90x Per Inch!!...How?

[andrew s]

1 minute ago, Stu said:

It was the ‘it confirms why I like reflectors’ comment afterwards which seemed to imply you weren’t impressed with it?

Just crossed wires I assume? 👍👍

Indeed. I was trying to be amusing. 

Regards Andrew 

[chiltonstar]

Well, I got up to 75x per inch last night with my classic (ie old) 4" f13 refractor and it was still pretty sharp. Saturn was glorious with a lot of detail, ditto Jupiter with I think a transit taking place, and even Mars at quite low altitude (23:30) was sharp enough to show the S polar cap well and a good bit of detail on the disk itself. A tribute to the seeing I think, odd really because the weather forecast was dismal with lightning flashes everywhere.

Chris

[Dave Lloyd]

Neither my ST102 or evo120(non ed) are much good over about 35/40x per inch. 

[jetstream]

Our H130 goes 53x/inch, the SW 120ED just over 60x/inch and the Stellarvue 90mm Raptor 88x/inch.

If I barlow the 2.4mm HR 1.5x I get 1.6mm for an unbelievable sharp 562x and an incredible 120x/inch in the TSA120.

Edited August 17, 2020 by jetstream

[John]

Top quality optics = top quality high magnification performance, when the seeing allows.

It's not rocket science

[chiltonstar]

58 minutes ago, John said:

Top quality optics = top quality high magnification performance, when the seeing allows.

It's not rocket science

...but of course within the limits imposed by the size of the Airy disk for that aperture, I presume?

Chris

[John]

17 minutes ago, chiltonstar said:

...but of course within the limits imposed by the size of the Airy disk for that aperture, I presume?

Chris

From some reports I read on forums, apparently not 

 

[Stu]

8 hours ago, chiltonstar said:

Well, I got up to 75x per inch last night with my classic (ie old) 4" f13 refractor and it was still pretty sharp. Saturn was glorious with a lot of detail, ditto Jupiter with I think a transit taking place, and even Mars at quite low altitude (23:30) was sharp enough to show the S polar cap well and a good bit of detail on the disk itself. A tribute to the seeing I think, odd really because the weather forecast was dismal with lightning flashes everywhere.

Chris

That’s what I’ve used with the Vixen 4” Fluorite, x300 and it seemed quite happy. I imagine it would take more on an excellent night.

[John]

38 minutes ago, Stu said:

That’s what I’ve used with the Vixen 4” Fluorite, x300 and it seemed quite happy. I imagine it would take more on an excellent night.

My earlier point was that's what top grade optics can do.

More "run of the mill" optics do seem to reach their useful limits at lower magnifications, even if the seeing conditions are good.

 

 

[Stu]

4 hours ago, John said:

My earlier point was that's what top grade optics can do.

More "run of the mill" optics do seem to reach their useful limits at lower magnifications, even if the seeing conditions are good.

Yep, don’t think I was contradicting you John?

I’ve certainly found the benefit of top notch optics with Solar observing for example. When you get every element optimised you can throw plenty of power at it and the contrast and detail is maintained. I’ve had stunning white light solar views with my Tak when the seeing co-operates, same as or better than I recall with my 120EDs with a Lunt wedge but in a much more convenient package.

[chiltonstar]

I think you made the point Stu about how large the Airy disks are with a small refractor (looking at doubles). How then do we see sharp views with a small frac at high magnifications when the Airy disks are so large at eg x300 and are blurring detail to that extent? Are we fooling ourselves.....?

Chris

[vlaiv]

18 minutes ago, chiltonstar said:

I think you made the point Stu about how large the Airy disks are with a small refractor (looking at doubles). How then do we see sharp views with a small frac at high magnifications when the Airy disks are so large at eg x300 and are blurring detail to that extent? Are we fooling ourselves.....?

Chris

I think so, but am happy to be proven otherwise.

[John]

Is it because a quality objective lens puts a very high % of the energy into the airy disk and very little elsewhere ?

 

[jetstream]

Regardless of what the scope does within limits the eye/brain presents a sharp image, if its "pleased". I think.

