Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Long-f/ratio instruments and insensitivity to defocus –– amendments to the "scientific proof"

  • Please log in to reply
60 replies to this topic

#26 daquad

daquad

    Surveyor 1

  • *****
  • Posts: 1,921
  • Joined: 14 May 2008

Posted 07 April 2021 - 11:02 AM

Well, Dom, I personally would find it quite relevant.

 

 

You see, my take since the beginning has been that enlarging a prime focus image X times (with an eyepiece, with a barlow, with whatever), does not produce an image identical (size apart) to the one natively that large at prime focus.

 

Your (and others) simulation -- trying to kindly explain that what I and others see is plainly wrong and/or nonexistent -- is based on formulas which calculates the Airy disks, and asserts that the two instruments should behave identically inasmuch as their respective Airy disks (both angular and linear) are calculated as perfectly matching after the enlargement.

 

Now, formulas are to be both descriptive and predictive. And, if valid, they are to be valid always, not only on odd days.

 

Mr James Smith, on the base of the results of your calculations -- supposedly describing what is exactly happening at the focus --, and given that the resulting linear Airy disks are calculated as being perfectly equal, is legitimately entitled to expect that -- given that the final Airy dimensions are now such to include not only the C-F defocus of the native f/30, but also the one of the native f/6 * 5x = f/30 equivalent -- his formerly-f/6 now-f/30 instrument should have the exact chromatic aberration level of the native f/30.  Not similar, exact.

 

While nothing of this happens, the CA remaining exactly the one of the original f/6. Only with an image 5 times larger.

 

And this happens both in even and odd days; and even on Sundays.

 

Therefore, if the description of what happens at the focus is not accurate (neither descriptive nor predictive) as per the C-F defocus, I am sorry but Mr Smith is understandably entitled to think that such a description is equally not accurate (neither in the description nor in the prediction) as per the behaviour of the seeing-induced defocus, no matter how nice the formula looks.

 

And, given that your humble servant (and not only him) has also repeated constant empirical evidences that a highly corrected 5" f/6 (triplet apo) is showing a severely more nervous and prone to defocus image than the nearby equally highly-corrected 5" f/31 (achro), which instead remains almost motionless and undisturbed, he is also understandably entitled to agree with Mr Smith perplexity.

 

And you cannot dismiss Mr Smith simply by telling him that "the inability of a barlow to correct chromatic aberration in an achromat is common knowledge", because:

 

a) this should not happen, IF f/30 native and f/6 * 5x = f/30 equivalent have the same linear Airy disk at the focal plane, AND what happened at prime focus has no importance; and

 

b) Mr Smith could equally well reply you that "the insensitivity of a high f/ratio instrument vis-à-vis a lower f/ratio one is also common knowledge"...

 

Forgive me, but here there seems to be a little impasse: on the one hand we have a heuristic theory that seems to accomodate two repeatedly constant empirical evidences (seeing-induced defocus and C-F defocus as being different in different f/ratio instruments); and on the other hand, we have a formula that does not seem to accomodate neither.

 

So, maybe, instead of systematically obliterating and dismissing repeated empirical evidences because they do not match with a formula,  we may think of a better descriptive formula and/or a more accurate and less simplistic modelling of the seeing behaviour vis-à-vis what is happening at a focal plane of the telescope?  Just a thought.

 

 

I hope I was able to explain why I would consider my remark as relevant.

 

Many thanks,
-- Max

Max, first of all, no one is saying what you are seeing is wrong.  We are saying the apparent insensitivity to seeing that you observe in long focus instruments is due to all of those reasons you checked in post #1, including one that you did not check (namely that the greater distance of the lens from the ground in long focus scopes helps improve the local seeing).  We are saying that the apparent insensitivity to the seeing is not due to the greater depth of focus of the long focus instrument.

 

Also my "calculations" as you call them, where I stated that a Barlow will reproduce the variations in focus of the native focal length by an amount equal to the magnification of the Barlow can hardly be disputed.  Please note that that variation is both spatial and temporal.  That is, the magnitude of the focus variation is increased by the effective power of the Barlow and the focus variations occur, obviously, at the same rate, which means that the  observed scintillations will be equally rapid.

 

Furthermore my "calculation" does not imply in any way that the use of the Barlow will correct the chromatic aberration to that of the new focal ratio provided by the Barlow.  That apparently is something you believe, not I. 

 

What you fail to realize is that not only will the Barlow magnify the Airy disc, it will also magnify the color blur. The color blur in a 6" f/6 achromatic refractor is 9 times the Airy disc, and it will be 9X the Airy disc with a Barlow inserted into the light path to magnify the image.  Your claim asserts that the Barlow will magnify the Airy disc, but not the out-of-focus light, which, clearly is absurd.  So Mr. Smith (who, I assume, is not a real person.) is not entitled to "legitimately expect that ... his formerly-f/6 now-f/30 instrument should have the exact chromatic aberration level of the native f/30."  Quite frankly, Mr. Smith's expectation is just plain wrong.  

 

Dom Q.


Edited by daquad, 08 April 2021 - 08:37 AM.


#27 Jim Curry

Jim Curry

    Mercury-Atlas

  • *****
  • Posts: 2,724
  • Joined: 29 Oct 2007
  • Loc: STL

Posted 08 April 2021 - 06:30 AM

As KB wrote, the greater depth of focus with slower focal ratios is not in contention. That's understood by all.

 

The contention here is that this somehow makes a longer focal length telescope less sensitive to seeing. 

 

The difficulty with this idea is that the greater depth of focus is counteracted by narrower depth of field, the net result being there is no effect.

 

I proposed a thought experiment to help understand why depth of focus depends on focal ratio and why depth of focus cannot be a factor in seeing senstivity..

