90x Per Inch!!...How?

[jetstream]

Peachs take on the ability of a telescope to use very high mag on the planets/moon is interesting. I find this to be the case.

http://www.damianpeach.com/simulation.htm

 

"Understanding Resolution and Contrast

Two points it is important to understand is the resolution a telescope can provide, and how the contrast of the objects we are imaging affects is related to what can be recorded. Its often seen quoted in the Dawes or Rayleigh criterion for a given aperture. Dawes criterion refers to the separation of double stars of equal brightness in unobstructed apertures. The value can given given by the following simple formula:

115/Aperture (mm.) For example, a 254mm aperture telescope has a dawes limit of 0.45" arc seconds. The dawes limit is really of little use the Planetary observer, as it applies to stellar images. Planetary detail behaves quite differently, and the resolution that can be achieved is directly related to the contrast of the objects we are looking at. A great example that can be used from modern images is Saturn's very fine Encke division in ring A. The narrow gap has an actual width of just 325km - which converts to an apparent angular width at the ring ansae of just 0.05" arc seconds - well below the Dawes criterion of even at 50cm telescope. In `fact, the division can be recorded in a 20cm telescope under excellent seeing, exceeding the Dawes limit by a factor of 11 times!. How is this possible?.

As mentioned above, contrast of the features we are looking at is critical to how fine the detail is that we can record. The Planets are extended objects, and the Dawes or Rayleigh criterion does not apply here as these limits refers to point sources of equal brightness on a black background. In fact it is possible for the limit to be exceeded anywhere up to around ten times on the Moon and Planets depending on the contrast of the detail being observed/imaged."

[jetstream]

The above goes a long way to explain how a truly good optic can go 90x/inch or more and pull out more detail than Dawes or others can explain. A truly good optic preserves MTF. IMHO.

[jetstream]

4 hours ago, vlaiv said:

It's not the point in looking at it like this, on SGL. Point is to open the image in another empty tab. That way it will be shown full size (about x3 the size of the image embedded here on SGL) - try seeing 17 on that image!

Yes thanks for pointing this out Vlaiv. When doing this I can see it from about 16 inches.

[mikeDnight]

12 hours ago, John said:

Then what ?

(the 17 popped out a little easier for me when I removed my reading glasses)

 

 

Same here, very obviously! And as I observe without glasses, does this play a part in seeing planetary detail more easily? 

[mikeDnight]

13 hours ago, jetstream said:

This is interesting... try seeing the "17"

https://www.imatest.com/docs/sqf/#csf

Like John I see the 17 quite easily without my reading glasses. When I wear them the 17 is difficult to see, but when I scroll the image up or down the screen the 17 becomes very obvious. This happens with a planetary image at the eyepiece. I don't wear glasses for observing, but often the technique of moving the image across the field will often cause subtle detail, or even unseen detail to suddenly reveal itself.

[JeremyS]

12 minutes ago, mikeDnight said:

Same here, very obviously! And as I observe without glasses, does this play a part in seeing planetary detail more easily? 

No: that’s your Vixen HR eyepieces and super high magnifications, Mike 🙂

 

[jetstream]

2 hours ago, mikeDnight said:

Like John I see the 17 quite easily without my reading glasses

Mike, did you click on it to make it larger like Vlaiv said? At what distance do you see the 17?

[mikeDnight]

30 minutes ago, jetstream said:

Mike, did you click on it to make it larger like Vlaiv said? At what distance do you see the 17?

No I didn't enlarge it. I was about an arm's length away from the image.

[jetstream]

5 hours ago, mikeDnight said:

No I didn't enlarge it. I was about an arm's length away from the image.

It really changes things when its enlarged, I'd giver a try.

[vlaiv]

10 hours ago, jetstream said:

Peachs take on the ability of a telescope to use very high mag on the planets/moon is interesting. I find this to be the case.

http://www.damianpeach.com/simulation.htm

 

"Understanding Resolution and Contrast

Two points it is important to understand is the resolution a telescope can provide, and how the contrast of the objects we are imaging affects is related to what can be recorded. Its often seen quoted in the Dawes or Rayleigh criterion for a given aperture. Dawes criterion refers to the separation of double stars of equal brightness in unobstructed apertures. The value can given given by the following simple formula:

115/Aperture (mm.) For example, a 254mm aperture telescope has a dawes limit of 0.45" arc seconds. The dawes limit is really of little use the Planetary observer, as it applies to stellar images. Planetary detail behaves quite differently, and the resolution that can be achieved is directly related to the contrast of the objects we are looking at. A great example that can be used from modern images is Saturn's very fine Encke division in ring A. The narrow gap has an actual width of just 325km - which converts to an apparent angular width at the ring ansae of just 0.05" arc seconds - well below the Dawes criterion of even at 50cm telescope. In `fact, the division can be recorded in a 20cm telescope under excellent seeing, exceeding the Dawes limit by a factor of 11 times!. How is this possible?.

