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# "BEST OF" the Binocular Forum - Start Here

46 replies to this topic

### #26 EdZ

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Posted 10 December 2004 - 06:36 PM

Depth of Field is the range of objects from foreground to background seen in focus at once without moving the focus dial. For instance a narrow depth of field might be all objects in focus from 30M to 50M. A wide depth of field might be all objects in focus from 15M to 75M.

This thread, once you read down into it, lays out some of the mathematics behind depth of field in binoculars. It started out with various users, including me, simply stating what we felt controlled DOF. Later on it gets into the real definition of how depth of field is determined.

Depth of Field start of thread
here are some extracts of the most important points

I suppose that z is the distance at which objects appear sharp to the naked eye at the same time that objects at infinity.
M is the binocular magnification
d is the distance at witch the binocular is perfectly focused.
Without accommodation of the eye, we can see objects perfectly sharp at the distance d'.
By definition, the depth of field is (d – d').
Now, if I suppose that the exit pupil is always larger than the eye pupil, I find the equation :

1/z = M².(1/d'-1/d)

.....So, the only optical parameter which determines depth of field is the magnification. Its influence is huge, because of the 2-power of M in the equation.
Now, why people find that binoculars with equal magnification have quite different depth of field ?
I think that the perceived depth of field in binoculars is determined by other parameters than optical ones.
JCB

focal length has almost none impact on the DOF. It is the magnification which dominates.

In summary it seems to be that only magnification and effective exit pupil are dominating factors for DOF. Focal length has some influence but not much. However, I am not sure how well the assumptions made for these calculations are satisfied. For example, a binocular is not made of thin lenses. Only professionals may be able to figure out the validity of these assumptions, maybe with the help of ray-tracing software.

With regards,
Holger

The results can be found with the formula I wrote in a previous post :

1/z = M².(1/d'-1/d)

Here d is infinity, z=b , M=V and d'=G.
We have therefore : b=G/V² which is nearly the same as the formula on your post, in which the negligible terms have been omitted.
(For V=10 and G=100000 mm, we find b=1000 mm)
My formula is not rigorously exact, but is more general because it is also valid when the binoculars are not focused to infinity, but to the distance d.

I think it's worth doing some applications of this formula :
We suppose that the binoculars are focused to infinity, and that with naked eye we can see sharply objects without accommodation if they are 1 m away. Then DOF are :
For a 7x binocular : 49 m to infinity
For a 8x binocular : 64 m to infinity
For a 10x binocular : 100 m to infinity
For a 12x binocular : 144 m to infinity
People more than 60 years old, lacking in eye accommodation, and who have to rapidly focus between two distances (like birders), have to very carefully examine the drawbacks of high power binoculars, considering their poor depth of field.

Jean-Charles

Often associated with Depth of Field discussion is what is known as the 3D effect. So, here is some discussion about 3D, what is seen, what cannot be seen and the mathematics related to how things seem to have depth of field or appear 3D.
3D or not,3D!

Binoculars with a greater objective lens separation WILL provide more depth perception than a binocular with a lesser separation, regardless of the prism types . . . PERIOD!

the separation between objective lenses is the most important factor: a Porro 8x30 will show more 3D view than a roof 8x30.

Magnification has effect on 3D view, a greater magnification increases the compression of the fields reducing the 3D view: a roof 10x30 will show less 3D view than a roof 8x30.

Also distance has a significant impact on the so-called 3D effect;

For a binocular with objectives 150mm apart, observing two objects placed at

10 meters and 12 meters distant, the angles of view (with reference to centerline) would be 26 arcminutes and 21 arcminutes, a difference of 5 arcminutes, easily perceivably by the human eye.

100 meters and 120 meters, the angles would be 2.6 arcmin and 2.1 arcmin, only one half arcmin difference or 30 arcseconds. Already this narrow angle is beyond the perception of the human eye.

But if the objects were at
1000 meters and 1200 meters, the angles are now 0.26 arcmin and 0.21 arcmin. The difference in these two angles is merely 3 arcseconds, not only beyond the ability of the human eye, but beyond the capabilities of diffraction limited optics smaller than 46mm.

Curvature and Depth of Field
Here's a discussion why some binoculars appear to have a greater depth of field than others, even when the lens equations won't account for any difference in depth of field. As noted above, there is more to perception of depth of field than just mathematics. In this case a strong correlation is shown between the aberration Field Curvature and Depth of Field. A lens that has strong Field Curvature has a shorter focal length in the center and a longer focal length towards the edges. This has the affect of making closer objects apppear in focus if the center is focused on a distant object and the closer objects are viewed further out in the field of view. Hence the binocular has a greater depth of field, but only for nearer objects.

### #27 EdZ

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Posted 10 December 2004 - 06:42 PM

This link was set up so all members could add links, all in one thread, to their own observing reports. By linking here to that thread, we've got a permanent access for you to get back to the thread that shows all of our Observing Reports. We add new links to new Observing Reports in this post. Here you can access the many Observing Reports from our members.

Now that the above thread handles all the observing reports, the rest of the links here will be to Observing Lists or links for lists. edz 7-19-07

Preparing to Run the Messier Marathon?
Messier List with 20x80 binos?

Observing Satellites

unusual use for binoculars

Links to Sattelite Observing - TiPS

DEEP SKY
Binocular Deep Sky Objects in Groups
a list of 100+ objects in about 40 groups, generally seen in binoculars in groups of two or three together. Like cluster pair M46 and M47 or galaxy three-some M65-M66-NGC3628. Some notes included giving an idea how much binocular it will take to see the targets. Also, some links to charts.

DOUBLE STARS
Binocular Doubles Seen in Pairs and Binocular Doubles within Clusters
about 30 doubles for various sizes binoculars, uniquely identified with another as a pair of doubles or apparent as a double within a cluster.

Binocular Doubles - List of about 75 Doubles showing magnitudes and separations
While the Trapezium is probably the most famous and decidely one of the most interesting multi-star systems to observe, there are many other double stars that can be observed with binoculars. This list has grown from the original list of 60 to about 80 doubles for binoculars. Some are very challenging even for large 100mm binoculars with variable power eyepieces.

Faint Binocular Doubles
a list of twenty mag 7, 8 and 9 near equal pairs, mostly for 20x80 or larger binoculars. Probably 5-7 for 15x, 12x and 10x binoculars.

How many stars should I be able to make out in the Trapezium through 11x56 obies?
For the answer to this question see the discussion at this link to the Binocular Forum
The Trapezium in Binoculars

Challenging Doubles for High Power Binoculars doubles mostly from 10" to 7"

Ten Binocular Doubles seen with 15x70s

COLORFUL DOUBLES

Ten Easy Colorful Double Stars see spreadsheet attachment

Common Doubles with Significant Color Contrast

THE SEARCH (for colorful doubles) GOES ON a list of 20 Doubles

LIMITING MAGNITUDE M45 - Binocular Observing Chart
See if you can observe stars down to mag12 using this chart and a table of over 250 identified stars. There are some asterisms identified on this chart. They are referred to in the post above "Binocular DSO Groups".

SURFACE BRIGHTNESS
The ability to Observe Deep Sky Objects will not always be indicated by the magnitude of the objects. For extended objects, Surface Brightness comes into play. Visual magnitude (Mv) of a deep sky "extended" object is almost always NOT the appropriate measure of how faint an extended object will appear. Determining which faint extended objects, such as galaxies, might be observed is complicated by the fact that books generally list the visual magnitude (Mv). That would be how bright the object would appear if all the light could be compressed in a small spot only 1 sq arcmin. More often, a better indicator of whether or not the object might be seen is Surface Brightness (Sb), which is usually much fainter than Mv, and even that can vary. Read this brief explanation.
See a preface on Surface Brightness in
Deep sky limits on 25x100s

Some Notes on Surface Brightness
If you took an object that is mag 8 visually (in a 1 arcmin area) and you spread the light out over an area of 10x10 arcminutes, or an area of 100 arcmin sq, then the Surface Brightness (per area) would drop by a factor of 100x from the visual magnitude. That equates to a drop of 5 magnitudes or now Sb=13. But few objects have equal brightness gradient over the full diameter. So, while we could say the entire object area aveage Surface Brightness = 13, if there is a brighter core, say Sb10, then the outer fringes might be Sb16. So usually we see only the core, and even then, that depends on the Sb of the core.

Does aperture rule in bino land?

Deep Sky Observing with 70 , 80 and 100mm Binocs

Tonight's objects with 25 x 100 IF Oberwerks

edz

### #28 EdZ

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Posted 16 December 2004 - 12:08 PM

Bill Cook explains the facts behind fixed focus and what value or what little value they really offer.

Fixed Focus Binoculars

### #29 EdZ

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Posted 29 December 2004 - 07:30 AM

What are the Issues with Handheld Binoculars?
Handheld Binocular Observing Issues
not only weight, but magnification is a major issue

Resolution mounted vs handheld
This is a resolution comparison of four binoculars; a 15x70, a 12x50, a 10x50 and a 8x42. I wanted to find out how resolution changes for mounted binocular versus a braced supported binocular, braced elbows and un-braced handheld.

Small Binoculars - Resolution
here's a list of 30 small binoculars from 7x to 12x. Both the best mounted resolution and the best hand held resolution are lists. Also listed is the percent drop in resolution from mounted to handheld. It's pretty consistent. Two thirds of all binoculars tested dropped 40%-50% in resolution when hand held, and the rest were either a step above or a step below that range.

50mm Binoculars handheld vs mounted
Here's a few comparative notes out of some recent observations. Here you can compare, first the difference in what a binocular can see handheld vs mounted and second, some differences between 7x50 and 10x50. Finally you can see a difference between the two 10x50s.

