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MrBill's new APM Apos....

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#51 Rich V.

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Posted 07 June 2014 - 10:16 AM

More troublesome is the off axis ghost images of bright stars just out of the fov which sweep through the field diagonally in the reverse direction of the sweep.


Bill, last year when we were comparing the three BTs, we noticed a similar effect in differing degrees.

How do the "semi-apos" which use a similar rhomb prism compare to the apos in this respect?

Rich

#52 Mr. Bill

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Posted 07 June 2014 - 02:13 PM

Bill, last year when we were comparing the three BTs, we noticed a similar effect in differing degrees.

How do the "semi-apos" which use a similar rhomb prism compare to the apos in this respect?

Rich


Check tonight...wonder if this ghosting is inherent in Miyauchi design.

#53 Mr. Bill

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Posted 07 June 2014 - 02:28 PM

Discussing lateral color issue with Markus. He tells me that the CA is in the eyepieces, not the binocular.

Need to check eyepieces with reflector which has zero color.

#54 GlennLeDrew

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Posted 07 June 2014 - 02:55 PM

Bill,
Close examination with eyepiece removed should tell the tale--as long as the eyepiece holder is not too tall. Look into the prism aperture from every available angle, until a full exploration through the 'anomalous' reflection/objective light path has been obtained. It seems natural that a prism edge is involved at some point, and you will discover which one. You could make sketches, too, for both the through-prism appearance and your interpretation in cross section; a valuable exercise for your own understanding.

#55 Mr. Bill

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Posted 07 June 2014 - 03:11 PM

Bill,
Close examination with eyepiece removed should tell the tale--as long as the eyepiece holder is not too tall. Look into the prism aperture from every available angle, until a full exploration through the 'anomalous' reflection/objective light path has been obtained. It seems natural that a prism edge is involved at some point, and you will discover which one. You could make sketches, too, for both the through-prism appearance and your interpretation in cross section; a valuable exercise for your own understanding.


Glenn....no disrespect, but I really don't care where it's coming from....I'm an observer first and foremost; I'm not an optical designer and rigorous analysis is tedious at best.

Question is....is this "normal" for this design? Could it have been avoided by more careful attention to detail in the design stage?

:question:

#56 Man in a Tub

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Posted 07 June 2014 - 03:46 PM

I'm an observer first and foremost; I'm not an optical designer and rigorous analysis is tedious at best.



:grin:

I haven't changed my signature for while. Thank you, Mr. Bill.

#57 Mr. Bill

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Posted 07 June 2014 - 03:59 PM

my "five minutes of fame"... :grin:

#58 GlennLeDrew

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Posted 07 June 2014 - 04:04 PM

Bill,
OK. After having designed and built your own Bino Box, I'd thought your proclivities might lean this way.

The shortness of the optical path length in these rhombs might be the issue. In order to minimize this dimension, the width is also kept minimal out of necessity. There is then no room in which to build in light traps such as transsecting grooves.

Yes, prisms are a liability. But their one advantage comes to the fore when faster objectives are used; they keep the front aperture of the erecting/folding subsystem smaller by virtue of pushing the focus farther rearward by just over 1/3 the optical path length tgrough the glass. This keeps the rear end more compact overall.

However, I get to mulling and wonder if, instead of a rhomboidal prism, a pair of mirrors might not be better employed here. The 'fingernail' would be gone, but the Schmidt prism would effectively be moved farther up into the light con (in real terms, the bino would be a bit shorter.) However, the aperture will not likely be reduced, there resulting a decrease in the circle of full illumination and greater fall-off toward the field edge. I would *happily* trade the 'fingernail' for a reduced circle of full illumination.

Mirrors and prisms both have their pros and cons, with differing balance depending on the mix.

#59 Man in a Tub

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Posted 07 June 2014 - 06:48 PM

my "five minutes of fame"... :grin:


Three hours and about two minutes, OK?

BTW, I am reading this thread.

:step:

#60 Mr. Bill

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Posted 07 June 2014 - 09:44 PM

my "five minutes of fame"... :grin:


Three hours and about two minutes, OK?

BTW, I am reading this thread.

:step:


Are you a covert moderator....my history hasn't been too good lately. :grin:

#61 JustaBoy

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Posted 07 June 2014 - 09:45 PM

"BTW, I am reading this thread."


Yes, but do you have any Proof that you are reading it, Todd?

#62 Vondragonnoggin

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Posted 07 June 2014 - 09:54 PM

Does the Kowa Highlander exhibit the same fingernail?

#63 Mr. Bill

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Posted 07 June 2014 - 10:57 PM

Does the Kowa Highlander exhibit the same fingernail?


Just been talking to Markus about this....he's going to get back to me when he checks it out.

Anyone else out there with one, chime in.

#64 Tamiji Homma

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Posted 08 June 2014 - 12:31 AM

Hi Bill,

Kowa Highlander at 21x shows 3 ghost Moons when the Moon just enters the FOV. When the Moon is about half way to center, you notice a dark green reflection in the FOV even though ghosts are almost gone. When the Moon is on axis, there is very little unwanted reflection seen. (or unable to see the ghosts due to the Moon brightness, maybe ghosts are overlapped) It produces the best low power Moon image among instruments that I've tried.