As mentioned above, contrast of the features we are looking at is critical to how fine the detail is that we can record. The Planets are extended objects, and the Dawes or Rayleigh criterion does not apply here as these limits refers to point sources of equal brightness on a black background. In fact it is possible for the limit to be exceeded anywhere up to around ten times on the Moon and Planets depending on the contrast of the detail being observed/imaged."

Problem with this explanation is understanding of what it means to resolve and what it means to detect.

Take any old double star - I'll detect it with even very small aperture. I'll say - there is a star there. You'll ask "Is it a double star", and I'll reply - "Dunno, I have not resolved it yet".

Take any image of Encke division and ask the same question - is it a single gap or dual gap? Or yet better - can you measure width of it from any of those images?

You can't cheat physics. Telescope aperture is about resolving not about detecting. Well, that too - you need large aperture to detect faint stuff - you won't be able to see mag15 star with 3" scope for sure, but in this context we are talking about resolving.

No amount of magnification, nor sharpness/quality of optics can make telescope resolve beyond what physics says it is capable / allows to be done.

[John]

10 minutes ago, vlaiv said:

.....You can't cheat physics. Telescope aperture is about resolving not about detecting. Well, that too - you need large aperture to detect faint stuff - you won't be able to see mag15 star with 3" scope for sure, but in this context we are talking about resolving.

No amount of magnification, nor sharpness/quality of optics can make telescope resolve beyond what physics says it is capable / allows to be done.

You can do quite a bit to ensure that you get as close to the best performance that is possible from a given aperture though and as often as the seeing conditions will allow. That's part of the fun of observing IMHO - pushing things as far as you can

My guess is that the capabilities of a lot of scopes are not really fully explored. Hopefully this thread and others like it will provide some tips on how observers might get a bit more from their instruments before succumbing to the urge to upgrade

 

[mikeDnight]

40 minutes ago, jetstream said:

It really changes things when its enlarged, I'd giver a try.

I enlarged it full screen and it disappeared until I took my glasses off, then I see it but not as obvious as before.

[mikeDnight]

Well I'm going to stick my neck out again and say you don't need a 20cm scope to see the Encke gap. Imaging may be a different thing, but visually it is visible in a good 120mm refractor when Saturn is high in the sky and the seeing is stable. I've seen it and sketched it numerous times over the years, and I'm not talking about the Encke minima which is easier still. The Encke gap is much closer to the outer edge of the A ring and reveals itself as an extremely fine sharply defined line in the ansae. You'll have to wait a while until Saturn once again rides high in our UK skies and you'll need a sharp scope, soft images will wash it out. For some reason it is usually easier to see in the preceding anse and you'll need to wait for those fleeting moments of perfect seeing, but once you glimpse it you'll wonder how you could have overlooked it.

[Geoff Barnes]

+1 @mikeDnight

I have posted before that I saw the Encke Division with my 12 inch Dob, only to be told that I had probably only seen the minima.

Now with Saturn at 65-70 degrees altitude here in Oz, on a night of good seeing the minima is almost always attainable with high powers, say 250x and above. But on those few nights of near perfect seeing the Encke Division is clearly there as the finest black thread near the edge of the A ring. I have seen it a couple of times but needed very high power with my 4mm SW Planetary EP giving 375x. In a couple of weeks I am hoping to see it with my new Vixen SLV 2.5mm EP at 600x. Good fun what! 😁

Edited August 14, 2020 by Geoff Barnes

[jetstream]

4 hours ago, vlaiv said:

You can't cheat physics. Telescope aperture is about resolving not about detecting.

I agree that we are governed by the laws of physics and as such each aperture is bound by a spacial frequency maximum. Now the question is- does Dawes apply to planetary detail or is it described better by MTF and spacial frequency?

and also each individuals contrast sensitivity function?

"Contrast sensitivity function (CSF)

Contrast sensitivity function

The human eye’s contrast sensitivity function (CSF) is limited by the eye’s optical system and cone density at high spatial frequencies and by signal processing in the retina (neuronal interactions; lateral inhibition) at low frequencies. Various studies place the peak response at bright light levels (typical of print viewing conditions) between 6 and 8 cycles per degree (around 4 for the CSF equation used for acutance). We have chosen a formula, described below, that peaks just below 8 cycles/degree.

You may learn something about your own eye’s CSF by viewing the image below at various distances and observing where the pattern appears to vanish. Contrast is proportional to (y/h)², for image height h. Although the image  appears to decrease in contrast linearly from top to bottom, the middle of the chart has 1/4 the contrast as the top."

[jetstream]

"The human eye's MTF, which is limited at high angular frequencies by the eye's optical system and cone density, does not tell the whole story of the eye's response. "

@vlaiv what I'd like to know is exactly what an individual aperture can show without the human eye looking through it and for this case planetary detail. Thoughts?

