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Strange reflection with Nikon Aculon

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Old Tuesday 8th October 2019, 09:40   #1
Steph barcelona
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Strange reflection with Nikon Aculon

Hello everyone,
I'm completly new in the binocular's world.
A month ago I purchased a pair of Nikon Aculon 8x42 for birds and nature observation and ocasional moon watching.
I was pretty happy with the quality of the observation. Sharp, bright image with a large field of view. My unexpert eyes didn t detect any kind of optical issues until I started watching at the moon.
I started to see like very small secundaries reflections of the moon in both sides of the binoculars. They are actualy quite small and I first thought it was dust but it s definitely a refelection. I couldn't apreciate that problem with other and less brighter luminous objects and neither during day time.
So, I was wondering if it s a commun and normal optical distortion or if it could be linked with a bad colimation? The global image is good and doesn t present a "double vision".
What do you think?
Thanks in advance for your help.
Regards
Stéphane
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Old Tuesday 8th October 2019, 12:26   #2
Mark9473
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The most common such reflection is occurring between your eyeball and the outside of the binocular eyepiece (and then back towards your eye, of course). You can identify it by its faster movement than the main image, when you shift the instrument around a bit.
If that's not it, then it's an internal reflection or ghost image internal in the instrument. Not related to optical distortion or collimation at all, just the quality of the coatings.
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Old Tuesday 8th October 2019, 13:34   #3
Binastro
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Hi Stephane and welcome.

Such ghost images are common, especially with uncoated surfaces, but also with coated surfaces.

Equal size moon ghost images, but much fainter than the real moon, are normal if viewing through double glazing.
In fact the detail seen in the fainter ghost image is often greater than in the over bright real moon.

I discovered a comet with a 12x45 Russian binocular through double glazing.
It took a ten mile round trip to get my 20x80 Celestron before I realised this comet was a ghost image of Jupiter well outside the field of view.

My 135mm refractor has such bad ghost images from between the uncoated objective elements that the scope is almost impossible to use.

Horace Dall recommended tilting optical windows on large scopes by one degree to avoid ghost images. There is no impact on the image quality, just removal of ghosts.

So one needs to be aware of ghost images, but recocgnise them for what they are.

I suppose glasses wearers also see ghost images.
A teacher in our school had eyes in the back of his head. When boys were unruly, he threw a chalk at them.
He identified the culprit from the reflection in his glasses.

Regards,
B.
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Old Thursday 10th October 2019, 08:57   #4
Steph barcelona
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Hi guys,
Thank you so much Mark and Binastro for your answers.
Great anecdocts Binastro about the ghost comet and the teacher. Very nice! Thanks for sharing. It ilustrates very well the ghost reflection effect.
According to the spec of my Nikon Aculon , the lenses are multicoated. I thought that this kind of treatment was suppose to prevent from secoundaries reflections.
The size of the reflection is very small compared to the original object. I realized that there is also a "second secundary reflection" (excuse the redondance), but there is also another reflection. Bigger than the other one but apparently much less brighter and less "predictable".
So you would say it s not a default of my binoculars? And that it s a common issue with entry level binoculars? Or should I ask for a repair, (still under garanty)?
Binastro, regarding the tip you mention of moving 1º the lenses. How do you do that exactly?
Thank you again for your tip, advices and explanations.
I really appreciate.
Regards
Stéphane
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Old Thursday 10th October 2019, 15:19   #5
Binastro
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Stephane,

I think that the 8x42 Aculon is not fully multicoated.
Some surfaces are single coated? and the prisms may be uncoated.
If there are any white reflections from say a 60 watt tungsten bulb or an old torch using white light then the surface is probably uncoated.
A prism flat surface will give an unmagnified image. It will act as a flat mirror, but only 4% to 5% of the light is reflected if uncoated. 1% if single coated and less if multicoated.
Smaller size reflections are from curved surfaces, i.e. lens elements.

There is nothing wrong with the binocular, a new one will be the same, unless the coatings have been improved.
It is only with higher price Nikons that all the surfaces are multicoated.

Looking at a nearby street light at night you will probably see the same ghosts as with the moon. A full moon will give brighter reflections than with a half moon.

The 1 degree tilt applies to astro telescopes of 200mm aperture upwards with a flat optical window at the front, i.e. a 'sealed' system.
It does not apply to small scopes or binoculars.

Some vague reflections can come from the inside tube wall or bright spots in lens cells, or just poor design. These are flare or glare. There can be multiple reflections from light bouncing around the inside of the tubes, or even bright spots outside the tubes.
These effects are mainly seen with bright light sources, and may be invisible in normal use.

Regards,
B.

Last edited by Binastro : Thursday 10th October 2019 at 15:25.
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Old Sunday 13th October 2019, 13:45   #6
Steph barcelona
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Hi Binastro,
Thank you very much for your didactic explanation. I understand better now how this residual reflections works.
I should have inform myself better before purchasing these binoculars since I d like to progressively go to stars observations. I guess as a all around binoculars it will do the job. I ll upgrade in a close future for a more appropriate and fully multicoated bins for sky observation.
Once more thank you very much for your help and for taking the time to explain me all of this.
That s very nice
Regards
Stéphane
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Old Sunday 13th October 2019, 16:30   #7
Binastro
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Stephane,
Actually, for stars the Aculon 8x42 is quite sufficient, as ghosting doesn't appear unless a bright object such as the Moon, Venus, Jupiter or street lights are involved.

Depending on the amount of light pollution that you have a 10x50 Aculon or better quality binocular might be useful for astro obsevations.
I also use 12x56, 15x70, 20x60 and 20x80 binoculars.

Regards,
B.
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