 

Consider to telescopes:

 

A 6 inch F/30 

 

A 6 inch F/6 with a 5X Barlow/Focal extender.  The important thing about This scope is the Barlow is not inserted in the focuser, rather it is part of the OTA and focusing is done after the Barlow on the F/30 light cone. 

 

Both have the same depth of focus since both are focusing on the same F/30 light cone.

 

The conclusion is then the observation that merely adding a Barlow cannot affect a system's sensitivity to seeing.

 

An example of a similar arrangement are the modified Petzvals like the TeleVue NP-101, a 4 inch F/5.4. These use the opposite of a Barlow, a focal reducer-flattener. 

 

The objective itself is a rather slow ED doublet, estimated to be F/9-F/12. The reducer/corrector is fixed to the OTA and does not move.

 

Focusing is achieved in the usual fashion, after the reducer. 

 

Anyone who has spent much time with one of knows that the depth of focus is not forgiving easy focus of an F/11, it's the more sensitive focus of the F/5.4. 

 

At F/5.4, the depth of focus is 0.0025".  A human hair is normally thicker. At F/11, it's about 0.010".. no two speed needed.

 

Jon

So Jon,

If the focal plane of the 6 f/30 is 180" from the lens, is the focal plane of the 6 f/6+5x barlow extended out to 180"?

 

The reason I ask is the few times I've used a 2x-3x barlow I don't remember needing any extension to achieve focus.


Edited by Jim Curry, 08 April 2021 - 06:49 AM.


#28 Max Lattanzi

Max Lattanzi

    Viking 1

  • *****
  • topic starter
  • Posts: 929
  • Joined: 27 Jun 2007

Posted 08 April 2021 - 07:01 AM

Your claim asserts that the Barlow will magnify the Airy disc, but not the out-of-focus light, which, clearly is absurd.  

Yes, Dom, you are totally right re the color blur, of course: shame on me for having put towards you something so stupid (there is no other way of calling it).

 

I should simply quit replying on the smartphone, when on the move and in between to RVs with totally other things in mind  (although it is something I am again doing right now...), so as to avoid indecent slips of logic. My bad, and due apologies for having even taken from you the needed time to reply to that.

 

As per the seeing-induced defocus, my total perplexities on the modelization and that formula remain intact.

 

This is something I (and not only me, of course) have been experiencing over the last 25 years, and you can get piles of witnesses and other people having similar long-focus instruments. We are talking tents, hundreds of instruments, used under various conditions (mine, trust me, in rigorous ones). With all other claimed variables being ruled out.

 

1) The greater distance of the lens from the ground does not apply. I already said it at the beginning. First, you may have the longfocus on a shorter mount and the short on a taller one so as to have their lenses side-by-side. Result doesn’t change. Second, that behaviour has been tested in horizontal position, with terrestrial targets. It’s the same. The heuristic power of that argument is irrelevant.

 

2) That slower (i.e., high f-number) optics are exponentially easier to fabricate to the same accuracy as faster optics would be applicable if I were comparing cheap department stores optics, not 0.99 Strehl apos from the top manufactures worldwide. This, again, does not apply.

 

3) The greater depth of focus of the high f-number telescopes makes them easier to precisely focus. True, but, again, we are talking razor-sharp optics with a FT3545 or similar behind (while on the long-focus you could spare that). Focus is perfectly achieved. This is not the matter. The matter is defocus fluctuation. One is visibly more stable; the other is visibly less stable. You may not notice it between f/6 and f/8, but between f/6 and f/30 is immense.

 

4) High f-number telescopes have a larger region of the focal plane that is diffraction limited, so off-axis performance is better. True but it does not apply: we are observing at the center of the field. So, once more, irrelevant.

 

5) Slower optics are easier to collimate accurately, and there are less detrimental optical implications to slight misalignments. True but, again, we are talking hi-end optics perfectly (and I mean "perfectly") collimated. That is certainly not a concern thus, again, the point is irrelevant.

 

6) Many eyepieces perform better with a higher f-number. Correct, of course. But we are not discussing better or lower contrast or sharpness, but defocus fluctuations.

 

And I am not entering into thermal equilibrium because, even though the curves of the optics are more gentle, in the long focus, apart from the glass, you have to handle a way more important column of air. So, if you do not manage it well you are in a mess -- that would be rather an argument in favour of short-focus instruments...

 

Once more this effect remains and that simulation and that formula cannot account for it. But not for this reason it disappears.

 

We cannot merely decide that something do not exist simply because we still do not have a proper explanation (and, consequently, a formalized explanatory formula) for it.

 

Mutatis mutandis, it is certainly not how the guys at Fermilab in Batavia are dealing with their Muons experiments exactly these days. They haven't shut down the lab and trashed the data or came up with turnaround explanations because their last results are not in keeping with the present theoretical models and formulas, rather are working on new more heuristic ones, exactly on the basis of those non-aligned and presently inexplicable data.

 

Science is done like this starting from Galileo; and I guess this is also what we are all trying to do here, don't we?

 

-- Max


  • pao likes this

#29 KBHornblower

KBHornblower

    Viking 1

  • -----
  • Posts: 723
  • Joined: 01 Jul 2020
  • Loc: Falls Church, VA (Washington DC suburb)

Posted 08 April 2021 - 08:08 AM

...snip

 

3) The greater depth of focus of the high f-number telescopes makes them easier to precisely focus. True, but, again, we are talking razor-sharp optics with a FT3545 or similar behind (while on the long-focus you could spare that). Focus is perfectly achieved. This is not the matter. The matter is defocus fluctuation. One is visibly more stable; the other is visibly less stable. You may not notice it between f/6 and f/8, but between f/6 and f/30 is immense.

