Slow scopes? (1 Viewer)

willemjacobusse · Mar 10, 2008

I have been reading lots about optics and "fast" versus "slow", slow being more forgiving as far as abberations are concerned.

So the obvious question, are there are any good slow scopes out there?
Lets assume for a moment that size is not a real problem.

ullihoeger · Mar 10, 2008

I don't understand your question. "Slow" and "Fast" are terms used for photo lenses in regards to their aperture (and possible shutter speeds). "Fast" lenses are bigger and heavier (and exponentially more expensive) than "Slow" lenses with the same focal length. Color fringing is a different problem, still more likely to occur in "cheaper" slower lenses.

In scopes the question is APO or non-APO when dealing with color issues, and aperture when dealing with light gathering ability (image brightness). Both can be independently addressed and both will bring up the price tag. A non-APO scope will be cheaper than the APO version, a bigger (i.e. 80mm) scope will be more expensive than a smaller (i.e. 60mm) version of the same kind.

Bottom line: it depends what you are willing to compromise on: color fringing or light gathering capabilities.

So you wonder if there are any good and cheap (meaning not to pricey) scopes?
Depends on your requirements and intended use!

UH

willemjacobusse · Mar 10, 2008

In scopes, speed has to do with focal ratio (aperture/focal length), fast is anything under 1/6 I believe.
Spotting scopes tend to be compact, i.e. relatively short and thus relativrly fast.

henry link · Mar 11, 2008

Yes, all things being equal telescopes with higher focal ratios have lower aberrations than telescopes with lower focal ratios, but of course all things are seldom equal. A few of the other other things to consider are the diameter of the objective lens and the design and glass types used. For instance, a doublet objective will need a focal ratio about 50% higher to achieve the same color correction as a triplet (once again all other things being equal), but triplets are harder to make well. A rough measure of the focal ratio required for achromatic correction in a doublet can be calculated by multiplying the lens diameter in inches by 3 (a 2.4" scope needs a focal ratio of f/7.2, a 3" scope needs f/9), so a larger scope needs a higher focal ratio for the same correction.

To answer your question in general terms it makes me happy to see a 60-65mm scope with an f/7 ED triplet objective. It would be hard to screw that up. It arouses my suspicions when I see an f/5.7 80-90mm scope. It's much harder to get that right.

willemjacobusse · Mar 11, 2008

henry link said:
To answer your question in general terms it makes me happy to see a 60-65mm scope with an f/7 ED triplet objective.

Thanks Henry, so which scope would meet those criteria?

henry link · Mar 12, 2008

Well, I'll mention one I have some experience with, compared only to it's siblings. The Swarovski 65mm scopes have f/7.1 objectives and the 80mm's have f/5.8 objectives of the same design. IMO the 65HD is easily the highest quality scope of the bunch, much better than the standard 80mm but also a higher quality scope (per mm of aperture) than the 80HD because of the lower aberrations from both the smaller objective and higher focal ratio. In that case, for once, everything besides focal ratio (and aperture) is equal (glass types, objective design, erecting system, eyepiece). As always, however, scopes have to be judged individually. A really good 80ED specimen would be better than a mediocre 65HD.

willemjacobusse · Mar 18, 2008

Thanks Henry, it seems that Uli is having the same sort of intuitive problem that I have. I would like Uli, imagine that exposure times are related to Objective diameters. The bigger the diameter, the more light, the shorter the exposure. I still fail to comprehend what focal ratio has to do with that.

Or does it have to with light being gathered by a faster scope from a larger field of view or being projected onto a smaller surface area?
You seem to know a lot about this sort of stuff and may you are able to explain. Sofar I have only found statement, not any satisfactory explanations and this is after all supposed to be science, not religion.

