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Slow scopes? (1 Viewer)
- Thread starter willemjacobusse
- Start date
willemjacobusse
Well-known member
Interesting indeed. I wonder about that $100 scope!
willemjacobusse
Well-known member
Is that $100 scope for sale by any chance Sout Fork?
henry link
Well-known member
Nice photos, Sout Fork, but as you probably already know they obscure rather than reveal the optical differences that would be visible at the eyepiece. After all, these are low resolution jpegs of completely different subjects, under different lighting conditions and different magnifications. Contrast is compressed compared to the real world and detail is probably limited by the medium, not the optics of the telescopes. A set of photos, all made at 60x, of the same test object under the same lighting conditions might give us a better idea about the relative quality of the scopes.
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Precisely my point:
The "real" underlying optical quality is always obscured by the the fact that each image is always viewed or taken of "completely different subjects, under different lighting conditions and different magnifications" and "limited by the medium".
Thus the final ultimate quality of any given image is, more often than not, limited by the optics by only a marginal extent compared to these other practical real world factors. That is assuming at least decent quality glass.
Answer to quiz:
Pict 1-Pentax 100mm spotter with CA-35 SLR adapter (38x) taken at 300 feet.
Pict 2-Orion 120mm achro barlowed to about 60x taken at about 120 feet.
Pict 3-Celestron 100mm achro (this is the $100 scope), prime focus, at 30 feet or so.
Pict 4-Astro-Tech 80mm "APO Triplet", prime focus, taken at about 20 feet.
Pict 1-Pentax 100mm spotter with CA-35 SLR adapter (38x) taken at 300 feet.
Pict 2-Orion 120mm achro barlowed to about 60x taken at about 120 feet.
Pict 3-Celestron 100mm achro (this is the $100 scope), prime focus, at 30 feet or so.
Pict 4-Astro-Tech 80mm "APO Triplet", prime focus, taken at about 20 feet.
henry link
Well-known member
Sout Fork,
Sorry, I don't see the point of showing us images with so many variables affecting their appearance that the true optical quality of the scopes is concealed. Surely there's no difficulty in visually distinguishing between your f/5 achromat and your f/7 APO at the eyepiece, especially in the real world.
Henry
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willemjacobusse
Well-known member
Hi Henry,
The new Kowa 88's are getting some great reviews as far as I can tell. havent been able to find their actual focal length yet but they must be "fast" by anybody's standard and certainly by yours. So have they achieved a breakthrough?
The new Kowa 88's are getting some great reviews as far as I can tell. havent been able to find their actual focal length yet but they must be "fast" by anybody's standard and certainly by yours. So have they achieved a breakthrough?
Willem, Henry L. contributed his take on the new Kowa optics a few insertions back and will respond, I am sure, but I would not anguish over focal lengths and ratios too much. No technological breakthroughs with the 883/4, but it is a decent performer if you want good resolution at great distances and low light conditions, and do not mind the price. The new Kowas are well corrected for chromatic aberration and astigmatism, but they are not perfect. They will work OK for astronomy if you cradle them on a heavy tripod.
The Swarosvski "HD" scopes are good choices too.
If you are interested in astronomical viewing, check the reviews and tech information in the Cloudy Nights site, which you can find on a search page.
Norm P, Massachusetts, USA
I forgot to add to the above, that TeleVue, I think, makes some quality crossover scopes that may work for both astronomy and terrestrial viewing. I would go to an appropriate user group on this forum, Cloudy Nights, or another to get more information. I don't know if this brand is distributed in Europe.
Norm P
Norm P
So would a similar reasoning apply to the new Kowa 77 and 88mm line of scopes as they both use the same eyepieces giving the same magnification, although the glass used in the objective lenses is different?
We hear a lot about the 88mm model but not much about the 77mm which is a bit more affordable for some of us.
Is there anywhere that lists the focal length of scopes new and older so in theory you could decide which one is more likely to be a good specimen?
The 88mm Kowas all seem to be decent performers but at a price! Perhaps the 77mm models perform just as consistently but no-one seems to be testing them.
Nev
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henry link
Well-known member
Nev,
Similar reasoning could be applied, but there are glass differences in the Kowas and the focal ratios and apertures are closer (88mm f/5.7, 77mm f/6.5.). My assumption when I first saw the Kowa specs was that the 77mm had been designed with a focal ratio just high enough to compensate for the difference between Fluorite and whatever "XD" glass really is, so that the aberrations in the 88mm and 77mm would be similar.
BTW you can compute the effective focal length of the Kowa scopes from the model numbers of the fixed eyepieces. The model number of the 25X for the 77/88mm scopes is TE-20H indicating an eyepiece focal length of about 20mm (probably rounded to the nearest whole number). Multiply the eyepiece focal length by the magnification to find the telescope focal length (about 500mm). For comparison the 82mm Kowa scopes are 450mm and the original TSN 1-4 are 420mm.
Let me caution again that no hard and fast assumptions can be made about the quality of a scope from focal ratio alone. There are too many variables and too much else that can go wrong unrelated to focal ratio. But, focal ratio can be a very general indicator of how difficult a task the designer faces in controlling aberrations.
Henry
Similar reasoning could be applied, but there are glass differences in the Kowas and the focal ratios and apertures are closer (88mm f/5.7, 77mm f/6.5.). My assumption when I first saw the Kowa specs was that the 77mm had been designed with a focal ratio just high enough to compensate for the difference between Fluorite and whatever "XD" glass really is, so that the aberrations in the 88mm and 77mm would be similar.
BTW you can compute the effective focal length of the Kowa scopes from the model numbers of the fixed eyepieces. The model number of the 25X for the 77/88mm scopes is TE-20H indicating an eyepiece focal length of about 20mm (probably rounded to the nearest whole number). Multiply the eyepiece focal length by the magnification to find the telescope focal length (about 500mm). For comparison the 82mm Kowa scopes are 450mm and the original TSN 1-4 are 420mm.
Let me caution again that no hard and fast assumptions can be made about the quality of a scope from focal ratio alone. There are too many variables and too much else that can go wrong unrelated to focal ratio. But, focal ratio can be a very general indicator of how difficult a task the designer faces in controlling aberrations.
Henry
