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Posts Made By: Milton Wilcox

March 22, 2004 02:38 AM Forum: Astro Binoculars

Which One's the Finder?

Posted By Milton Wilcox

My messier tours with my Canon 18x50's and old Nikon 12x36's are about complete (within the bino's and my ability). I thought that I had hit the wall with the Nikon's for awhile until I tried laying them on top of the mounted Canons, and found out they pointed in the same direction! This has been an immense help in finding threshold objects for the little guys.

My counts now stand at 78 for the Nikons and 95 for the Canons. The 18x50's will easily break 100, failing only at M109 and a few of the Virgo cluster galaxies in my mag 5.5 skies. The 12x36's have probably gone about as far as they can go, even with the finding help. cool

Notice the different colors of the coatings in the picture. Nikon's were green 20+ years ago - anyone know if they still are? In comparison, the Canon coatings are distinctly blue in color. Yet, the CA is not objectionable in either bino!?

Clear Skies,
Milt

April 21, 2004 07:56 PM Forum: Binoviewers

Binoviewing's Dark Secret?

Posted By Milton Wilcox

Back in his message posted 1/5/2004 1:38:09 PM, Tom Hole wrote:

"I have been reading a little on beamsplitters and every site I have read talks about these [spherical or chromatic] aberrations that are introduced by beam splitters/prisms. Can you see it at the eyepiece? Dunno."

My answer is "Yes" Tom, much to my disappointment.

My 8" f/4 mirror has decent edges and correction as evidenced by the in-focus and out-focus diffraction patterns being nearly identical. However when I insert my premium highly-rated binoviewer & OCS that you now also own, the in-focus pattern becomes concentrated in the outer diffraction ring, while the out-focus pattern is soft and fuzzy. Classic spherical aberration, and a lot of it.

How have I seen it? Well, in a high magnitude difference double that I resolved in mono mode but could not split with the binoviewer, and in noticeable softening of the core of a bright globular cluster compared to mono mode. Just to name two so far.

Surprisingly, this has seldom been brought up on this forum, and none of the CN binoviewer comparison reviews even mention it. (In another review I found "through the binoviewer stars are slightly more fussy.") In an ATM thread it was stated that the problem is a function of the steepness of the light cone entering the beam-splitter/prism, so maybe most folks are using much slower scopes than I am. But I doubt it.

On the manufacturer side, TeleVue and A-P seem to be the only ones who own up to the problem and try to do something about it. The Zeiss/Baader with Roland Christen's Glaspath compensator might be the only combination to have completely solved this problem (ref: 'Sky&Telescope' Sept. 2002 binoviewer comparison). But it wouldn't focus in my Newtonian.

Referencing the latest thread: Roland, if it will focus in Newt's AND is still "completely free of spherical aberration and astigmatism" (Alan Dyer's words in S&T) you will have hit a home run.

Milt

April 27, 2004 02:19 AM Forum: Takahashi

Mewlon Questions

Posted By Milton Wilcox

Hello Mewlon Owners,

I am not an owner myself, but was talking to a couple of visitors to our star party last weekend who owned Dall-Kirkham's. One said that the largest TFOV he could get was 26'. I didn't think to ask them if they were Tak's. The Mewlon is not on my dream list (yet) because of the long f.l., but one of my most stunning views ever was the core of M13 through an M250.

The excellent on-axis performance prompts the question, how well do large field stop 2" ep's work in these scopes? For example, my calculations indicate that you should be able to get a little over 1° in an M210 using a 41Pan. This would greatly extend the usefulness of the scope. However, don't D-K's exhibit off-axis coma? If so, does a Paracorr help?

Someone also said they binoviewed with a Mewlon. I would be curious to know what kind of BV it is and how it affects resolution. Yes, I know what curiosity did to the cat. :-)

Thanks,
Milt

May 9, 2004 02:37 AM Forum: Astro Binoculars

Binocular Illumination Tests Part I: Setup

Posted By Milton Wilcox

My wife and I have a glass-top coffee table in our living room. This gave me the inspiration for a way to move my laser around a binocular objective without changing its incident angle. After some experimenting, I found that a small hydraulic jack would hold the binoculars square against the underside of the glass so that I could slide the laser around on top of the glass over the objective. The padded jack seats against the unused eyepiece. A box under the tested side raises a piece of white paper close enough to the exit pupil to make measurements on the laser spot position in the field.

To keep the laser square against the top of the glass, I used a 2" to 1-1/4" adapter as a base. When I initially tightened the screw in the 1-1/4" adapter, just the play in the adapter was enough to cause the laser beam to be slightly off-axis. This can be checked by rotating the laser in place over the objective. If the laser axis is not precisely parallel to the optical axis of the binocular, the dot will rotate. Once I shimmed the adapter with thin strips of Scotch tape, the dot remained fixed in position as I rotated the laser.

With the binocular set up, I positioned the laser in the center of the objective and manually tilted it to locate the field edges on the paper. I found that as long as the laser was on-axis and centered over the objective, the holographic grid could also be used to show the true field. However, this was the only thing I used the grid for. Below is a picture taken under the table of my elderly Ofuna's ready for illumination testing.

Welcome to the discussion, Rick! (Mr. Shaffer is one of my mentors from whom I learned much of this stuff).

May 9, 2004 02:45 AM Forum: Astro Binoculars

Binocular Illumination Tests Part II: Methodology

Posted By Milton Wilcox

The first test was to slide the laser around the objective to check for on-axis (center of field) illumination. The exit pupil dot should remain stationary in the center of the field during this test. If the dot moves as the laser nears an objective edge, it indicates aberration(s) that will affect the star image. If the dot disappears before the laser reaches an objective edge, it indicates vignetting and less than 100% illumination at the field center.

Once the on-axis work was done, I taped a shim onto the edge of the laser holder to incline it at a known angle relative to the optical axis. I used two shims, one 0.6mm thick including tape, and the other 1.2mm. The spacing between a shim edge and far base edge measured 45mm. This allows the tilt angle to be calculated: arctan(0.6/45)=0.76° for the thinner shim and arctan(1.2/45)=1.53° for the thicker.

An off-axis laser beam shifts the exit pupil dot away from the center of the field. Again, it should remain stationary as the laser is moved around the objective, providing the laser is not rotated while being moved. Knowing the incident angle of the laser and measuring the dot position on the paper allowed me to confirm the TFOV of each binocular.

As I moved the laser over the objective in the direction it was inclined, I found that the dot would disappear before it reached the objective edge, indicating some vignetting of the off-axis beam. However, there was no vignetting at the opposite objective edge and seldom along either perpendicular edge. This makes the lost objective area a crescent, which is difficult (for me) to calculate. To come up with % illumination, I estimated the usable objective area from sketches on graph paper. As a sanity check, if one half of the objective is contributing full illumination and the other more than 50% of its half, we know the total illumination must be better than 75%.

The next picture is of the Canon’s from the top of the table. The red tape marked the edges of the laser base with the beam at the objective edges.

May 9, 2004 03:06 AM Forum: Astro Binoculars

Illumination Tests Results

Posted By Milton Wilcox

I tested three binoculars:

1) Early 1960's Ofuna Kogaku (Tower) 7x50
The on-axis (field center) tested 100% illuminated, but the exit dot moved significantly as the laser approached the objective edges, indicating aberrations. The TFOV measured slightly larger than the claimed 7.1°.

The 0.76° shim (21% from field center to edge) caused 3mm max. vignetting along only one edge. The 1.53° shim (43% from center) caused 6mm max. vignetting along one edge and less than 1mm along the perpendicular edges. Estimated illuminations 94% and 87% respectively.

2) Canon 18x50IS, IS turned off
The on-axis (field center) tested 100% illuminated and the exit dot had much smaller movement than the Ofuna's as the laser approached the objective edges. The TFOV was about 3.5°, as advertised.

This binocular has some strange goings-on inside. Non-symmetrical vignetting dependant on the orientation of the incident angle was observed with both shims. Also, in two orientations illumination was actually regained in the direction opposite the vignetted side. This made it very difficult to estimate illuminations, but here goes: for 0.76° off-axis (43% from center), 91% and for 1.53° off-axis (87% from center), 80%.

Here is a picture of a Canon 1.53° off-axis dot with the laser positioned just inside a perpendicular objective edge. The drawn circle has a 3° diameter.

May 9, 2004 03:11 AM Forum: Astro Binoculars

Remaining Test Results & Conclusions

Posted By Milton Wilcox

3) Nikon 12x36

The on-axis (field center) is not only 100% illuminated, but the exit dot looked anchored in place as I moved the laser around the objective. It has very well figured optics - probably why it cost $250 over 20 years ago. The TFOV measured 4.7°, slightly less than the claimed 5°.

The vignetting was completely symmetrical and always along only one edge. For 0.76° off-axis (32% from center) I estimate 92% illumination and for 1.53° (65% from center), 75%.

Conclusions:

We shouldn’t lose any sleep over off-axis illumination in most binoculars. Although Ed Z's eyes and patience might be able to discern the reduction in limiting magnitude at 75% illumination, most of us could not. And besides, all you have to do is re-center the object and you're back to 100%.

Clear Skies,
Milt

May 27, 2004 06:40 PM Forum: Astro Binoculars

Rick's Exit Pupil Tutorial

Posted By Milton Wilcox

Fellow Binocular Observers,

If you were at all confused by the recent discussion of exit pupil illumination by Ed, Rick, Alan, Kenny and myself, make sure to read Rick Shaffer's first 'Wild Card' column in the articles section of AM. (Even if you are certain you understand exit pupils, I can almost guarantee that you will learn something new - I know I did).

I want to be the first to thank Rick for sharing his easily understood tutorial on this important subject. In particular, his optical path diagram that traces rays from the objective through the image plane and on out the exit pupil is what originally "showed me the light" in Rick's book.

We should all feel privileged to have Rick, who is also an avid binocular observer, in our corner.

Clear Skies,
Milt

May 27, 2004 07:06 PM Forum: A Day in the Life of the Administrator

The Ever Improving AM

Posted By Milton Wilcox

I want to thank Herb and anyone else involved in turning Astromart into a world class astronomy site.

I just finished reading Rick Shaffer's first 'Wild Card,' which was in part prompted by a long thread on the Astro Binoculars forum and maybe by my nagging Rick as well. To everyone who already knows how an exit pupil is formed, I say read Rick's column anyway...

There is probably a Mewlon in my future, definitely to be purchased through ATWB. But for now I need to recover from the new 31NT5. Choke. Gasp.

Best Regards,
Milt

May 29, 2004 01:38 AM Forum: Binoviewers

Stillborn Comparison

Posted By Milton Wilcox

Hello all,

Not that anyone was waiting with bated breath, but after accumulating 25 pages of observing notes and email exchanges with other binoviewer users and Russ Lederman at Denkmeier, I have decided to abandon my binoviewer to binoscope comparison article.

As I progressed farther into the project, it became increasingly apparent that my experience was the exception not the rule. From what I have been able determine, my Vixen R200SS f/4 Newtonian had two issues (maybe more) that affected its use with binoviewers. They are 1) The image plane is brought 5" outside the short tube, requiring an exceptionally large 37% C.O., and 2) The primary is actually slightly over-corrected, i.e. past a parabola into a hyperbola. Although the scope produces stunning widefield mono views (2.6° with Paracorr & 31NT5), it does not like binoviewers at all.

It simply would not be fair to cast doubt on binoviewers in general for the results I got from this specialized scope. My only advice would be that if you have a non-mainstream scope, check with the binoviewer manufacturer first before you order your package.

I will continue to monitor this forum and, who knows, may give binoviewing a try again in the future - but with a different scope.

Clear Skies,
Milt