Another question;
Apart from being relatively short, do "faster" scopes have other advantages?

iporali · Mar 19, 2008

Willem,

To be specific, spotting scopes (with eyepieces on) actually don't have focal ratios (they are afocal) - they only have power/magnification. In practice, however, the primary image is formed by the scope objective, which is focal (ie. has f-length and f-ratio). If you forget the eyepiece, the scope objectives behave almost like the camera tele lenses. "Fast" focal ratios ("apertures") produce almost always less sharp pictures than the same lens "stopped down" a bit. The trade-off of stopping down is - as you indicated - a darker image. Fast focal ratio means large front lenses which usually work well at the center but not as well closer to the edges where the optical aberrations start to degrade the image. "Less fast" focal ratio made by stopping down prevents the aberrated light rays from spoiling the good image formed by the center of the lens. The same effect can be achieved using a small objective lens in the first place...

Pros of fast spotting/tele/scopes:
a) At given focal length (eg. Swaro 65 (slow) vs 80mm (fast)) - better resolution, brighter image (large exit pupil)
b) At given power (Swaro 65 (slow) vs Leica 62mm (fast)) - smaller size.
Ability to use low powered wide angled eyepieces. In good light eye pupil does the stopping-down.

Cons of fast spotting/tele/scopes: more aberrations, which become visible in poor light and/or high powers.
a) At given focal length - larger size, more expensive (ED-glass more important),
b) At given power - inferior image (loss of ER when zooming).

Hope you can figure out something useful from all this :t:

Ilkka

EDIT: In Finland where dusk and dawn last very long, many birdwatchers prefer large (fast) scopes, whereas in conditions where there is more light and a rapid transfer from brightness <-> darkness a slow scope may actually perform better.

greg g · Mar 19, 2008

All,
If I understand this correctly does the Kowa 88 equal a F/5.68 ? If this so is it also a major concern for abberations similar to the 80HD swaro? Based on the mathmatical formula on five major scopes (Zeiss, Lieca, Swaro, Kowa, Nikon) in the 60mm to 90mm range is the the f/7.1 swaro 65 considered the one that meets this lower abberations by a manufacturer with the proper quality control the best based strictly at this math criteria?
Thanks, Greg

willemjacobusse · Mar 19, 2008

I came across an example concerning telephoto lenses.
One wld be 100, f/4, there for a diameter of 25mm.
The other wld be 135 also f/4, therefore a diameter of 33.75mm

Despite the different diameters (apertures) both wld have identical shutter speed. Thats what doesnt seem to make sense to me!

henry link · Mar 19, 2008

Imagine photographing a very simple object with these two lenses; for instance a white circle on a dark background. If the white circle forms an image 2.5mm in diameter on the film or camera sensor using the 100mm lens then photographing that same circle will form an image 3.375mm in diameter on the sensor when the 135mm lens is used. The surface brightness of the circles formed at the sensor plane is identical because the 135mm image circle is larger than the 100mm image circle in the same ratio as the lens apertures. The surface brightness of the image circle formed at the sensor would be the same for any f/4 lens, but the size of the circle would vary with the focal length of the lens. The unvarying surface brightness of different size image circles formed at the sensor is why the shutter speed doesn't vary.

willemjacobusse · Mar 19, 2008

Thanks Henry, I think I am finally getting it!

Trinie · Mar 19, 2008

Henry,

What type of glass is better for reducing or eliminating purple fringing? Would you know which scopes do a good job at, starting from the best downwards?

willemjacobusse · Mar 19, 2008

Last Friday, Mooreorless posted a link to a clearance sale at www.optcorp.com for an 80mm ED Celestron. It was an f/7.5 and looked accordingly long, probably more aimed at the astronomy crowd. Looks like I wasnt the only one interested since they were already sold out by Monday!
According to Henry's criteria it should be at least an f/9 but interesting all the same.

npos · Mar 19, 2008

willemjacobusse said:
Last Friday, Mooreorless posted a link to a clearance sale at www.optcorp.com for an 80mm ED Celestron. It was an f/7.5 and looked accordingly long, probably more aimed at the astronomy crowd.

