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Old 24-01-2019, 06:27 PM
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silv (Annette)
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mirror mystery: how the diagonal murdered the perfect eyepiece

Hi all,

<tl;dr>
When I look through an eyepiece and refractor with or without diagonal, not only will the produced image suffer from a reflective impurity on the mirror surface, those missing 6-10% lambda, but also from the flattening, dimension-substracting effect a flat mirror has on rays in a bundle of rays in relation to each other.

</tl;dr>

Still pondering this thread http://www.iceinspace.com.au/forum/s...d.php?t=172903 . Mental4astro says that there are eyepieces designed specifically for the convex image a refractor produces and also eyepiece designs for concave images from a newt's primary mirror.

I'm on my own quest of starting my first eye piece collection for f4.9 achromat plus TS-Bino with 1.6x barlow, and I’m doing a bit of general “getting the hang of optics and perception”. Without any previous knowledge other than reading the occasional forum post here on IIS. Mental4astro's and Wavyton's for example


The mirror mystery is my first obstacle. What does a flat polished mirror like a refractor’s diagonal or a newt’s secondary do to the convex or concave curvature of the entering image ray bundle?
Does the mirror murder that curvature? Does the mirror murder the eyepiece design?

I read on wikipedia: one incoming light ray reflects off the mirror surface in the same angle as its impact angle.
Okay. Hm. Why is it then, when I hold a ping pong ball in front of the mirror and look at the mirror surface, what I see is not a 3d ball but a flat image? It’s only my brain knowing it’s actually a sphere and not a mere circle. But what I see as reflection IS a flat circle. Correct?

So my train of thought goes like this:
when the bundle of light rays from a curved object reflect off a mirror surface, the distance or time difference between ray 1 from the edge of the ball [further away from mirror surface] and ray 2 from the middle of the ball [closer to mirror surface], this difference is lost during reflection. Hence the loss of depth perception. A 3d object becomes a 2d image.
This would mean, a mirror's effect on an image can't be discussed looking at one single light ray. It's the rays in a bundle in relation to each other which produce a sphere or a flat circle.

In this thought direction it then follows that the 3D curvature, convex or concave, (not the image of an object but its curved representation produced by scope design) is lost when being reflected off a secondary or a diagonal. Like Wiki says, the individual rays ARE reflected at the same angle. But the produced image is not mirroring the convex nature of the image. Like the pingpong ball is no longer a sphere but a circle, the convex curved image from the refractor becomes a flat 2D round circle or maybe shaped slightly elongated, I don't know.

So what enters the eyepiece and comes out of it at our eyeball MUST differ from the image we would see through an eyepiece if there were no diagonal in between.

If this is true, then even with a super top-of-the-shelf reflective diagonal mirror, the generated image rays bundle would still be blunted and flattened when they enter and traverse the lens construction of an eyepiece.
Meaning: an eyepiece working for a refractor’s convex image curvature, would be surprised by the flattened, 2D, non-convex ray bundle coming off the mirror. The eyepiece would still attempt its eyepiece magic, being designed for refractor's convex curvature, but in a mirror-falsified work place, the produced image at the eye ball won't match the quality of the image intended by they eyepiece designer.
The reflective quality of 95% lambda or whatyamaycallit silver or dielectic mirror surface is not the only limitation of a flat mirror in an optical path.
The way I am thinking, being a mirror in the first place is a more prominent limitation.



Let’s forget about the newt’s secondary mirror for the moment because I see no way of testing or verifying without a secondary.

But when I look through an eyepiece and refractor with or without diagonal, not only will the produced image suffer from a reflective impurity on the mirror surface, those missing 6-10% lambda, but also from the flattening, dimension-substraction effect a flat mirror has on rays in a bundle of rays in relation to each other.

? Is that so?

This question is not directly related to my actual quest of eyepiece purchase. But it's an open question in my head which hinders progress in decision making.

Thank you so much for any input!

Annette

Last edited by silv; 24-01-2019 at 06:37 PM.
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Old 24-01-2019, 08:54 PM
sharpiel
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Originally Posted by silv View Post
Hi all,

<tl;dr>
When I look through an eyepiece and refractor with or without diagonal, not only will the produced image suffer from a reflective impurity on the mirror surface, those missing 6-10% lambda, but also from the flattening, dimension-substracting effect a flat mirror has on rays in a bundle of rays in relation to each other.

</tl;dr>

Still pondering this thread http://www.iceinspace.com.au/forum/s...d.php?t=172903 . Mental4astro says that there are eyepieces designed specifically for the convex image a refractor produces and also eyepiece designs for concave images from a newt's primary mirror.

