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Five-Way Shootout of Zoom Eyepieces Solar Viewing

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#1 mloffland

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Posted 20 October 2005 - 12:03 PM

Five-Way Shootout of Zoom Eyepieces for Solar Viewing

By: Jim Richberg

#2 Starman1

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Posted 20 October 2005 - 02:02 PM

This review was for solar viewing.
However, had he done only deep-sky viewing, he would have mentioned some other facets of performance:
--star images at the edge vs. the center vs. focal length
--the curve of AFOV increase with power differs substantially from one zoom to the next. This is an important feature, since it determines the FOV in the mid ranges of the zoom.
--chromatic aberration vs. focal length and the control of same at all FL's
--sharpness vs. FL in each zoom. Some do quite well at low power, and fall apart at higher powers. Others don't. This feature is important.
--range of change in AFOV. Some zooms have less variation from short to long FLs (like the Antares Speers-Waler 5-8 zoom, or the two Nagler Zooms), while others vary hugely (like the Vixen Zoom)
--degree of parfocalism in the eyepiece. Some vary hardly at all, while others require major refocusing. If you focus the aforementioned Vixen Zoom at 8mm, it stays in focus all the way to 24mm. In contrast, the Nikon requires refocusing at nearly every step, in my experience. Others, like the Speers Waler, have no pretense at parfocalism at all.

I think the review was detailed. I think the author needs to do a Part II on deep-sky only.

Zooms are a "fringe" element to the eyepiece offerings of most companies except in spotting scopes. That's too bad, because they could be so much better than they are.

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Posted 20 October 2005 - 03:39 PM

A fairly objective review, especially for the solar viewing. I did a similar comparison when trying to decide whether the TV 7-21 or Nikon 7-21 would work for me. My goals were to minimize eyepiece inventory with two portable scope setups:
a Nexstar 5 and Televue Oracle 3 on a Nexstar 80GT.

The Oracle is my primary solar scope with a SolarMax 40. I also use it with a Canon A70 and Coolpix 995 for imaging.

The third eyepiece in my test was the Orion Explorer II with a Hands-On minus violet filter. For me, the Nikon came in first for solar viewing with the TV Oracle and night viewing with the Nexstar 5. The TV zoom did not offer the contrast the Nikon provided in solar. With the five inch for night viewing it never did provide any "wow" factor. Suprisingly, the Orion zoom does provide a slightly better solar view with the five inch. But, like the TV zoom eyeball placement is not as forgiving as the Nikon.

I tend to use the Nikon zoom for 90% of my viewing. My night time targets are doubles and the Nikon is the clear winner for my eyes in terms of color and sharpness, especially with the Oracle 3.

#4 David Knisely

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Posted 21 October 2005 - 02:00 PM

There are a couple of problems with the review, mostly in an understanding of the terminology. First off, in H-alpha, what you are viewing is mainly the solar CHROMOSPHERE and NOT the Corona. The true Corona is not really all that visible in narrow-band H-alpha filtering systems (despite the term "Coronagraph" being used for some systems designed for viewing prominences). It is a more broad-band emission feature that is much fainter than the common chromospheric emission features. We are seeing features which are sometimes located in the corona, but we do not see the things which make the corona a distinct and noticable feature, like the faint coronal streamers or other structure. The glow reported in the two eyepieces is very probably more scattered light either from the telescope or in the eyepieces, than the light from the Corona itself.

Second, when mentioning the work "coma" in eyepieces. What is being seen is very likely not coma, but astigmatism induced by the eyepiece itself. Coma would be present more in the paraboloidal mirrors of short focal length Newtonian telescopes, and is not usually seen in eyepieces. Astigmatism gets worse at shorter f/ratios, and it is no surprise that it reared its ugly head for some eyepiece designs below f/5. Clear skies to you.

#5 JBR

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Posted 21 October 2005 - 06:28 PM

Thanks for the comments, Don.
I started out intending to do a full review under both solar and night observing conditions, but for a variety of reasons (especially available viewing time, and weather) found I was only able to make much progress on the solar end. Rather than hold things up, since I knew a number of folks in my club (NOVAC.com) who were especially interested in the solar results, I opted to publish those on my new web site (http://users.adelphia.net/~jbrspace) along with my extremely preliminary "first impression" nighttime results.

You have hit on a number of the dimensions that are important for a night time test-- I think I may even have forgotten to edit a couple of them out of the preface to the current review. Will try to do the DSO version as time permits.

BTW, I did test for parfocalism-- as I indicated in the review, I'm pretty sure the Nikon 9-21mm is parfocal, and that the slight correction I had to make after zooming is an artifact of my vision. (I understand that unless you have perfect 20/20 vision, as you change the focal ratio of an eyepiece you will have to re-focus. I've got ~20/15 vision, and hence had to do a bit of adjustment, just as I have to do with the parfocal Nagler 3-6mm zoom). I don't get why you found the Nikon to require significant refocusing after even a minor zoom and not the Vixen --which I understand to be a clone of the TV8-24 I tested. I can understand our discrepency on the Nikon, if we didn't test the same model (the Apogee clone of an older non-multi coated Nikon is far from parfocal, for example) but our opposing experiences with the Vixen puzzles me.

Again, thanks for the comments.

