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Why do really expensive scopes sell and what attracts us to them ?


John
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13 minutes ago, jetstream said:

You need 2 scope of the same focal length and aperture- one that has a fast snap focus and one that has a longer focus range. This will eliminate f ratio as the factor and confirm or deny this idea.

Eagerly waiting test!

In that test you'd probably actually be testing more for spherical aberration,which has the effect of creating a larger region of "least-bad" focus, so my first suspicion would be that the longer-focal-range instrument is the worse-corrected scope of the two.

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16 hours ago, Stu said:

What do you mean by a triplet having a longer focus range? I

The LZOS does snap to focus and stay's there. Where as with a the F7.7 doublet I need to adjust more, the focus point is over a shorter range hence hard to find/keep.

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11 hours ago, jetstream said:

Fundamentally, depth of focus depends on the focal ratio of our eye's own lens. This is why the exit pupil is important in this respect for telescopes in the afocal configuration (where it directs essentially parallel light to the eye as from a great distance.) Note the implication; the telescope objective's f/ratio is of no import, while the eye's own f/ratio is. "

Great so I'm reporting how my own eye works and not the scope... 😃😆

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48 minutes ago, Deadlake said:

The LZOS does snap to focus and stay's there. Where as with a the F7.7 doublet I need to adjust more, the focus point is over a shorter range hence hard to find/keep.

You are saying two contradictory things here I think!

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To me, term snap to focus means following:

It's defocused - you turn focusing knob, turn, turn and then - suddenly it's in focus :D - snap to focus

I'd say effect has more to do with mechanics of focuser and seeing conditions than quality of optics.

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So many variables involved it is perhaps impossible to come up with a precise objective definition of many of the things discussed here so of course we fall back on the good old subjective opinion. 🤔

And yes if we all had identical eyeballs it would make things easier.

 

Edited by johninderby
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Do we see more or less with our eyes than the camera sees? That depends very much on what we're looking at, I suspect. As a rule of thumb I'd suggest that the brighter it is, the better is the eye and the fainter it is, the better is the camera. In a given scope - say a 4 inch refractor.  It's ludicrously easy to photograph the Horsehead nebula and ludicrously difficult to see it. But it's ludicrously easy, at the eyepiece, to resolve four stars in the neighbouring Trapezium while it's  moderately tricky to resolve it in images. The moment you want to observe the fifth and sixth stars you'll find it easier at the eyepiece than the camera, where you'll need an exceptional site and, probably, a tight H alpha filter. 

In amateur instruments I'd rather observe stars than photograph them but I'd rather photograph the faint things.

Olly

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The often quoted formula for the maximum magnification a particular aperture telescope can handle is 2D, where D is in mm. Personally and through experience at the eyepiece, I think a more conservative 3/4 or even 2/3 of 2D is more appropriate and will lead to less disappointment and sharper, more defined views, albeit on a smaller scale. However, the 2D formula is interesting because at that point in the optical system the exit pupil will be 0.5mm and with a 0.5mm exit pupil everything that the optics have to offer will be on show. The good and the bad. If you own a telescope which will still snap into a crisp focus at an exit pupil of 0.5mm then you have got a good'n. Atmospheric conditions play a big part in whether or not you can push a scope to it's theoretical limits.

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That 2D for D in mm or x50 per inch in inches - is really not founded in science. Even term - maximum useful magnification is very strange.

If you want to go by science - you'll actually get much lower values for "magnification that shows all there is to be seen".

How much more magnification is needed for comfortable observing - depends on observer. It is a bit like text on computer screen. At certain font size - you'll be able to read the text. This depends on your eyesight. But that is not font size that you'll use to read the text - you'll probably want a bit larger text than smallest one that can be read.

There is no upper limit of how large text can be - similarly, there is no upper limit to how magnified image can be - with one exception. Magnified image gets darker and that can start to cause issues at some point (sooner with smaller apertures than with larger).

 

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Back to expensive scopes- many fracs will support 80-100x aperture and my own 15" dob supports 60x aperture on the moon, with the very fast in and out "snap" focus- thats 900x or so. The little Chinese optic 90mm Raptor goes over 80x aperture on the moon and has a vg but not the best snap.

I think that expensive scopes should ensure vg optics and the process of ensuring this adds cost, not to say less expensive scopes like my 90mm arent right up there either.

Would I pay $15000 for a refractor around 150mm-160mm? thought about it, but no, I'd go for more aperture in different design scopes, but to each their own.

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26 minutes ago, jetstream said:

In my 2 best scopes, one f4.8 the other f7.5 the focus is BAM in & BAM out... my SW120ED has a broad focus range and also doesnt support really high mag, 300x on the moon, less on Jupiter, 250x or a bit better maybe.

Lol my stella lyra appears the exact opposite. Very broad long focus range. And it does support high mag

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2 minutes ago, vlaiv said:

How much more magnification is needed for comfortable observing - depends on observer

True, it all boils down to the limit of resolving power within the human eye. Whittaker's rule has maximum magnification at 1D in mm and states that definition starts to break down from diffraction effects when 1D is exceeded. Experience at the eyepiece says otherwise IMO.