How to Hand Hold a Binocular
This thread has some users notes on methods for best results

Handheld holding technique to dampen vibration?

But visit this site by forum member Stephen Tonkin for the best explanation of various hand holding techniques

How to Hand-hold a Binocular

### #30 EdZ

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Posted 30 December 2004 - 12:07 PM

From time to time we go through this. It is generally a hard lesson for people to learn, but with few exceptions the results are usually the same. You get what you pay for.

Don't expect much from an \$11 or \$29 pair of binoculars when that cost is not much more than what it might really cost to just produce the objective lenses. When you think about it, binoculars are a complex instrument comprised of aligning the light path from the objective thru a pair of prisms on each side and keeping it all aligned as it passes thru two eyepieces and enters two eyes. How many of you purchase \$11 eyepieces for your telescopes?

So just to remind you of the "you get what you pay for" mantra, here a a few of the recent adventures of some on this forum. Are you feeling lucky?

edz

Barska 15x70 Nightmare a deal is not always what it seems.

good price on Meade Tarvelview not when they end up in the garbage!

Mixed opinions about these from various users More on the Meade Travelviews. Some won't go this route at all but a few people who have done so got lucky and are happy.

Meade Travelview 12x50 for \$19.99 any thoughts? Most users thoughts are, if you don't expect much you won't be disappointed. As some who have gone this route point out, it's pretty much hit or miss.

Super Zoom 20-125x binoculars !

Cheap zoom binculars?
"would they be any use"?
"Oh definitely -- as paperweights, window ornaments, something to hurl at the neighbour's dog when it attempts to foul your lawn ... but not as an optical instrument". Stephen
"AND, if they very expensive ZOOM binoculars, just expect them to be expensive junk". Bill

Things to watch out for in cheap binoculars

### #31 EdZ

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Posted 15 January 2005 - 09:29 PM

We got this question not long ago from an interested forum participant.

“I was wondering if someone could point me to a web page that describes how to adequately test binoculars in a store before buying?”

Lots of people responded. This is a collection of the advice that was given. There was other discussion.. You can see the original thread here.
Binocular Testing

There is no doubt that some things cannot be checked in a short period of time. Personally, I'm of the opinion that to check out a binocular takes weeks of use. Some things just don't come out at you until you've used it for a while. It takes quite a while to really evaluate a pair of binoculars.

If you are looking at binoculars for astronomy, then I think a respectable business would provide a means for you to somehow have a look at night.

I think the best you can do in stores is to judge the brightness and resolution and to see if the ergonomics are to your liking. A \$1 bill taped to a wall 15 ft or more distant can be a way of checking resolution. Larger binoculars like 12x50 need about 25 feet, 15x70 need 60 feet and 20x80 might need 100 feet.

Look at some reflections of lights in the coatings. In the objective lens you should see several reflections and ALL of them should be color reflections, none white. Generally a white reflection indicates some uncoated surface.

Better coatings reflect less light back at you. Look for coatings that reflect the least amount of light back.

Look down inside the binocular through the objective lens. In the circular metal ring that surrounds the prism face, you should not see any prism edges exposed in better binoculars. Also you should not see any sides of the other prisms protruding into the light path. A very small clipping of the exit pupil is often seen in mid-priced binoculars.

Hold the binoculars 6 inches to a foot in front of you and look at the exit pupil, the little beam of light that comes out of the eyepieces. It should be perfectly round. If it is noticably misshapen (oval) in any way, reject it.

Look very closely at the exit pupil for any straight edges cutting into the round exit pupil. Extrememly small edges cut off of the round exit pupil won't do much harm, but in a premium binocular reject any with big edge cuts.

Once more look at the exit pupils. If you see a diamond shape around the exit pupil where the light is dimmed in the diamond shape, these are Bk7 prisms. these will probably show the dimmed view in use.

Check to see if the binoculars can come together narrow enough for the width of your eyes. Inter-pupilary distance, IPD, can range from 52mm to 78mm on some binoculars, but on others only 60mm to 72mm. If you have eyes that are set apart by only 56-58mm, you must spend the time to find a binocular that adjusts narrow enough to fit. If you are endowed with a large nose, you may need a binocular with a very wide setting.

If the binocular is to be used by children as young as 7 or 8, the IPD will need to accommodate very narrow set eyes. Of the hundreds of young kids that I've had out to view, quite a few had a problem at 59mm IPD (Orion Ultraview, Minolta Activa), most were served well with a 57mm IPD. Only very few need a binocular with IPD narrower than 55mm. Two young 4th grade girls out of 13 kids in my recent class needed less than 55mm.

Press against the eyepieces while holding them up to your eyes. If the eyepieces are loose and move in allowing you to change the focus, reject that binocular.

Make sure the right diopter has enough range to accommodate your prescription. Some binoculars do not have enough right diopter adjustment.

Look AT the edges of the body outside of the front objective lens. Look to see that both objectives are recessed from the front of the binocular by the same distance. If they are substantially different it will throw off the diopter settings when you focus and you may not get the use of all the diopter adjustment that you need for your eyes.

Focus at some distant sign. Check the image across the field of view, not only at the very center of the field, but also at the edges and midway between center and edge. How does the image look, clear and sharp or blurry? A binocular should have at least 60% to 70% of the central view clear and sharp. Any less and I would reject it.

Edge sharpness of binos is quite easy to tell when you get a pair under the stars but difficult to evaluate when viewing cityscape in the daytime. Binoculars that seem perfectly sharp in daylight can have miserable off-axis performance on the night sky, even costly premium instruments. Pinpoint star images are a much more critical test for sharpness than anything that can be done in daylight.

Shake the binocular. If you hear anything rattle, reject it.

Must you wear glasses while observing? Check to see if you can observe the entire field of view with your glasses on. If not, there is not enough eye relief.

Even non-glasses wearers should check the eye relief. If you can't see the entire field of view without glasses, pick another binocular.

Observe some very high contrast object, like a black pole against a brightly lit background, or a white post against a dark background. Do you see a lot of color fringing around the edges of the object? Do you find it objectionable?

If you don't like the way a binocular feels in your hands, it probably isn't a good choice for you no matter what the specs indicate. One of the primary advantages of binoculars over telescopes is comfort and ease of use. No matter how outstanding a binocular is optically, if it isn't comfortable for YOU, it's probably not a good choice.

Lighter is not always easier to hold for longer. After you've tried a few different binocs, you will find some that feel good and some that feel not-so-good. Not-so-good will usually get worse the more you use them.

An outstanding binocular has to be excellent optically AND ergonomically.

I've never seen ANY daytime test that gives as precise a collimation check as does checking on a star. Checking collimation at 100-200 ft. is not the same as collimation at infinity.

Certainly you can give this a try. With you eyes about an inch or two behind the binoculars, allow your eyes to let the images in each side go loose and watch to see if they come back together or merge. If collimation is really way off, you can tell in an instant.

Move every hinge and turn every dial, feel for sticking spots. Any bad humps or real sticky spots, try out another pair.

Turn the focus dial so the eyepieces are half way out. Now grab the whole eyepiece bar assembly with your fingers and try to rock it. Does it require some force to make it rock, or does it seem flimsy and rock in and out real easy? Reject flimsy.

If you don't know the differences between what the fields of view (fov) are like, then you need to try about three different sized pair with different fov. Put one edge of your view on the exact same spot for each binocular and make a note of how far across the wall it can see. Compare with the others. Every 1° field of view is 1.75 feet across at a distance of 100 feet.

You can test for pincushion distortion on a distant high-contrast sharp line, distant TV antenna or power lines or a vertical post and moving it to the edge of the field. Look for a curve in the vertical or horizontal edge. This is not necessarily bad for astronomy, but might be a reason to reject if terrestrial use is your priority.

While looking through the binocular, see what other items are for sale in the store. If the shop is full of items such as thermos flasks, thermal socks, walking boots and cheap compasses, you are probably in the wrong store.

For waterproof binoculars, it can be very useful to take along a bucket full of salty water. Ask the assistant to dunk the binocular in it for 3 minutes. Then remove it and see if there are any signs of water ingress.

### #32 EdZ

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Posted 10 February 2005 - 11:52 AM

These links will take you to the several POLLS we have conducted here in the Binocular Forum. You can participate in the poll, or if you have already taken it, you can check out the results as of today. You may add comments/replies to the Poll threads.

edz

Poll: How many binocular forum visitors?
The binocular forum has a number of regulars. But, how many visitors do we get? Perhaps some non-members will join CN just to cast their vote. thanks, edz.

Poll: Binocs - Image Stabilized vs Conventional
What do you use for handheld? At what size do you use a mount?

Poll: Did You Use our Best Of Forum Resources?
Share your opinion with us if you find the "Best Of" links useful.

Poll : Largest Handheld Binocular
the next three sizes added together are still less than 10x50s

Poll : How Many Binoculars Do You Own?
not surprisingly, most people own 1-2-3 or 4 pair.

Poll: What Size Binocular Do You Use Most Often?
the three most commonly used sizes in order are 10x50, 15x70 and 8x40/42
the three most commonly used sizes in order are still 10x50, 15x70 and 8x40/42

How Big are Your Eye Pupils?
most people who responded picked eye pupils 6mm or larger.
Hmmm, not the typical bell curve results we would expect to see.
Also sort of flies in the face of decades of medical studies. salt anyone.

### #33 EdZ

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Posted 10 February 2005 - 11:53 AM

This is your first stop for a comprehensive explanation of Binocular Summation, how binoculars, telescopes and telescope/Binoviewers all compare and the benefits that can be derived from each.
Binocular Vision Summation
This comprehensive thread includes a compilation of all I have written on Binocular Summation. In addition several noteworthy posts by other members add some imporant information on vision.

