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Unidentified artefacts while observing the Moon with Vixen VMC 110L and NPL 6mm/SW 7.5mm eyepiece

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#1 Magnetic Field

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Posted 25 August 2019 - 08:52 AM

How do you do?

 

The link to my Vixen modification can be found here:

 

https://www.cloudyni...i-prism-fitted/

 

1. The prism greatly reduces glare around the moon and planets.

2. The outstanding issue is only a concern while observing the Moon.

 

The Takahashi prism has reduced the effect but not fully eliminated it.

 

The following Figure on the left shows what I see when observing the Moon with a 6mm Vixen NPL eyepiece. This is also there while observing the Moon with the 7.5mm Sky-Watcher Ploessel.  There is a brighter outer ring and darker inner regions. Often I think it is the other way round. It cannot be the secondary because the inner shadow is much larger than then 37% obstruction of the secondary.

 

The Figure on the left shows what I see glancing at the eyepiece from some distance (e.g 20cm or so). They can also be seen in the Sky-Watcher 7.5mm eyepiece. First I thought this are multiple internal reflections of something (e.g baffle tube) and due to the eyepiece design. I dismantled the Vixen NPL 6mm eyepiece to check if the lens edges are blackened and they are (I haven't checked it for the Sky-Watcher).  The strange thing: when I rotate the eyepiece  those dots stay at the place and do not  rotate with the eyepiece (it cannot be lens glass imperfections in the 6mm NPL or Sky-Watcher 7.5mm).

 

EypieceMoon.jpg

 

 

I have have included a Figure that shows the interior of the OTA. I was thinking of flocking it. But would be surprised this causes the Moon artefact.

 

VixenOTAFront.jpg

 

 

Has anyone any ideas what is going on here? Again the prism has greatly reduced glare around the Moon (and planets in the 6mm eyepiece look really great) but not the shadow effect as seen in Figure 1 while observing the Moon.


Edited by Magnetic Field, 25 August 2019 - 08:54 AM.


#2 Hesiod

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Posted 25 August 2019 - 09:00 AM

As for the dots, could they be some kind of damage of the prism?
The other issue seems the so-called "edge of field brightening" which, as far as I know, usually is a sort of illusion

#3 Magnetic Field

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Posted 25 August 2019 - 09:23 AM

As for the dots, could they be some kind of damage of the prism?
The other issue seems the so-called "edge of field brightening" which, as far as I know, usually is a sort of illusion

 

It has nothing to do with the prism.

 

As I said the effect was already there with the rubbish flip mirror. But at that time I always thought it is because of the crap flip mirror.


Edited by Magnetic Field, 25 August 2019 - 09:24 AM.


#4 Starman1

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Posted 25 August 2019 - 09:59 AM

When you view something as bright as the Moon, a couple things happen:

--the pupil of your eye constricts.  If your eye is sensitive to light, it constricts a lot.  This has the effect of making the secondary shadow portion of the star image

a much larger portion of the light going into your pupil.  It can make your eye see the center of the field as only partially illuminated, as if in shadow, while the outer portion of the field

still appears quite bright.  This normally occurs at low powers with the eye at the exit pupil, but can be seen at other powers as well.

--some eyepieces which are otherwise good performers on deep sky display serious internal reflections.  One classic example is Vixen's SLV eyepieces, which are, essentially, not recommended for Moon viewing

because the outer field reflection is severe.

--some designs of eyepieces suffer from a characteristic known as EOFB, or edge of field brightening, which is not an illusion (there is even a post here on CN where someone photographed it).  Plössls

do not suffer from this characteristic, so that is not the case here. (the NPL is a Plössl).

 

When you backed away from the eyepiece, you were seeing a smaller and smaller portion of the field of the eyepiece.  You got to a distance where the shadow of the secondary became a huge portion of the image and that is, in a nutshell, what you were seeing in the first illustration above.

 

As for the second illustration, there are multiple possibilities, but the most likely is simply floaters in your eye.  You don't see them normally, but when the exit pupil behind the eyepiece shrinks to the size of a laser beam (and it is very tiny in that scope with really short focal length eyepieces), little bits of flotsam and jetsam in the eye (technically, small agglomerations of protein in the vitreous humor) become visible.