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19 minutes ago, ollypenrice said:

Do we see more or less with our eyes than the camera sees? That depends very much on what we're looking at, I suspect. As a rule of thumb I'd suggest that the brighter it is, the better is the eye and the fainter it is, the better is the camera. In a given scope - say a 4 inch refractor.  It's ludicrously easy to photograph the Horsehead nebula and ludicrously difficult to see it. But it's ludicrously easy, at the eyepiece, to resolve four stars in the neighbouring Trapezium while it's  moderately tricky to resolve it in images. The moment you want to observe the fifth and sixth stars you'll find it easier at the eyepiece than the camera, where you'll need an exceptional site and, probably, a tight H alpha filter. 

In amateur instruments I'd rather observe stars than photograph them but I'd rather photograph the faint things.

Olly

That may well be true Olly but my comments regarding camera detail on jupiter, are pretty valid i think. This is not something i am guessing i just noticed a couple of times here. But 15 years worth of noticing it.. 

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1 minute ago, Franklin said:

True, it all boils down to the limit of resolving power within the human eye. Whittaker's rule has maximum magnification at 1D in mm and states that definition starts to break down from diffraction effects when 1D is exceeded. Experience at the eyepiece says otherwise IMO.

I'm not even sure what does "start to break down" really mean.

I've seen that used countless times - but I never really understood the term. I do agree that 1D is all that is needed for sharp eyed, and I personally enjoy the most that sort of magnification (maybe a bit more - at about 1.2-1.4 D - maybe I'm not as sharp eyed as one can be :D ).

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Hello all.  I've been really enjoying this thread and where it has meandered since @John OP kicked it off..  Must admit the technical discussion has gone somewhat above my head. Here's my take for what it's worth. Apologies if not in any particular order of merit. I am a mark 1 eyeball observer. 

Enjoyed the hobby for many years now but it has always been tempered by a few limitations that I have stuck to. Some by choice, some not.

Time, skill, funds, fluctuating enthusiasm and my limited knowledge. I'm more than happy that I have some nice kit. From what I've  read on the site via posts and threads, I count just one piece of gear I have, rightly or wrongly,  - a 2nd hand, 2 in, Green and Black Panoptic 27mm EP, as "high end". My other gear is to me as much as I can willing afford with a clear conscience, weighed against my observational skills, how well the kit works for me and the weather.  I am happy to experiment with it all and it doesn't tax my skill level. 

I "trade up " kit as and when I see fit and it gives me nearly as much fun as being out in the field, as to get my hands on a new to me, EP for example.  If I think a particular piece of gear will be a nice step up the food chain from what I currently have, then I will open the c card account. 

The 5%  "extra" that I seem to see quoted quite regularly on the forum, that a top quality scope can give to the user I'm sure is true but I do not think that I would ever make use of that premium kit to it's full potential. This is where the buck stops for me. I think that our weather has a huge bearing on this 5% too. Does it play ball to merit the cost. 

However. I'm more than happy to read about the views such gear gives to forum member's and it's a case of get the best use out of your scope as regularly as you can. That's where the "worth" of your scope lies in my mind. I love to read about and watch tv programmes about objects I will never be able to afford - cars, planes et al -  but where's the harm. There is none.  

I hope everyone can do this as much as they can with there equipment and enjoy this somewhat frustrating hobby of ours to the full. 

Thx  John 

 

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9 minutes ago, jetstream said:

How high razor sharp?

The design itself is comfortable at high mag. Razor sharp will be more seeing dependent. And i should add collimation dependent. Which i am now getting closer to as far as good collimation goes

Edited by neil phillips
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1 minute ago, neil phillips said:

The design itself is comfortable at high mag. Razor sharp will be more seeing dependent. 

So what mag?

most every design scope can be comfortable at high mag, given proper execution of the design. I evaluate my optics many times, under my best seeing before drawing conclusions.

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33 minutes ago, Franklin said:

The often quoted formula for the maximum magnification a particular aperture telescope can handle is 2D, where D is in mm. Personally and through experience at the eyepiece, I think a more conservative 3/4 or even 2/3 of 2D is more appropriate and will lead to less disappointment and sharper, more defined views, albeit on a smaller scale. However, the 2D formula is interesting because at that point in the optical system the exit pupil will be 0.5mm and with a 0.5mm exit pupil everything that the optics have to offer will be on show. The good and the bad. If you own a telescope which will still snap into a crisp focus at an exit pupil of 0.5mm then you have got a good'n. Atmospheric conditions play a big part in whether or not you can push a scope to it's theoretical limits.

That limit varies depending on the scope size I think, certainly for the U.K. take a 10”/250mm scope. For that, a 1mm exit pupil is a perfectly sensible maximum on most nights. Apply that to a 100mm apo and you would leave a lot of performance on the table; down to 0.5mm and even further, taking you to x200 or x300 is perfectly useable under the right conditions.

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