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


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What does high mag reveal to you about the telescope?

I really don't understand that - "holds up well at high mag" and "breaks down at high mag". Can someone explain that to me?

Here, look at this as an example. Contender is this, probably one of the most affordable 80mm triplets on the market:

image.png.dd0bfb590e4fd05cc8b0f9d92619c749.png

When I got it - I did a test shot of some chimneys on house down the street (from where I lived at the time):

image.png.66e1e2f72aca7be0f1d0486028bbdd91.png

That is FOV with APS-C sized chip. Here is close up at 100% zoom level:

image.png.1614ac97d02caf168ac03d74977bda32.png

Second chimney - top part / gravel detail

And this is part of it zoomed further 300%:

image.png.6a18cf251d44b87ea5c8391b7aa89bf3.png

Does this fall under definition of "falls apart" or "holds together"?

I know that this is "digital" zoom - but in reality, image formed at telescope focal plane has certain definition irrespective of what sort of focal length eyepiece you use to magnify it (how much you magnify it). If EP is good enough - it will be just magnified as is - it can't turn better than it is?

 

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As we’ve said, anything observational is subjective and only the person observing can really say if the image is good to them or not. There are many factor which would go into that, human, equipment and environmental.

I recall once being enthusiastically shown a view of Jupiter at something like x500 through a large dob. Yes, it was large, but to me it was horrid, fuzzy and not showing crisp detail. It would have looked much better at lower mag given the conditions.

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3 minutes ago, vlaiv said:

What does high mag reveal to you about the telescope?

I really don't understand that - "holds up well at high mag" and "breaks down at high mag". Can someone explain that to me?

Here, look at this as an example. Contender is this, probably one of the most affordable 80mm triplets on the market:

image.png.dd0bfb590e4fd05cc8b0f9d92619c749.png

When I got it - I did a test shot of some chimneys on house down the street (from where I lived at the time):

image.png.66e1e2f72aca7be0f1d0486028bbdd91.png

That is FOV with APS-C sized chip. Here is close up at 100% zoom level:

image.png.1614ac97d02caf168ac03d74977bda32.png

Second chimney - top part / gravel detail

And this is part of it zoomed further 300%:

image.png.6a18cf251d44b87ea5c8391b7aa89bf3.png

Does this fall under definition of "falls apart" or "holds together"?

I know that this is "digital" zoom - but in reality, image formed at telescope focal plane has certain definition irrespective of what sort of focal length eyepiece you use to magnify it (how much you magnify it). If EP is good enough - it will be just magnified as is - it can't turn better than it is?

 

As a genuine question @vlaiv, with no offence meant, do you do much visual observing? I guess I’ve considered you to be much more of an imager. I think if you spent time observing you would likely know what is meant by the image breaking down. It may not be a scientific term, but it is a real enough effect visually.

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3 minutes ago, vlaiv said:

I really don't understand that - "holds up well at high mag" and "breaks down at high mag". Can someone explain that to me?

Take the sharpest, clearest view you see through your scope and keep upping the mag, if the view worsens in any way youve reached the limit or breakdown. This needs to be done under vg seeing IMHO.

OR the limit of your eyes have been reached.

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I may be mistaken, but from what I can gather from the science behind all of this the resolution of the telescope aperture limits the upper limit on magnification. According to Dawes limit to resolve 1sec of arc an aperture of just under 5" is required. As the limit of the resolving power of the human eye is 3-5arc min, say 4 for average, then that is 240arc sec. Which is why a magnification above x240 in a 5" scope is said to break down. This is where the logic behind the 2D rule comes from, it is based on the laws of optics. You can magnify more and if your optics are good then the image will still be sharp, but you won't see anymore detail because it is beyond the resolving limits of your eye. To see more detail at higher power you need more aperture.

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2 minutes ago, jetstream said:

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.

 

2 minutes ago, jetstream said:

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.

 Agreed its only a early observation Fair point. If that changes with more use i will update. But it was very comfortable with a 9mm eyepeice and then a 2x barlow. So 271 x without the barlow. The low elevation of jupiter not really supporting the X2 as well though. it was a night of excellent seeing. As was posted on the imaging page recently 

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5 minutes ago, Franklin said:

That is 2D.