I'm on my own quest of starting my first eye piece collection for f4.9 achromat plus TS-Bino with 1.6x barlow, and I’m doing a bit of general “getting the hang of optics and perception”. Without any previous knowledge other than reading the occasional forum post here on IIS. Mental4astro's and Wavyton's for example


The mirror mystery is my first obstacle. What does a flat polished mirror like a refractor’s diagonal or a newt’s secondary do to the convex or concave curvature of the entering image ray bundle?
Does the mirror murder that curvature? Does the mirror murder the eyepiece design?

I read on wikipedia: one incoming light ray reflects off the mirror surface in the same angle as its impact angle.
Okay. Hm. Why is it then, when I hold a ping pong ball in front of the mirror and look at the mirror surface, what I see is not a 3d ball but a flat image? It’s only my brain knowing it’s actually a sphere and not a mere circle. But what I see as reflection IS a flat circle. Correct?

So my train of thought goes like this:
when the bundle of light rays from a curved object reflect off a mirror surface, the distance or time difference between ray 1 from the edge of the ball [further away from mirror surface] and ray 2 from the middle of the ball [closer to mirror surface], this difference is lost during reflection. Hence the loss of depth perception. A 3d object becomes a 2d image.
This would mean, a mirror's effect on an image can't be discussed looking at one single light ray. It's the rays in a bundle in relation to each other which produce a sphere or a flat circle.

In this thought direction it then follows that the 3D curvature, convex or concave, (not the image of an object but its curved representation produced by scope design) is lost when being reflected off a secondary or a diagonal. Like Wiki says, the individual rays ARE reflected at the same angle. But the produced image is not mirroring the convex nature of the image. Like the pingpong ball is no longer a sphere but a circle, the convex curved image from the refractor becomes a flat 2D round circle or maybe shaped slightly elongated, I don't know.

So what enters the eyepiece and comes out of it at our eyeball MUST differ from the image we would see through an eyepiece if there were no diagonal in between.

If this is true, then even with a super top-of-the-shelf reflective diagonal mirror, the generated image rays bundle would still be blunted and flattened when they enter and traverse the lens construction of an eyepiece.
Meaning: an eyepiece working for a refractor’s convex image curvature, would be surprised by the flattened, 2D, non-convex ray bundle coming off the mirror. The eyepiece would still attempt its eyepiece magic, being designed for refractor's convex curvature, but in a mirror-falsified work place, the produced image at the eye ball won't match the quality of the image intended by they eyepiece designer.
The reflective quality of 95% lambda or whatyamaycallit silver or dielectic mirror surface is not the only limitation of a flat mirror in an optical path.
The way I am thinking, being a mirror in the first place is a more prominent limitation.



Let’s forget about the newt’s secondary mirror for the moment because I see no way of testing or verifying without a secondary.

But when I look through an eyepiece and refractor with or without diagonal, not only will the produced image suffer from a reflective impurity on the mirror surface, those missing 6-10% lambda, but also from the flattening, dimension-substraction effect a flat mirror has on rays in a bundle of rays in relation to each other.

? Is that so?

This question is not directly related to my actual quest of eyepiece purchase. But it's an open question in my head which hinders progress in decision making.

Thank you so much for any input!

Annette
I suspect a flaw in your logic in that the secondary (flat) mirror being inherent in the design of the Newtonian still produces a concave image for which some eyepieces are more suited. As referred to by Alex and Nick.

Likewise inversely for the refractor’s image after diagonal reflection as many users incorporate a diagonal into the optical path and still prefer eyepieces more suited to the convex image.

As the ray paths don’t vary in length after being reflected by a flat surface (secondary mirror) or refracted by a prism diagonal they will still converge at the same distance from either the primary mirror or final refractive surface of a lens and should still conform to the original concave or convex image originally produced.

Just my thoughts.
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Old 24-01-2019, 10:12 PM
Ukastronomer (Jeremy)
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Too much to read, sorry, but my comment is

Murdered is a complete overstatement.

I challenge anyone to look through any eyepiece with an identical scope with and without a decent "DECENT" diagonal and see any difference
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Old 24-01-2019, 10:36 PM
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silv (Annette)
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Quote:
I suspect a flaw in your logic
I do TOO!

Quote:
As the ray paths don’t vary in length after being reflected by a flat surface ... still converge at the same distance from either the primary mirror or final refractive surface of a lens and should still conform to the original concave or convex image originally produced.
they do, do they? I still don't get that. Maybe in another 50 years. Until then, I'll re-add that question to the collection of "Things I don't understand".

Nick, your post amounts to attempted bullying. Very bad. But you seem to enjoy it once a fortnight or so, trying to bully forum members, don't you. Like a hobby? Well. I enjoy trying to understand things I don't understand. It's a less mean pastime. In the future, you could do us all a favor and ignore my posts. As I will do with yours.

Jeremy, I don't own a really good diagonal nor eyepiece. I don't know any person who does, either. So your assumption might be correct. But I can't confirm it.

"tl;dr" at the beginning of my post stands for "too long, didn't read" - an acronym used on the internet to summarize a following, much longer version.
I know my posts are often too long. Especially with stuff I don't understand. I need to learn boiling down my thoughts better. Until I can do that properly, I add a "tl;dr" short version to lessen the negative impact on others.
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Old 24-01-2019, 10:55 PM
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silv (Annette)
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My meandering thoughts and long posts have a bit to do with still living in two languages. On many days it seems, I can only express with a reduced vocabulary in either language, German, my native one, or English. Like having a bad hair day - but in regards to active vocabulary.
That's when thoughts don't form in words but in something like blurry images seen through a gel pad, just out of reach.
On bad language days I can't switch off and wait for better times.
On other days, when vocabulary depth is available, thoughts appear as the precise chain of words. That's when I can think what I want to think - and am not reduced to think what I can think. Sort of. -ish.
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Old 24-01-2019, 10:59 PM
sharpiel
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Originally Posted by silv View Post
My meandering thoughts and long posts have a bit to do with still living in two languages. On many days it seems, I can only express with a reduced vocabulary in either language, German, my native one, or English. Like having a bad hair day - but in regards to active vocabulary.
That's when thoughts don't form in words but in something like blurry images seen through a gel pad, just out of reach.
On bad language days I can't switch off and wait for better times.
On other days, when vocabulary depth is available, thoughts appear as the precise chain of words. That's when I can think what I want to think - and am not reduced to think what I can think. Sort of. -ish.
Your posts seem quite lucid to me.
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Old 24-01-2019, 11:20 PM
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silv (Annette)
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Really? The long one about the mirrors and bundle of rays, too?
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Old 24-01-2019, 11:21 PM
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Really? The long one about the mirrors and bundle of rays, too?
You seemed to incorrectly explain your idea quite clearly
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Old 24-01-2019, 11:26 PM
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chuckles
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Old 24-01-2019, 11:56 PM
Ukastronomer (Jeremy)
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I do TOO!

they do, do they? I still don't get that. Maybe in another 50 years. Until then, I'll re-add that question to the collection of "Things I don't understand".

Nick, your post amounts to attempted bullying. Very bad. But you seem to enjoy it once a fortnight or so, trying to bully forum members, don't you. Like a hobby? Well. I enjoy trying to understand things I don't understand. It's a less mean pastime. In the future, you could do us all a favor and ignore my posts. As I will do with yours.

Jeremy, I don't own a really good diagonal nor eyepiece. I don't know any person who does, either. So your assumption might be correct. But I can't confirm it.

"tl;dr" at the beginning of my post stands for "too long, didn't read" - an acronym used on the internet to summarize a following, much longer version.
I know my posts are often too long. Especially with stuff I don't understand. I need to learn boiling down my thoughts better. Until I can do that properly, I add a "tl;dr" short version to lessen the negative impact on others.


I agree 100% that NO rudeness high or low level is of benefit to anyone here, I am AUTISTIC, I have Co Morbid ADHD and Autism, and very often say when I PERSONALLY believe to be ok, but others may wrongly see as either blunt or rude, we all have a right to our comments and just because others may not agree, or we may be wrong does NOT entitle us to be rude when responding.

It is just as easy to state the person is wrong in a POLITE way

JR
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Old 25-01-2019, 12:35 AM
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Hi Silv

I'm no optics expert and and am far from a genius just to put my credentials out there. My assumption would be, referring to your ping pong ball that unless the mirror was the exact inverse curvature as the object being observed complete flattening would not occur given he angle of incidence of the incoming rays from the varied surfaces of the object. Furthermore I would then expect the flat mirror to give the opposite angles of reflection.

I could be completely wrong but that's just my 2 cents
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Old 25-01-2019, 01:20 AM
Ukastronomer (Jeremy)
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Hi Silv

I'm no optics expert and and am far from a genius just to put my credentials out there. My assumption would be, referring to your ping pong ball that unless the mirror was the exact inverse curvature as the object being observed complete flattening would not occur given he angle of incidence of the incoming rays from the varied surfaces of the object. Furthermore I would then expect the flat mirror to give the opposite angles of reflection.

I could be completely wrong but that's just my 2 cents
WOW! that's way over MY head
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Old 25-01-2019, 10:33 AM
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Annette,

A flat mirror doesn't introduce any aberrations into a light bundle, it merely changes its direction. The field curvature, for instance, will remain exactly the same. A prism diagonal is a different matter, because that involves refraction as well as reflection and will have some effect.
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Old 25-01-2019, 10:55 AM
N1 (Mirko)
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Ok, I'll have a crack at this, using the ping pong ball analogy:

The ping pong ball's reflection in a flat mirror still looks 3D to me, assuming both my eyes can see that reflection, of course. Also, the outside of the ball's image is still convex and the inside concave (if the ball was made of transparent material I could see the inside's image directly). So now if I were to cut a section out of said ball (I understand that would be called a spherical cap) to make a model of a curved focal plane, and held that in front of the mirror to resemble, say, the curved focal plane of a lens (as opposed to that of a concave mirror) with the "bulge" facing away from the mirror, then the reflected image would still have the same shape as the original, sides reversed. The centre would still appear to be further from me than the rim. In fact, a laser measurement would confirm exactly that. I would conclude that if the "ball section's" centre was bulging away from me regardless of whether I viewed its inside directly or via a mirror, then the same must be true for anything else of that shape, including focal planes.

The uncut ball does not turn into a flat object when viewed in the mirror, nor does it get turned inside-out (as would need to happen if the mirror turned convex into concave and vice versa).

If it was that easy to remove field curvature, then field flatteners and diagonals/prisms would achieve exactly the same thing. Alas, they don't.
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Old 25-01-2019, 12:30 PM
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Originally Posted by silv View Post
My meandering thoughts and long posts have a bit to do with still living in two languages. On many days it seems, I can only express with a reduced vocabulary in either language, German, my native one, or English. Like having a bad hair day - but in regards to active vocabulary.
That's when thoughts don't form in words but in something like blurry images seen through a gel pad, just out of reach.
On bad language days I can't switch off and wait for better times.
On other days, when vocabulary depth is available, thoughts appear as the precise chain of words. That's when I can think what I want to think - and am not reduced to think what I can think. Sort of. -ish.

Interesting - this is exactly what bothers me as well (sometimes.. when it seems my internal bio-HD is corrupted or not accessible ...)

Then I try with joke - usually only to make things worse.
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Old 25-01-2019, 02:20 PM
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Hi Annette,

The focal length of the primary (lens or mirror) will not change if a flat secondary mirror or diagonal is used. The curved focal plane is produced by the curvature of the primary. The light path will simply be deflected in the direction set by the secondary mirror or diagonal. The focal plane will similarly be shifted. The curvature of the focal plane can be changed by adding another curved reflective surface to the light path.

The light entering the primary (lens or mirror) is effectively 2D to start with. Example: the flow of photons off the side of a sphere (planet or Sun) or from the centre of its face is constant so there is no depth perception which can be measured as a function of time. Even though photons off one side of the Sun may take about 2 seconds longer to reach us than those emitted at the same time from the middle of its face (radius 700,000km).

I think most telescopes produce an image that has positive curvature i.e. the edges of the field are curved towards the primary. An EP with negative curvature for its focal plane (relative to the observer) will have a focal plane preferably bending in the same direction as the telescope primary.

Regards, Rob
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Old 25-01-2019, 08:31 PM
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Great point Rob. So what does this mean for the realistic position of the stars in a given galaxy as we observe them given the light from the side or the rear of an almost edge on galaxy may well have left its origin several thousand years before the light from the front and yet all are observed at the same point in time ? Does this give us ( although marginally in relitive terms ) a skewed sense of what the real positions of the stars within those galaxies are ?
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Old 26-01-2019, 01:01 PM
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Apart from the nearer galaxies you wouldn't see individual stars anyway. I believe individual stars can be seen in the Andromeda Galaxy (2.5 million light years away) by the HST. But you are right, stars near the left/right side or rear of the galaxy would be seen older in time than stars at the front (near end). For a galaxy 100,000 light years across, the stars at the rear could be seen 100,000 years later than the ones at the front (nearest end). Therefore, at any point in time the stars at the rear might be seen at an observed position which is much further away from their actual position in the galaxy relative to the stars at the front end.
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