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Old Friday 9th February 2018, 08:33   #1
MacHector
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The Objective Lens

Hi All

All other things being equal, what will help you see better, a mid-range scope (for example, and Opticron ES 80 GA with an HDF 20-60 eyepiece) with an 80mm objective lens, or a high-end scope (for example, a Swarovski ATX 20-60) with a 65mm objective lens?

Presumably the mid-range 80mm objective lens will theoretically let in more light, but will the higher quality optics in the high-end 65mm lens compensate for that? What is it that objective lens does but let in light (and increase weight)?

Advice welcome.

Thanks

MacHecor
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Old Friday 9th February 2018, 10:33   #2
Binastro
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Hi Machector.

Theoretically, a larger objective resolves finer detail, and lets in more light than a smaller one if the transmission is the same.
The transmission depends on coatings and the glass material.

Although theoretically there shouldn't be much difference, longer focal length, i.e. slower, objectives resolve better.

A 3 inch aperture (76mm) doublet objective of good quality needs to be f/9 to reduce CA to good levels. However, Conrady's stricter limits are f/15 for the same doublet.
Some objectives are a bit undersized. Actually 76mm when stated as 77mm, and sometimes larger differences. For instance my original 3 inch diameter telescope was actually 73mm aperture.
With binoculars there is a lot of cheating. A 50mm binocular can be as small as 39mm because of vignettingg.
Very cheap 25mm scopes used to be as small as 10mm and single glass or even plastic because of a stop behind the objective.
One needs to actually measure the aperture.
The new Zeiss Harpia scope is said to be much smaller at low magnifications because of unusual characteristics.

By using triplets or ED glass shorter focal lengths can be used, but there are few spotting scopes that can equal a long focal length astro doublet or triplet.

My Pentax 100mm f/12 doublet refractor could easily and usefully handle 300x and the image did not break down at 400x. I have not heard of any spotting scope that could do this, although I suppose a best sample Swarovski 95mm might get close.

Having said all this, spotting scopes are relatively low magnification instruments. Few are used at high enough power to use the full resolution, unless the observer has particularly fine eyesight.

Also most don't seem to be made to high quality astro standards.

The only way to know which scope is better is to test them side by side by yourself.

Last edited by Binastro : Friday 9th February 2018 at 10:57.
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Old Friday 9th February 2018, 11:36   #3
bioscope
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Hi Hector -

the bigger lenses gather more light and that leads to a bigger exit-pupil. That's only physics and has nothing to do with quality. I think, the Opticron 80 isn't not a bad scope, and in low-light it can be better used than the 65mm.
I own a 85mm Zeiss DiaScope and a 60mm MM4 - the 60mm is tack-sharp and with the Gitzos a light and satisfying combo - in bad light conditions and higher magnifications the exit pupil is too small and the Zeiss give me more observing time at the end of daylight. If you observe more in dawn or under very cloudy skies, you need perhaps the bigger lens. But you have to mount it on asturdier tripod and have to carry the combo.

Bigger is not always better - but sometimes helpful.

Manfred

http://www.juelich-bonn.com/jForum/read.php?9,436519

Last edited by bioscope : Friday 9th February 2018 at 11:40.
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Old Friday 9th February 2018, 17:31   #4
Wetstuff
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MacHecor... I'm glad you asked that question. I am going thru one of those 'weight vs usability' decisions. I can buy anything but am a practical sort.

May I add to MacHector's question.. 'using those same examples, 65m vs 80mm: What if you do not anticipate doing long distance viewing?

I am in 'high humidity zone' during summer and assume I will not get a clear view at a distance even with 200mm. (I don't know...) Thanks.

Jim
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Old Friday 9th February 2018, 20:45   #5
etudiant
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Quote:
Originally Posted by Wetstuff View Post
MacHecor... I'm glad you asked that question. I am going thru one of those 'weight vs usability' decisions. I can buy anything but am a practical sort.

May I add to MacHector's question.. 'using those same examples, 65m vs 80mm: What if you do not anticipate doing long distance viewing?

I am in 'high humidity zone' during summer and assume I will not get a clear view at a distance even with 200mm. (I don't know...) Thanks.

Jim
Ease of view really matters if one expects to use a scope extensively.
I can get good looks with my ED50 travel scope, but it is not as comfortable a view as through a big scope. Not sure why, perhaps better light gathering.
So in the situation outlined, where light weight and small footprint are not major factors, the big glass is hands down preferable imho.
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Old Saturday 10th February 2018, 08:18   #6
Troubador
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Quote:
Originally Posted by Wetstuff View Post
MacHecor... I'm glad you asked that question. I am going thru one of those 'weight vs usability' decisions. I can buy anything but am a practical sort.

May I add to MacHector's question.. 'using those same examples, 65m vs 80mm: What if you do not anticipate doing long distance viewing?

I am in 'high humidity zone' during summer and assume I will not get a clear view at a distance even with 200mm. (I don't know...) Thanks.

Jim
Jim
You ask a good question and I am not making fun of it if I say that, if you aren't viewing long distances then binoculars win easily. The benefits of viewing with both eyes makes them the clear choice for short and medium distances.

Lee
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Old Saturday 10th February 2018, 08:34   #7
Troubador
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I think the OP's question is impossible to answer definitively. So much depends on what is meant by 'see better' and some lower priced optics perform above expectations while some top-end scopes have been known to attract criticism.

One thing is relatively clear. A bigger objective giving a bigger exit pupil gives a more easily accessible view. Zeiss's Harpia has an EP limited to 2.5mm but compensates for this by having the same angle of view at all magnifications which is so relaxing to use.

Binastro has outlined the theoretical advantages of bigger objectives but clearly these advantages will only be realised if the lenses are made in the right way and have good quality coatings.

A 65mm or similar definitely comes with disadvantages but it comes with portability which shouldn't be under-estimated. If you plan a hike along a Scottish coast hoping for Sea Eagles you might well be reluctant to commit to carrying an 80mm scope whereas a 65 would not be such a burden. Even with scopes already set on tripods I would bet a 65 will be faster out of a car and viewing than an 80.

As to whether a 65 could beat an 80 optically I am sure this is possible but the 80 would have to be pretty substandard and don't forget that although the objectives gather the image, you examine the image using the eyepiece. If the eyepiece is rubbish then rubbish is what you see never mind how big the objective is.

Lee
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