing that wasn't the case.

But the unwanted light that does get into the field by reflection gets into the field of view by passing through a part of the field that is inside the field stop.

Light that is outside the field stop (think of a ray reflected from the side of the tube that does not get through the eyepiece's field stop) does not make it to the eye. You can think of the field stop as the final baffle in the system.

Ergo, though unwanted light from outside the field of the scope or eyepiece CAN get into the field of the eyepiece, all that light will be IN the field of the eyepiece and inside the exit pupil for the eyepiece. Light outside the exit pupil is outside the field stop of the eyepiece and does not make it through the eyepiece.

In the two-refractor example, you essentially prove that. You also show that long ratio or short, unwanted light can get through without proper addressing of the issue of scattered light.

So the issue I have had with the discussion is that, all along, there was an implication that scattered light from outside the field gets to the part of the pupil of the eye that is outside the exit pupil of the eyepiece if the exit pupil of the eyepiece is smaller than the pupil diameter of the eye, and there is no way for that to happen.
[Excepting reflection from the cornea to the eyepiece and back]

I fully agree that light from outside the field of view can make its way into the exit pupil, for sure.

In the example of the 2 refractors, reducing the size of the eyepiece or field stop therein would also reduce the unwanted reflected light from entering the eyepiece or exit pupil. That implies that there would be LESS effect from scattered light at high powers, and this conforms to my experience, too.


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Starman1]
      #5857913 - 05/13/13 04:50 PM Attachment (8 downloads)

Don,

Quote:

So the issue I have had with the discussion is that, all along, there was an implication that scattered light from outside the field gets to the part of the pupil of the eye that is outside the exit pupil of the eyepiece if the exit pupil of the eyepiece is smaller than the pupil diameter of the eye, and there is no way for that to happen.




Yes there is, explained many times in my previous posts, but let's see if another picture helps


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Starman1]
      #5858007 - 05/13/13 05:21 PM Attachment (5 downloads)

Don,

Quote:

Light outside the exit pupil is outside the field stop of the eyepiece and does not make it through the eyepiece.




This is not correct - as demonstrated in Fig 1 in my previous post above.

The field-stop cannot mask off any of the area outside the exit pupil. That would be geometrically impossible. All it can do is restrict the maximum angle that a light bundle can travel through the exit-pupil.

Again, an illustration is in order:

Edited by great_bear (05/13/13 05:36 PM)


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Pushing the magnification new [Re: great_bear]
      #5858064 - 05/13/13 05:35 PM Attachment (6 downloads)

But junk light is not focused as your drawing shows. Junk light washes everything it can reach. The attached drawing show this and it doesn't really matter what eyepiece you use. Longer eyepieces use wider field stops and bigger lenses so they are washed in a greater amount of junk light.

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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: dan_h]
      #5858148 - 05/13/13 05:56 PM

Hi dan_h

Quote:

But junk light is not focused...




Agreed.

Quote:

...as your drawing shows.




That's not what it shows. It's not actually showing the junk light focused if you think about it.

Look again - Fig 1 in my diagram (not your version) is showing two light paths from unrelated areas of the junk light being refracted onto the same point. This happens because all light paths - related or otherwise - which cross a specific point on the focal plane will meet again at a specific single point on the retina.

In the case of junk light, different light paths from the same part of the junk get scattered and refracted all over place. But then you know that, since that's exactly what you've drawn.

Quote:

Longer eyepieces use wider field stops and bigger lenses so they are washed in a greater amount of junk light.




Depends on the angle - look again at fig 2 of my diagram that you reference. It's pretty hard to draw a straight light path from the junk light to the eyepiece lens - and if the focal length was even longer, it would be impossible.


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GeneT
Ely Kid
*****

Reged: 11/07/08

Loc: South Texas
Re: Pushing the magnification new [Re: great_bear]
      #5858193 - 05/13/13 06:09 PM

Quote:

It's always been a source of frustration for me (and no doubt a source of confusion for newbies) that even the most experienced and respected of commentators here cannot agree on what the maximum useful scope-relative magnification is for planetary observation.

Some say telescopes max out at 25x per inch of aperture and others say 30x per inch is closer the mark, however I remember EdZ saying that its possible to squeeze out more detail up to about 39x p.i. (0.65 Ex/Pupil)




Here's how I came up with my working answer/solution to your question. For my F5, 12.5 inch Dob, I consider high power when moving to a 10 mm focal length eyepiece, and smaller. When moving into the 'high power' range, I have eyepieces going in smaller, 1 mm increments to my 4 mm Radian. In other words, I have eyepieces ranging from 10, 9, 8, 7, 6, 5 and 4 mm. Some nights, due to seeing, or other issues, I can't go lower than my 12 Nagler. When viewing the planets, I keep my working my way down to smaller and smaller focal length eyepieces. At some point I hit the wall, i.e. the view will not support more magnification. Rarely can I use my 4 mm, and it does not give me any better views of the planets than my 5mm Pextax XW and XO eyepieces. You might consider buying one of the better zooms. Many people report that they do great on the planets, and would save you from having to buy several eyepieces.


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: GeneT]
      #5858204 - 05/13/13 06:14 PM

I did consider it yes - but I use binoviewers and didn't like the idea of two zooms getting out of sync.

I've read on forums that some people seem to do it though...