[chiltonstar]

7 hours ago, John said:

Is it because a quality objective lens puts a very high % of the energy into the airy disk and very little elsewhere ?

 

...but if that Airy disk is large and easily visible at the mag used, does this make any difference?

Chris

[vlaiv]

16 hours ago, chiltonstar said:

...but of course within the limits imposed by the size of the Airy disk for that aperture, I presume?

Chris

 

16 hours ago, John said:

From some reports I read on forums, apparently not 

 

@John you are actually saying that according to some reports, people are resolving more than theoretical limit for perfect aperture of a given size?

Can you quote / link single example of such claim?

[iPeace]

Just to see whether I get this; if I were to explain this (to someone vaguely interested) in very simplified terms (expressly taking into account the arbitrary nature of the selected criteria on which the given size of exit pupil is based, so not as 'rule of thumb'), could it be like this?

• compare to an image comprised of pixels displayed on a computer screen
• aperture determines the number of available pixels
• exit pupil determines the extent to which the available pixels are involved in the actual image
• above a certain 'optimum' (bear with me, here) size exit pupil, not all of the available pixels are used to display the image (more magnification will result in more of the available pixels being involved in the actual image)
• at the 'optimum' size exit pupil, all of the available pixels are involved in the actual image (it's as resolved as it's going to get)
• below the 'optimum' size exit pupil, you're not involving more pixels (as there are no more), you're making the individual pixels appear larger
• all things being equal (conditions, design and quality of optics, experience/expertise of observer), making the individual pixels appear larger can help with detection of the details of the resolved image, thus enriching the image subsequently formed by the human brain (= a potential benefit of using higher magnification), at the cost (possibly) of perceived sharpness (= a potential drawback of using higher magnification; 'looking at larger pixels')
• in practice, perceived results will vary according to all things not being equal, one of them being personal taste

(With particular thanks to @vlaiv for the detailed explanations.)

Edited August 18, 2020 by iPeace

[Stu]

10 hours ago, chiltonstar said:

I think you made the point Stu about how large the Airy disks are with a small refractor (looking at doubles). How then do we see sharp views with a small frac at high magnifications when the Airy disks are so large at eg x300 and are blurring detail to that extent? Are we fooling ourselves.....?

Chris

I guess it’s a matter of presentation Chris. Most of the light is concentrated into the airy disk, so what we see are nice clean (large) disks which do hide a lot of mess I guess? So long as the scope is capable of resolving sufficiently to separate the disks then you will be able to split the stars cleanly.

I’m not necessarily an advocate of refractors being able to perform beyond their theoretical limits, but I do think larger newts and compound scopes often perform well below their theoretical limits due to cooling, collimation and seeing issues. Their albeit smaller airy disks are often hidden in a mess of diffraction spikes and heat plumes so I find you rarely actually see them to the same degree, and actually stars are often harder to split than in a smaller scope.

I agree that a larger scope will theoretically split tighter doubles, but, real world performance is what counts to me.

A genuine question though, have advances in refractor optics design and coatings changed anything in terms of performance reaching closer to theory than was the case in the past?

[vlaiv]

23 minutes ago, iPeace said:

Just to see whether I get this; if I were to explain this (to someone vaguely interested) in very simplified terms (expressly taking into account the arbitrary nature of the selected criteria on which the given size of exit pupil is based, so not as 'rule of thumb'), could it be like this?

• compare to an image comprised of pixels displayed on a computer screen
• aperture determines the number of available pixels
• exit pupil determines the extent to which the available pixels are involved in the actual image
• above a certain 'optimum' (bear with me, here) size exit pupil, not all of the available pixels are used to display the image (more magnification will result in more of the available pixels being involved in the actual image)
• at the 'optimum' size exit pupil, all of the available pixels are involved in the actual image (it's as resolved as it's going to get)
• below the 'optimum' size exit pupil, you're not involving more pixels (as there are no more), you're making the individual pixels appear larger
• all things being equal (conditions, design and quality of optics, experience/expertise of observer), making the individual pixels appear larger can help with detection of the details of the resolved image, thus enriching the image subsequently formed by the human brain (= a potential benefit of using higher magnification), at the cost (possibly) of perceived sharpness (= a potential drawback of using higher magnification; 'looking at larger pixels')
• in practice, perceived results will vary according to all things not being equal, one of them being personal taste

(With particular thanks to @vlaiv for the detailed explanations.)