Northeastern State University College of Optometry, Tahlequah, OK
Vision Science course module by Thomas O. Salmon, OD, PhD
Vision Science Home
Salmon's Current Lectures
Old Lectures - Binocular Vision Series
Lecture 10 Binocular Summation

Predicting Binocular Visual Field Sensitivity from Monocular Visual Field Results
(Investigative Ophthalmology and Visual Science. 2000;41:2212-2221.)
© 2000 by The Association for Research in Vision and Ophthalmology, Inc.
Jacqueline M. Nelson-Quigg1, Kimberly Cello1 and Chris A. Johnson2
1 From the Optics and Visual Assessment Laboratory, Department of Ophthalmology, University of California, Davis; and 2 Discoveries in Sight Research Laboratories, Devers Eye Institute, Legacy Health Systems, Portland, Oregon.

Older threads that helped generate some of the information that eventually got included above.

The question gets asked, "What scope would provide the equivalent of the view thru a given size binocular?" You will find the answers here. Along with that you will find some links to some great information on the internet that will help you understand the concept of Binocular Summation. Also see the thread here at the top of the binocular forum that provides links to internet articles for more links to info on binocular summation.

Informal Shootout: 5" APO vs. Saturn III this more recent thread covers the topic in detail

Binocular Summation - Aperture Equivalency
This thread discusses what equivalent aperture would be needed in a mono view scope to produce the same amount of light and contrast as a binocular.

Binocular vs Binoviewer
in this thread the comparison takes a twist. Comparing to a binoviewer, you need to account for beam splitting and then the resultant binocular summation in the binoviewer.

This new thread Jan 2007, gives data collected from three different models binoviewers and shows how clear aperture restricts field of view. A summary is given for lowest power acceptable field of view for binoviewers and then that is also give as a binocular equivalent.
Binoviewers and Binoculars (equivalents)

Wikipedia explanations of Binocular Summation

### #34 EdZ

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Posted 28 March 2005 - 11:05 AM

This is a collection of Binocular and Binocular Mount photos posted by members who have established a Personal Photo Gallery and have linked their photos into the Binocular Photo Gallery

Binocular Photo Gallery

edz

### #35 EdZ

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Posted 03 May 2005 - 06:45 PM

These observing charts were created for studying limiting magnitude in binoculars and in small scopes. Some of the charts have from 100 to 250 stars labeled and identified with an accompanying table listing all the star magnitudes. Some charts go to mag 13.0, some only go to mag 12.5, but you will find that really goes deep for binoculars. As additional charts are developed they will be posted here.

edz

Chart of M44 to mag 13 (The Beehive)

no table of magnitudes yet for this chart

Chart of Cr399 to mag 12.5 (The Coathanger)

Table of stars with magnitude listed for this Cr399 chart

Chart of M45 to mag 13.0 (The Pleaides)

table of stars with magnitude listed for this m45 chart

### #36 EdZ

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Posted 02 June 2005 - 06:46 PM

This question came up recently and as I looked back through the forum history it comes up repeatedly. This space will soon be filled with links to all the great suggestions and book reviews provided by the outstanding contribututors to the binocular forum.

edz

the newest binocular book on the market
Binocular Astronomy by Stephen Tonkin
released December 2006. Stephen is a regular in the CN binocular forum, and a presence in Astronomy, earlier I'm sure, but at least since I've been reading his stuff on the web, since 1998. This book spans the spectrum from beginner to experienced binocular user.

single best book for a binocular astronomy beginner there probably isn't just one, but this thread will help you by asking what are your needs and will highlight a few depending on what they are, learning, deep sky, science or repair.

Well then, next clear night I'll point at Sagitarius. (Where is that, exactly?) A few suggestions to help you find your way around.

Southern binocular objects... need some ideas Carsten Doehring suggests The Observers Sky Atlas by E. Karkoschka

Binocular observing books? this thread is a goldmine of contibutions by many members of the forum. Books and Charts for observing.

I would give strong recommendation to two books.
I recommend both Craig Crossen's Binocular Astronomy AND Norton's Star Atlas, not either/or.

Crossen's will give you:
a selection of objects specific to both large and small binoculars;
some very good detail localized maps, but not a full set of charts;
an outstanding explanation of the structure of the Milky Way;
a presentation of astronomy by seasons;
some excellent black and white photos close to what you see thru binoculars.

Norton's will give you:
A set of full sky charts, the Bright Sky Atlas, but no localized detail charts;
detailed lists of objects on each of those charts, some beyond binoculars;
some excellent technical explanations of the physics of optics;
some excellent technical explanations of astronomy, galaxy types, star evolution, etc.;
some detailed moon maps.

Books for Lunar Observing

and

"Modern Moon"

Star atlases suitable for binocular astronomy

Binocular Stargazing "Reynolds gives a very good overview of binocular observing" but seems to be a little light on stargazing info.

Observing Lists
"110 extraordinary objects to see with an ordinary pair of binoculars"
This link, provided by John Flannery, from the Irish Federation of Astronomical Societies (IFAS). The binocular handbook features a half-page about each object along with a chart of the field. The introduction says that you must be a member of an IFAS club, however they dropped this requirement. Feel free to get in touch with any errors or comments. Also, the handbook can be freely circulated. See comments from the author here.

Technical References

Choosing, Using & Repairing Binoculars by J. W. Seyfried, president of University Optics. Has some good, but also some outdated, information on repair maintenance and adjustment of binoculars. Has good explanations and diagrams to help the amatuer understand binocular assembly and adjustment.

BASIC OPTICS AND OPTICAL INSTRUMENTS prepared by the Naval Education and Training Development Center. recommended by William J. (Bill) Cook, Chief Opticalman, USNR-Ret., Founding Editor, Amateur Telescope Making Journal
The book has a great section on understanding basic optics principles. Other than that, the very short bino collimation section may very well be all that interests you. It is full of metal work, what kinds of greases to use for this or that, and a bunch of boilerplate that pertains to Navy instruments that haven't been made in 40 years. Please keep in mind that the collimation section deals only with instruments that are collimated with eccentric rings.

MIlitary HandBooK 141 - Optics, Vision and Optical Design
this arrangement of PDF files is hosted by the University of Arizona and represents an immense resource on Optics and Vision.

### #37 EdZ

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Posted 25 August 2005 - 09:50 AM

Red Light vs Green Light

Pupil Size

technical paper by C. J. R. Lord FRAS
a technical paper that plots reduction in percentage illumination as affected by eye pupil smaller than exit pupil and reduction in percentage illumination as affected by eye pupil larger than exit pupil.

technical paper on optics and vision
this in a huge reference work published by the DOD. Here is a link to the 630 page document hosted by UofA Optics Department

Poll: How Big are your Eye Pupils?

### #38 EdZ

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Posted 29 October 2005 - 10:41 AM

CHROMATIC ABERRATION (CA)

Achromats and False Color Blur Circle

The visual appearance of the ratio of color blur is low limit eye dependant. While color blur is always larger than the low limit of the eye, the resolution disks are almost always limited by the eye. Therefore in all cases we need to use the low limit of the eye for resolution.
there is a lot more visible color in a f/4 or f/6 100mm binocular than there is in a 8x40 or 10x50 and to some lesser degree even a 15x70. That's because the combination of small aperture and low power has a dramatic affect on reducing color blur.
for any given aperture, higher f# (a slower scope) provides better color correction. Most knew that. But what is not so readily known is that small apertures have very little effect on false color, or color blur as it is referred to here. An 8x40 achromat binocular has the same color error as a 120mm ED scope at 25x. The color blur in a 7x32 or 8x32 achromat, although still not apo quality, is extremely good without any further manipulation of the optics, the 7x32 being better than and the 8x40 or a 120mm ED semi-Apo scope and the 8x32 being equal. It doesn't take ED glass in the 8x40 or 7x32 to get that low color blur ratio.

The most important thing the user should understand is that it is impossible to completely eliminate CA from a lens system. It can be reduced to an almost imperceptible level, but cannot be eliminated altogether.

This is an explanation of what chromatic aberration is and how it affects the overall image
Chromatic Aberration

This explains some of the math of CA
Chromatic Aberration on planetary disks

Chromatic Abberation is one of the least worrisome aberrations. In an astro binocular, it is only present in about 1% of the objects viewed. So it should be one of the least critical aberrations on which to base a decision. Judge your choice of binocular on all the features and known aberrations that make a difference in the view 99% of the time. Honestly, the planets are not binocular targets, so the moon is the only astro target that would be a consideration for judging whether one needs to consider CA in the choices of astro binocular.

For terrestrial viewing CA becomes an important factor. CA becomes a problem in extremely bright conditions, not in low light conditions. However, there are not too many people using 15x or 16x binoculars for terra viewing.

A Table Plotting Chromatic Aberration vs Objective Size and Focal Length
see this interesting post in the refractors forum that discusses CA.. From the post...
" It illustrates the relationship of aperture and f/ratio to displayed chromatic aberration in achromatic refractors. Roughly, telescopes with similar CA ratios can be expected to display similar levels of visual CA; as CA in achromatic systems is a function of objective diameter and focal ratio.

Using this chart, one can get an idea of how another refractor will perform based on the performance of whatever model one already has. "

A discussion of CA in premium and well corrected optics

A discussion related to chromatic aberration in several model binoculars took place embedded in this thread.
Obie vs Fujinon CA

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There are two different forms of CA

Logitudinal CA is caused by the fact that, even with precisely on-axis light, in the focused image from an achromat all the wavelengths do not have the same focal length. Typically light in the image of an achromat varies from blue to red focal length by 1/1800th to 1/2000th of the focal length of the objective lens. So, for a f/4 100mm binocular, the focal length of red to blue might vary by as little as 0.2mm. That may seem inconsequential to you, but that might represent a 15°-20° turn of a center focus dial on a binocular.