 

...snip...

Have you observed the reputed difference in a rigorously controlled side by side comparison of such instruments?  Or is this a second hand account of assertations from other observers who may or may not have been exercising the necessary rigor?



#30 daquad

daquad

    Surveyor 1

  • *****
  • Posts: 1,921
  • Joined: 14 May 2008

Posted 08 April 2021 - 09:51 AM

Yes, Dom, you are totally right re the color blur, of course: shame on me for having put towards you something so stupid (there is no other way of calling it).

 

I should simply quit replying on the smartphone, when on the move and in between to RVs with totally other things in mind  (although it is something I am again doing right now...), so as to avoid indecent slips of logic. My bad, and due apologies for having even taken from you the needed time to reply to that.

 

As per the seeing-induced defocus, my total perplexities on the modelization and that formula remain intact.

 

This is something I (and not only me, of course) have been experiencing over the last 25 years, and you can get piles of witnesses and other people having similar long-focus instruments. We are talking tents, hundreds of instruments, used under various conditions (mine, trust me, in rigorous ones). With all other claimed variables being ruled out.

 

1) The greater distance of the lens from the ground does not apply. I already said it at the beginning. First, you may have the longfocus on a shorter mount and the short on a taller one so as to have their lenses side-by-side. Result doesn’t change. Second, that behaviour has been tested in horizontal position, with terrestrial targets. It’s the same. The heuristic power of that argument is irrelevant.

 

2) That slower (i.e., high f-number) optics are exponentially easier to fabricate to the same accuracy as faster optics would be applicable if I were comparing cheap department stores optics, not 0.99 Strehl apos from the top manufactures worldwide. This, again, does not apply.

 

3) The greater depth of focus of the high f-number telescopes makes them easier to precisely focus. True, but, again, we are talking razor-sharp optics with a FT3545 or similar behind (while on the long-focus you could spare that). Focus is perfectly achieved. This is not the matter. The matter is defocus fluctuation. One is visibly more stable; the other is visibly less stable. You may not notice it between f/6 and f/8, but between f/6 and f/30 is immense.

 

4) High f-number telescopes have a larger region of the focal plane that is diffraction limited, so off-axis performance is better. True but it does not apply: we are observing at the center of the field. So, once more, irrelevant.

 

5) Slower optics are easier to collimate accurately, and there are less detrimental optical implications to slight misalignments. True but, again, we are talking hi-end optics perfectly (and I mean "perfectly") collimated. That is certainly not a concern thus, again, the point is irrelevant.

 

6) Many eyepieces perform better with a higher f-number. Correct, of course. But we are not discussing better or lower contrast or sharpness, but defocus fluctuations.

 

And I am not entering into thermal equilibrium because, even though the curves of the optics are more gentle, in the long focus, apart from the glass, you have to handle a way more important column of air. So, if you do not manage it well you are in a mess -- that would be rather an argument in favour of short-focus instruments...

 

Once more this effect remains and that simulation and that formula cannot account for it. But not for this reason it disappears.

 

We cannot merely decide that something do not exist simply because we still do not have a proper explanation (and, consequently, a formalized explanatory formula) for it.

 

Mutatis mutandis, it is certainly not how the guys at Fermilab in Batavia are dealing with their Muons experiments exactly these days. They haven't shut down the lab and trashed the data or came up with turnaround explanations because their last results are not in keeping with the present theoretical models and formulas, rather are working on new more heuristic ones, exactly on the basis of those non-aligned and presently inexplicable data.

 

Science is done like this starting from Galileo; and I guess this is also what we are all trying to do here, don't we?

 

-- Max

OK, Max.  I don't doubt your observations, but I'm not convinced that greater depth of focus is the reason for them.  I think we agree that the focal shift is proportional to the focal ratio.  (And a simple ray trace diagram shows the blur circle to increase linearly with distance from the focal point.) So here is the crux of the matter. 

 

Let's say the seeing causes a 0.2 mm focal shift in the f/6 instrument; then the focal shift in the f/30 instrument should be 1.0 mm.  So the question becomes is the focal shift of 0.2 mm in the f/6 scope more detrimental to the image than the 1.0 mm shift in the f/30 scope?  Or, to put it the other way, is a focal shift of 1.0 mm in the f/30 instrument less damaging to the image than a 0.2 mm shift in the f/6 instrument?  Basically, that is how you are interpreting your observations.

 

To test this in a controlled manner we would have to take both instruments indoors that will allow us to observe an artificial star at a long distance (maybe a large warehouse?).  Then defocus each instrument in proportion to its focal ratio and examine the results, perhaps photographically.  If the f/30 instrument shows a consistently "better"  ("better" to be defined elsewhere) Airy disc, then I would accept your claim. 

 

But until someone can perform a precisely controlled experiment, such as the one I suggest*, to prove beyond doubt that the larger focal shift in the long focal ratio instrument, is, in fact, less harmful to the image than the proportionally smaller focal shift in the shorter focal ratio instrument, then, although I don't disagree with your observations, I must respectfully disagree with your interpretation of them.

 

* (I'm sure others on this forum can devise more convincing experiments than my suggestion.)

 

Dom Q.


  • bobhen, Kevin Barker and eblanken like this

#31 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 08 April 2021 - 11:15 AM

Depth of focus seems straightforward enough:

https://en.wikipedia.../Depth_of_focus

 

Depth of focus is approximately proportional to  N  (the f-number)........

    ....so a longer barrel (same aperture)  has a greater depth of focus,

    and thus less sensitivity to eyepiece location.


