Posts Made By: Ed Zarenski

Zach,

Check back with Cory to confirm your discussion. The difference in coatings, if all surfaces are coated MgF vs FMC will be substantially more than 10%. The difference in lost reflected light can be 0.5% per surface coated vs 4% per surface uncoated. There can be up to 14 surfaces in a binocular.

I tested and published results on Oberwerk models that were FMC vs Oberwerk that had MC on Objectives but only MgF on prisms and eyepieces. I recorded differences of several tenths of a magnitude in Limiting Magnitude, differences in amount of faint diffuse light that could be seen and noted significantly greater ghost images in the nonFMC version.

Personally I recommend to all my forum visitors on CN that if budget allows then do NOT purchase nonFMC binoculars.

An 8x56 binocular is a special condition binocular. Most users will never be able to use it to the full potential of its aperture because it has a 7mm exit pupil. Two reasons that's not a wise choice follow.

For a person with eye pupils that dilate to only 5mm or 6mm, that 8x56 will be an effective aperture of 40mm or 48mm, not 56mm. Few people can utilize a 7mm exit pupil. I recommend if their eyes can only dilate to 6mm don't get anything larger than an exit pupil of 6mm.

In moderately light poluted skies of around mag 4.0 to mag 5.0. it will do no better than an 8x40 binocular. A larger exit pupil binocular in poor skies has such washed out images that it performs no better than a much smaller binocular. I recorded data to show this relation and published the results from a comparison of Fujinon 10x70 vs Fujinon 16x70.

So my point about 8x56 being such a special condition binocular is this; it needs to be matched to eyes that can fully utilize the large exit pupils and it needs to be used under very dark skies to prevent the washed out effect. Personally, I would place an 8x56 uncoated binocular on the shelf right next to my 30 year old 7x35 Sears $29.95 binoculars. That would be precariously close to the trash bin. Single coated is a bit better.

My suggestions, go with fully multi coated throughout to pass more light and eliminate internal reflections and go with the 11x56 for the 5.1mm exit pupil, an exit pupil that can be utilized by a broader user group.

edz

Zach,

I corrected my previous post to reflect % reflectance of coatings.

It should have read the difference in coatings can be 0.5% reflectance per surface for coated lenses versus 4% per surface for uncoated lenses.

Obvoiously then, the most significant difference is between coated and uncoated, not between coated and multicoated.

edz

As a buyer (much more than a seller), I look for binoculars priced about 60% of new, but I do also consider that some higher quality (and some new) binoculars don't get much lower than 70% of new.

edz

Well that is an interesting response. I've seen many explanations for resolving power, but I've never seen that value of 125 used before.

In general most references you will find use Dawes Limit to define the resolving power of a lens. Dawes Limit is 116/Dmm. What Dawes really defines is an empirical value for two stars of equal magnitude that have a separation where the peak brighness of the central disks are far enough apart to detect that there are two disks. This will almost always result in a double star that will show an overlap of about 20% or 30% of the bright central disks.

The resolution of a lens is defined by the ability to separate two point sources. The finest point of light that a lens can make is the Airy disk. The Airy disk size is calculated by the Airy disk formula or the Rayleigh Limit. Therefore Rayleigh Limit defines resolution.

Kitchin: The angular resolution is equal to the Rayleigh limit, where separation between two stars is considered as achieved when the stars are just touching.

Suiter: “The Rayleigh resolution criterion is met when the separation of the two objects is precisely at the radius of the theoretical Airy disk. In other words, the second star is placed on the valley between the first star’s central disk and the first diffraction ring.”

These two quotes above help explain resolving power. Clearly both of these texts refer to Rayleigh limit. Any physics text that provides the forumula for the resolution of a lens is refering to Rayleigh's formula for the radius of the Airy disk. However there are a great many astronomy texts that ignore the physics of lens resolution and simply use Dawes limit to define resolving power. I have explained up above what Dawes Limit really represents.

The resolution of a lens is determined by the Rayleigh Limit or the forumula for the radius of the Airy disk which is dependant on the wavelength of the light and the diameter of the lens. Rayleigh limit is 1.22 lambda D, or reduced itis 138.4/Dmm or also the well known 5.45/Dinches and the answer gives arcseconds. As mentioned in a post below, these values are for yellow light (lambda = 550nm).