[chiltonstar]

I think the issue of seeing the Encke division with a smaller scope can be looked at in a different way, as used in my "day job" as an atomic spectroscopist. Many features seen by eye or recorded with an imaging system are of course below the Dawes limit for the aperture, even Cassini. What we see is actually a convolution of the actual true appearance of the object with the instrument function (here Dawes, but in my area of work slit profile). A large, high resolution scope will broaden the feature a little but not degrade too much the intensity of the feature, but a small scope with low resolution will broaden the feature a lot and reduce its intensity a lot. In this latter case, the feature will only be seen with a scope delivering high contrast that can see the slight darkening produced by a convolution of eg Encke with an eg 1 arcsec Dawes function. My small Mak (127) has certainly shown Encke up on occasions when Saturn was high above the horizon, but the width of the feature as seen would have been very close to the 1 arcsec or so dictated by the Dawes limit for the scope (120mm).

Chris

[andrew s]

Spot on @chiltonstar the Airy disk is not the image of a star but the  convolution of a point source with the telescope aperture function.  I sometimes wonder what these debates would be like if the "natural " form of a lens had been square or hexagonal rather than round.

Regards Andrew 

[vlaiv]

5 hours ago, jetstream said:

"The human eye's MTF, which is limited at high angular frequencies by the eye's optical system and cone density, does not tell the whole story of the eye's response. "

@vlaiv what I'd like to know is exactly what an individual aperture can show without the human eye looking through it and for this case planetary detail. Thoughts?

I think that we need to separate two things. Image that scope forms at focal plane and image that we see and/or record.

Point a telescope to a planet or a star and image will be formed at focal plane of that telescope regardless of who is watching or if there is camera attached to the telescope. That image behaves as described above in post by @chiltonstar - it is original image convolved with PSF of telescope and projected at certain scale.

Rayleigh criteria is equally applicable to point sources or low contrast features in what it describes. You can apply it to two stars (same or different intensities) or you can for example apply it to line pairs (again same or different intensity). It tells you how close these features need to be in order for image to show them as separate features. It boils down to whether a graph of light intensity will have a single peak or double peak.

Only after all of this we can talk about ability of detector - be that eyepiece + human eye or camera sensor at prime focus (or any other configuration) - to record / detect above resolved feature.

Human eye requires certain conditions to be able to capture details in the image that are important. It needs certain magnification and certain level of contrast - and these two are not in trivial relationship as we have seen from above graphs and examples with number 17.

Similarly - camera sensor also needs certain conditions. It needs adequate pixel size compared to image scale at focal plane. It also needs enough integration to achieve good SNR for contrast of the features and so on.

In above sense, aperture is the natural limit of what can be resolved and yes, if we talk about resolving (even very fine contrast details) - a very specific term different from detecting - it will resolve in focal plane according to Rayleigh criteria.

If we on the other hand talk about recording that light intensity function without a loss, we can't use Rayleigh criteria any more, we need to use Nyquist. Similarly, if we talk just about detection - again we need to use other criteria in that case. What we should not do is mix criteria and "cross apply" as it might lead us to wrong conclusions.

[John]

Back in 2016 I purchased two telescopes which I hoped would deliver pretty much as good performance as you can get from their respective apertures. I'm pretty confident that they both live up to that expectation.

While I can't control the seeing conditions I think that I can recognise what they are reasonably well now and adjust my observing accordingly.

I'm less certain that my eye is able to take full advantage of what these scopes can do when the seeing conditions are good enough to allow them to perform to their full capability thought. I'm the weak link in the optical system I expect now

There is a slight irony that it takes until the latter part of ones life to be able to afford excellent optics which you might be better able to exploit more with younger eyes and younger energy levels.

But I soldier on anyway

Sorry - nothing scientific in this post

 

 

 

[andrew s]

@John I agree In the end visual astronomy has to consider the human visual system  even as its power wains.

Regards Andrew 

[rl]

30 minutes ago, John said:

There is a slight irony that it takes until the latter part of ones life to be able to afford excellent optics which you might be better able to exploit more with younger eyes and younger energy levels.

John...You have taken the exact phrase out of my brain that I was going to post! 

[chiltonstar]

46 minutes ago, rl said:

John...You have taken the exact phrase out of my brain that I was going to post! 

Reminds me of the quote...."youth is wasted on the young"

Chris

[Stu]

I’m certainly finding that my ability to enjoy smaller scopes at high power is becoming more limited. Floater visibility at small exit pupils is more of an issue than it used to be unfortunately. I am countering this by using binoviewers where possible, but increased aperture is the only real fix. That’s the main reason why a 130mm apo is on my wanted list at some point!

[markse68]

3 hours ago, John said:

There is a slight irony that it takes until the latter part of ones life to be able to afford excellent optics which you might be better able to exploit more with younger eyes and younger energy levels.

You maybe get more patience though to partially compensate?