Edited by MartinPond, 08 April 2021 - 11:18 AM.

  • Bomber Bob likes this

#32 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • Posts: 19,042
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 08 April 2021 - 12:18 PM

z) Slower instruments (i.e., high f-ratio) produce final images that are less affected by atmospheric turbulence, showing larger insensitivity to defocus and increased tranquillity.

 

I have observed that in Side-By-Sides between 3 of my vintage 4" achromatic refractors:  1970s Jaegers F5 & Dakin F10, and 1950s Edmund F15.  But, it's only at the higher magnifications, like 40x - 60x per inch (160x - 240x), with Jupiter as the target during the same session.  To my mind, a controlled test would require 2 or more fracs from the same maker, with the same OTA build quality & hardware, and using the same accessories.  How many CNers have all that in their collections?

 

I think that the observed differences have multiple factors, but I can't prove that.  I've just learned over 5 decades which refractor to use on a given night for the object(s) I want to enjoy...


  • doctordub and eblanken like this

#33 russell23

russell23

    Voyager 1

  • *****
  • Posts: 11,701
  • Joined: 31 May 2009
  • Loc: Upstate NY

Posted 08 April 2021 - 12:31 PM

I don't know about the premise of this thread, but I have seen that it is easier to get my 102mm f/11 to sharpest focus than my 102mm f/7, 72mm f/6, 120mm f/7.5.   I saw this in particular last week on a night of poor seeing conditions.  The f/11 scope had tighter stars than the f/7 scope.  Exit pupil/magnification didn't matter.



#34 daquad

daquad

    Surveyor 1

  • *****
  • Posts: 1,921
  • Joined: 14 May 2008

Posted 08 April 2021 - 01:47 PM

Depth of focus seems straightforward enough:

https://en.wikipedia.../Depth_of_focus

 

Depth of focus is approximately proportional to  N  (the f-number)........

    ....so a longer barrel (same aperture)  has a greater depth of focus,

    and thus less sensitivity to eyepiece location.

Yes, you are correct with respect to eyepiece position, holding the focal point constant.  If the eyepiece is 0.2 mm from the perfect focus of the f/30 instrument, it is relatively closer to the perfect focus point than in an f/6 instrument and its out-of-focus blur will be relatively smaller than the in-focus blur than in the f/6 scope.

 

However, that is not the situation with respect to shift of focus due to seeing, because the focal point is not constant.  The focus displacement in the f/30 instrument will be 5X greater than in the f/6 instrument.  So if the eyepiece is held stationary, the out-of-focus blur will be 5X greater in the f/30 scope than in the f/6 scope, with the result that its out-of-focus blur will be the same size relative to the in-focus blur.  Hence the conclusion that long focal ratio scopes have no advantage in "cutting through the seeing,"  to use the common phrase.

 

Dom Q.



#35 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 91,250
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 08 April 2021 - 02:15 PM

3) The greater depth of focus of the high f-number telescopes makes them easier to precisely focus. True, but, again, we are talking razor-sharp optics with a FT3545 or similar behind (while on the long-focus you could spare that). Focus is perfectly achieved. This is not the matter. The matter is defocus fluctuation. One is visibly more stable; the other is visibly less stable. You may not notice it between f/6 and f/8, but between f/6 and f/30 is immense

 

 

It always comes back to understanding depth of focus and why focusing on an F/30 light cone will have the same depth of focus regardless of how it is achieved.

 

In terms of depth of focus, there is no fundamental difference between the light cone produced by a F/30 Mak, an F/30 refractor, an F/30 Newtonian or an F/15 Newtonian with a 2X Barlow.

 

As long as you are focusing at F/30, the on axis optics work out the same.

 

The chromatic aberration is a red herring, it has nothing to do with depth of focus.

 

In all cases, the depth of focus is 2.0 mm.

 

To assert that depth of focus is the cause of stability requires that the exact method of focusing matters. And yet once focused, the light cone of an F/15 with a 2x Barlow behaves exactly the same as an F/30.

 

As a matter of interest, an F/30 Newtonian has a coma free field of 600 mm, an F/15 of 74 mm.

 

Jon


  • bobhen and eblanken like this

#36 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 09 April 2021 - 08:57 AM

A highway of red herring?   Talking about anything but seeing itself.

Equating power also has the same washing-out effect as Barlows...

And if there are some differences (chromatics remaining good or bad

  for a barrel despite Barlowing),   it is a sidetrack to seeing stability....a big sidetrack.

 

If we grant some credence to real experiences,

   there is a major issue.

Putting the explanation entirely in the barrel

  is ignoring the most important part of the path: the seeing cells.

 

There is a tendency to apply what we know to explain something,

   when a phenomenon is mostly from something else.

   Fiddling with theory without looking at seeing is.......not seeing.

 

There are experiments you can do, like probing the seeing alone

   with a laser target,  or crossing different color lasers to adjust target distance

   (that one is tricky for measurement)...so you get seeing-vs-distance. 

 

 

  What if something is actually happening

     but we don't really know what it is?   

     That's the beginning of a scientific search,

     not the end (the in-barrel speculations).

     

"Keep looking up!"  ---Jack Horkheimer


Edited by MartinPond, 09 April 2021 - 08:58 AM.

  • eblanken likes this

#37 KBHornblower

KBHornblower

    Viking 1

  • -----
  • Posts: 723
  • Joined: 01 Jul 2020
  • Loc: Falls Church, VA (Washington DC suburb)

Posted 09 April 2021 - 02:34 PM

Here is how I would compare two telescopes side by side for sensitivity to poor seeing.  The scopes would be premium apo refractors with the same aperture but different focal lengths.  I would mount them with the objectives side by side to minimize slight differences in seeing at different positions.  Since we are looking at transient disturbances I would not be satisfied with going back and forth between two eyepieces.  I would use suitable imaging devices in place of the eyepieces and put video displays side by side on a screen and monitor them for a long time in comfort.  I would match their image scales to show the Airy disks the same size in moments of good seeing.