Light that is not focused precisely does not make the minimum sized point of light in the image. If the minimum diameter circles of light for each wavelength is at a slightly different focal length, that means when focused precisely at any one color (usually yellow) that the Airy disks of all other wavelengths (colors) have slightly larger sizes and they overlap the yellow image, forming CA in the on-axis image. This is not generally seen by the observer unless you slightly defocus the image. But rest assured it is there and it has an affect on contrast of fine detail. This may possibly be seen in the inability to bring the focus to a precisely small point of light. An achromat that is not corrected to even 1/1800th f may have a difficult time forming a precisely small image of even a moderately bright point source.

Lateral CA is caused by the fact that the light rays forming the image across the lens all have slightly different lengths to a focal plane but the focus mechanism wants to focus across a flat plane. For all the points of light in an image to be in focus at exactly the same time, our focus mechanism would be required to focus across a slightly curved image plane. That doesn't happen. Since the system is incapable of providing that precision, we see the light spread across that plane at varying amounts of out-of-focus.

If you were to draw several half inch wide concentric circles over the field of view, you would see that in the outer fov ring you see all one color, green, in the next ring in all another color, purple. The color you see is dependant on how far off-axis the image is in the plane. The further off axis, the longer the physical length of the light ray beam to the plane. The varying lengths of light rays causes each color to be out of focus more-so than some other color at different distances off axis. This form of CA is entirely due to where the light falls across the plane.

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This comment recently generated a discussion about Achromat doublets, Triplet objectives, Semi-Apo and Apochromatic lennses, means of achieving better color correction or lower CA.

I've learned what it means with telescopes that are Doublet and Triplet...but how is that terminology used for the 20x80 triplet bino's? are they true apo or semi-apo or something entirely different?

I doubt you will find any binocular anywhere on the market that is a true Apo, whether it is labeled as such or not, unless possibly you are up in the range of several thousands of dollars.

The fact that it is a triplet should indeed lead to some degree of better color correction, but it is not a gaurantee. While it usually takes a triplet to make an Apochromat, triplet is no indication that any instrument, binocular or telescope, is an Apochromat. It simply means that a third element of glass is incorporated into the design to give a slightly better color correction. In a binocular, it is probably still an Achromat.

A typical Achromat has two colors corrected, usually red and blue, so their focal lengths will be within approximately 1/2000th the focal length of yellow. An instrument would need to be corrected to within 1/8000th to 1/10,000th the focal length of yellow to be considered an Apochromat.

Generally the term Semi-Apo is applied to instruments that have been corrected to within 1/4000th F. It is very likely some of the instruments on the market, both telescopes and binoculars, are not corrected to 1/4000th F, and yet are still called Semi-Apo. It could be these instruments are better than 1/2000 corrcted, perhaps 1/2500 or 1/3000. Frankly, any improvement over the level of correction of a standard achromat should be appreciated as it is not particularly easy to achieve and rather expensive to incorporate into the design. I would question if any of the triplet binoculars are reaching a level of correction to 1/4000 F.

ED Glass in Binoculars

edz

### #39 EdZ

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Posted 29 October 2005 - 11:27 AM

Binocular Resolution Testing Using 1951 USAF Line Pair Charts
In addition to test results for about a dozen binoculars, this thread shows some comparative results for a very good semi-apo 80mm telescope. Also included is a discussion on how to use the charts and links to web access for the charts and tables.

See the chart and the tables in my gallery here
USAF 1951 Resolution Chart and Table of Constants
It is extremely important that the chart be exactly sized.
The caution here is the user must know exactly how to size the chart.
You will find sizing notes below in this post and also in my gallery.

Source for commercially available resolution charts
Edmund Optics

Conflicting Arcsecond Test Results Explained
an excellent discussion explaining the differences in various arcsecond resolution values posted various places around the internet, and how to interpret them. This discussion is relevant to understanding how to use the USAF resolution charts and how the values determined by using them is so different than point source resolution. It explains why resolution values determined by the USAF charts are significantly closer than can be achieved observing point sources.

Understanding Resolution
a comprehensive technical report on the various criteria affecting resolution

Double Stars for Binoculars - a list of doubles for all sizes from 7x35 to 40x100
a list of double stars from which you can select observation targets providing you with good indications of the maximum visual point source resolution of your instrument.

Lens Resolution Testing
and a link to a USAF 1951 Chart with the Table of Constants.
Chart with Table of Values

I find it interesting that few if any current web links provide ALL the information necessary to use these charts.

Using this table of values linked here which shows the constants for the Groups and Elements of the Line Pairs Chart,
the forumula is 8121 / (D x LPM) where D is the distance to your target measured in inches and LPM is the value from the Constants.
The constants are the actual number of line pairs per mm for the mimimum you can see.
For instance a 10x binocular used at a distance of 125 feet that can see line pairs in Group-1, Element2 has a value of 0.561. Therefore it's resolution is 8121/(125x12x0.561) = 9.65 arcseconds.

Line Pairs Resolution CANNOT be directly compared to point source resolution. See the forum discussion.
It is extremely important to have the target sized properly. See the discussion in the forum thread.
Binocular Resolution Testing Using 1951 USAF Line Pair Charts
included is a discussion on how to use the charts and links to web access for the charts and tables.

It is extremely important that the chart be exactly sized. As a check, the three bars and two spaces in Group-2 Element 1 should measure exactly 10mmx10mm. Both the sets of bars in G2-E2 and the solid black square top center should measure 9mmx9mm.

http://medfmt.8k.com...resolution.html
Lens Resolution Testing by Robert Monaghan
which has posted the following notation along with the information
[My understanding is that the USAF chart etc. is in the public domain as it is a government produced chart and related documentation...]

### #40 EdZ

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Posted 29 October 2005 - 11:28 AM

Limiting Magnitude Articles and Resources

Limiting Magnitude tests on M45 - More than a dozen binocular sizes
this thread has links to charts fro M45, M44 and Cr399. it also has the record of the best observations I've recorded with dozens of binocular attempts at reaching BLM.

Bino Magnitude Efficiancy - Handheld vs. Mounted

Binoculars Limit Star Magnitude with Small Scopes Comparisons

Call for Participants in Binocular Study of M45
This post includes links to charts you can use on M45 and provides an explanation of this study.

Some results from the initial Limiting Magnitude Study and a brief summary of some of the factors that will affect LM results

First Study of Limiting Magnitude on Cr399

Limiting Magnitude in Binoculars - Lab Report by EdZ
Explains the importance of magnification to achieve limiting magnitude.

Naked Eye Limiting Magnitudes in Taurus
includes a broad selection of stars identified from mag3 to mag6.5

NELM chart Little Dipper and Polaris

NELM chart area around Sagitta, M27 and Cr399

NELM chart of Delphinus

NELM chart Circlet of Pisces

Binocular Limiting Magnitude chart to mag 13.0 in M45 - The Pleaides
includes Naked Eye Limiting Magnitudes from 2.8 up to mag 7

Binocular Limiting Magnitude chart to mag 11.0 in CR399 - The Coathanger
includes Naked Eye Limiting Magnitudes from 5.1 to 7.3

Binocular Limiting Magnitude chart to mag 12.5 in CR399 - The Coathanger
same chart as in previous, updated to include deep magnitudes for BIG binoculars 100mm and larger,
or for use in mag6+ skies where even 20x80 binoculars might be able to go deeper than mag 12.0

Limiting Magnitude chart of M44 - the Beehive magnitudes not listed but symbols provide a guide from brighest to faintest stars

This post in the binocular forum has links to the above charts, tables of star magnitudes for use with the charts and reports on some data collected with many binoculars comparing the limits of magnitude that can be reached with various sizes of binocs.
Limiting Magnitude Charts and Tables

Southern Hemisphere Several LM Charts

See these threads for discussions of Surface brightness

Does aperture rule in bino land?

Deep Sky Observing with 70 , 80 and 100mm Binocs

Tonight's objects with 25 x 100 IF Oberwerks

edz

### #41 EdZ

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Posted 29 October 2005 - 11:29 AM

To have your review published in the CN Reviews (and not just posted here in the forums) attach a word document and send it to submit@cloudynights.com

How to Write a Binocular Review
this thread starts by explaining that format and grouping of content is important to get your point across. Then it provides a list of links for reviewing many of the aspects that can be measured and reported as important information in a binocular review.

This is a list of suggestions on aspects you can cover in a review
Topics List for a Mini Review
this thread contains the history of the start of the MiniReviews.

And this list prepared by Milt Wilcox,
Astronomical Binoculars Figures of Merit
is a another very good crib sheet of things to consider when writing a review.

Read the CN Editorial Commentary here.
It explains that CN as a whole wishes and strives to remain fair-handed in its reviews.

And see these brief guidelines for submitting a review to the CN Reviews
We ask all submissions to agree to some simple guidelines

Here is an article published in the CN Reviews on
How To Write a Product Review

The fact that some people question how or why they should attempt to write a review is not new.
Look at this discussion that took place well over a year ago.
Don't Forget How Cloudy Nights Got it's Start.

edz

### #42 EdZ

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Posted 07 March 2006 - 09:52 AM

These links have been moved here to their own library card, as previously they were buried in the Binocular tests link and were quite a bit more difficult to find. The resources provided here are far more worthy than to be buried somewhere.
edz 3-7-06

The following links to posts, compilments of Arek, are the shared information provided to the CN binocular forum on all the work that has been published in what we now refer to as the Polish Binocular Reviews. From Arek's first post on this subject, I'd like to mention that credit is due to the following organizations and people. thank you Arek. edz

Our tests are made by the group of people connected with the Polish
Comets and Meteors Workshop (CMW), Nicolaus Copernicus Astronomical
Center in Warsaw and Warsaw University Observatory. The people belonging
to this group are: Michal Jurek, Lukasz Kowalski, Krzysztof Mularczyk,
5-7 persons and their marks are averaged.