This problem grows worse as you get older.  Many older observers don't use eyepieces with exit pupils below 1mm for that reason (1mm exit pupil is where the eyepiece focal length equals the f/ratio of the scope).  You see them when looking at very small details on planets or moon when the eyepiece used yields an exit pupil that is very small.

They often appear as dots, as you drew, and sometimes amoeba shapes or bacillus shapes and, if really close to the retina, dark spots.  They would not rotate with the eyepiece.  If they did rotate with the eyepiece, then they'd simply be dust motes on the bottom lens of the eyepiece.

 

This scope is f/9.4, so such short eyepieces have very small exit pupils.  And, the secondary is large and yields a shadow which is a large % of the field of the scope.  And, because you are using an external diagonal, the f/ratio and focal length of the scope are longer than the parent f/9.4 (maybe as high as f/11), this makes the exit pupils yielded by the eyepieces even smaller.  So I'm fairly certain the spots issue is merely making the floaters in your eye more visible.  Want to see them in the daytime?  Open your eyelids as wide as you can and stare at the daytime sky.  Without moving your head, move your eyes back and forth.  You will see the floaters move back and forth as subtle shadows moving back and forth.  They kind of resemble eyelashes in your vision, which is why I said to open your eyes as much as possible to get the eyelashes out of the field of vision.

 

Since you don't actually view through the telescope with your eye in the position where you saw these things, they are not really of concern.

Sorry about the floaters.  Once you see them, you will always see them, especially in Moon or bright planet viewing.  But they can be ignored.  I have lots of them, but I'm unaware of them 99.9% of the time.


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#5 Magnetic Field

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Posted 25 August 2019 - 10:31 AM

When you view something as bright as the Moon, a couple things happen:

--the pupil of your eye constricts.  If your eye is sensitive to light, it constricts a lot.  This has the effect of making the secondary shadow portion of the star image

a much larger portion of the light going into your pupil.  It can make your eye see the center of the field as only partially illuminated, as if in shadow, while the outer portion of the field

still appears quite bright.  This normally occurs at low powers with the eye at the exit pupil, but can be seen at other powers as well.

--some eyepieces which are otherwise good performers on deep sky display serious internal reflections.  One classic example is Vixen's SLV eyepieces, which are, essentially, not recommended for Moon viewing

because the outer field reflection is severe.

--some designs of eyepieces suffer from a characteristic known as EOFB, or edge of field brightening, which is not an illusion (there is even a post here on CN where someone photographed it).  Plössls

do not suffer from this characteristic, so that is not the case here. (the NPL is a Plössl).

 

When you backed away from the eyepiece, you were seeing a smaller and smaller portion of the field of the eyepiece.  You got to a distance where the shadow of the secondary became a huge portion of the image and that is, in a nutshell, what you were seeing in the first illustration above.

 

As for the second illustration, there are multiple possibilities, but the most likely is simply floaters in your eye.  You don't see them normally, but when the exit pupil behind the eyepiece shrinks to the size of a laser beam (and it is very tiny in that scope with really short focal length eyepieces), little bits of flotsam and jetsam in the eye (technically, small agglomerations of protein in the vitreous humor) become visible.

This problem grows worse as you get older.  Many older observers don't use eyepieces with exit pupils below 1mm for that reason (1mm exit pupil is where the eyepiece focal length equals the f/ratio of the scope).  You see them when looking at very small details on planets or moon when the eyepiece used yields an exit pupil that is very small.

They often appear as dots, as you drew, and sometimes amoeba shapes or bacillus shapes and, if really close to the retina, dark spots.  They would not rotate with the eyepiece.  If they did rotate with the eyepiece, then they'd simply be dust motes on the bottom lens of the eyepiece.

 

This scope is f/9.4, so such short eyepieces have very small exit pupils.  And, the secondary is large and yields a shadow which is a large % of the field of the scope.  And, because you are using an external diagonal, the f/ratio and focal length of the scope are longer than the parent f/9.4 (maybe as high as f/11), this makes the exit pupils yielded by the eyepieces even smaller.  So I'm fairly certain the spots issue is merely making the floaters in your eye more visible.  Want to see them in the daytime?  Open your eyelids as wide as you can and stare at the daytime sky.  Without moving your head, move your eyes back and forth.  You will see the floaters move back and forth as subtle shadows moving back and forth.  They kind of resemble eyelashes in your vision, which is why I said to open your eyes as much as possible to get the eyelashes out of the field of vision.