Only objection to this very nice explanation that I have is that it can be misleading to anyone knowing anything about pixels

Pixel is often considered to be a unity / single quantity / binary object. It is either lit or not. It is a dot that can either be seen or not. If you compare detail in a telescope to something like that, there is a chance that people will think something along the lines:

"if feature is smaller than telescope resolving capability - it will not be seen" (it's smaller than single pixel).

Or, as the opposite of that - I'm seeing this thing (let's say crater on the moon) it does look like a featureless blob (not resolved - you can't even tell it is crater it is just dark "dot") but I know it is crater that is smaller in size then what my telescope is supposed to resolve - therefore my telescope "resolves beyond theoretical limit".

This is not how telescopes and resolution works - every star that we observe in our night sky is smaller than what our telescope can resolve / smaller than a single pixel - yet we see each of them.

Probably best way to "remedy" this pixel analogy would be to explain that pixel will be "lit up" even if there is something very small inside that pixel. Whole pixel will be lit up. And when we increase magnification - we won't see actual pixels - like little squares

 

[andrew s]

Just a word of caution. The theoretical resolution is a convention i.e. a definition.

It is used in optical analysis  for example to set the cut off of the MTF.

However, if you presented a telescopic image of two perfect disks separated at the limit then some would see the split and others not. Similarly some could "split"  closer pairs others would need them to be wider. 

Regards Andrew

Edited August 18, 2020 by andrew s

[vlaiv]

6 minutes ago, andrew s said:

Similarly some could "split"  closer pairs others would need them to be wider. 

I'm not sure how this could happen.

If we have correct protocol - i.e. Call them split if and only if you detect ....

How can someone see what is not there?

[iPeace]

49 minutes ago, vlaiv said:

it can be misleading to anyone knowing anything about pixels

I'm safe, then.

50 minutes ago, vlaiv said:

Probably best way to "remedy" this pixel analogy would be to explain that pixel will be "lit up" even if there is something very small inside that pixel. Whole pixel will be lit up.

Re-thinking this, yes, I had assumed the 'full-pixel-triggering' without realising it, let alone expressing it.

Way out of my depth here with regard to the actual science of this, but it's very interesting.

34 minutes ago, vlaiv said:

How can someone see what is not there?

Happens continuously. (But your question is not answered by this observation...)

[andrew s]

56 minutes ago, vlaiv said:

I'm not sure how this could happen.

If we have correct protocol - i.e. Call them split if and only if you detect ....

How can someone see what is not there?

It is there.

If you use a photometer you can measure the exact depth of the dip.

If you use the human visual system you can't. If you look at the old post war papers you will see they used very large samples to determine human contrast detecterbility.

Basically humans vary in visual acruity.

Regards Andrew 

 

Edited August 18, 2020 by andrew s

[andrew s]

1 hour ago, vlaiv said:

I'm not sure how this could happen.

If we have correct protocol - i.e. Call them split if and only if you detect ....

How can someone see what is not there?

It's not even true for a measured system. A central obstruction narrows the PSF (at the expense of increasing the intensity of the rings) and so even by the technical definition the cut of frequency of the MTF should not be the classical "Airy disk" limit.

For details see https://telescope-optics.net/central_obstruction_effect.htm

 

Regards Andrew

[vlaiv]

3 minutes ago, andrew s said:

It is there.

If you use a photometer you can measure the exact depth of the dip.

If you use the human visual system you can't. If you look at the old post war papers you will see they used very large samples to determine human contrast detecterbility.

 

Regards Andrew 

 

No it is not. I think you are confusing things here.

Look at this image:

I think you are thinking in terms of dashed lines here but you need to look at their sum rather than individual Airy patterns. At some point, when patterns are close enough, their sum is larger than the dip formed by individual patterns and dip disappears.