I dont believe in hard and fast rules when it comes to visual observing or the eyes. Years ago I had a great article with a chart showing the effect of exit pupil/entrance pupil vs resolution. I then found my "best" exit/entrance pupil  (.8mm) and then designed my 15" dob around it which gives about 460x. I can go lower in illumination but after time this will strain the eyes so I dont go there other than to test.

Edited by jetstream
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29 minutes ago, neil phillips said:

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.. 

This is a fast frame camera, however, which again changes the situation. A fast frame camera beats the average seeing while a long exposure one does not. I won't dispute your observations on planetary observing.

Olly

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5 minutes ago, Stu said:

As a genuine question @vlaiv, with no offence meant, do you do much visual observing? I guess I’ve considered you to be much more of an imager. I think if you spent time observing you would likely know what is meant by the image breaking down. It may not be a scientific term, but it is a real enough effect visually.

Not sure how to best answer that question. I don't do much astronomy at the moment at all. Neither imaging nor observing. I don't really consider myself to be experienced observer, but I do like to observe and have done so on many occasions.

What I don't understand about image break down is this - This term sounds to me that image somehow changes. This does not happen to my eyes. I've pushed my 8" to silly mags like x500+ - and only effect I ever saw when pushing magnification higher is just that - more magnified image, but image remains the same. Once level of detail is reached - it remains like that and image just gets magnified - like images on computer - image gets bigger and no additional detail is seen. Detail that is there is blurry - simply because image is bigger without additional detail.

This process usually starts at about x1D in my scopes to my eyes. Up to x1D image is very sharp. Past that point - image sort of stays the same in terms of detail seen. I do find it more comfortable to observe at say x1.4D although no additional detail can be seen - but what is there is just a bit bigger and still not soft enough to be distracting.

That is why I don't understand the term break down - for me, image never really changes - it only becomes larger and dimmer but it stays the same in terms of contents.

 

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3 minutes ago, jetstream said:

I then found my "best" exit/entrance pupil  (.8mm)

Yes I agree, which is why 3/4 or even 2/3 of 2D gives better results in practice. But the 2D rule is based on optical physics, beyond 2D the aperture will not resolve any finer detail, so raising the magnification above 2D will just make the view larger and dimmer for the given aperture.

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12 minutes ago, Franklin said:

I may be mistaken, but from what I can gather from the science behind all of this the resolution of the telescope aperture limits the upper limit on magnification. According to Dawes limit to resolve 1sec of arc an aperture of just under 5" is required. As the limit of the resolving power of the human eye is 3-5arc min, say 4 for average, then that is 240arc sec. Which is why a magnification above x240 in a 5" scope is said to break down. This is where the logic behind the 2D rule comes from, it is based on the laws of optics. You can magnify more and if your optics are good then the image will still be sharp, but you won't see anymore detail because it is beyond the resolving limits of your eye. To see more detail at higher power you need more aperture.

If we take Dawes limit - say 5" will have resolution of 1" - or two stars will be seen as two stars if they are separated by 1" and we take visual acuity into account:

https://en.wikipedia.org/wiki/Visual_acuity

image.png.2de589fe55133966cf340e018e7e50e8.png

Person with 20/20 vision has 1 arc minute visual acuity - that means they can see two stars as being two distinct stars if they are separated by 1 arc minute.

This implies that magnification of only x60 is needed for 5" telescope to resolve two stars at 1" apart - or x60 is all that is needed. That is just x12 as minimum needed magnification in inches - much lower than x50.

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5 minutes ago, Franklin said:

Yes I agree, which is why 3/4 or even 2/3 of 2D gives better results in practice. But the 2D rule is based on optical physics, beyond 2D the aperture will not resolve any finer detail, so raising the magnification above 2D will just make the view larger and dimmer for the given aperture.

Part of getting the most out of observing is finding the best avg mag seeing will support and then buy a scope that gives that mag at "your" optimum exit pupil. Herein lies the reason the 100mm Taks perform so well in the UK.IMHO.

Btw, I usually observe Jupiter at around 300x-400x in my 15" when the planet is high and the seeing good. Staggering views actually.