The first post is a list of tests and descriptions of what might be considered closer to labratory tests of binoculars.
Testing methods in Polish tests of the binoculars
the methods are those employed by a group of people mentioned above. Some of these procedures cannot be undertaken by the average observer without the use of digital equipment.

The actual reviews can be found here at this website showing the results of the tests

Arek has provided links here to a number of other reviews/test results

Arek has translated and posted some of these reviews here to the CN Binocular Forum. Rather than point you to each individual review, I will point you to a link page showing all of Arek's posts so you can search out any of those reviews you would like to read in english.

Also see this post for more comments on this work.please refer to this post about Polish binocular reviews for links to Arek's work.

### #43 EdZ

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Posted 17 March 2006 - 04:00 PM

Surface Brightness of Faint Extended Objects

A Preface on Surface Brightness

Finding book references for Observing of faint extended objects such as galaxies is complicated by the fact that books generally list the visual magnitude, but a better indicator of whether or not a faint extended object might be seen is Surface Brightness (Sb). Even that is not the best indicator of how easy it will be to see as you will see. But at least read this brief explanation to gain some understanding of how Surface Brightness works.

Visual magnitude of a star has all the light concentrated to a singular point. Not so for extended objects. Extended Objects have varying sizes, some can be very large. Visual magnitude of an extended object would be the magnitude you would see IF you could compress all the light of the object to a size of 1 arcminute area, about the size of M57, the Ring nebula. Surface Brightness of an object and size together gives an indication of how spread out the light is and how faint it will really appear. An object that has a a Sb = 12.0 with a diameter of 10 arcminutes is going to be much easier to see than an object that has a Sb = 12.0, but has a diameter of 25 arcminutes.

The formulas we use to find what limiting magnitude an instrument might reach do not always tell us how well an instrument will perform on all types of objects, especially faint entended objects, which have a whole different set of requirements. While total light gathering (usually measured by limiting magnitude of stars) is important, to differentiate a low surface brightness object from the background sky takes good contrast.

If you have an object that is mag 8.0 visually (in a 1 arcmin area) and you spread the light out over an area of 10x10 arcminutes, or an area of 100 arcmin sq, then the Surface Brightness would drop by a factor of 100x from the visual magnitude. That equates to a drop of 5 magnitudes. Therefore it would have an average Sb of mag 13.0.

Likewise, if an object surface brightness is given as Sb 15.0 and its area is given as 17'x15' or 255 square arcmin, we can figure its visual magnitude for an area of 1 sq arcmin. What magnitude corresponds to 255x brighter? Well its easy enough to get to 250x brighter. 100x is 5 magnitudes and it's 2.51x for each magnitude, so 251x is almost exactly 6 magnitudes. So 255x the area would be close to 6.0 magnitudes difference. Of course, you could never condense the object to a size of 1 arcmin, so it's not as useful to calculate Mv. It's much more useful to have Mmv given and know how to calculate the fainter Sb.

edz

See these threads for discussions of Surface brightness
Does aperture rule in bino land?

Actually no it does not. Dark Skies Rule. Here's Why
An offsite paper by Bill Ferris explaining contrast threshold
Lowering the Threshold

Deep Sky Observing with 70 , 80 and 100mm Binocs

Tonight's objects with 25 x 100 IF Oberwerks

Rain Delayed Observing

An Explanation of Contrast Ratio by Bill Ferris

Surface Brightness (Sb)

Visual magnitude of a deep sky "extended" object is almost always NOT the appropriate measure of how faint the object will appear. Observing faint extended objects such as galaxies is complicated by the fact that books generally list the visual magnitude, but a better indicator of whether or not the object might be seen is Surface Brightness (Sb), and even that can vary due to being brighter towards the middle and fainter towards the edges.

For an object like M101 that has a visual magnitude of mag 7.7, but an area of diameter 26 arcminutes, that light is spread out over 530 square arcminutes. Hence is has a very low Sb = 14.7. That is averaged. As you get out towards the extremities of the object, it is fainter, in towards the center it is generally brighter. So for instance a galaxy with a bright core might be visible, but it would appear much smaller than its full size because you can see the core but not the extension.

M101 has some brightening towards the core, so the core area actually has a little brighter Sb than 14.7, while the extremities are fainter than the average Sb of 14.7. Another example is M33 in Tri, at Sb 14.0 it's fairly easy. This one also has a broad brighter core, so in these cases we generally see just the brighter core area and it's generally brighter than the average Sb that's listed.

******************************************
Occasionally, someone comes along and asks, "Why can't I see this galaxy, the book says it is mag 9, that should be easy enough?" The answer lies in the definition of Surface Brightness. Surface brightness will be referred to as Sb.

The criteria we use to determine the limiting magnitude (faintest stars seen) of an instrument do not always give a good indication how well an instrument will perform on all types of objects, especially faint entended objects. These have an entirely different set of requirements. Total light gathering (aperture) is important, but, to differentiate a very low surface brightness object from the background sky takes good contrast. Lower surface brightness objects have very little contrast with the background sky, hence can be very difficult to see.

The magnitude scale works like this. A difference of 5 magnitudes (from mag1 to mag6) is a difference of 100x light, brighter or fainter. Therefore, each magnitude is 100 to the 5 root from the next. That is, one magnitude is approx 2.51x brighter or fainter. So 1 mag difference is 2.51x, a 2 mag difference is 2.51 x 2.51, and so on.

For extended objects, (any object that produces an image larger than the Airy disk of the aperture), the visual magnitude Mv, is given as the magnitude the object would appear if all of the light from the object could be condensed into an area 1 square arcminute. Almost all extended objects have a size much larger than that. For the purposes of figuring Sb, each increase in area of the object by 100 to the 5th root (or 2.51x) will result in a decrease in the apparent brightness of the object by 1 magnitude.

Let's use an example of a DSO listed as visual mag 9. If the object size is 2.5'x2.5' then it has an area of 6.25 sq arcmin. The light would be spread over an area 6.25x greater than the compressed area used to determine the Mv value. It would actually appear 2.5x2.5, or 6.25x fainter than the visual magnitude. From above, we know a light difference of 6.25x is equal to 2 magnitudes, so the Sb of this object would be Sb = 9 + 2 = Sb mag 11.0. BUT what if the object size is 10'x10', then it has an area of 100 sq arcmin. The light would be spread over an area 100x greater than the compressed area used to determine the visual magnitude measurement. It would actually appear 100x fainter than the visual magnitude. A light difference of 100x is equal to 5 magnitudes, so the Sb of this object would be Sb = 9 + 5 = Sb mag 14.0.

The second concept that must be considered is brightness gradients. Look at the photos of almost any galaxy or globular cluster. Often the central core is much brighter than the outer fringes. Our 10x10 example object might appear with the central 50 sq arcmin at Sb 13.0 and the outer edges 50 sq arcmin at Sb 15.0, for an "average" of Sb 14.0. What we would see is the brighter central area. We may not see the outer fringes at all.

Some example galaxies as observed in 25x100:

Using 25x100 binoculars, I have observed several faint extended galaxies that have little to no brightening of the core (meaning the core was not significantly brighter than the spiral extensions). Many of the faintest Surface Brightness galaxies are face-on. Under mag 5.4-5.7 skies, in approximate order of difficulty, some I was able to see were:

M 33 in Tri has a visual magnitude (Mv) of mag 5.7 but it has Sb = 14.0. Its size is a very large 62'x39'. The area of this galaxy makes its Sb about 9x fainter than its Mv value. But it's actually fairly easy to see! Why? because it has a broad brighter core and wispy faint extensions, so we only see the brighter core.

NGC 3628 in Leo near M65/M66, at Sb 13.5 was pretty easy, more edge-on than face-on, it measures 4'x15' and has a bright core.

M 95 in Leo is 7'x5' and has Mv about 9.6. About fout magnitude fainter its Sb 13.6 was not as easy, but a small 3' core is brighter, so makes the center easier to see..

M 101 in UMa is Mv 7.7, but has Sb 14.6 and it is not very easy. Its size is 27'x26', but it shows only about 15' dia. with a slight brightening to the core. This means the 15 arcmin diam. that we see is brighter and the outer fringes are much fainter than Sb 14.6.

M 74 in Psc at Sb 14.4 is difficult, weakly brighter center, size is 10'x10', so visual magnitude is Mv=9.0, but this is no indication of how difficult this is to see.

NGC 6946 in Cyg (near Cepheus) has Mv 8.9, but it's size 11'x10', and the fact it has no brighter core at all makes it a very even lit and difficult to see Sb 13.8.

IC 342 in Cam at Sb 15.0, with little brightening in the core, broad face-on wispy galaxy, difficult even in the best conditions. It's 18'x17' means it's Mv is labeled about 6.25m brighter than its Sb. Whereas Sb=15.0, Mv = 8.75.

For me, IC342 has been one of the most difficult galaxies in all the sky for my small instruments. It has a surface brightness of mag 15.0. As a comparison M74 has a surface brightness of mag 14.4. NGC 2403 has a surface brightness of 13.9, about the same as M33, easier than M74 and much easier than IC 342.

What can you roughly predict from this. In NELM mag 5.4 skies, with 100mm objectives I can gather enough light that I can see faint extended objects with surface brightness 8.0 to 9.0 magnitudes fainter than NELM. Using this instrument under this sky I gather enough light to get enough contrast to see about 9.0 mag fainter than sky. That is the limit of contrast detection I can get.

