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Porro prisms at night

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#1 btschumy

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Posted 29 July 2004 - 08:33 AM

As you know, I have been investigating constructing a "binocular" by pairing two spotting scopes. I have also been looking at using two astronomical scopes such as the new William Optics Megrez II. With the latter option I would need a porro prism in each tube to both correct the image and use the light path offset to control the interpupillary distance.

So I've been researching porro prism image erectors. I found the following on the Astronomics.com site:

"Some people are tempted to use a 45-degree viewing image erecting system for astronomical observing, feeling that a correctly-oriented image that matches the orientation of a star chart would help them locate objects more easily. This use is not recommended, however, as the vertex in the prism puts a small spike of light on every bright point in the image. This is of no consequence in the daylight, as the spike is dim and the image is usually lit brightly enough to wash out the spike. At night, however, when the background is dim and dark, the spike of light is quite visible and reduces the resolution of binary stars, star cluster, and subtle lunar and planetary features."

I had known that prisms slightly degrade the image and are not recommended for high power astronomical observing. However, this is the first I've heard about it introducing a spike on point sources (or more likely on the airy disk).

Almost every binocular I've looked through has not given me as pleasing an image as the low power views through my Apo refractor. Depending upon head position, I always seem to get small flares coming of the stars. I had always chalked it up to the objectives of the eyepieces not being up to the same level of quality as my refractor. Now I'm wondering if this is just inherent in porro prism binoculars because of the prisms.

So I'm curious, does everyone else see small flares around brighter stars when using binoculars? Is this just porro prism in action? Would using roof prisms correct this?

#2 lighttrap

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Posted 29 July 2004 - 08:37 AM

Well constructed, well specified binoculars will not show prism spiking. However, I've experimented a good deal trying to use various prisms with a TV Ranger and even the best Amici 90* prism does in fact show spiking, and the vertical line in the middle of the prism is visible even in daylight. I've always been curious how spotting scopes and binoculars get away from this problem. But, it's not a problem in most quality binoculars or spotting scopes.

#3 btschumy

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Posted 29 July 2004 - 09:05 AM

Then I wonder why I still get small flares with my Nikon 10x42 SEs? I have also noticed them with Fuji 16x70s. It is possible there is something unusual with my eyes or my glasses, but I don't see this when using small refractors at low power.

Do you get comparable image quality through quality binoculars and quality refractors?

#4 btschumy

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Posted 29 July 2004 - 09:42 AM

I was doing a bit more research on porro prisms and I found the following statement by Clive Gibbons (someone whose opinion I value) in sci.astro.amateur :

"Actually Ross, there is a way to make an image erector which doesn't produce diffraction spikes off of bright objects. It's called a Porro Prism erector, but instead of bending light through a 90-degree or 45-degree angle, the eyepiece axis stays inline with the main scope. The diffraction effect you're noticing is caused by the "roof angle" in the roof prism of the 45-degree diagonal."

So maybe the problem mainly exists in the 45 degree prism setups.

#5 Guest_**DONOTDELETE**_*

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Posted 29 July 2004 - 10:29 AM

Bill, I get the small flares through every pair of binos ive looked through.

#6 KennyJ

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Posted 29 July 2004 - 03:31 PM

Bill,

First of all , could you clarify what you mean by your following statement: ?

< I had always chalked it up to the objectives of the eyepieces not being up to the same level of quality as my refractor >

In other words , what do you mean by " objectives of the eyepieces ? "

This alleged "spiking" is certainly not a problem I've noticed "on axis" with my Zeiss 85mm Flourite scope with 20 -60x zoom lens ,at any power , which of course provides a " correct image " - so I suspect the cause of this cannot be the erecting prism per se.

As you probably know Bill , very few binoculars are constructed to the standards of a genuine APO refractor , mainly because it rockets the cost out of most folks' sensible price range , so unless you are comparing APO refractors to APO binoculars , with respect , you are comparing apples and pears , not apples and apples.

I cannot speak from personal use , but I've read that Takahashi 22 x 60 fluorite , Zeiss 15 x 60 BGAT , and Nikon 7 x 50 Pro -Star , which incidentally are all PORRO -PRISM models , do not suffer from this spiking.

Neither have I read about it being a problem with Kowa Highlanders or the 150mm Fujinons , so I'm guessing there might be another reason for this.

The "two -eyed" as opposed to "one" could also be a factor here , as you suggest yourself.

I wonder if you ALWAYS perceive a "spike -free" image with EITHER eye through your APO refractor ?

Even if you do , I would not neccesarily conclude beyond all doubt that it MUST be the " porro - prism binocular syndrome" to blame.

Even though I much prefer using two eyes to one ( as my signature implies ) I must admit that strange things DO happen when using two eyes which are unevenly matched , and I don't believe such a simple device as a "diopter adjuster" can compensate for ALL differences in optical performance between two eyes.

Human eyes and vision are more complex than that.

By the way , I too am giving consideration to creating a GIANT binocular , in my case by pairing two 102mm Synta f5 refractors , but I'm planning on by -passing the common IPD adjustement problem by making it specifically for MY IPD of 68mm.

I too am giving thought to 45 degree or 90 degree oculars, but why not go for BOTH -- having the option of either with the added option of various magnifications too , using standard 1.25 inch eps ?

My initial plan is to go for 33 x 102 with 45 degree angle and take it from there.

I wish you success with your project Bill , and hope you will keep us informed of developments.