  • daquad, Astrojensen and eblanken like this

#38 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 09 April 2021 - 02:54 PM

....

....

However, that is not the situation with respect to shift of focus due to seeing, because the focal point is not constant.

  The focus displacement in the f/30 instrument will be 5X greater than in the f/6 instrument.  So if the eyepiece is held stationary, the out-of-focus blur will be 5X greater in the f/30 scope than in the f/6 scope, with the result that its out-of-focus blur will be the same size relative to the in-focus blur.  Hence the conclusion that long focal ratio scopes have no advantage in "cutting through the seeing,"  to use the common phrase.

 

Dom Q.

And if you reduce the power to the same as the f/6 instrument (another EP, longer fl),

  to achieve apples to apples....the resultant blur will be the same.

 

Now, this assumes that that the effect of seeing can completely be  

    accounted for by the focal plane distance.  For both of youz-guys

    (pro and con long-barrel-good).

 

But we know that seeing has an effect where the entire image is

   deflected all over the place. The image is darting about.

   The cell isn't a fixed lens, to say the least.  It slides across the path.

   Indeed, there are times when the cell, as a lens, prevents any scope

   from focusing at all.   Long vs. short scope FL would only matter if the cell

   were, say, ~30 FLs away, or less.  So...I'm still not a champion of the long barrel..

".

The curiosity approach seems to suggest we do not know enough

    about seeing to say, theoretically, what is going on.  

    If someone actually saw a difference, neither they nor you or I

    has a handle on this.  Which is not to say "give up",

     maybe it's more to say "something to explore!"..

 

Columbus set out for India and China.  He found North America.

   Of course, he was a bit stubborn and called the locals "Indians"

   ...but something had been found!

 

   Given my sloppy skies, this might be an interesting thing to probe.

 

   I find it productive sometimes to question someone's theory, but not their experiences.


Edited by MartinPond, 09 April 2021 - 02:56 PM.

  • eblanken likes this

#39 daquad

daquad

    Surveyor 1

  • *****
  • Posts: 1,921
  • Joined: 14 May 2008

Posted 09 April 2021 - 06:59 PM

And if you reduce the power to the same as the f/6 instrument (another EP, longer fl),

  to achieve apples to apples....the resultant blur will be the same.

 

Now, this assumes that that the effect of seeing can completely be  

    accounted for by the focal plane distance.  For both of youz-guys

    (pro and con long-barrel-good).

 

But we know that seeing has an effect where the entire image is

   deflected all over the place. The image is darting about.

   The cell isn't a fixed lens, to say the least.  It slides across the path.

   Indeed, there are times when the cell, as a lens, prevents any scope

   from focusing at all.   Long vs. short scope FL would only matter if the cell

   were, say, ~30 FLs away, or less.  So...I'm still not a champion of the long barrel..

".

The curiosity approach seems to suggest we do not know enough

    about seeing to say, theoretically, what is going on.  

    If someone actually saw a difference, neither they nor you or I

    has a handle on this.  Which is not to say "give up",

     maybe it's more to say "something to explore!"..

 

Columbus set out for India and China.  He found North America.

   Of course, he was a bit stubborn and called the locals "Indians"

   ...but something had been found!

 

   Given my sloppy skies, this might be an interesting thing to probe.

 

   I find it productive sometimes to question someone's theory, but not their experiences.

How does what you are saying resolve the question as to what the apparent insensitivity to seeing is with long focal ratio optics?  The curiosity approach?  What does that mean?  Of course we are curious, otherwise why would we be spending the time to resolve the issue?  Curiosity is central to understanding the world we live in.  It is the motivation for our actions.  

 

Something to explore? That is exactly what we are trying to do.  So how would you explore this issue?  KB has suggested a method to explore this question (Post # 37) ; you can read my suggestion in post #30.  What is your recommendation to resolve this issue? Namely, why do some observers believe that long focal ratio scopes are less affected by seeing than short focal ratio scopes?  

 

That seeing is a difficult phenomenon to quantify theoretically, does not mean we should not try.  You say we "do not have a handle on this."  But getting a "handle" on it is what we are about.  What about you?  Sounds to me like you have given up, even though you say "something to explore."  Explore is exactly what we are doing, yet you seem to be as inconsistent and "all over the place" as you put it,  as the seeing.

 

What exactly, and I mean EXACTLY, are you trying to say?  Because, frankly, your ramblings are confusing and seem to have no focus.  Are we not being productive because we are questioning observers' experiences?  BTW we are not questioning their observations, but their interpretation of them.  If you want to question the theory then come up with an explanation that refutes the current theory.

 

Dom Q.


  • eblanken likes this

#40 KBHornblower

KBHornblower

    Viking 1

  • -----
  • Posts: 723
  • Joined: 01 Jul 2020
  • Loc: Falls Church, VA (Washington DC suburb)

Posted 09 April 2021 - 07:00 PM

My remarks inserted in boldface.

 

And if you reduce the power to the same as the f/6 instrument (another EP, longer fl),

  to achieve apples to apples....the resultant blur will be the same.

 

Now, this assumes that that the effect of seeing can completely be  

    accounted for by the focal plane distance.  For both of youz-guys

    (pro and con long-barrel-good).

 

But we know that seeing has an effect where the entire image is

   deflected all over the place. The image is darting about.

   The cell isn't a fixed lens, to say the least.  It slides across the path.

