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Posts Made By: Ed Zarenski

May 19, 2006 09:28 AM Forum: Astro Binoculars

Fujinon Techno-Stabi 14x40

Posted By Ed Zarenski

For the $1100 price that's not a very practicle choice for astronomy, given the 40mm objectives. Even a hand held 12x50 would show just about the same. A hand held 12x50 Nikon SE would probably show more, for half the price.

edz

May 25, 2006 03:42 PM Forum: Deep Sky Observing

Deep sky with binoculars

Posted By Ed Zarenski

Hmmm, I find Sky Atlas 2000.0 the easiest tool to use for binocular observing. And i do an awful lot of binocular observing.

edz

June 11, 2006 04:40 PM Forum: Binoviewers

Binoviewer Light path

Posted By Ed Zarenski

Did you account for the fact that the focal length thru a prism is not the same as the physical light path length thru a prism.

When you measured with your mirror diagonal there was no shortening or lengthening of the light path vs the focal length. The length of the light path is the focal length.

When you measure with a prism in the path, to get the focal length you must know the physical dimensions of the prism. You must determine the physical distance the light travels thru the prism.

Light slows down thru glass. So the focal length thru the prism can be found by dividing the length of the prism light path by the index of refraction, typically about 1.52 to 1.56. If the physical length of the path thru a single prism is 50mm, the the focal length used up thru the prism is only 50/1.56 = 32mm.

edz

June 21, 2006 10:57 AM Forum: Ask rating questions here

Re: How do I retract good review? after getting ri

Posted By Ed Zarenski

Go To Your

Account Menu

Ratings I have Given

Click on the Delete this Rating next the the appropriate rating.

edz

December 12, 2006 07:22 AM Forum: Astro Binoculars

Fujis--worth it?

Posted By Ed Zarenski

The 10x70 Fujinon would give you the needed eye relief, but as you guessed, may be just too wide an exit pupil for your eyes. Check your eyes and your skies. I find this particular binocular is best for someone with pupils over 6mm and skies darker than mag6. If your eye pupils never get larger than 5mm, then that 10x70 will never be more than a 10x50 for you.

The 16x70 Fujinon has an effective usable eye relief of only about 10mm. I wear eye glasses with them. While I would not trade them, I do see only about 90% of the fov. But the view is one of the best.

The 12x50 Nikon SE performs like a slightly larger binocular. So the 10x70 isn't going to do a lot more for you unless you have very large eye pupils AND you get out to very dark skies. On the other hand the 16x70 is a significant step up.

edz

December 20, 2006 08:47 AM Forum: Binoviewers

Bino V Mono

Posted By Ed Zarenski

Nice comparisons. If you cover your SV102 with a mask to reduce it to 96mm, you would even up the theoretical playing field. As it stands, your 102 has a 13% LG advantage.



edz

January 8, 2007 12:43 PM Forum: Eyepieces

Meade 26mm 5K plossl

Posted By Ed Zarenski

Meade 26mm 5000
10-07-06

Well, I picked up another pair of eyepieces recently, looking for a better pair of low powered eyepieces for my BT100. new model Meade 5000, 26mm plossls with a 60° AFOV. These sell for $99.

On axis the Meade 5000 was pretty good, but off axis it deteriorated with coma (and astigmatism I think) very rapidly. By 50% out, it started to show. At 70-75% out coma was responsible for deterioration that prevented seeing a 36" double separated. That's poor. By 80-85% out, you couldn't tell it was a double. The TV and Meade SP LP can see doubles of 22" or closer all the way to the edge.

A Televue 26mm and the Meade 26mm SPLP allow seeing closer doubles completely separated all the way out to the edges. So both of them have the full use of the entire 50° AFOV without much noticable deterioration of the image quality. The TV26 has a slight edge in throughput over the Meade SPLP in the outer 25% ring.

Just so you can put this in perspective, in this same instrument, the TV26 can see a 14" double still clearly separated within 5% of the edge. A TV20mm plossl can see a 7 arcsec double at 95% out and the 14mm Radian can see a 5 arcsec double still clearly separated as it passes off the edge of the field stop. These are apparent separations of 330 to 220 arcseconds undistorted (without much coma, SA, curvature or astigmatism) at the edge of the FOV. The Meade 5000 26mm point image is distorted to an undesirable 450 arcseconds at 60% out and to an unrecognizable 850 arcseconds at 80-85% out. (Smaller apparent values are better).

This new Meade 5000 26mm has use of maybe at best 65-75% of the AFOV and in part even that is compromised. Effectively, that's no more than about 42-45° undistorted AFOV. At f/6.2, Coma in the outer 20% of the FOV was deplorable.

The 26mm 5000 did perform better with a wider corrected fov in the f/11 C5, that was as expected. But how much better? Are they any better than a standard plossl?

I measured the TFOV and calc that the Meade 5000 measures about 61-62° AFOV. Previously, I measured both the TV26 and SPLP26 and both of those have a 50° Afov. At 80% out in the Meade 500, the Tfov (not the image) is equal to the entire filed of the TV and Meade plossls. For the Meade 5000 to equal the plossls, I should at least be able to see the same image quality at a point 80% out from center.