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: astro_baby]
      #5858243 - 05/13/13 06:22 PM

Quote:

Unless I have missed it no one has raised atmosphere and seeing as an issue. In the UK seeing is most often restricted to a maximum of x235 and very ofetn lower than that which most often down to a very wobbly atmosphere and tough seeing conditions.




Also in the UK, many observe from housing estates where - no matter where you point the scope - you're always aiming above the rooftop of a nearby house.

When buying our house in London my wife let me choose one that was well-positioned for astronomy. The gardens of our street go in a line running south-east/north-west, so that's a great house-free observing corridor in one direction.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Pushing the magnification new [Re: dan_h]
      #5858342 - 05/13/13 06:51 PM

Please refer to the following link's illustration as I explain a few things about exit pupil and how the eyepiece produces an image.
http://www.telescope-optics.net/eyepiece1.htm

1) Your drawings do not show the exit pupil, but only the center of the field of the instrument illustrated. The rays coming back away from the eyepiece start out the width of the eye lens and converge on a small flat plane known as the exit pupil. It is the narrowest point of convergence of the rays exiting the eye lens. That is why, as your eye moves away from the eyepiece the visible field narrows and why, when you are too close to the eyepiece, the iris of your eye cuts off the field. Even on eyepieces of small exit pupil, this is true, and you can hold your eye too close to allow all the rays from the eyepiece to enter the pupil of your eye. It is in focus at all distances from the eyepiece.
2) The objective has a field of view and it is illustrated in the diagram to which I link. The farther off axis, the farther toward the edge of the field stop do the rays become, until, at the field stop, a maximum off-axis field is defined. Note that rays can continue to diverge after the field stop, but the field stop defines the edge of the field visible by the objective for that eyepiece.
3) Rays from farther afield cannot make it to the exit pupil because they do not make it past the field stop.
Nowhere in this illustration does it preclude the possibility of the reflection of light off an internal surface that would make it through the field stop and into the exit pupil behind the eyepiece.
4) the exit pupil contains all the light that makes it through the eyepiece. It utilizes all the light that makes it through the field stop of the eyepiece, unwanted or not. Light that does not make it past the field stop (at whatever angle) is not part of the exit pupil. Seems simple enough. If the field of the eyepiece is 50 degrees, you are seeing no light from 30 degrees off axis.
5) Ergo, if the exit pupil of the eyepiece is smaller than the pupil of the eye, all the light passing through the exit pupil goes into the eye. This may occur, depending on the exit pupil size, at a small range of distances from the eyepiece.
6) The error in your first illustration is that it shows all rays leaving the eye lens of the eyepiece parallel (and what you mark as the exit pupil is only part of the exit pupil). They are not. They converge on the exit pupil and diverge thereafter.
7) Therefore, and I will word this as carefully as I can, light outside the diameter of the exit pupil (if that exit pupil is smaller than the pupil of the eye) but inside the diameter of the pupil of the eye and outside the exit pupil of the eyepiece, did not come through the eyepiece. It cannot have. It could have come from the field peripheral to the eyepiece as, for instance, the outside surface of the scope or the top surface of the eyepiece reflecting the sky or some extraneous light. But it did not come through the eyepiece. If it had it would be the equivalent of seeing a 60 degree field through a 50 degree eyepiece.


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Starman1]
      #5859173 - 05/14/13 05:44 AM Attachment (3 downloads)

Hi Don,

The diagram you refer to is dual-point, mine are all single-point (kinda obvious, I thought?)

No matter... It makes no difference. Whether the diagram is single-point, dual-point or even triple-point for good measure (as shown below), it's still the case that junk light is being transmitted outside of the exit-pupil, and there's nothing that the field-stop can do about it.

You might not like this. You may even want to put your fingers in your ears and scream "no! no! no!", but the fact remains it's clearly illustrated in this diagram for all CN members to see - and anyone with a ruler can validate it.

Clear skies to you.

Fig 4:


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planet earth
Pooh-Bah
****

Reged: 09/07/10

Loc: Ontario Canada
Re: Pushing the magnification new [Re: great_bear]
      #5859186 - 05/14/13 06:09 AM

"it's still the case that junk light is being transmitted outside of the exit-pupil, and there's nothing that the field-stop can do about it."

Not to be rude but:
If my Newtonian get's superb planetary views, does it matter if this unwanted light exists?
To me it's really a non issue.
Sam


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: planet earth]
      #5859199 - 05/14/13 06:40 AM

No, clearly not an issue in your case - but it's a problem in some telescopes unfortunately.
So your question's not rude; it's a perfectly reasonable one.


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planet earth
Pooh-Bah
****

Reged: 09/07/10

Loc: Ontario Canada
Re: Pushing the magnification new [Re: great_bear]
      #5859202 - 05/14/13 06:45 AM

Just to note:
I do find this thread very interesting, and educational.
Even though it went a bit OT.
Sam


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Pushing the magnification new [Re: planet earth]
      #5859217 - 05/14/13 07:05 AM

For Newts, it's pretty easy to prevent "junk light" ...

1) Fully flock the interior of the OTA.
2) Fully flock the focuser.
3) Make or buy a light shield about the same length as the aperture of the telescope, up to a foot long or so.
4) Fully flock the interior of the light shield.
5) Clean the mirrors.
6) Clean the eyepieces.
7) Use eyepieces with high light transmission, low scatter, good baffling and good coatings.

Then don't worry about it.


Mike


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