Edited by jetstream
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1 minute ago, jetstream said:

Part of getting the most out of observing is finding the best avg mag seeing will support and then buy a scope that gives that mag at "your" optimum exit pupil. Herein lies the reason the 100mm Taks perform so well in the UK.IMHO.

Btw, I usualy observe Jupiter at around 300x-400x in my 15" when the planet us high and the seeing good. Staggering views actually.

15" what scope is that ? sounds interesting

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2 minutes ago, vlaiv said:

This implies that magnification of only x60 is needed for 5" telescope to resolve two stars at 1" apart - or x60 is all that is needed.

That is correct, but I don't think there are many people whose eyes can resolve at 1'. All the references I've seen take the resolution of the human eye to be 3'-5'.

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Just now, neil phillips said:

15" what scope is that ? sounds interesting

Years ago I bought an Astrsosytems dob out of Colorado and had Terry Ostahowski make me a nice mirror. Its actually "low spec" 1/5 PV but also tested at .92 Strehl ( I believe Terrys numbers) and is obstructed 21% and with no offset. This thing gives wicked lunar and planetary views under good conditions. The mirror is very smooth, a requirement of mine.

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23 minutes ago, jetstream said:

ps, my 90mm will show detail in a flys wings at 700 meters lol! and across the lake, under vg conditions.

What sort of fly is that?

If we take that fly's wing is 5mm in length on average fly - at 700 meters it will subtend about 1.5" and Dawes limit for 90mm scope is 1.3". I'm not sure if you'll be able to resolve the shape of fly's wing at that distance let alone detail on it :D

 

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2 minutes ago, jetstream said:

Years ago I bought an Astrsosytems dob out of Colorado and had Terry Ostahowski make me a nice mirror. Its actually "low spec" 1/5 PV but also tested at .92 Strehl ( I believe Terrys numbers) and is obstructed 21% and with no offset. This thing gives wicked lunar and planetary views under good conditions. The mirror is very smooth, a requirement of mine.

Wow sounds great. Is that driven ?  Have you ever imaged with it ? 

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1 minute ago, vlaiv said:

What sort of fly is that?

If we take that fly's wing is 5mm in length on average fly - at 700 meters it will subtend about 1.5" and Dawes limit for 90mm scope is 1.3". I'm not sure if you'll be able to resolve the shape of fly's wing at that distance let alone detail on it :D

 

lol! Dragon flys among others sitting on an island across the bay, btw my eyes are 20/10 the doc says. Does the Dawes limit apply to flys? :grin:

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Just now, neil phillips said:

Wow sounds great. Is that driven ?  Have you ever imaged with it ? 

No but Vlaiv is helping me to be able to image someday, trying with 90mm first if I ever get going on it. No, not driven other than by the wheel barrow handles :grin:

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7 minutes ago, jetstream said:

Btw, I usually observe Jupiter at around 300x-400x in my 15"

The 15" scope can support these high powers. 15" aperture scope according to Dawes has a resolving power of about 0.3". So in theory should support magnifications upwards of x650. 300x-400x in a 15" scope is around 1D and your exit pupil around 1mm, that is why it looks good.

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2 minutes ago, Franklin said:

That is correct, but I don't think there are many people whose eyes can resolve at 1'. All the references I've seen take the resolution of the human eye to be 3'-5'.

You can test your eyesight right now if you know pixel pitch of your computer screen.

Make image that consists out of three pixels - two white ones and one black in the middle. See what sort of distance from your screen you need in order to see two distinct pixels.

Use angle / size / distance calculator to figure out angular size:

http://www.1728.org/angsize.htm

also look at this article:

https://en.wikipedia.org/wiki/Snellen_chart

It is about this chart:

192px-Snellen_chart.svg.png

Letters in that chart are each 5x5 elements - look at first E on the top - space between top and bottom notch on right side of E is one element - top side is 5 elements - white space between top and bottom bars are 3 elements. Each letter is like that - and if you can recognize the letter - you have certain vision - for 5' letter size - you have 20/20 vision - which means that you can see all the notches and spaces on 5x5 grid that is 5' on the side - smallest element that you can see as distinct is 1' with 20/20 vision.

 

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