Either a smaller aperture or a brighter sky may not allow that limit. So, as far as NELM goes, if mine were let's say 4.9, a half magnitude brighter, it would be likely I would lose sight of probably all but NGC 3628.

So, What happens to faint Low Surface Brightness Objects as we vary aperture and magnification?

OK, let's try increasing magnification to darken sky background and increase contrast. This works great on stars since it doesn't decrease the brightness of the stars but it does decrease the brightness of the background sky, which is an extended object, so you get to see fainter stars. However, with an extended object such as a faint face-on galaxy, the problem you will have with trying to darken a bright background sky by increasing magnification is that you have a very narrow range where that will produce a darker sky and yet still provide a large enough exit pupil or enough brightness to the eye to keep the image bright enough for the eye to see the very faint object.

If you increase magnification in hopes of darkening the sky background, you reduce exit pupil and you will also darken the extended light of the galaxy. If you increase the aperture too much without increasing magnification, you increase exit pupil and you will brighten the entire image, both object and sky background, and you may get a washed out image. However if you increase aperture AND increase magnification, maintaining constant exit pupil, you may improve the contrast detection of the faint extended object because you have gathered more light. Of course this will only work when sky is dark enough to permit at least reaching the lower magnitude limit of contrast detection.

So then, 'under brighter local light pollution, magnification increases in importance relative to aperture' would always be true for viewing stellar targets and is usually true also for very bright extended objects but is may not help at all for very faint, extended, low surface brightness objects.

On the other hand 'under darker skies, there is more gain from increasing aperture relative to magnification' is probably always true, at least up to the limit of the eye pupil. Exceptions to this would be very small extended objects that require increased magnification to increase image scale or to reach ODM, optimum detection magnification. (see Binoc Web Links - Mel Bartels).

How are you supposed to know what works? I guess the answer to that is go out and observe as many objects as you can in all kinds of conditions with as many different instruments as you can get your hands on. You'll find out what works.

edz

### #44 EdZ

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Posted 26 January 2007 - 01:46 PM

It is necessary to understand all of this information to get a clear picture of the equivalents from binoculars compared to scopes and finally compared to scopes with binoviewers.

This thread gives data collected from three different models binoviewers and shows how clear aperture restricts field of view. A summary is given for lowest power acceptable field of view for binoviewers and then it is also given as a binocular equivalent. The comparison is made to show lowest power widest field of view available in stock bargain binoviewers as compared to binoculars primarily in the range 70mm to 100mm.
Binoviewers and Binoculars (equivalents)

Binocular Vision Summation - Two Eyes vs One Eye

Binoviewers and Clear Aperture
If selecting one of the 20mm CA binoviewers on the market, not only is there a light loss at the outer edges of the field stops, but also the Maximum True Field of View that you can get will be restricted to eyepieces with field stops about 22-23mm max. You can purchase a more expensive binoviewer with a 26mm or more clear aperture to get wider BV field of view.

Is there a scope that's best for binoviewing?
This post I put in the Binoviewer forum starts out " A few things you should know about binoviewers and scopes". I notice people in discussions about their binoviewer equipment have a few misconceptions. What I try to do in this post is explain not only how clear aperture affects the field of view, but also how the various attachments to the binoviewer nose change from the specified magnification factor when used in different scope/diagonal configurations. It is my experience, some people that are using binoviewers with an SCT scope have not attempted to determine what magnification they are using or what they are getting for true field of view. It is not as simple as assuming all the stated nominal factors and focal lenghts control. This post helps explain some of the above.

edz

### #45 EdZ

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Posted 26 January 2007 - 01:47 PM

What do you recommend for a first binocular? What can I expect to see?

This is not an uncommon question at all, but one that usually should at least stir up another 10 questions. Before anyone can make any recommendations to you, YOU need to answer a lot more questions about yourself and your needs. For instance, What do you intend to See with this binocular? When you have developed some thoughts on each of these questions, and you begin to understand some of these basic things about binoculars, then we can reasonably recommend a binocular that will work for you.

Here’s some of what we don't know about you that would be helpful for making a recommendation:

Do you wear eyeglasses?
If so, will you always wear them when using the binocular? For eyeglasses, consider a binocular with about 14-16mm eye relief. 11-12mm is tight and you will lose some of the field of view. 8-10mm of eyerelief is extremely tight and if you have eyeglasses on, your eyes will be so far behind the proper eye relief point that you will not be able to see a large portion of the field of view.

Do you know your Inter-pupilary distance (IPD)?
Some binoculars are not made to fit people with narrow set eyes. If you have eye pupils that are set only 56mm apart, a binocular that has a minimum of 58mm is no good for you. The Swift Ultralite 8x42 gets down to 51mm, but the Orion Vista 8x42 only closes to 59mm. All the Nikon Action Extremes get down to 55mm. But some of the larger 20x80 and 25x100 binoculars have a smallest (closest) setting of only 60mm. Most children cannot use binoculars that don’t get down to 56-58mm.

Do you know how large your eye pupils are?
Under dark conditions, if your eye pupils open to 7mm, you would be able to use the full exit pupil from most all binoculars. BUT, if your eye pupils don't get any larger than 4mm or 5mm, it won't do you any good to carry around the extra weight of the glass needed to provide a very large exit pupil. 12x60, 10x50 and 8x40 are all examples of 5mm exit pupil binoculars. 20x80 has a 4mm exit pupil. 10x60 has a 6mm exit pupil. 10x70 has a 7mm exit pupil. 7x50 has a 7.1mm exit pupil. If you have maximum 5mm eye pupils and you use a 10x70 binocular that has a 7mm exit pupil, your smaller eye cuts the effective aperture to 50mm and eliminates all the light outside that. You can pretty easily have someone help you measure your eye pupils in a fairly dark room. An easy method is to use a strip of paper with a series of dot sizes, sliced thru the middle of all the dots.

Will you observe in very low light conditions?
A 10x50 has only a 5mm exit pupil, but an 10x70 has a 7mm exit pupil providing a far brighter image, and that would be useful IF your eye pupils open wide enough to take in all the light. Generally, binoculars with such large exit pupils are used by people who can view from very dark skies. Or for example for terrestrial observing the choice might be between a 8x42 or a 8x56. The 8x56 would give brighter images and be more useful under low-light conditions. Actually, I consider binoculars with 7mm exit pupils as specialty binoculars and I would rarely recommend them to a first time buyer. Pay attention to eye pupils here again. As far as light goes, you can only use an exit pupil as large as your eye pupil.

Do you demand a waterproof binocular, or simply a weatherproof binocular?
There are waterproof binoculars, water resistant binoculars and binoculars that have no resistance to water infiltration. If you plan often to use your binoculars down by the seaside, you will most certainly want the ability to wash off the salt spray. For that you need waterproof. For astronomy it may not be needed at all. However, it will keep moisture from ever fogging up the insides, and that’s a good thing.

Have you any experience hand-holding binoculars?
You need to give some consideration to how much weight you can hold for long periods or how much magnification you can hold for a steady view? My 7x35 Minolta Activa weigh only 24oz. The Swift Ultralite 8x42 weighs about 22 ounces. The Nikon Action Extreme 10x50 weighs in at 36 ounces. The Oberwerk Mariner 10x60 and the William Optic ED 7x50 each weigh over 48oz. Generally holding anything over 10x is not an issue with the weight as much as it is an issue holding 10x power steady enough to stop the image from swimming around in your view. Some 15x70 binoculars weigh only a little more than 3.5#, but at 15x, every little movement of your hands or arms or even your heartbeat, will show up in the magnified image. I’ve handheld my 4#12oz Fujinon 16x70 for brief periods, but I wouldn’t want to use them handheld for a full night. Some recommend the consideration of IS binoculars.

Have you any experience with wide-field versus narrow-field?
Things in motion put more demands on how much field of view you use. 15x70 binoculars usually have a little more than 4.0° field of view. The Nikon SE 12x50 has 5°. Many 10x50 binoculars have about a 6.0° field of view. The Nikon Action Extreme 8x40 has an 8.0° field of view. A 25x100 or 20x80 binocular will generally provide only 2.5° to 3.0°. If you don't know the differences between what the fields of view (fov) look like, then you need to try about three different sized pair with different fov. Put one edge of your view on the exact same spot for each binocular and make a note of how far across a wall it can see. Compare with the others. Every 1° field of view is 1.75 feet across at a distance of 100 feet. You need a 5-6° field of view to fit the Hyades. You need only 3° to fit the Pleiades with room to spare.

Here’s some of what people will talk about that you should know at least a little bit about:

What difference do multi coatings make?
Coatings are a very important product in the transmission of light. In this age of products, no one should purchase binoculars that have uncoated surfaces. An uncoated lens surface can reflect 4% of the light hitting it. Uncoated internal surfaces contibute to more internal reflections and lowered contrast. Magnesium Flouride (MgF) single coating can reduce reflected light to 1.5%. Properly applied multicoatings can reduce that reflectance to 0.5% or 0.25% per lens surface. A binocular may have as many as 14 lens surfaces. With the exception of extremely high level of performace of some superior coatings, even a properly muticoated binocular can transmit at best only about 93% to 96% of incoming light. The same binocular with all 14 surfaces only single coated with MgF may transmit only 79% of the light. Subtract a few tenths of a magnitude from the “how faint” numbers above for any binoculars that are not fully multi-coated.