   Indeed, there are times when the cell, as a lens, prevents any scope

   from focusing at all.   Long vs. short scope FL would only matter if the cell

   were, say, ~30 FLs away, or less.  So...I'm still not a champion of the long barrel..

".

I don't think it matters how close or far the cell is from the objective.  If the incoming rays are deflected, say, one arcsecond, the image at the focus will undergo an angular displacement of one arcsecond.

 

The curiosity approach seems to suggest we do not know enough

    about seeing to say, theoretically, what is going on.  

    If someone actually saw a difference, neither they nor you or I

    has a handle on this.  Which is not to say "give up",

     maybe it's more to say "something to explore!"..

 

What is there not to know in theory about poor seeing.  It is fluctuating refraction of incoming light by non-homogeneous air, period.

 

If an observer asserts that he sees consistent differences between the two instruments, let me do another thought exercise.  Equip my scopes with beamsplitters so he can look through one scope or the other while I am monitoring the video display.  My hypothesis is that he is seeing something else and possibly misdescribing and/or misinterpreting whatever he is seeing.  Perhaps discomfort with the shorter focus eyepiece in the short scope is inducing some sort of illusion.

 

Columbus set out for India and China.  He found North America.

   Of course, he was a bit stubborn and called the locals "Indians"

   ...but something had been found!

 

   Given my sloppy skies, this might be an interesting thing to probe.

 

   I find it productive sometimes to question someone's theory, but not their experiences.



#41 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 09 April 2021 - 09:20 PM

How does what you are saying resolve the question as to what the apparent insensitivity to seeing is with long focal ratio optics?  The curiosity approach?  What does that mean? 

You have barely begun to explore the nature of your foe.

 

I mean.......you should be thinking about the interaction of the light plane and barrel with the atmosphere.

It's a bit  un-curious to treat seeing as a simple change of the focal distance.

Shifting the focal plan a little bit is a tiny fraction of 'seeing conditions'.   

And a very temporary one at that.

 

"I suppose it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail."...A.Maslow

 

 I could say "be more open-minded, consider the whole system".

That is a lot harder.   


Edited by MartinPond, 09 April 2021 - 10:35 PM.

  • eblanken likes this

#42 Paul Hyndman

Paul Hyndman

    Apollo

  • -----
  • Posts: 1,321
  • Joined: 13 Jul 2004
  • Loc: Connecticut Shoreline USA

Posted 09 April 2021 - 10:21 PM

FWIW, I used focusing masks to aid in imaging setup and found shorter focal lengths presented more of a challenge to attain/maintain critical focus ("faster" scopes required more fidgeting to maintain critical focus). The following illustrates the concept (Note: I no longer have my website, but a clearer image PDF can be found at Wayback:

https://web.archive....SolarPrimer.pdf pages 27-28)

Attached Thumbnails

  • Focusing Techiques1.jpg
  • Focusing Techiques2.jpg

Edited by Paul Hyndman, 09 April 2021 - 10:38 PM.

  • Bomber Bob and eblanken like this

#43 KBHornblower

KBHornblower

    Viking 1

  • -----
  • Posts: 723
  • Joined: 01 Jul 2020
  • Loc: Falls Church, VA (Washington DC suburb)

Posted 09 April 2021 - 10:28 PM

You have barely begun to explore the nature of your foe.

 

I mean.......you should be thinking about the interaction of the light plane and barrel with the atmosphere.

It's a bit  un-curious to treat seeing as a simple change of the focal distance.

Shifting the focal plan a little bit is a tiny fraction of 'seeing conditions'.   

And a very temporary one at that.

 

"I suppose it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail."...A.Maslow

 

 I could say "be more open-minded, consider the whole system".

That is lot harder.   

It might help if you can illustrate your line of thought in appropriate mathematical detail.



#44 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 09 April 2021 - 10:44 PM

It might help if you can illustrate your line of thought in appropriate mathematical detail.

That's funny.

I have to prove something that neither of us know about at the moment....?

 

Things pass from explorer to scientist to engineer to technician to craftsman to consumer.

At the tech or craftsman level, you assume all the lore is already known.  

 

I say:  It isn't.  At least, not about 'seeing'.

 

That   Hartman Mask entry is awesome.

Not just on axis, but also off to the sides......that is the type of thing

    that might really add to understanding!

 Off to the sides is far more common with 'seeing'...

Cool.


Edited by MartinPond, 10 April 2021 - 12:15 PM.

  • eblanken likes this

#45 KBHornblower

KBHornblower

    Viking 1

  • -----
  • Posts: 723
  • Joined: 01 Jul 2020
  • Loc: Falls Church, VA (Washington DC suburb)

Posted 10 April 2021 - 01:35 PM

I am belatedly digging into the OP to find errors, and I acknowledge that Mr. Greer did not complete the presentation.

...snip...

Latter part of Bryan Greer's presentation:

...snip...

 

"Air lens" strength     150 mm f/5          150 mm f/10
weak                     0.021 mm            0.082 mm
moderate                 0.056 mm            0.227 mm
strong                   0.069 mm            0.275 mm

 

Table 1. Shift in best focus for atmospheric "air lenses" of varying strength.
 

..>>

 

 

And here the "demonstration" stops.

 

Please do note that I am buying the demonstration “as such”, thus not entering into any meta-analysis of the way in which OSLO makes simulations and, more important, was here utilized by the author. It could be done, as I personally have a lot of remarks on the severe reductionist way in which such a simulation was carried out; but it would be really long and complex and, further, that is not the crucial point. The banana peel is much easier to spot.

 

So, at this point, without going any further, the author derives the following:

 

<<..