 

Since you don't actually view through the telescope with your eye in the position where you saw these things, they are not really of concern.

Sorry about the floaters.  Once you see them, you will always see them, especially in Moon or bright planet viewing.  But they can be ignored.  I have lots of them, but I'm unaware of them 99.9% of the time.

I am not convinced those dots are floaters.

 

I know what floaters are. I suffer a lot of them (especially at high powers).

 

Those dots appear more like a static "honeycomb".

 

In contrast: my floaters randomly move through the field of view (and believe me they always multiply when seeing gets better).

 

The prism has greatly reduced the glare around the moon though. However, it still feels I am observing through a dirty eyepiece when not using a moon filter. This was the reasons why I dismantled the NPL Ploessl and cleaned it (and angst led me  to also blacken the 2-element lens edges with a permanent marker although they are already factory blackened).

 

I was tempted to buy a 6mm TeleVue maybe on the second hand market. But as you say it is probably not the eyepiece design. The TeleVue will have a smidge of better coatings but the design I assume is still the same: 2-elements on each side.



#6 Starman1

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Posted 25 August 2019 - 11:27 AM

Well, as I said, there could be other issues:

--dust on the prism.  It wouldn't rotate with the eyepiece.  Test it by rotating the prism to see if the spots move then.  That dust would be close to the focal plane, too.

--extreme rough spots on prism or mirror surfaces.  Unlikely.

--oxidation spots on the primary or secondary mirror, possibly indication a coating failure.  No rotation with the eyepiece here, either.

--corneal scratches, caused by dry eyes and blinking, can yield vision anomalies.  The pattern might vary from eye to eye.

--cataracts in the lens of the eye.  Again, no rotation with the eyepiece.  The pattern would be different in each eye.

--small spots on the retina of the eye.  Could indicate retinal hemorrhaging.

 

If it's in the scope, another observer could duplicate what you see.

If it's in your eye, though........



#7 csrlice12

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Posted 25 August 2019 - 11:35 AM

Or the moon just fell apart....RUN!



#8 Magnetic Field

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Posted 25 August 2019 - 01:45 PM

Well, as I said, there could be other issues:

--dust on the prism.  It wouldn't rotate with the eyepiece.  Test it by rotating the prism to see if the spots move then.  That dust would be close to the focal plane, too.

--extreme rough spots on prism or mirror surfaces.  Unlikely.

--oxidation spots on the primary or secondary mirror, possibly indication a coating failure.  No rotation with the eyepiece here, either.

--corneal scratches, caused by dry eyes and blinking, can yield vision anomalies.  The pattern might vary from eye to eye.

--cataracts in the lens of the eye.  Again, no rotation with the eyepiece.  The pattern would be different in each eye.

--small spots on the retina of the eye.  Could indicate retinal hemorrhaging.

 

If it's in the scope, another observer could duplicate what you see.

If it's in your eye, though........

 

(Oxidation spot is actually an interesting speculation worthy closer examination).

 

As I said: prism has to be ruled out. The effect was also noticeable with the Micky Mouth Frankenstein flip mirror.

 

I hate the shadowing artefact with brighter outer ring  the most (Figure 1, left). Sure it is only an issue for the full Moon at high magnifications without a moon filter.

 

By the way: My 5mm TMB Sky-Watcher clone ocular doesn't show the honeycomb effect on the Moon, if my recollection is right. Will check when I am back in the UK.

 

I will flock my OTA in the next few months.



#9 Starman1

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Posted 25 August 2019 - 03:53 PM

 

I hate the shadowing artifact with brighter outer ring  the most (Figure 1, left). Sure it is only an issue for the full Moon at high magnifications without a moon filter.

 

 

And, it is likely only something you see with the eye a zillion miles from the eyepiece.  When you're at the exit pupil, you won't see it.

If seeing it when your eye is not in the position to use the eyepiece bothers you, I foresee a refractor in your future.

A refractor won't see that because there is no secondary mirror.




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