You will almost always NOT be able to completely separate a pair of stars at the Rayleigh Limit. Besides the color, which has a significant affect, also the magnitude of the stars has an affect on the resovability. Brighter stars will not be resolved at the Rayleigh Limit. However, Rayleigh represents the best condition and may be a value some scopes could achieve on some stars. A scope might easily exceed Rayleigh Limit if observing a pair of blue stars. Usually, a scope will not completely separte two stars at the Dawes limit.

edz

G5 125mm F1250mm f10 scope (although by drift testing I have discovered it is operating at about F=1375mm = f11).

16mm UO Konig
12mm UO Konig
6mmUO Ortho
5mmUO Ortho and
4mm UO Ortho

Log notes from recent observing sessions.

6-2-02 very cold (this night was so transparent that I recorded sighting M101 with my 15x70 Oberwerks)

M81-M82
8.8UWA has sharpest view to edge by far, out to 90-95% fov
16K is sharp only 70-80% out from center
12K sharper than 16, out to 80%+
14R sharp out to near edge

18SWA allowed seeing both M81 and M82 in same field but not quite high enough power to see bright detail
16K allows seeing both across 75% of fov, power getting better (86x, 45’fov)
14R both M81 & M82 are just at edge of fov 36’, power 99x allows M81 extensions to be seen to about 1/5 of total fov
12K M82 outstanding, seen brighter with averted vision, fills 1/5 to ¼ of fov 31’
8.8UWA M81 core takes on dimension, also I can see a dbl star to SW of M81
6Ortho splits that dbl star to SW of M81, it is faint equal mag

6-3-2002
M13
16K very nicely framed between the two 7mag stars with plenty more surrounding frame
14Rad counted 35* total
12K very well resolved with cloudy glow extensions
8.8UWA glow with resolved stars in extension to SE, brightest part of core shifted off center to W, core gains in brightness by averted vision, also total # of stars resolved surrounding edges of core increases with averted vision
6Ortho saw 25* outside the core 11* within, faint glow seen along S extension more dark seperation seen between core and outer diffuse area along S curving W to E
5Ortho core begins to become diffuse with many stars resolved within, counted 55*, resolved core becoming indistinct from overall view, at 9’ fov M13 fills more than the entire fov of the 5mm, had to dial back and forth to do the count.

M57 Ring Nebula
8.8UWA glimpsed 13.1m star on east
6Ortho 13.1m star remained in constant view

I own an Orion ST90 and a Celestron G5. The G5 is the C5 tube on a G3 equatoral mount. The G5 puts the ST90 to shame. I don't care that the ST90 is capable of a much wider fov. The views thru the G5 are superb. Cool down is an issue. I leave my G5 set up all the time. When I'm planning on a night of viewing I carry it outside around an hour early. When I've tried to view right after taking it outside (only if there is a large temp differential) the views are moving around for about 45min to an hour. I'd go with the C5. If the temp diff is +/- 10* you could be outside viewing in less than 2 minutes.

I would encourage anyone considering planetary eyepiece selection to read this Planetary Eypiece article at CloudyNights.com

http://www.cloudynights.com/eyepieces2/planetary.htm

I use my orion Ultrablock filter in front of the eyelens of my 15x70's. I was viewing M16 and M17 for the last 2 nights and both nights the filter resulted in a better view of the nebula on each of these. Only get to cover one eyepiece with the filter, but it works like a charm. M16 begins to show up so you don't have to ask yourself, Do I really see a nebula here? M17 is outstanding and the upsidedown swan shape can be seen more readily with the filter than without.

Orion sells adaptors. Are you trying to gain fov or just want to use some short f eyepieces that otherwise sit in your box? I think that unless you have a 2" visual back, you will not gain any field of view. I just looked at my G5, same scope. It doesnt look like the exit is big enough to accomodate a 2" visual back.

IF all is well with your dob, then in addition to two eyes are better than one, I'd be thinking it's probably because of the larger exit pupil provided by the binoculars. I have 15x70's, exit pupil is 4.6mm. To get a 4.6mm exit pupil out of my F8 scope I'd have to use a 36mm eyepiece. I could use a 40mm. To get the same with my F10 scope, I'd have to go to a 46mm eyepiece. The best I could do would be my 40mm = 4mm exit pupil. edz