What’s Chromatic Aberration?
Binoculars can suffer from a number of aberrations but this one is often discussed. Chromatic Aberration (CA) is the result of the various wavelengths of light not reaching focus at the same precise point in the image. CA is a function of refraction. Light from different wavelengths is refracted differently. Blue, yellow and red wavelengths of light reach focus at slightly different points along the focal length axis of the lens. The most important thing the user should understand is that it is impossible to completely eliminate CA from a lens system. It can be reduced, but cannot be eliminated altogether. In an astro binocular, it appears prominently in less than 1% of the objects viewed. So it should be one of the least critical aberrations on which to base a decision, unless of course you are buying your binoculars just to look at the moon. Judge your choice of binocular on all the features that make a difference in the view 99% of the time. Honestly, the planets are not binocular targets, so the moon is the only astro target that would be a consideration for judging whether one needs to consider CA in the choices of astro binocular. For terrestrial viewing, CA becomes an important factor. CA becomes a problem in extremely bright conditions, not in low light conditions.

What’s all this talk about sharp field of view?
Sharpness of field image is deteriorated in the outer field of view by a combination of other aberrations, the scope of which is beyond this discussion, but they are coma, spherical aberration, curvature and astigmatism. Generally (but not always), a wider Afov binocular will suffer from more aberrations in the outer portions of the field of view. Much of those aberrations are contributed by the eyepieces, and it takes a very high quality eyepiece to successfully eliminate the aberrations in a wide field. Think about the cost of the binoculars you have in mind, and then think about the cost of a pair of good 18mm to 22mm widefield eyepieces. Usually, binoculars don’t have the highest quality eyepieces. Therefore, somewhat narrower Afov binoculars, usually in the range of afov 60° and lower, have less aberrations in the outer field of view. Many Pentax binoculars use very narrow Afov eyepieces, resulting in a narrow but extremely well corrected and sharp field of view. It doesn’t do you much good to purchase a wide-field 20x80 with an Afov of 70° if the outer 30% is degraded. You may as well get a 20x80 with a 60° Afov with less aberrations.

Why do these binoculars look the same, but one is so much cheaper?
Probably the most important differences in any two binoculars that look the same but sell for much different prices is (A)the mechanical construction, workings and range of adjustment, and (B)(and very likely more important) the degree to which all the optical surfaces have been figured, polished and coated. There are other things that make a difference in the quality and most of them are not visible on the outside. Usually, cheaper is not just cheaper for no reason at all.

What should I consider for a mount?

Here's some idea about what you might be able to see with some different sizes of binoculars:

How faint are the stars you can see with binoculars?
First judge how dark are your skies naked eye. If you can only see 6 of the stars in the Little Dipper, then you have no better than mag 4.5 skies. If you have skies that allow you to see all seven main stars in the Little Dipper naked Eye, then you have mag 5.0 skies. If you are able to see 9 or 10 stars in the Pleiades naked eye, then you have mag 5.5 skies. That’s pretty decent. Under mag 5.5 skies you can see stars just fainter than mag 10 with a 10x50, stars to mag 10.5 with a 12x50, stars to mag 11 with a 15x70, stars to mag 11.5 with a 20x80 and stars to mag 12 with 25x100. It is very difficult even with the best 70mm binoculars to see stars beyond mag 10 when handheld. Seeing stars at mag 10 requires a completely quieted binocular on a stable mount, viewing without touching the eyepieces. For mag 4.5 skies subtract a full magnitude. For handheld binoculars subtract another full magnitude or even more.

What can some various sizes binoculars see?
Keep in mind, you will not see as much handheld as you will with mounted binoculars. Assuming at least mag 5.5 skies:

In an 8x40, many of the smaller open clusters or fainter globular clusters and nebula such as M76, M26, M9, M10, M14, M71, M18, M1 and M78 are barely seen as a smudge or cannot be seen at all. 8x is a bit low for astronomy and it could be seen readily when compared to 12x or 15x on clusters. However 8x did perform very well on some objects. I easily found M13, M27, M29, M11, M35, M36, M37 and many other open clusters, but M11 was barely a smudge. On M36 you will see the glow of the cluster, but no stars will be resolved. M38, M46 and M67 might be missed. Doubles 20” can be split, but none smaller. Even the smallest binoculars will see the 4 moons of Jupiter and can see bright Titan, although you would have difficulty even telling that Saturn is elongated.

10x50 binoculars can see many open clusters and in a good dark sky will find a large number of the brighter galaxies and nebula. Both M81 and M82 can be seen handheld in a 10x50, but M51 needs a mount. A few bright globular clusters can be seen, but they will be small, however they will appear distinctly non-stellar. M5 can be seen handheld, M10 is seen but very small and M12 was just barely seen. In the Nikon AE 10x50 held braced, I saw M15 as a small but obvious globular. Doubles can be resolved down to about 16 arcseconds. M11 is still just a faint patch. You will be able to see the Ring nebula, but only just barely be able to see that it is a non-stellar spot. The broad galaxy M33 can be seen, but a similarly broad galaxy M101 will not be seen as it is too faint for 10x50 in mag 5.5 skies. Using a 10x60, M78 was just barely detected. The beauty of binoculars like 10x50 with a 6° fov is that objects like the Hyades can be seen all in one view.

12x50 binoculars will capture nice views of M42, M45, M11, M27, M71, Alberio, Sagitta, Delphinus, CR399, and the Cygnus Milky Way. Bright globular clusters can be seen, but they will still be small, however more will begin to appear as obvious globular clusters. Doubles can be resolved down to about 13 arcseconds. You will be able to see the Ring nebula distinctly as a non-stellar spot. 12x50 is still not enough to see M101 in mag 5.5 skies, but M1 easily and M78 just barely were detected handheld using a Nikon SE 12x50.

In 15x70s and 16x70s, objects like the galaxies M65 /M66 can be seen. However M65 is difficult in a 15x70 and just a bit easier in the Fujinon 16x70. The companion NGC 3628 is not be seen in either. Portions of the North America nebula are visible. You can split doubles down to 12", and sometimes 10" with difficulty. M11 takes on the appearance of a glow with a faint star. On the steadiest nights, although you might suspect resolution of all four components in the Trapezium, most people can only positively see three components. On M36, a 15x70 can separate only 4 to 6 stars, while Fuji 16x70 saw about 8 stars resolved. When small and faint M78 was seen with difficulty in a fine handheld 12x50, it was found quickly and easily in the 16x70. M101 can be seen under the best conditions, mag 5.5+ and excellent transparency. Saturn can be seen with a little black space between the rings and the disk.

20x80s are solidly into the range of giant binoculars. Image scale grows. The field of view narrows to about 3°. You see more stars in clusters just due to the increase in magnification. In clusters like M44 the Beehive, an 8x40 will see only about 40 stars, in a 10x50 you might see 50 stars, in a 15x70 maybe 80 stars, but in a 20x80 over 100 stars. On M36, a 20x80 can resolve about 6-8 stars. You can sometimes pick out all four stars in the Trapezium, the closest being 8.7". You can see magnitude as deep as 11.2 to 11.5 and occasionally elongate some not-so-bright equal doubles in the 7" range. Some of the fainter galaxies such as M95 and M101 can be seen on good nights. On a night of mag 5.0 skies, M76, the Little Dumbell nebula in Perseus was not seen in a 16x70 but in a 20x80 it was very faint and obviously not star-like.

In a 25x100 binocular the field narrows to 2.4°. These binocular see stars at mag 12.0. When you view faint clusters in these binoculars you see stars that just were not there in any smaller binocular. On M36, a 25x100 can see about 20 stars. In M44 I’ve counted over 150 stars in a 25x100. Doubles of 7" are clearly separated. M105 and its companion NGC 3384 make a nice pair. M57 is an obvious torus. M81 and M82 were both very easy and bright and both show extension beyond a much brighter core. Fainter galaxies seem to be easy pickings for these 100mm binoculars. While M66 is seen bright, and M65 is difficult in 16x70s, both are readily seen in the 25x100s. In addition, the companion NGC 3628, not seen in any smaller binocular, was visible several times in the 25x100. 25x100s make globular cluster M12 look like it is on the verge of resolution in the outer edges. 16x70s could see but could not resolve M12 at all.

Do you want to scan around and see the sky in general? Do you still need to learn your way around the constellations? If so, get a low power wide field binocular. You might be content with an 8x40, but I’d go for a 10x50.

Do you want to observe mostly open clusters and some of the brighter deep sky objects? Maybe you have no desire to lug around a 8#-10# binocular mounted on a 10# tripod, and sometimes you might prefer to just ahndhold your binocular for a few minutes. A low to moderate power and light weight 12x50 up to a 15x70 is a great choice for this.

Do you want to see both large and small clusters, hunt down faint objects and see extension in galaxies? A 15x70 might do it for you. But if you want the most out of deep sky viewing and you will consider investing in a good mount, then you should be looking for a 20x80 or 25x100 size.

If you're looking for a handheld binocular, then consider this; If you don't like the way a binocular feels in your hands, it probably isn't a good choice for you no matter what the specs indicate. One of the primary advantages of binoculars over telescopes is comfort and ease of use. No matter how outstanding a binocular is optically, if it isn't comfortable for YOU, it's probably not a good choice. If your looking for a large mounted binocular, you must consider whether you currently have enough of a mount or will you invest in a proper mount.

Don't rush into accepting recommendations because someone thinks this or that is great. Consider all the information provided to you, but get a binocular that fits you like a good pair of shoes. When you wear them, they get the job done right, and you can't even tell they are there.

These models mentioned here are not necessarily recommended choices, but in the context of getting to the questions that need answers, they help make the point.

Testing Binoculars in the Store Before Buying

and

These Look The Same, are they?
This post summarizes a lot of the quality differences that might exist among various brands and price levels of binoculars, most of which would never be evident by looking at the outside of the binocular. With this post we try to do our best to provide you the information you need to dispense with the often heard internet myth that they are all the same. It's just not so.

See more on this subject thru this Best Of link
If They Look the Same, What Could be Different?

Affect of Eye Pupil on Binocular Aperture THREAD

Affect of Eye Pupil on Binocular Aperture CN REPORT

What makes a particular binocular good for astronomy?

edz

### #46 EdZ

EdZ

Professor EdZ

• topic starter
• Posts: 18820
• Joined: 15 Feb 2002
• Loc: Cumberland, R I , USA42N71.4W

Posted 26 January 2007 - 01:48 PM

Welcome. Some posts have been assembled here that will help the beginner get going on the right path. For the most part, this post is no different than every other post here in the Best Of thread. It provides you with links to all the relevant information on this specific topic. As you progress in your quest for knowledge, you can go to each of the specific topics in this Best Of section and read much more related to each specific topic.

Thanks to Steve Napier for suggesting this topic.

Thanks also to Stephen Saber for getting us started with this post.
Beginners and First Time Buyers Start Here for links to offsite sources with lists of definitions

I can't tell you how often someone comes new into this forum and asks, "What do you recommend for an all-around astro/terrestrial binocular?" It is our collective experience here that there are questions you need to ask yourself before people can make good recommendations for you. This list of questions has evolved into a brief article that will help the "first time" buyer answer some questions about themselves, their needs and their goals and will help them understand what can be expected from some various sizes of binoculars. See the "Best Of" post here titled
First Binocular Purchase? things to consider
for some ideas about things you should consider and what you might expect to see.

This offsite link takes you to a very well written and illustrated article
Binoculars - A Basic Guide for Astronomy
by Dennis Simmons

Testing Binoculars in the Store Before Buying

and

These Look The Same, are they?
This post summarizes a lot of the quality differences that might exist among various brands and price levels of binoculars, most of which would never be evident by looking at the outside of the binocular. With this post we try to do our best to provide you the information you need to dispense with the often heard internet myth that they are all the same. It's just not so.

See more on this subject thru this Best Of link
If They Look the Same, What Could be Different?

One question that a beginner often asks about is exit pupils. There is so much talk about exit pupils that it can be thoroughly mind-boggling. This primer will set you on the right path. When you begin to yearn for more information visit the Best Of links dedicated to Exit Pupil and Affect of Exit Pupil on Binocular Aperture.
Binocular Primer - An explanation of Exit Pupil, Eye relief and Field of View

Exit Pupils 5mm vs. 7mm, or Should I get a 7x50 or a 10x50?
this will help explain why a large 7mm exit pupil is good only in certain conditions.

What are the Issues with Handheld Binoculars?
Handheld Binocular Observing Issues
not only weight, but magnification is a major issue

Advertised Eye Relief versus Effective Eye Relief
the depth of the recess to the lens affects how much eye relief you really get to use. Usable eye relief is often a lot less than advertised.

Sometimes eyerelief is a little too long, especially for eyeglass wearers when the eyecups are folded down. This may cause blackouts. There are some simple fixes; Here's one shown with pictures.
Solution to the Nikon 10x42 SE blackout problem

The best place we can direct you to understand more about the importance of COATINGS is the Best Of thread dedicated to the topic of coatings. There is a very good summary right in that single post that should help you understand the importance of coatings. If you wish, there are links in that post that will take you into more involved discussion.

Looking for a good book to get started?
single best book for a binocular astronomy beginner
there probably isn't just one best book, but this thread will help you by asking what are your needs and will highlight a few books depending on what they are, learning the sky, deep sky, science, technical aspects or repair.

Binocular observing books? this thread is a goldmine of contibutions by many members of the forum. Books and Charts for observing.

Here is a post that has some good recommendations of binoculars for a first time buyer. Of course this assumes you've at least visited several of the links above here and read the primers and have answered some questions for yourself. In this link you will find a well organized presentation of suggested binoculars put together by Mike Swaim. While originally prepared for the purpose of recommending to beginners various binoculars for specified use or price ranges, Mike's gathering and organization of information leads us to a collection of short mini-reviews within product lines. Mike touches on over 30 different binoculars in this very worth-while post that groups binoculars by power and by price ranges. Along the way, he explains a few simple things the beginner needs to know when shopping for a good first binocular.
Light Trap's beginning binocular suggestions

TAKE A POLL
Binocular Forum Polls Page
take about 30 seconds each or less to answer and see what everybody else says

edz

### #47 EdZ

EdZ

Professor EdZ

• topic starter
• Posts: 18820
• Joined: 15 Feb 2002
• Loc: Cumberland, R I , USA42N71.4W

Posted 24 May 2012 - 05:39 AM

--Binocular Telescope Beginnings

The basic concept of a “binocular telescope” goes back in history to around 1608 when Hans Lippershey, a maker of eyeglasses, combined two small telescopes of his own original design into a binocular configuration. It wasn’t until the 19th century, though, that the aperture of these instruments and prismatic design approached what we now associate with large astronomical binoculars.

While larger aperture binoculars are relatively new to the modern amateur astronomy scene, there have been a number of “giant” or “big eye” binoculars developed for use by military forces over the years for battle front and naval applications. These giant binoculars used large aperture objectives from 80mm to 250mm at magnifications from around 10x to as high as 50x. Because of limited available light and/or the rolling motion on deck, these binoculars featured large exit pupils that made it easier to locate targets under adverse conditions and keep them in view once located. Many of these binoculars also featured angled eyepieces to make sweeping the skies for aircraft more comfortable.

Over the years, a few of these giant binoculars remained in production for civilian use; it was only logical that someone would turn them up to the sky and realize that they gave wonderful views of the heavens as well. Probably the most well known of these today are the Fujinon 25x and 40x150 giant binoculars. Available in straight-through as well as angled designs over the years, these single magnification binoculars have become the gold standard of giant binoculars used for astronomy.

In the 1990s, a Japanese optical company called Miyauchi developed large, lightweight binoculars specifically for astronomical viewing. They featured both 45° and 90° viewing angle models to alleviate the neck strain caused by looking up at high angles with straight-through designs. These angled binoculars were made as small as 60mm and as large as 141mm. All provided the non-reversed, correct view that we have come to expect from binoculars. What really set these Miyauchis apart from the crowd was their provision of several interchangeable proprietary eyepiece pairs rather than fixed eyepieces so that users could change magnifications with these binoculars just like they were already accustomed to doing with their telescopes. Vixen Optics of Japan also brought out their BT80 and BT125 45° binoculars, first with fixed eyepieces and later using interchangeable 1-1/4” eyepieces. By definition, at least in our circle here at Cloudy Nights, this binocular form has become known as a Binocular Telescope (BT). While not intended for high magnification use, BTs can be used comfortably up to around 75x and can give astounding views of Milky Way starfields, dark nebula as well as many DSOs.

While Miyauchi may have developed the modern Binocular Telescope, they never had a large market due to high costs and limited production runs. Their mechanical and optical quality was held quite high but because of that they were never able to break into the mass optical market that has become increasingly more cost conscious. Sadly, of late, Miyauchi no longer produces BTs. Vixen still makes their 80mm and 125mm BTs. The Japanese firm Kowa, of camera and spotting scope fame, makes an ultra- premium BT of similar design to the Miyauchi but suffers similarly with proprietary eyepieces and even higher cost. It’s a great performer if you can afford it.

In the last decade, larger Chinese manufacturers copied the Miyauchi design and, while having to make some compromises to save cost, have hammered out most of their early problems and now produce a pretty solid product. As a plus, they have adapted the design to accept standard 1-1/4” eyepieces which has made it easier to appeal to every viewer’s personal needs. These Miyauchi “clone” BTs are now available from dealers such as Garrett, Telescope Service and APM in apertures ranging from 70mm to 150mm. A 100mm BT of this type is a relatively lightweight 13-15 lbs and mounts easily on a sturdy photo tripod and video head.

Not to be omitted here, another popular BT offering the same abilities but of a slightly different, heavier but likely more robust “unibody” design is based on Chinese military border binoculars. This design has now evolved into the Oberwerk BT100/45°. It’s about twice the weight of the competition so needs a sturdier tripod and head to support it. Many CNers use this BT and are quite happy with its performance.

Expect a maximum 2.5° FOV at around 25x in the 100mm versions of any of these BTs.

--What Is The Advantage of a Binocular Telescope ?

Any binocular, due to the summation of two eyes instead of one, will give you a view with greater brightness and contrast than a single objective/eye. There are a number of informative posts in the Binoculars “Best Of” section on the particulars of binocular summation. Comparing a binocular telescope view to that of a telescope/binoviewer combo, the BT will be equivalent to a telescope of 1.4x greater aperture. This means that a 100mm BT will deliver the same amount of light to each eye as a 140mm scope/BV combo.

You may ask “why should I get a binocular telescope when I can just put a binoviewer on my telescope?” The simple answer is this; a binocular telescope using two 100mm objectives will have a shorter focal length than an equivalent 140mm refractor used with a binoviewer. Since most BVs are in the 1-1/4” format, you cannot use wide field 2” eyepieces to make up for this longer focal length. The shorter focal length of the BT obviously results in a lower magnification, wider field view than with the scope/BV combo. For wide field Milky Way sweeping it’s hard to beat the binocular telescope. The two-eyed wide field vistas we see are oriented in the sky exactly as we see them naked eye or on our charts; it’s as if suddenly you are thrust deep into space where you are immersed in the area you are studying. Using two eyes stimulates both hemispheres of your brain and you can’t help thinking you are there among the stars! You can sit comfortably in a chair and the angled, interchangeable eyepieces of a BT allow you to view at high angles with a simple, light mount that needn’t weigh any more than the BT alone.

Try looking through a binocular telescope if you have the opportunity; I think you will be pleasantly surprised by what you see!

Garrett BT70-90 MK II's arrived!

Convenience, 45 ° or 90 °?

Mr. Bill's recent DIY project - 6/8/12

127mm f/5.5 binocular

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