Fujinon 16x70 vs 4" refractor

I have a 4" refractor that I use of deep sky observing from my suburban location and wondering how the Fujinon 16x70's would compare in light gathering ability. I mainly use a 24 panoptic in a Borg 101ED which gives ~26x and gives a nice 2.6 deg fov.

By the following calculation for comparison of light gathering; 70^2 + 70^2 = 9800 square mm in the Fujinon 16x70's while the Borg gives 101^2 = 10201 square mm which are about the same for light gathering. I know the above is a big oversimplification and wondering if anyone has compared views between binoculars and refractors of similar sizes to above. It is a hard comparison I'm sure with different magnifications and the factors of single eye vs both eyes. Basically there open clusters viewable from my location with the Borg 101ED and wondering how much more difficult they will be to detect and view with Fujinon 16x70's. Thanks.

Dave Adams

Dave
If you look up some of the excellent posts by Ed Zarenski on cloudy nights and in the binocular observing forum you will find much information germane to this issue. The claim has been made that using binoculars produces an image that is equivalent to a single aperture about 20% larger than the aperture of the binoculars--this statement would be true if the magnification were the same in the binoculars and the scope. I am quite sure if you perform Ed Zarenski's exercise of counting the number of stars you can see in the Pleiades with your 4 inch refractor at 26X you will count more stars and see stars of fainter magnitude in your 4 inch refractor than in the Fuji 16X70's. You can find the finder charts for this exercise in the files section of the Yahoo group called "binocular astronomy." They may be available on the cloudy nights site as well or by contacting Ed Zarenski. I think Ed's compilation of results of limiting magnitude in the Pleiades and number of stars counted in the field he has outlined for the Fuji 16X70's are also available on the Yahoo group files and at Cloudy Nights. The Fuji's will give you a wider field of view than the 2.6 degrees you get from your 4 inch refractor at 26X.
To equal your 4 inch refractor for limiting magntude or star counts in clusters you would probably have to go to something like 20X100 binoculars which would have a little more "aperture equivalence" and a little less magnification.
I have a Nikon 20X120 binocular that would probably give your "26X102" monocular a run for its money but is undoubtedly more unwieldy to transport, handle, and set up than your refractor.

The best instrument (between the 101ED and the 16x70 Fujinons) for viewing open clusters depends on the cluster. Some clusters view better at lower powers (i.e., 16x). These include many of the showpieces of the sky (M45, M44, etc.). In these clusters, the binocular effect of viewing with both eyes would make the Fujinon's the instrument of choice (although the Borg would not be a shabby second choice by any means!).

A lot more clusters are better viewed at medium power (including some showpieces also such as M11), which leaves the binoculars a distant second for those objects.

For detecting, I'd give the edge to the binoculars due to their wider field of view.

There is also the effect of a larger aperture being able to better resolve the faintest stars in a cluster (which I believe is independent of the equivalent light gathering power of the binoculars, but am not enough of an optics expert to know for certain).

Hugh Bartlett

Hi Dave, I'll chime in here rather than reply to your email.

IMO the biggest drawbacks with binocs are (1) straight-through view aka pain in the neck, (2) fixed eyepiece/magnification, so you can't optimize for a given object or conditions (such as sky brightness). There are exotic binoculars (Apogee RA88 -- one was listed just yesterday) that more-or-less address these two issues.

In my own light-polluted skies, I've found that a magnification around 35-40X FOR ONE EYE really helps to darken the sky background, bringing out fainter stars and objects. In my experience, two eyes should enhance perceived contrast fairly significantly so 16X might be enough -- but I have not looked through a Fujinon. Another thing to note is that the Fujinon's EPs aren't nearly as wide and flat-field as your 24Pan. While I can spot open clusters with a nice pair of 10X binocs, the power is too low.

When I first learned the skies I was fortunate to have a TeleVue Genesis with its excellent red-dot finder. Sometimes 10-mile-distant San Jose would become fog-covered, making my sky dark enough to show (barely) the Milky Way. On those rare nights scanning the sky was FUN. I think my first such night I managed to find nearly all of the visible well-known open clusters -- in most cases seeing them for the first time.

A few months later, I ran into the limitations of a 4" scope: globular clusters and galaxies were no fun. I managed to buy an 8" Dob with an exquisite mirror (made by an Astromarter) and, on another good night, picked off most of the Messier globs and galaxies visible. (Orion UltraBlock filter definitely helped.) In a moment of weakness I sold the Genesis and still miss it. (Then at some point I got into binoviewing but that's another story!)

Nowadays I have a few projects, including Big Dobs and Binoscopes.

-- William (Saratoga, CA)

Re: binoculars

Although I haven't used an Apogee RA88, based on the concept and people's reviews, that's the one I would recommend. It's quite heavy, but mounts like an alt-az telescope.

The SkyWindow concept has potential issues such as mirror flatness (they do sell a more premium version, which raises the question of how premium), dust on the front-surface hard-to-clean mirror, and the fact that you have to do mental geometry to figure out where in the sky you are pointing! (Remember that the image is upside-down and the angle of reflection is not constant, IIRC.)

If my skies were darker, I think I would invest in a nice large pair of binocs. But like I said, 35-40X makes a huge difference in darkening the sky background.

Another thing in the RA88's favor is the possibility of using light-pollution filters.

-- William

p.s. And as you know, I'm building a Borg 100 Achro binoscope :-)