Using Gamma Delphinus and Struve 2725 as my targets, I checked for furthest point out where I could still see the pairs resolved, even with some aberration present. When the aberrations take over and I can no longer see that the pair is a double star, that's the limit just beyond the point I record here.

Also, I previously measured my C5 and have found that it operates much closer to f/11. The standard figure of f/10 for an SCT is generally given for the plane of the visual back. I use a diagonal. I'm using F=1350. All three of these eyepieces give 52x in this scope. The Meade 5000 gets a 1.2° TFOV while both the others get almost 1°.

In the Meade 5000, Gamma Del is NOT seen as two stars when Stf 2725 is on the edge of the field stop. That places Y Del 55% out in the FOV. So, at 55% out the Meade 5000 looses a 9.8 arcsec double from view. At that magnification, 52x, that represents an apparent separation of 509 arcseconds. At 85-90% out in the view this eyepiece exhibits significant astigmatism. While the view is comatic, I could turn inside and outside of focus and easily watch the elongated shape of the out-of-focus image flip orientation 90°.

In both the TV26 and the Meade SPLP26 the views were nearly identical. This time, with Gamma Del on the edge of the field stop, Stf 2725 is still seen as separated. In fact I was able to move Y Del out past the field stop to reduce the edge distance by nearly 50% of the space between these two doubles. So that places Stf 2725 just beyond 70% out from center. At 70% out, both the TV26 and SPLP26 reach the limit, and that is the limit where they can both see this 5.8 arcsec double as separated. That means both these can see an apparent separation of 52x5.8 = 302 arcsec at 70% out.

Meade 5000 = 509 arcsec at 55%. AFOV 62° x 55% = 34° Afov produces 509 arcsec correction.

TV26 and SPLP26 = 302 arcsec at 70%. AFOV 50° x 70% = 35° Afov produces 302 arcsec correction.

Samller apparent separation is better.

All three of these eyepieces allowed some ability to refocus the image, at least to a small degree, with the image placed 50-70% out. That would indicate some Curvature present that could be focused out. In no case was that the predominant aberration. None seemed to be plagued by Spherical Aberration, at least not to any great degree. In the Meade 5000, the dominant aberration seemed to be coma, overwhelmed by astigmatism out beyond 80%. In the TV26 and SPLP26, it seemed to be mostly coma with little astigmatism.

Although the Meade 5000 is a very nice mechanical eyepiece, and I like the twist-up eyeguard even better that that of the Radians, it's correction for aberrations in the outer field does not match even that of standard plossls. The plossls provide a wider corrected view.

edz


August 16, 2007 04:21 PM Forum: Solar System Observing

Mars to be its brightest on 8/27!!

Posted By Ed Zarenski

It all came from a very good article that showed and stated that 'if you keep one eye to a telescope and look at Mars at about 100 power, and use your other eye unaided to view the full moon (at the same time), then mars in the telescope will appear the same size as the full moon. Of course the wording that was "cut out" of the original article left the very important critical information on the next line sitting behind.

I read the original article.

edz

October 6, 2007 08:34 AM Forum: Astro Binoculars

Nikon 16 x 50 "Action Extreme" binos?

Posted By Ed Zarenski

I would expect it to have smaller fov by 6%-7%, same as the other two 50 Action EX ATB. However , both do measure up very close to the speced eye relief. Only the 7x50 and 12x50 have a field flattener lens, so I would expect this one to have good filed sharpness out to only 60% of fov.

edz

February 22, 2008 03:43 AM Forum: Eyepieces

Eyepieces for Binoculars

Posted By Ed Zarenski

125mm at f/5? Hmmm, that sounds really quite short. The Oberwerk BT100 is 100mm F=620mm, so it's f/6.2, or F=598 depending on which model you have, straight or angled.

I presume you are looking at 1.25" eyepieces? I've not seen any stock binocular telescopes that use a larger size eyepiece. You might want to start looking at field stop diameters rather than stated Afov. Stated Afov includes pincushion and therefore true fs dia produces a smaller Tfov than the Afov would indicate. Your 65 or better requirement would eliminate the 24Pans, as they actually only have a 27mm fs, which really gives only 64°, not the 68 which includes pincushion. Maximum fs diameters all top out at about 27.5mm. Some eyepiece hav NO field stop, which means inside barrel sleeve diameter of about 28mm controls fov. It gets you a bit wider, but the view is usually an indistict greyed ring in the outer 5%fov.

These will all give nearly identical Tfov, except the Ultimas, would be about 3-4% smaller, negligable really in the grand scheme of things. Probably far more important would be selecting for minimal outer fov aberrations, in which case, I'd eliminate the Meade 5K, which IMO has the most. I have an Oberwerk 24.5mm eyepiece with fs=27.3mm, that gives a similar WF as these, but outer field aberrations ar poor.

24 Panoptic fs=27mm
26mm Meade 5K pl fs=27.4mm
30mm Ultima pl fs=26.2mm
32mm TV pl fs=26.9mm

Once you get a list of eyepiece field stops that max out your fov, select the magnification/exit pupil you want to live with.

edz