Jim

#6 JBR

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Posted 21 October 2005 - 06:48 PM

Dave,
:o Oops-- I do know the difference between the contribution of eyepiece astigmatism and focal ratio coma to the visual defect I saw in the Pentax in faster scopes at night. Many in my experience tend to refer to both indiscriminately as "coma" in conversation, and I fell into this trap of using the sloppy nomenclature in writing as well. Mea culpa, and I've fixed it in the version of the review on my web site. (http://users.adelphi...olar_review.htm)
I'll submit an update of the CN review once I've taken all the comments in.

Regarding the chromosphere/corona issue you raise, I take your point, but would note the following:
a) what I saw in the Nikon and Pentax almost certainly was not normal glare, since I've been able to produce off-axis telescope/eyepiece glare even with these eyepieces under the right (or maybe, wrong?) circumstances, and it looked distinctly different than this
b) the artifact I saw displayed structure --looking more like the corona visible in eclipse photos than anything else.
c) this was only visible under conditions of good to excellent atmospheric clarity, and was relatively faint and subtle even under those conditions.
d) When it was visible, at low power its' extent seemed to larger than the chromosphere, since around some parts of the solar disc it visibly extended one or more diameters of the sun beyond the limb.
:question:
Jim

#7 Starman1

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Posted 21 October 2005 - 06:52 PM

Jim,
It's been 15 years, but the Nikon I had was a 1-1/4" adapted spotting scope zoom, and it was very tough to turn.
The Vixen required refocusing unless I first focused at 8mm, then turned it down to lower powers. I guess the ability of my eye to accommodate corrected the image at lower powers. That was 15 years ago. I'd love to re-test it with my older eyes. In the same time, I've gone from no glasses to trifocals, so my ability to accommodate has certainly lessened. At infinity, though, I'm 20/20, so perhaps that has something to do with it.
The main complaint I had with every zoom was the narrow field of view at low power, where wide field is the most important. It's the reason why some seasoned zoom users say the minimum eyepiece collection contains one zoom and one low-power, widefield, eyepiece.

#8 JBR

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Posted 21 October 2005 - 06:55 PM

Just out of curiousity, which Nikon 7-21 do you have, the "MC2" (multicoated one) or the older Apogee one? I didn't have the $ to splurge on an MC2 for testing purposes...

However, since even the Apogee came out ahead of the TV in my test, either your eyes have the same flaws mine do, or there's some reassurance to be had in the fact that we both came to the same conclusion.

:grin: Jim

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Posted 21 October 2005 - 09:17 PM

Very good review. I'm at a loss as to how there seems to be a significant discrepancy between what was observed in the TV and what I see in my Vixen LV 8-24 unless the assumption that the two are identical is incorrect. In my PST, I see no internal reflections (double images) nor do experience blackout issues (e.g., 12mm Radian, 14mm Scopetronix Maxview)--mind you, I almost invariably use the eyeguard "up" (but that's true of the Radian too).

Cheers!

Armand

#10 David Knisely

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Posted 22 October 2005 - 01:45 AM

Jim posted:

Regarding the chromosphere/corona issue you raise, I take your point, but would note the following:
a) what I saw in the Nikon and Pentax almost certainly was not normal glare, since I've been able to produce off-axis telescope/eyepiece glare even with these eyepieces under the right (or maybe, wrong?) circumstances, and it looked distinctly different than this
b) the artifact I saw displayed structure --looking more like the corona visible in eclipse photos than anything else.
c) this was only visible under conditions of good to excellent atmospheric clarity, and was relatively faint and subtle even under those conditions.
d) When it was visible, at low power its' extent seemed to larger than the chromosphere, since around some parts of the solar disc it visibly extended one or more diameters of the sun beyond the limb.


Well, you don't specifically mention the scope you were using, but you did mention a PST, and that *does* have some issues concerning a halo of light around the solar disk (I know, I own one). The amount of scattered light in the eyepiece may also show variations and will be less the farther the sun is from the center of the sun (this is also true for the telescope at least to some degree).

As for seeing the true Corona in H-alpha, that is probably not possible (especially if you are not using an occulting disk). You might have seen some faint arches (arcades) seen almost end-on, some faint surges, or other chromospheric prominences, but probably not anything directly linked with the Corona. I have seen surges occasionally send some material in a narrow stream up to nearly a solar radii above the limb. I have also seen debris from disparition brusque eruptions and coronal rain go well above the limb, but not the fine diffuse coronal features themselves. The Hydrogen is almost fully ionized there, so almost the only light one sees in the K Corona is the nearly continuous spectra from Thompson scattering (photospheric light scattered off of free electrons). A narrow-band sub-angstrom filter would kill this light almost completely. I have used a 0.7 Angstrom DayStar T-Scanner on a 3.5 inch aperture solar telescope during a solar eclipse to try and detect the moon well prior to first contact and did not seen the moon until it actually began to hit the chromosphere's dense spicule fringe at the limb. Any glow of the "corona" should have been blocked by the moon well before that and this was not observed. Professional observatory ground-based coronagraphs often use the bright "coronium" spectral line of Iron (5303 Angstroms) to observe the inner corona as well as the upper chromosphere. Also, you mention the solar wind, but again, the solar wind is not really a visible phenomena although it extends outward from the outermost reaches of the solar corona. What you see in H-alpha is material above and in the chromosphere, primarily some of the many types of prominence and prominence-like limb phenomena. Clear skies to you.

#11 ccs_hello

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Posted 13 November 2005 - 08:06 AM

Jim,

Nice review!

I'd like to point out the Apogee 7.4-22mm (long gone long time ago) started the upban legend. The new one is 7.3-22mm which looks similar to the old 7.4 one, but probably not coming from the same factory.

Clear Skies!

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