And if ANYBODY can give me ANY advice on how best to go about MY fixed parallel train model , PLEASE don't hesitate to do so. I'm in no rush with this --it's a "medium term project" -- but I want to "get it right" first time if I can !

Regards and clear skies to all -- Kenny.

#7 btschumy

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Posted 29 July 2004 - 05:53 PM

Kenny,

Thanks for commenting.

My confusing line:

< I had always chalked it up to the objectives of the eyepieces not being up to the same level of quality as my refractor >

should read:

<.... the objectives or the eyepieces ...>

I unfortunately tend to make a fair number of typos like that.

I realize that most binoculars are not up to the snuff of an Apo. However, even lowly achromats will give great low power views on objects of moderate to low brightness. My Nikon SEs are not Apos, but given what they cost you would think they did not skimp too much on the optics. Nevertheless, I do get flaring around stars with them.

I have since learned that the spikeing is in the roof prisms used in 45 degree and 90 degree erecting prisms. Apparently a true porro prism will not give the spikes. They will, however, introduce spherical and chromatic abberation unless the eyepieces are designed to compensate for it.

I hear you that some people don't see spikeing. There are a couple of possibilities for this:

1. There is something perculiar about my eyes (and presumably others also). This is certainly a possibility.

2. Maybe others are not as sensitive to this abberation as I am. Am pretty picky about my optics <g>.

You speculate that it may be a "two eyed" as opposed to "one eyed" issue. Maybe, but even if I cover one objective so I'm only using one eye, I still see it.

So it's still a mystery to me. I guess I'll just keep collecting data and see if I can figure out what's going on.

#8 lighttrap

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Posted 29 July 2004 - 06:35 PM

This topic is interesting to me. I will look for spiking if it ever clears in NC long enough for nightsky observing to again be an activity. I'm thinking by October, that should come to pass. Anyway, as to the sensitivity issue, I think you're probably right. I was never sensitive to relatively minor chromatic aberations, until I started really looking for them. Now, there are certain binoculars, like the Fujinon FMT-SX 16x70 and Swift Kestrels that I just can't help but be bothered by CA every time I pick them up. What's always been a real consideration for me is clarity and resolution. I know for a fact that not all are as picky in this regard as I am. Otherwise, we wouldn't have folks praising a whole slew of binos that I've frankly rejected as not being sharp enough for my tastes. (That's not a slight against anybody, just indication of vast differences in the way we all perceive things.)In fact, I think having these various "super-sensitivities" is a bad thing, because it means that it's ever tougher to be satisfied. I know that I'm always mentally comparing the view through various binoculars with the view through the Nikon SE 12x50s, and as you may imagine, hardly anything comes close. So it becomes difficult to be satisfied. I've heard similar from various high end Zeiss and Swarovski users.

As for spiking, I'm so used to the diffraction spikes on my motley collection of reflectors, that I may just unconsciously ignore very minor ones from binos and spotting scopes used for astronomy. Thinking about it, it's odd how often I never really look at individual stars. They're just pointers to something more interesting. (To me, no slam on those who enjoy splitting doubles and that kind of thing, it's just not my game.)

#9 craig_oz_land

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Posted 30 July 2004 - 02:14 AM

lighttrap,

Which Amici 90* prism?

I am just waiting for a Baader T2 Amici to arrive. They are suppose to be one of, if not the best. I already have their 90* star diagonal and am also getting the dielectric mirror.

Cheers, Craig.


Well constructed, well specified binoculars will not show prism spiking. However, I've experimented a good deal trying to use various prisms with a TV Ranger and even the best Amici 90* prism does in fact show spiking, and the vertical line in the middle of the prism is visible even in daylight. I've always been curious how spotting scopes and binoculars get away from this problem. But, it's not a problem in most quality binoculars or spotting scopes.



#10 lighttrap

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Posted 30 July 2004 - 06:17 AM

lighttrap,

Which Amici 90* prism?


I've got one of the older University Optics 90 degree Amici prisms. I'm not even sure if it's still listed on their website. They show one, but I'm not sure if it's the same one or not, because this one cost more than the one currently on their site. Either way, I would not recomend this one.

#11 Guest_**DONOTDELETE**_*

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Posted 30 July 2004 - 07:10 AM

Like TPalmer, I have gotten some degree of light spiking in all the binos I have looked through. I'll admit that I have not looked through as many binos as EdZ or Barry or Bill Cook, but the few I have used have shown some spiking. The BT100 at high power, 63x, shows some relatively small amount of light scattering...particularly in the right EP for some reason... :question:. I can say that it is not my right eye despite the coincidence that my right eye is the "weak" one of the two.

#12 Guest_**DONOTDELETE**_*

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Posted 30 July 2004 - 11:43 AM

Mike, you put it perfectly. It is easy to become supersensitive and never be satisfied. I dont mean to get off on another topic, but its been very revealing to me to finally look through the Ober 22x100s after reading every review and opinion I could find on them for the last year.

Overanalyzation and too much research can lead one to be pre-programmed to look for faults. Myself for example, im certain it would have taken a long time to even notice false color, but ive read so much about it that now im looking for it. However back to the subject..flaring on bright objects I did notice right away, only natural to try and focus the flaring out of the image.

On the sensitivity issue...I think it pays off to start humbly and work your way up. Someone here said(I cant remember where) when speaking of equipment...its best to always have something to look forward to. In other words dont start with a Fujinon or high-end Nikon, give yourself room to grow and enjoy the satisfaction of progressively better equipment.
Not bad advice.


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