Hi, may I ask what your intended use is? If not astronomy, I presume a telescope for wildlife and general terrestrial viewing, and most of these instruments embody a set of compromises to achieve field worthiness such as weather resistance, portability, robustness, wide views and so forth. Perhaps consider the big picture rather than focal ratios alone?? An expensive "fast" instrument may have been rather more corrected for image aberrations than a low-end low-cost "slow" one.

Just a thought. Please correct if I am off here.

Norm P

willemjacobusse · Mar 19, 2008

Good questions: we have a house on the coast with a great view, so a scope in a fixed spot wld be great, not much need to move it around. And using it once or twice for stargazing maybe. So not your typical mobile scope use I would say. On the other end of the spectrum, my girlfriend wld probably want something ultraportable. So who knows we might end up with two.

henry link · Mar 19, 2008

You're not off, Norm. I may have created some misunderstanding about this. No reliable prediction can be made about the quality of a telescope from the focal ratio alone. In birding scopes low focal ratios are often a limiting factor, but there are plenty of other things to go wrong. That's why I chose the Swarovski example. I assumed that the 65mm and 80mm Swarovskis are identical as to glass types, optical design of the objective, erecting prisms, eyepieces and quality of construction. The only differences would be aperture and focal ratio. Everything changes if the Swarovskis are compared to another brand.

I would say that glass is mostly useless as a predictor of color correction in "APO" type birding scopes because the glass types actually used are unknown. Marketing terms like ED, FL, UD, HD, etc. are not glass types. Even if we knew what those really are we still wouldn't know the mating glass types which are just as important.

npos · Mar 19, 2008

henry link said:
I would say that glass is mostly useless as a predictor of color correction in "APO" type birding scopes because the glass types actually used are unknown. Marketing terms like ED, FL, UD, HD, etc. are not glass types. Even if we knew what those really are we still wouldn't know the mating glass types which are just as important.

Yes, marketing hype is something to consider and stand wary of. I use a birding scope that advertises "pure flourite crystal," but that may be just one element in the objective lens group, and which one I am not totally certain of. I presume the first element only mated with conventional glass, though the pointer in this picture points to the second air-spaced element.

I could stop my 88mm aperture down by punching a 60mm hole in the lens cap.

http://kowa-prominar.com/catalog/pdf/tsn880.pdf

Norm P

henry link · Mar 19, 2008

Norm,

Thanks for the link. I had only seen that page before with Japanese text. Like so much marketing information this is in turn frustrating incomplete and a little misleading. For instance, can you tell from this whether the Kowa has a doublet or triplet objective? The illustration is not clear. The text implies doublet when it mentions a convex element paired with a concave element. The first element is certainly convex and what follows may be a single thick concave element or a perhaps a cemented doublet. The mating element is said to be made of glass "with special dispersion properties to reduce the chromatic aberration to a further degree". I think that phrase probably means a glass type with high dispersion that is more nearly complimentary to Fluorite than common crown glass, so that the combination is more color free, but I'm not certain. Two low dispersion elements used together would make the CA worse. The slightly misleading part is the implication that Calcium Fluoride stands alone as a low dispersion lens material. I think they are probably aware at Kowa that there are several glass types available now that have characteristics almost identical to Fluorite, but it's not in their interests to mention that.

I suppose I shouldn't complain. This is actually more information than we usually get.

Henry

Sout Fork · Mar 20, 2008

OK.
Now that we have had a course in optics 101 it's time for the final exam.

The following were taken with a wide range of scopes:

1. 120mm, f/8.3, 1000mm fl, achro astro scope barlowed to 60x?
2. 100mm, f/5 500mm fl, achro astro scope (a hundred buck scope)
3. 100mm, f/12, 620mm fl, "Ed" spotter.
4. 80mm, f/7, 560mm fl, "APO triplet" astro scope

Anyone care to quess which pic was taken with which glass?

Slow scopes? (1 Viewer)

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