 

Conclusion

 

Telescopes of equal aperture are affected the same by atmospheric turbulence, regardless of focal ratio.

 

The error in the hypothesis is that it was assumed that the same atmospheric distortion will cause the same shift in the best focus position in the two telescopes, and this is not true.

 

While the high f-number telescope does enjoy a greater depth of focus, unfortunately the shift in best focus caused by turbulence is also greater.

 

In fact, the two are locked together; the instrument with four times greater depth of focus also has a four times greater linear shift of the best focus position.
 

..>>

 

That's it.

 

"Unfortunately" (to quote the author), in his enthusiastic demonstration of what is true and what true is not, he overlooked that the "focus shift" seems to "neutralize the benefits of the greater depth of focus" of the longer f/ratio instrument only if we examine the images at the focal plane the size they are.

 

But, in the 150 f/5 instrument we have a focal length of 750mm, which produces at the focal plane an image of any object (its turbulence-induced focus-shift included) that is half in linear size (and 1/4th in area) than that of a 150 f/10, whose focal length in 1500mm. And the gap obviously keeps increasing vis-à-vis an f/15 (FL=2250mm), and so forth for longer f/ratio instruments.

 

Now, let's see if we can make good use of good old layman logic and common sense.

 

• Say, I am testing the stability of two mounts with a slam test, and report that they are "equal", but I've tested one looking into the telescope at 100x, and the other at 200x, would anyone consider this test actually valid?

 

• Say, I am testing two telescopes sharpness, and report that they are "equally sharp", but the image in the first is at 200x while in the second is at 400x, would anyone consider this as a valid test to actually assess the sharpness of the two instruments?

 

• Say, I am testing the sharpness of two photographic lenses and, to do so, I print the picture of the first at 10x15cm (4x6"), while the picture of second at 20x30cm (8x12"), would anyone consider this as a valid test to assess their actual respective sharpness?

 

• Say, I am testing the amplitude of the oscillation of two objects (let's say two undermounted telescopes shaken by the wind), and I report that the mounts are "equally capable" because they shake the same *to my eye*; but, alas, I am observing one of the instruments at five meters, while the other is at ten meters from me… would anyone consider this a valid test?

 

 

I may go on, but I guess by now the slip in logic -- the banana peel -- is clear to everyone.

 

In the demonstration above, the author obliterated (or simply forgot) that was drawing hasty conclusions by evaluating images of different linear size; images that, during the astronomical observation, need to be enlarged differently (in a 2:1 ratio in that case; 3:1 if the comparison was with a f/15 instrument; and so forth).

 

And apparently all the many readers of the "essay" -- maybe seduced by the beautiful graphs and formulas (all things that make so much "scientific paper") -- failed to notice this (involuntary) sleight of hand in the demonstration logic.

 

So, if now we enlarge the image of the f/5 instrument to match the magnification of the f/10 (or f/15, etc.) one, we are again enlarging the amplitude of those focus-shift effects that seemed to have been equalized in the truncated "demonstration" above.

 

And we shall be able to see again that the longer f/ratio instrument -- whose larger prime focus image needs to be enlarged less -- re-gains and retains its inherent defocus advantage (read: larger insensitivity to defocus and tranquillity of images).

 

As easy as this.

 

...snip...

Max, I think you stepped on a banana peel, which anyone could have done, in your quest to complete what Mr. Greer was presenting.  It is resolved that a refractive disturbance from the turbulent air will shift the focus 4 times as far in the f/10 as in the f/5.  Since the f/10 light cone has half the angular width, we get a blur circle twice the diameter as the one in the f/5.  Using twice as strong an eyepiece with the f/5 to get the same magnification in both scopes, we get equal angular diameter of the blur patterns as seen through the eyepieces.  In moments of good seeing the f/10 presents an Airy disk of twice the linear diameter as that of the f/5, so the equalized magnification gives us disks with the same angular diameter in the two scopes.  Summing up, we see the same appearance in either scope, good seeing or bad.  Those who assert otherwise need to be more literally specific rather than using vague qualitative terms such as tranquility. 


  • eblanken likes this

#46 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 10 April 2021 - 01:36 PM

z) Slower instruments (i.e., high f-ratio) produce final images that are less affected by atmospheric turbulence, showing larger insensitivity to defocus and increased tranquillity.

 

I have observed that in Side-By-Sides between 3 of my vintage 4" achromatic refractors:  1970s Jaegers F5 & Dakin F10, and 1950s Edmund F15.  But, it's only at the higher magnifications, like 40x - 60x per inch (160x - 240x), with Jupiter as the target during the same session.  To my mind, a controlled test would require 2 or more fracs from the same maker, with the same OTA build quality & hardware, and using the same accessories.  How many CNers have all that in their collections?

 

I think that the observed differences have multiple factors, but I can't prove that.  I've just learned over 5 decades which refractor to use on a given night for the object(s) I want to enjoy...

 

I would be looking for strong tendencies....

  otherwise, the value of the distinction is lost..

 

I just did an experiment...

    ---from "just near fuzzy", though sharp focus, to "just far fuzzy" (lost the tiny font legibility)

        ---on an 80mmx800mm barrel (F10),  with a 30mm eyepiece,  I travel 3mm.

        ---on the 80x500 (F5), withb a 15mm eyepiece, I travel  ~1.5 mm   

   So at the same power,  the twice-long barrel travels twice as long in focus.

   I suppose that makes sense, because it's the same portion of a longer-FL eyepiece.

   A wobbling distance image might move the image plane twicw as far in the long barrel, of course.

   Hopefully, people equalize power when doing these comparisons.

    That's the 'static' picture, empirically.

 

 

I won't bother with math at this point.  There is a mighty bee's nest of maths above,

   one way and the other.

