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Posts Made By: Ed Zarenski

May 19, 2003 03:25 AM Forum: Deep Sky Observing

Visual Acuity - What's Yours?

Posted By Ed Zarenski

Visual Acuity is a limiting factor in the eye’s ability to achieve resolution and magnification is employed to make an image large enough for the eye to perceive. If your visual acuity is 200 arcseconds, you need 12x mag to see a 17” double or 80x mag to see a 2.5” double.

Acuity is the apparent separation results for a range of varying doubles measured with one or more pieces of equipment. The results will fall into a fairly narrow range. This would be the observer's acuity range.

“The resolution of the eye is 3 to 5 minutes of arc; therefore a minimum magnification must be utilized to enlarge the image sufficiently to exceed the eye’s resolution.” C. R. Kitchin, “Telescopes and Techniques” Springer-Verlag, 1995. Other sources will quote lower limits. My personal records of many doubles puts me right in the middle of this range. Possibly some observers can achieve better results.

In my best binoculars I can rarely achieve apparent separations of less than 200 arc-seconds. I’ve recorded many observations between 210 and 240 arc-sec. Some telescope results are slightly better, possibly attributed to the better resolution of telescopes. The best results I’ve achieved with very good optics was about 160 arc-sec, equivalent to a 2” double at 80x. The closer the doubles or the wider the mag delta, the harder it becomes to achieve these results.

So my usual visual acuity is 3 to 4 arc minutes. I am not capable of seeing separation in a 4 arc-min pair naked eye; I cannot see e1-e2 Lyra (207") as separated. Optics improve the point source resolution and contrast making it easier to see separation.

I've read one binocular observer was able to split doubles of 4 seconds magnified 20x = to apparent separation 80 arcsec. My results do not indicate this cannot be accomplished, but they indicate it’s clear if we wish to compare our ability to perceive resolution with our instruments, we must first establish the visual acuity of the individuals. Anyone ever split Castor 4” or Algeiba 4.4” with 20x or 25x binoculars?

We could test acuity with a list of doubles. Some that might get better success with binoculars are 17CrB at 6” or 7CrB at 6.3”, E(sigma)953 in Mon at 7.1”, Mesartim y Ari at 7.8” or y Del at 9.6”. A wide range of binocs might be able to see these close matched doubles. Smaller binocs try 100Herc at 14.2” or any one of many around 20”. With telescopes, for each double record the lowest magnification that splits it.

Some may achieve acuity of 2.5 arc minutes or 150”, or less, requiring a magnification of only 150”/9.6” or 16x to see separation in yDel.

I’m very interested in collecting results from all observers that might lead to some realistic conclusions concerning acuity. Having determined acuity, we should be able to compare results of resolution tests from various individuals, as long as all other measurement criteria are held constant.

Please Email results to

June 7, 2003 01:37 PM Forum: Astro Binoculars

Oberwerk 15x70s 2002 vs 2003

Posted By Ed Zarenski

Just a little over a year ago, I purchased a pair of Oberwerk's widely known 15x70 binoculars. They have been well reviewed and compared over the past year by many and in fact my own 15x70s are the subject of two articles in the Big binocular review section on CN.

There are many aspects of performance to discuss in a comprehensive binocular review. This post is not that comprehensive. But just recently, in some internet posts, I had an exchange with Kevin Busarow from Oberwerk about the ability of the 15x70s to "see" and how they compare to some other binoculars. Kevin related to me that the 15x70 binoculars have improved. I now have in my hands a new 2003 model of the Oberwerk 15x70 and I'm comparing them to my May 2002 model.

There is a significant difference in the coatings between the 02 and 03 models. While I had noted the 02 model has the most reflections off the lenses of various binoculars I was testing at the time, the new 03 model has significantly less reflections off the objective lenses. At the eye lens, the 02 models appears to be single MgF coated and the 03 model seems to have the same multicoating as the objective.

In use tripod mounted on a nice clear night, the 03 model shows significantly improved performance over the 02 model. In detailed observation of every star group I tested, the 03 model sees a few more stars than my 02 model. On a few double stars, most in the range of 16" to 13", the 03 model showed a marginal improvement. No huge differnces here, but a few doubles easier to see or suspected where the 02 model did not see it at all. The limit for my eyes with these binoculars still seems to be an even 14" double, but somewhat easier to see in the 03 model.

The overall view thru the new 03 model seems to be noticably brighter than the older 02 model. Viewing around the area of the Cygnus Milky Way, there was more and brighter faint star background visible in the new 03 model.

These 15x70s, just like my 02 model, need to have the right diopter adjusted about 90% into the minus range to focus even with my glasses on. That did not take away from the views I got with these binocs, but if you are right eye near-sighted and you want to use these without glasses, it will be a problem.

I had my 16x70 Fujinons out also while I was comparing the 02 vs 03 15x70s. I would say, while the newer 03 Oberwerks still are not Fujinons, the gap has narrowed since the 02 model. Those of you who are buying the latest Oberwerk 15x70 are getting a product which has been improved over the pair I bought a year ago.


June 21, 2003 05:51 AM Forum: Deep Sky Observing

M92 and M13

Posted By Ed Zarenski

As it got later last night, it got better.

M92 in my G5 SCT with a 7.5mmTak LE 180x 17'Afov. The halo of M92 took up about 1/3 or a little more of the 17'fov. Many stars were resolved all around the halo and somewhat into the core.

M13 in my G5 SCT with a 6mmUOortho 225x 11'Afov. The halo of M13 fills the entire 11'fov. Counted 22 stars resolved in the halo with many more too difficult too isolate in the count. Counted 10 stars in the core which took up about 1/4 of the 11'Afov. Estimated 30 to 40 stars in the halo and 20 stars resolved in the core. What a fine sight it was!

The 6mm UOorhto allowed resolving more stars than the 7.5mm TakLe, I believe just due to the magnification that permitted deeper penetration.


July 2, 2003 03:04 AM Forum: Astro Binoculars

Rare night in New England! 15x70Oberwerk2003

Posted By Ed Zarenski

I started out by carrying a scope and my 20x80 Oberwerk Deluxe outside. Right away I spotted a few things with the 20x80s and notice how easy things were. Mizar and Cor Coroli were easy, M51 was seen, M81 & M82 were easy, M3 and M13 very bright. I never even set the scope up. I went in and grabbed the 15x70 Oberwerk/2003 on loan from Kevin Busarow. They were easier to focus than the 20x80s and were mounted on a better tripod. I used the 15x70s for the rest of the night.

By 10PM, naked eye, the Little Dipper had 11 stars. 11 and 15 UMi at 5.0mag and 5.2mag were seen directly, 19 Umi at 5.5mag was seen averted. First time I’ve ever recorded seeing 19UMi. By midnight the sky was aglow overhead with Cygnus near zenith.

The new Oberwerk 15x70s proved their value for the rest of the night. Tfov measured = 4.3°

Mizar 2.3-4.0/14.4” was instantly easy, Cor Coroli 2.9-5.5/19.4” faint but easy, M51 seen, M81 and M82 seen bright, M63 and M94 seen, M3 bright core large halo, M13 brighter overall larger core, 100Her 5.9-6.0/14.2” very easy, E2259Her 7.3-8.3/19.6” near 94Her faint but seen, E2474Lyr 6.7-8.8/16.2” easy, E2470Lyr 6.6-8.6/13.4” more difficult but seen, M71 glow, M27 large bright, M29 brite, yDel 4.5-5.5/9.6” squinting to see, M10 bright, M12 faint, M9 small, 6539 glob near M9 very faint like a galaxy, M16 with neb, M17 nebula, M18 o.c. brite, M24 very large mixed with brite background, M23 dense wide, M11 looks like a globular, M26 seen, 15Aql 5.5-7.0/38”, 61Oph 6.0-6.5/21” easy, I4756Oph o.c. large mixed, 6633Oph bright mixed, I4665Ser vast dense faint stars, Alya theta Ser 4.5-5.0/22” very easy.

When I realized how good a night it was I decided to try for M101. At first, I had it in the field of view of the 15x70s, but I didn’t see it. I was looking at the wrong edge of the field. I got out my charts and verified the star pattern around it and there it was. I walked away and came back to swing right to it three more times. Once I knew where it was, it stood right out. I have only seen M101 a few times in my years of viewing. This final item was not the last seen but quite early on in the evening it was the best jewel to catch.

Not long ago I made some comments about the limitations of the 15x70 Oberwerks. Kevin asked me to look at the coatings on these new 2003 model of the 15x70 Oberwerk as compared to my one year old model of the 15x70. Fact is with these newer 15x70s, I was able to see maybe a dozen objects I never saw before in my older 15x70s. Limitations aside, if 15x70s work for you, I don't think you could spend your $150 on any better than these.


July 12, 2003 08:04 PM Forum: Astro Binoculars

Kathadin trip plan

Posted By Ed Zarenski

Well gentlemen, a lot has been written in the past couple of days. As much as I might say I’m sorry I haven’t been a part, you all do very well without. It never ends, does it? Mike, very well done.

My weekend has been filled with buying last needed items and preparing for next week into the Kathadin wilderness with my brothers. Shopping was the order of the day, as I needed a new jacket to insure preparedness. A new North Face waterproof will now fill the bill since with recent weather it may get more use than the binoculars I have finally decided to take on the trip.

Originally, I planned to take just one pair and the decision was difficult. I wanted the highest power but the least weight. Those two criteria don’t exist in the same binocular. On the last trip I took my 10x50 Ultraviews, at 32oz. and 10x they worked well, but I always wanted just a little more power. With the recent addition of a 4th man to the trip this past week, everything changed with now the ability to spread the primary loads between 4 packs. So a new plan developed today and we will make the trip with two pair of binoculars.

The big binocs are all way too heavy, Oberwerks20x80 8#0, Fujinon16x70 4#12z. The small binocs have the right weight, but power is less than I wanted, Minolta Activa 7x35 1#8z, Ultraview10x50 2#0. The 12x50s 2#2z seemed light enough but not big enough. So a decision has been made.

I’m taking the Oberwerk 15x70s 3#2z for midweight but high power, and my brother will carry the Swift Ultralite 8x42 1#4z. The Oberwerks should provide the boost for power at night and the Ultralites, which are finally staying together after two tries at repair now, will do nicely on the trail and fill the second handheld spot at night.

I’ve got a small 7” tri-leg plastic camera mini-pod with a ball joint head that weighs about 8z. It gets velcro-strapped to the top of my ski-pole/walking stick. The pole gets jammed securely into the ground. This serves well as a binocular mount for night viewing. Stability issues can be overlooked somewhat when the equipment must be carried 30 miles for 5 days. It works with at least the same effect as a monopod, and is sometimes completely still.

Given the 4th man to distribute load, I find with my share now a few pounds lighter, the decision to take the 15x70s is just a little easier to make. The added power and aperture will be tremendous over 10x50s. The 8x42s coming along (without me having to carry them) is a big bonus. These are the lightest binocs I’ve ever held and won’t add much to our collective load. Hopefully they will serve well day and night.

Last trip, the night sky was so dark and filled with wonders, I found it a little difficult to navigate around. I could only hope to get the same type of skies once again on this trip. Moon phase will be excellent (18th thru 25th). I’m in the can’t wait mode now.


August 1, 2003 05:51 PM Forum: Astro Binoculars

Deep Sky Binocular Views at Mt Katahdin 1

Posted By Ed Zarenski

Mt. Katahdin Maine trip July 2003

This was an annual backpacking trip with my two younger brothers. We’ve been doing trips like this for over 25 years. The mountains in Baxter State Park, Maine are considerably more inaccessible than the mountains on some previous trips we have taken in New Hampshire and New York.

Campground the first night was the furthest point that we could drive in to prepare for the trip that would begin the next morning. I was glad for the opportunity to test out gear before we actually started backpacking. Once you begin the backpacking trip, you carry what you take with you, even if it doesn’t work, and there is no going back for more.

At first I tried to mount my binoculars on a small folding tabletop camera mini-pod strapped to the top of my walking pole. When used like a monopod, it worked OK, but in this manner I could not show anyone else what I was looking at. With the pole securely stuck in the ground I could not completely stop shake and it made it very difficult to view.

I had my Bogen 3211 and had to carry the full weight of 7# for the 30-mile trip. This tripod pushed my pack load up over 60#, but several good nights of observing during this 6-day trip made it worth the effort to carry the equipment.

The most notable difference in the sky from a dark location is how readily the Milky Way stands out. On my first night I momentarily mistook the Milky Way for a cloudbank. From the Great Rift in Cygnus to the bright star clouds in Scutum and Sagitarius, the Milky Way shown like I have never seen it before. The varying intensities of brightly shining background stars that define the Milky Way gave the impression of seeing through the spiral arms that make up our galaxy.

When I finally did see my first cloudbank roll in, it also was like I have never seen before. The absence of ground light meant that clouds were not lit from below. Clouds could not be seen. Only the obscured section of sky hidden by the clouds allowed a hint that clouds were present.

Just recently at home in Rhode Island, I recorded one of the best nights of seeing I have written in my logbook. On that night around 2:00 A.M., I counted 11 stars down to mag5.6 in the Little Dipper, the most I had ever seen. On the first night of the Katahdin trip, viewing at Roaring Brook campground, at about 10:00-11:00 P.M., I counted and plotted 16 stars. I repeated the count again several nights later at Chimney Pond and plotted 18 stars. There were others nearby that I did not count, but they were off to the side and not the faintest. I have been trying to verify the mag limit of the 18 stars. 17 are shown in the mag6.5 Cambridge Star Atlas, so I assume I reached at least mag 6.6 naked eye. The undetermined star, the faintest of them all is just east of and about a half degree north of 21UMi.



August 20, 2003 09:06 AM Forum: Astro Binoculars

Limiting Mag 16x70 Fujinon & 15x70/03 Oberwerk

Posted By Ed Zarenski

Last night I spent considerable effort to determine limiting magnitude with my 16x70 Fujinon and my 15x70/03 Oberwerk. I consider this preliminary as it is my intent to perform this evaluation several times.

"The Backyard Astronomer's Guide" has a naked eye limiting magnitude chart of the Little Dipper on page 117. That's the chart I use for determining NELM.

Covington's "Astrophotography for the Amatuer" has two limiting magnitude charts (for use with binoculars or small scopes) on ppg 127 and 128. The Coathanger and the Pleadies show stars to mag 12.

The limiting magnitude in binoculars (or scopes) will increase as NELM increases. There is a tremendous effort to explain these limits in the Clark/Blackwell papers.

Some other factors that have an affect on lim mag are:

As aperture increases, lim mag increases;
As magnification increases, exit pupil decreases and sky background gets darker, contrast with background improves, resulting in deeper achievable lim mag;
As contrast provided by the components of the optical system improves, i.e., higher quality coatings and baffles, lim mag increases;
As the eye is kept to the eyepiece for a longer duration, dark adaptation improves to the level of the light provided by the eyepiece only, increasing observed lim mag.

Last night's efforts:

NELM direct 5.2mag direct and 5.6mag averted on Ursa Minor and at zenith.
Transparency good to very good.
Seeing good to very good, (6-8? out of 10).

I used the Coathanger (CR399 Vul) lim mag chart. I carefully noted the star patterns as compared to the chart, taking special note to visually compare distance between stars in the pattern to confirm which are seen. I did each pattern in each binocular twice.

The variety of labeled stars may not be enough to show any minor differences that might exist between the two models. There were no stars noted as mag 10.4 and only one at 10.5. There were none between 10.5 and 10.9. I would like to have a chart with stars labeled every 0.1mag between 10th and 11th mag.

Although the sky background in the 16x70 Fujinon looked slightly darker and it seemed the faintest stars were slightly easier to see, I got the same results with both binoculars.

It was very difficult to see stars beyond mag 9.7. A passing glance in the eyepiece would not show stars beyond 10th mag. It required a concentrated period at the eyepiece, allowing gaze to move around. Once glimpsed, stars beyond mag10 could then be seen directly.

Even stars of 9.5mag could not be seen if the binoculars were moving. Seeing stars of this magnitude required a completely quieted binocular on a stable mount, viewing without touching the eyepieces.

The binoculars were 16x70 and 15x70. I was not able to see any stars at 10.5 mag or beyond. The faintest stars observed (labeled and identified on the chart) in both binoculars were 10.2 and 10.3mag.

Efforts to be continued.

September 30, 2003 10:51 AM Forum: Astro Binoculars

Limiting Magnitude in Binoculars

Posted By Ed Zarenski

In July 2003, an active discussion took place concerning the topic “star magnitude and binoculars”. (Sorry google link won’t fit).

Based on previous studies I performed in July 2002 and again in winter 2003 , I questioned some of the results that were being predicted.

I set out to find some answers. It took a lot longer and a lot more work than I thought before I was convinced I had enough information to answer these questions for myself. The end result will soon be a published article on CN addressing Limiting Magnitude in Binoculars.

Based on testing eight binoculars on many different nights representing a range of conditions, this is some of what I found:

Binocular Limiting Magnitudes for a given size aperture are significantly less, approx. one full magnitude, than a scope of equal aperture. This is due to the inability of the aperture in binoculars to reach full potential because of low magnifications in use.

Two-eyed viewing vs. one-eyed viewing contributes only a small fractional gain in magnitude. There is no 40% gain realized because you have two apertures of the same size versus a similar sized scope. Gain may be more like 15% to 20%.

Naked Eye Limiting Magnitude does not act linearly on Binocular Limiting Magnitude. BLM does not increase in step equally as NELM increases. For the tested range with a variance of 1.5+ mag NELM, Binocular Limiting Magnitude varied by less than 0.5 mag.

When binocular magnification and binocular aperture are each tested separately, for various sizes and powers of binoculars, magnification produces results about twice what Carlin’s formula predicts and aperture produces results about half of what Carlin’s formula predicts.

When binocular magnification and binocular aperture are each tested separately, by incremental changes in magnification and aperture, it is found for each equal increment that magnification has approximately three to four times the influence as aperture on increases in limiting magnitude.

In binoculars much more limiting magnitude gain is realized from increases in magnification than from aperture. This is also related to the fact that aperture is under-utilized in binoculars. Unless optimum magnification is employed, the abilities of the aperture to put an image in the focal plane are never fully delivered to the eye.

Based on my results, for commonly used binocular magnifications in mag 6.5+ skies, I approximate the maximum limiting magnitude for a 100mm binocular at mag 12.0, for a 60mm binocular at mag 11.0 and for a 40mm binocular about mag 10.0. For mag 5.0 skies, all limits are about 0.5 mag lower.

The ultimate limiting magnitude reached for any given aperture is significantly dependant on the magnification in use.

The full article that has been submitted should be available on CN within the next week or two.


November 12, 2003 09:55 AM Forum: Astro Binoculars

Re: Opinions on these 3?

Posted By Ed Zarenski


Erik makes the most valid of points. You need to know your maximum dark adapted pupil. If it's only 5.5mm no sense going for 8x56s or 7x50s. Your pupil if smaller will reduce the effective apertture.

I had Orion Vista 8x42s for a while. The 8x42 is the same as Carton Alderblick and almost the spitten image of the Swift Ultralites. Smallest binocular I've ever held. They were a little loose for me in the focus and had much too short 11mm measured eye relief. They also had small prism obstruction. Excellent coatings tho. Don't know if this translates to the larger pair.

The Swift Ultralite 8x42 by comparison weighs slightly less 20oz., is a little tighter, had no prism cutoff, has 19mm eye relief, same fov, ip distance gets 5mm closer and 3mm wider. same price, beats Vista hands down.

I have 12x50 PCF III and 16x60 PCF V. Coatings are better on the PCF V. Not as good as the Swift.

Coatings on newer models Oberwerk are improved. Similar to Pentax PCF V, not in color but in performance.

Are the Pentax 7x50s PCF V waterproof? None of the others are WP.

Do you have a star chart? Photo copy a page with the dark circles showing the sizes for the star magnitudes. Cut the photcopy right down the middle of those star sizes. have someone hold the half circles up to your eyes in a fairly darkened room. You need some light to see what you are doing. looking in a mirror to do it yourself constricts you pupils when you try to focus on your own eyes in the mirror. get a good read on your own pupils. Assume potential for 1mm increase once outdoors under your dark skies.


May 5, 2004 11:23 AM Forum: Astro Binoculars

Apparent Brightness Comparisons

Posted By Ed Zarenski

In the previous thread, (Low power - Zeiss Victory / Fujinon impression), the discussion relative to apparent brightness struck my attention.

Recent studies have shown that binoculars do not deliver all the light expected based on the simple calculations of aperture area and exit pupil. The brightness of the exit pupil is considerably affected by vignetting in the binocular system. Even for some of the best binoculars, vignetting is present. The amount present can cause considerable differences between models of the same sizes. Brightness cannot simply be assumed based on light gathering area and exit pupil.

Without getting into how this is measured (you can read all of that if you wish in the CN binocular forum), I will give a few examples of light loss in some very good binoculars. This is a factor that must be taken into consideration when comparing the image brightness from one model to another.

Fujinon FMT-SX 16x70
Light entering at 70% out on the diameter of the objective lens is providing 100% illumination to only the central 50% of the diameter of the exit pupil, 25% of the exit pupil area.

The light entering the objective at 90% out from the center, produces an exit pupil only 50% the width of full size. This is the furthest point out on the objective that is fully illuminating any portion of the exit pupil. The effective aperture of full illumination is 90% x 70mm = 63mm.

At the outer edge of the objective, light entering produces an exit pupil only 40% the width of a full size exit pupil. If you rotate that 40% width exit pupil about the central axis, you would find that the central 20% of the exit pupil receives no light at all from the outer edge of the objective.

This is one of the better examples. Most others have proven less. With the exception of a binocular telescope, all binoculars I tested ranged between 70% and 90% effective aperture of full illumination.

Fujinon FMT-SX 10x70
Light at 70% out on objective diameter illuminates only the central 25% dia of the exit pupil.

The furthest point out on the objective that is fully illuminating any portion of the exit pupil is 85%. The effective aperture of full illumination is 85% x 70mm = 60mm.

From the outer edge of the objective, light entering produces an exit pupil only 35% full width. The central 30% of the exit pupil receives no light at all from the outer edge of the objective.

A Minolta Activa 7x35 has a 70% effective aperture of full illumination, only 24mm. Light from the outer edge does not reach the exit pupil at all.

The brightness of the image in the exit pupil is entirely dependant on the light delivered from the objective. The % illumination has an effect on this outcome.

For a more complete post see,2,3,4,5,8,9,10&Number=102901&page=0&view=collapsed&sb=5&o=&vc=1