With Swarovski ATX 95 EMS binoscope with Panoptic 27, you hardly notice the Moon is coming up to view until it hits FOV if sky is clear (very little sky glow, high transparency). It has excellent unwanted light suppression.

I think it is hard to eliminate unwanted reflection in complex prism system if you are looking at the Moon. It is waaaay too bright :)

Tammy

#65 garret

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Posted 08 June 2014 - 05:42 AM

I think it is hard to eliminate unwanted reflection in complex prism system if you are looking at the Moon. It is waaaay too bright



The moon is bright because its large, if you take the intensity of a first magnitude stars and make as large as the moon then you have bright object :roflmao:

What is the point-intensity of the Moon btw?

garrett van der veen

#66 GlennLeDrew

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Posted 08 June 2014 - 07:12 AM

A brief review...

The 'fingernail' is a clipped, *separate* exit pupil for a *separate* image for a field lying some distance off axis from the primary image's field. It's rather like sending the image from a *separate* cat's eye'-shaped objective, mounted beside the 'proper' objective, its optical axis not parallel to the 'proper' objective, through the one eyepiece.

If one had a *really* long sun shade/dew cap, it would block that far field light. Depending on the angular separation between the closest points of these two fields of view, the shade could end up somewhat vignetting outer field light of the 'primary' image.

If the two fields partially overlap (possible? at lower powers, and depending on the geometry defined by the objective focal length and physical distance off axis for the place on the prism wbere the offensive reflection occurs), a shade can never completely mask off the unwanted light.

I'd be curious, Bill, if an object's ghost can be seen while the object itself is still in the primary FOV. If not, how close to the field edge (or optical axis) can it get while remaining visible? You can check with a flashlight set up at a distance in twilight if cloudy.

#67 Mr. Bill

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Posted 08 June 2014 - 09:17 AM

Glenn...I'll look tonight.

I remember this conversation from my thread about the ghosting in the Fuji 10x50. The real long sun/dew shield was talked about.

What it boils down to, is this inherent in prism/lens systems or can it be corrected by more careful design specifically to prevent this ghosting.

This off axis light leakage is there regardless if you see it or not, and must have a veiling effect that degrades the contrast, so this is an important issue.

#68 Mr. Bill

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Posted 08 June 2014 - 09:47 AM

Here are the fingernails in the exit pupil of the Fuji 10x50...this is from thread a couple of years ago when I reviewed them and caused a ****storm from other owners and was in effect accused of being way too critical.

This shows fingernails on axis, with the APOs you have to look off axis, so I guess that's an improvement.

Attached Thumbnails

  • 6570400-Fuji 10x50 prism leakage.JPG


#69 Mr. Bill

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Posted 08 June 2014 - 10:36 AM

Next....

Vixen BT81 with eyepiece removed, on axis. This is why I sent them back, almost unusable for starfield sweeping. Very poor design, IMO.

Way undersized prisms was the reason here.

Discussions with Vixen US rep showed total ignorance and indifference to problem....just wanted me to go away :mad:

Attached Thumbnails

  • 6570470-BT maximum fingernails.JPG


#70 Mr. Bill

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Posted 08 June 2014 - 10:54 AM

Next... APO on axis with no eyepiece.

This is interesting in that besides the fingernails, there is a flat surface that appears matte black and other than a prism reflection.

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  • 6570514-On axis (2).jpg


#71 Mr. Bill

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Posted 08 June 2014 - 10:59 AM

Another looking off axis...

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  • 6570529-off axis -1 (2).jpg


#72 Mr. Bill

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Posted 08 June 2014 - 11:04 AM

Other side....

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  • 6570541-off axis-2 (2).jpg


#73 Mr. Bill

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Posted 08 June 2014 - 01:07 PM

Bill,
OK. After having designed and built your own Bino Box, I'd thought your proclivities might lean this way.(analyzing optics)


Really didn't enjoy designing and building the Binobox; it was a huge relief when I finished it.

Much more fun to use it. :grin:

Really didn't have a choice as there was nothing on the market that would work ( and probably cost way too much if there was) and I wanted to get away from this issue of prisms and off axis light leakage by using first surface mirrors.

#74 Rich V.

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Posted 08 June 2014 - 01:59 PM

FWIW, here's an off axis shot showing the worst case scenario of a "fingernail" looking into the focuser of my Miyauchi Saturns.

I've got to go way off axis for this "fingernail" to appear outside the exit pupil in the eyepiece. There appears to be a baffle of some sort that blocks unwanted reflections nearest the exit pupil.

Rich

Attached Thumbnails

  • 6570799-fingernail Image1.jpg


#75 Rich V.

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Posted 08 June 2014 - 02:16 PM

An image on axis into the focuser:

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  • 6570830-no fingernail Image2.jpg



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