 

So...nothing special, and so, I think an actual seeing situation is key.

The bottom of the loop in Science is: corroboration....or...discovery.

 

 

sorry...a little math creeping in:

 

1/(obect distance)  + 1/(image distance) = 1/(focal length)

 

---on an ideal night, no 'seeing' disturbance, both object distance(infinity) and the telescope focal length (what you bought) are fixed.

 

----so, what would it take to disturb ithe presumed-steady  mage distance?

     ---the [apparent] object distance would have to vary,

            from [beyomd imfinity (??)]  to "far less than infinity"

      ---OR: that equation and presumed 1 element are not correct at all.

 

A lens of wobbling value far from the scope would be one approximation.

Otherwise, ordinary in-barrel lens equations would be useless.


Edited by MartinPond, 10 April 2021 - 01:51 PM.

  • eblanken likes this

#47 KBHornblower

KBHornblower

    Viking 1

  • -----
  • Posts: 723
  • Joined: 01 Jul 2020
  • Loc: Falls Church, VA (Washington DC suburb)

Posted 10 April 2021 - 01:38 PM

That's funny.

I have to prove something that neither of us know about at the moment....?

 

Things pass from explorer to scientist to engineer to technician to craftsman to consumer.

At the tech or craftsman level, you assume all the lore is already known.  

 

I say:  It isn't.  At least, not about 'seeing'.

 

That   Hartman Mask entry is awesome.

Not just on axis, but also off to the sides......that is the type of thing

    that might really add to understanding!

 Off to the sides is far more common with 'seeing'...

Cool.

My bold.  If you don't know about it, whatever it might be, you are not going to be able to tell me what to look for in a quest to analyze it.



#48 daquad

daquad

    Surveyor 1

  • *****
  • Posts: 1,921
  • Joined: 14 May 2008

Posted 10 April 2021 - 02:50 PM

I would be looking for strong tendencies....

  otherwise, the value of the distinction is lost..

 

I just did an experiment...

    ---from "just near fuzzy", though sharp focus, to "just far fuzzy" (lost the tiny font legibility)

        ---on an 80mmx800mm barrel (F10),  with a 30mm eyepiece,  I travel 3mm.

        ---on the 80x500 (F5), withb a 15mm eyepiece, I travel  ~1.5 mm   

   So at the same power,  the twice-long barrel travels twice as long in focus.

   I suppose that makes sense, because it's the same portion of a longer-FL eyepiece.

   A wobbling distance image might move the image plane twicw as far in the long barrel, of course.

   Hopefully, people equalize power when doing these comparisons.

    That's the 'static' picture, empirically.

 

 

I won't bother with math at this point.  There is a mighty bee's nest of maths above,

   one way and the other.

 

So...nothing special, and so, I think an actual seeing situation is key.

The bottom of the loop in Science is: corroboration....or...discovery.

 

 

sorry...a little math creeping in:

 

1/(obect distance)  + 1/(image distance) = 1/(focal length)

 

---on an ideal night, no 'seeing' disturbance, both object distance(infinity) and the telescope focal length (what you bought) are fixed.

 

----so, what would it take to disturb ithe presumed-steady  mage distance?

     ---the [apparent] object distance would have to vary,

            from [beyomd imfinity (??)]  to "far less than infinity"

      ---OR: that equation and presumed 1 element are not correct at all.

 

A lens of wobbling value far from the scope would be one approximation.

Otherwise, ordinary in-barrel lens equations would be useless.

Max, but that is just the point.  The only constant in the thin lens equation is the focal length.  Your assumption that the object distance is constant is false. The plane wavefront emanating from a star at infinity is no longer plane after passing through a turbulent cell in earth's atmosphere and so can no longer be considered at infinity. 

 

The wave has been distorted to something irregular, but in the ideal case a wavefront that is spherical, either concave or convex.  Greer's model assumes a thin lens at the distance of the cells in the atmosphere that are distorting the original plane wave front.  Granted, models are often simplified versions of reality, but they do aid greatly in understanding complex phenomena.  

 

The fact that the distortion is probably not spherical, does not detract from the analysis.  We know that in variable seeing a star appears to go in and out of focus even if we are not at the exact infinity focus of the telescope.  In other words, the atmospheric cell is acting like a thin lens that changes the object distance and, therefore can come to focus at other than the infinity focus.

 

Dom Q.


  • eblanken likes this

#49 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 10 April 2021 - 02:53 PM

My bold.  If you don't know about it, whatever it might be, you are not going to be able to tell me what to look for in a quest to analyze it.

You are supposed to know more than me.

I'd wager, having been where they track objects with  telescopes,

I know a little more. But....press on.

 

You are the one who claims to know what's happening

 with seeing...you constantly treat it as a  minor fluctuation in focal point.

 That, your premise, is not proven at all.


  • eblanken likes this

#50 MartinPond

MartinPond

    Fly Me to the Moon

  • *****
  • Posts: 6,038
  • Joined: 16 Sep 2014

Posted 10 April 2021 - 02:57 PM

 

....  Your assumption that the object distance is constant is false. The plane wavefront emanating from a star at infinity is no longer plane after passing through a turbulent cell in earth's atmosphere and so can no longer be considered at infinity. 

 

 

 

Dom Q.

Yes...that's precisely my point, actually.

I make the assumption, figure the results,

   and then show how a fluctuation in focal point

     destroys my assumption (of infinite object distance).

 

In fact, half the time, there is no adequate number for object distance,

   not for that equation (which is based on the tracing that all the bickering centers on).


Edited by MartinPond, 10 April 2021 - 03:11 PM.

  • eblanken likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics