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Eyepeice Abberations

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#51 Vla

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Posted 27 July 2019 - 10:25 PM

Norme, if you see the Airy disc intact, and the ring structure partially, as come and go, your D/r0 is about 2. With D=150mm aperture it is little better than 2 arcsec seeing. At D/r0=1 most or all of the ring structure is there all the time. Even if you would have the pattern shown for excellent seeing, there is no meaningful periods of nearly unchanging error, it is rapidly changing 90% of the time (ballpark figure). The average seeing pattern actually shows significantly longer periods of relatively small error variation (still typically not more than a few seconds). Seems what you are doing here is trying to fit whatever you can into the "defocus hypothesis". Like the need to refocus slightly after some time while making a sketch of Jupiter, despite you probably knowing that the focal length changes with the temperature. That's not a scientific approach. In it, in order to prove something, you start with doing your best to disprove it, and if it doesn't succeed, you can go on to actually try to prove it.



#52 Asbytec

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Posted 28 July 2019 - 02:53 AM

Okay, like I said, I have no idea what 2" arc seeing means for visual. Imaging makes more sense. But, yes, my approach is not scientific, just describing what I see. I am pretty familiar with the seeing in this area and agree, even in the best seeing I'd rate as Pickering 8 or better, there are small variations on the surface of Jupiter as small details roll in and out of view. Better moments evident even in the best seeing. But, it never drives the urge to refocus. Not even in lesser seeing. 

 

Truthfully, I am not sure what is going on, all I know is I can defocus onto a nicer seeing induced image only to watch the increasing blur over a minute or two. Then refocus and repeat. But, If I leave focus alone for a minute, the image does return to normal, at least for a short while. It does behave like seeing induced image degradation, except that it's possible to chase a good focus. The only thing I am certain of it's not the focuser under load. It may be seeing, it acts like seeing, but it may be mechanical cooling. 

 

It can be irritating to focus precisely only to hold it for a short while. This phenomenon drives an urge to chase focus as the whole image steadily degrades, including stars. As I recall (maybe Don can confirm), they look like defocused stars and not seeing induced point sources. It really does look like the entire field is out of focus instead of degraded by seeing. Maybe it's mechanical and slight variations in mechanical adjustments affect the small dept of focus at faster focal ratios, but that does not explain why the image returns to good focus. 

 

Much appreciated Vla, I'll keep an eye on it out of curiosity.


Edited by Asbytec, 28 July 2019 - 03:07 AM.


#53 Vla

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Posted 28 July 2019 - 09:35 AM

Norme, here's an illustration, from Aberrator, of what the seeing-deformed wavefront looks like. Those are just snapshots frozen in time, but in reality it is in a constant random change. Bottom left is defocus error, something that you can actually refocus on. Best focus change in the turbulence-affected wavefront is generally negligible vs. magnitude of aberration, and even if you could catch it, it wouldn't make noticeable difference. But what we are talking about here is the atmospheric turbulence, which generally does not include air conditions just above (within a few meters) the ground, nor inside the telescope. We know that tube currents do affect the wavefront, with part of it accelerating through warmer layers of air, although that form of deformation generally can't be refocused. What about plums of warm air raising off the (usually warmer than air) ground? Pretty sure they can affect wavefront shape, just don't know how. What about body heat getting between the diagonal and eyepiece in open tube Newtonians? That's kind of unexplored area.

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#54 Starman1

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Posted 28 July 2019 - 09:47 AM

 

It can be irritating to focus precisely only to hold it for a short while. This phenomenon drives an urge to chase focus as the whole image steadily degrades, including stars. As I recall (maybe Don can confirm), they look like defocused stars and not seeing induced point sources. It really does look like the entire field is out of focus instead of degraded by seeing. Maybe it's mechanical and slight variations in mechanical adjustments affect the small dept of focus at faster focal ratios, but that does not explain why the image returns to good focus. 

 

 

This is what I see too.  You CAN refocus sharply, but it will vary again.

When the oscillation is fast, it is usually accompanied by some periodic blurring and lateral movement, too.

I think this is all seeing. 

 

When you look up "chasing focus", though, you will be treated to many different descriptions from many causes, from moving targets (binos) to mirror settling (SCTs), etc. etc.

 

See also:

https://www.handprin...RO/seeing1.html

https://www.handprin...RO/seeing2.html

https://www.handprin...RO/seeing3.html


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#55 Jon Isaacs

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Posted 28 July 2019 - 09:57 AM

Just a comment:

 

Norme lives in the Philippines. His seeing is very different from what those of us living in northern latitudes experience. From following his observing reports, my guess is that it tends to be very good to excellent.

 

In terms of the sensitivity to defocus, at what point does one decide the focal ratio.

 

Is an SCT an F/2 scope with an F/5 mirror-Barlow or a F/10 telescope. And how would that differ from an F/4 Newtonian with an 2.5x Barlow?

 

Maybe the Afocal device assumption applies?

 

Jon



#56 Asbytec

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Posted 28 July 2019 - 11:40 AM

Vla, "Best focus change in the turbulence-affected wavefront is generally negligible vs. magnitude of aberration, and even if you could catch it, it wouldn't make noticeable difference."

Yes, that makes perfect sense. It does not behave like roughness component of seeing. Its almost like a long slow wavelength or tilt component that passes overhead. Not unlike the "air lens" used in the article Jon linked above. That's a good description, whether it's the answer or not.

Of course I am guessing because I really do not know what causes it.

As Don said, it behaves like a seeing effect, except on seemingly much larger scales. It does not just blur the image of Jupiter and turn stars into Pickering simulations. They all appear defocused across the FOV. Jupiter still blurs just like with seeing, except we can refocus it nicely.

That's the kicker, we can attain good refocus that holds for a bit. I do not (often) bother trying to refocus in Pickering 6/10. No urge to do so and, of course, trying to chase focus in seeing is not going to help. But, in this case, it does. That is what's weird.

It really behaves like the focuser is slipping very slowly, but it's not. So, there is a natural urge to chase focus, its not something I normally have an urge to do even in modest seeing. Sure, I might try to refine the focus when seeing is modest, but not nearly as much and without the innate urge to chase it.

In the case we're discussing, any observer would likely recognize the scope needs to be focused and not because seeing is degrading Jupiter (even though it is degrading Jupiter, we can see that during periods of good focus). Its almost intuitive as a defocus problem. That's why we intuitively reach for the focuser knob.

Jon, we're well into our southerly monsoon rainy season. Surely the atmosphere is behaving differently with warm moist air and thunderstorms off in the distance. I observe between them. Seeing as indicated by star test indicates 5 or 6/10 pickering and sometimes better. And more variable than usual, probably due to the many storms in the area.

I did not notice this in our NW monsoon dry season (Pacific airmass which brings generly better seeing conditions) at more modest magnifications. It was only recently noticed after checking out the focal extender and cranking it up a bit. That's two variables and doesn't make it easier to lock down.

It may have been present in the dry season, but it was not noticable at more modest magnifications observing galaxies with the normal focus touch up. It wasn't readily a problem in the Mak during many years observing at f/13 and frequently higher magnification.

Can't think of a better way to explain it and include as much detail as may be pertinent. Maybe the one who figures it out can have a phenomenon named after them. :)

#57 Starman1

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Posted 28 July 2019 - 11:50 AM

Ever see the atmosphere running over the surface of the Moon like water waves?

Pieces of the Moon shrink and grow as if the magnification is varying, as well as shift from side to side.

And portions go in and out of focus, but don't move.

I think if you look at the Moon, you can easily see just about every seeing effect over just a few seconds--blurring, flashing, lateral movement, multiple images of the same thing, etc.


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#58 Jon Isaacs

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Posted 28 July 2019 - 11:52 AM

Norme:

 

I normally resist the urge to refocus. If I chase focus, I will be continually chasing focus. It seems best if I decide on best focus on a star and just let the seeing come to me.

 

I am no one who pushes high magnifications viewing the planets and doubles. If the seeing is not allowing decent views, I choose other targets and let the seeing come to me.

 

Jon



#59 MartinPond

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Posted 28 July 2019 - 12:06 PM

Yeah...the edge of the Moon is a great way to really see the 

     way the seeing is tonight!  (if it's at least partway there)

 

A note: 

The chances of re-focusing for atmospheric lensing are slim, 

      since the atmosphere doesn't make ideal lenses.

    Mostly it's sort of "ripples of astigmatism".

 What can happen, though, is that you don't know what

    the best focus is and pick one that's not clear for much time.

.  If you  tweek it a little one way or the other

    and look for a while, you might find the position that is

    best for the longest time.

 

 

 

I was looking into the 'seeing', and it seems there are three different consequences:

   1) your target is moving around and de-focusing off and on

   2) You look up and see stars 'twinkling'.   A star can be de-focused enough

       that it seems to disappear/reappear from the scattering.

   3)  "Scintillation"

           This is often conflated with 2) , but it is different...

           when there is an area of stars that are packed closely (in the field),

           they can merge with each other or make an empty spot.

           You end up seeing a small field where the lights are blinking.

           If you take a photo over a a fraction or whole seconds,

           you will see a mottled patch.

           It has been compared to the patterns of moving light and darkness

           you can see at the bottom of a pool when the sun shines on it.

 

With 3) , if you are looking at something in the foreground,

        the contrast will be spoiled a bit. 

 

Awesome graphics, vla !

"...What about plums of warm air raising off the (usually warmer than air) ground?."..

Plumes make trouble, but it's mostly atmospheric "mixing", like along

  weather fronts, or when rising heat plume catches the winds aloft, gets spread out.

Cooling ground can generate hug slabs and gobs of warm air at night that then

   have to mix into the air above.  Even view from 3200 feet is far less

   winky at night than the view at 200 ft above sea level.  Much less mixing.


Edited by MartinPond, 28 July 2019 - 12:16 PM.


#60 Vla

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Posted 28 July 2019 - 07:42 PM

This is what I see too.  You CAN refocus sharply, but it will vary again.

Don, let's address the obvious. If the seeing error is significant, it will make difficult to find best focus a good portion of time. So when you "refocus sharply" that can be broken down to two main components: (1) your focusing action coincided closely enough with the reduction in aberration magnitude, and (2) it at the same time enabled you to come closer to the best focus. That *is*, in part, refocusing, but it is not something that you reduce the seeing error with.


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#61 Asbytec

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Posted 28 July 2019 - 07:48 PM

Norme:

 

I normally resist the urge to refocus. If I chase focus, I will be continually chasing focus. It seems best if I decide on best focus on a star and just let the seeing come to me.

 

I am no one who pushes high magnifications viewing the planets and doubles. If the seeing is not allowing decent views, I choose other targets and let the seeing come to me.

 

Jon

I learned to resist the urge, too, Jon. I am a stickler for good collimation and focus, so chasing focus is frustrating and futile. I just find a good focus and wait for it. I like your idea of letting the image come to you, I use the same philosophy even with faint fuzzies. I relax and let the image come to me instead of mentally struggling to see it, if that makes any sense. The image will come to you. 

 

On magnification, I usually push as high as the scope (er, my eyes) and seeing will allow, but not higher. That seems to be the most productive image. Normally seeing is good enough the image dims too much before the image becomes too blurry. To me, that's ideal. However, if seeing is not cooperating, I'll move onto something else, too. Not gonna see much on a blurry planet, anyway, when seeing is not cooperating and good focus is not possible. Same with the moon. Some folks drop down in magnification below the level of seeing  for a nicer, apparently higher contrast, and small image. I usually do not, if I can get it at the optimum magnification, then I won't spend much time with a smaller image. 

 

Pieces of the Moon shrink and grow as if the magnification is varying, as well as shift from side to side.

 

 

Don, I cannot say I have noticed the magnification effect on the moon in seeing. I never paid much attention, it's just an ugly wavering image. Like looking at the moon sitting on the bottom of a swimming pool. For me, it's just difficult to make anything of it or to decipher any pattern. But, maybe it does. 

 

 

 What can happen, though, is that you don't know what

    the best focus is and pick one that's not clear for much time.

.  If you  tweek it a little one way or the other

    and look for a while, you might find the position that is

    best for the longest time.

 

 

 

I was looking into the 'seeing', and it seems there are three different consequences:

   1) your target is moving around and de-focusing off and on

   

Yes, find the best focus location and stick to it, until you find another one. But, that's how you find the best location. But, are you saying you see this phenomenon of achieving good focus at different points for longer periods rather than determining the one position of best image quality? 

 

Where did you find the reference for defocusing on and off? I recall some discussion about defocus error, but not of actual focus drift. Does this imply it's possible to chase focus in seeing? Normally in moderate seeing, and in my experience, there really is no good focus location. The entire image is blurry. I mean sometimes you can get a lucky snapshot to know you are in focus or not, but once there no amount of refocus ever repairs the damage. This is what Vla is driving at and I agree. 

 

But, the weird thing that seems to be happening lately is the ability to chase focus in seeing and actually find a better image with just a slight turn of the focuser knob and do it often. That's weird and not normal seeing conditions I am familiar with or understand where you can not successfully chase focus.You just find the best focus location and wait for it. 


Edited by Asbytec, 28 July 2019 - 07:52 PM.


#62 MartinPond

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Posted 28 July 2019 - 10:29 PM

"------------------

Where did you find the reference for defocusing on and off?

--------------------"

 

I thought that would be self-evident....I guess.

It is also inherent in the sensor technique of taking image over a short time.

A clearer image is built up with these selections.

That would be impossible if the focus drifted out for long periods of time.

 

The strange phenomenon seeking rferences ....... is the assertion

   that you keep refocusing over a number of seconds.

  That.....the ideal focus is varying.

 

What I suspect is that you find a local minimum to the 

    disturbance and things seem to be in focus for a number of 

    time slices, when a similar thing would happen elsewhere

    in the focusing as well.   There is an inherent "frequency"

   (average) to scintillation disturbances:

https://www.scienced...g/scintillation

  https://ieeexplore.i...ocument/752997/

But is is not currently known to last over seconds

  as you seem to say.   


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#63 Starman1

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Posted 28 July 2019 - 11:27 PM

Don, let's address the obvious. If the seeing error is significant, it will make difficult to find best focus a good portion of time. So when you "refocus sharply" that can be broken down to two main components: (1) your focusing action coincided closely enough with the reduction in aberration magnitude, and (2) it at the same time enabled you to come closer to the best focus. That *is*, in part, refocusing, but it is not something that you reduce the seeing error with.

I totally understand the point, the theory and the math.

But I have direct, first-hand, evidence that seeing causes the image to move in a direction both across the focal plane and perpendicular to it.

This is in addition to flares, splikes, and flashing.

 

I can point out one situation I noticed:

The double star went out of focus while I watched, so I moved the focuser in to achieve a sharper focus.

The double star went out of focus again, so this time I moved the focuser out to achieve a sharper focus.

This repeated several times. 

Each time I was able to refocus and obtain a sharp image of the double star I was viewing.

All the while, the image was shifting around as well.

Had I merely left the focuser at one setting, the stars would have been in focus only a small portion of the time.

How could I not conclude that seeing had a longitudinal as well as lateral component?



#64 luxo II

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Posted 29 July 2019 - 04:42 AM

Vla, all fine and dandy but I have an issue with this.

Because the eyepiece is close to the final image plane the light cone forming an image of any object is very narrow at the eyepiece. In this respect it is inappropriate to consider the waterfront error due to the aperture of the eyepiece - it must be computed for the light cone passing through.

For the same reason, the correction of a newtonian is not seriously degraded by a secondary mirror that has a significant departure from flat.

#65 MartinPond

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Posted 29 July 2019 - 06:05 AM

I totally understand the point, the theory and the math.

But I have direct, first-hand, evidence that seeing causes the image to move in a direction both across the focal plane and perpendicular to it.

This is in addition to flares, splikes, and flashing.

 

I can point out one situation I noticed:

The double star went out of focus while I watched, so I moved the focuser in to achieve a sharper focus.

The double star went out of focus again, so this time I moved the focuser out to achieve a sharper focus.

This repeated several times. 

Each time I was able to refocus and obtain a sharp image of the double star I was viewing.

All the while, the image was shifting around as well.

Had I merely left the focuser at one setting, the stars would have been in focus only a small portion of the time.

How could I not conclude that seeing had a longitudinal as well as lateral component?

 

So...you are the second observer of a long-term (single to tens of seconds?)

 re-focusable phenomenon.   The credibility of two jumps way up from one!

Probably:   

    1) it has to be a fairly "round" effect, that would actually act lens-like...

             no turbulence at the moment.

    2) It can't too far above the scope ....that would make it much more changeable

   I would theorize that it is a fairly smooth dome of different temperature over the

    land.... 100 to 1000 feet up.  An airflow following contours could do that

    for a fairly long time.  Then it would break up, make another  curve, and

    settle into that.  Seems like altitude would reduce that a lot, and higher powers

    would see it more "smoothly", seeing a smaller section of the dome or

    valley in the air..   



#66 Starman1

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Posted 29 July 2019 - 08:49 AM

Martin,

I think you are right it is close to the scope.

The parking lot is asphalt and surrounded by tall pine trees that block the horizon up to about 20° all the way around.

We often hear the wind moving across the trees but don't feel it.

Early in the night, the heat given off by the asphalt could easily create a warm bubble of air that acts like a lens until it dissipates.

And, it's dissipation wouldn't be uniform, either.

I have to give this more thought.  Have I seen the phenomenon elsewhere?



#67 Asbytec

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Posted 29 July 2019 - 11:06 AM

I searched for this topic and found nothing on it, yet at least one other person has seen it. I've heard of people chasing focus, though.

#68 Starman1

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Posted 29 July 2019 - 11:28 AM

One site suggested seeing variation could be in line with the focuser but that this would be local, rather than high altitude.

That backs up Martin's supposition, too.



#69 Vla

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Posted 29 July 2019 - 12:12 PM

I can point out one situation I noticed:

The double star went out of focus while I watched, so I moved the focuser in to achieve a sharper focus.

The double star went out of focus again, so this time I moved the focuser out to achieve a sharper focus.

This repeated several times. 

Each time I was able to refocus and obtain a sharp image of the double star I was viewing.

All the while, the image was shifting around as well.

Had I merely left the focuser at one setting, the stars would have been in focus only a small portion of the time.

How could I not conclude that seeing had a longitudinal as well as lateral component?

Don, stop for a second and think how the seeing error is generated. It is deformation after miles of turbulent air which at the same times moves sideways, carried by winds. How likely it will produce wavefront deformation close to that shown above for defocus, let alone keep it for any appreciable amount of time, even changing its magnitude back and forth? I'm sure there is a probability that could be assigned to it, but practically speaking it's a pure fiction. Even your eye having focusing twitches is much more likely scenario than that, but local turbulence is, as Martin says, the most likely explanation. All it takes is a thin layer of turbulent air cells of the right size. One degree Celsius difference in air temperature changes the light speed in air by about 0.00011%, which means that air cell of about 10 inches in diameter with that temperature differential between the center and edges could generate about 1/2 wave p-v wavefront deformation that could be, at least partly, corrected by refocusing.


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#70 Asbytec

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Posted 29 July 2019 - 12:36 PM

...think how the seeing error is generated. It is deformation after miles of turbulent air which at the same times moves sideways, carried by winds. How likely it will produce wavefront deformation close to that shown above for defocus, let alone keep it for any appreciable amount of time, even changing its magnitude back and forth?

 

All it takes is a thin layer of turbulent air cells of the right size...which means that air cell of about 10 inches in diameter with that temperature differential between the center and edges could generate about 1/2 wave p-v wavefront deformation that could be, at least partly, corrected by refocusing.

Vla, yes. At least I see the normal seeing higher altitude effects when the image is focused. Nothing changes in that regard.

 

One site suggested seeing variation could be in line with the focuser but that this would be local, rather than high altitude.

That backs up Martin's supposition, too.

Maybe Martin is right, maybe it is a low altitude phenomenon. I could see the ground cooling and mixing in such a way as to create "cells" of air above the scope that may drift slowly across the aperture in a condition of little or no wind. Or if whatever conditions are just right, say, a varied surface of vegetation, bare ground, and maybe some pavement in the area. Maybe as the air itself is cooling a bit and falling at the same time as warmer air is rising, Something has to give way and yeild to the circulation.

 

There are cloud formations, open cell convection, that exhibit this behavior. No reason it could not happen in the right conditions near the ground normally stormy, cool weather, possibly with an inversion layer. The nights I observed it were pretty much these conditions. Thunderstorms in the area and fairly cool outside with some uneven surface heating. Probably a lot of cooler air in the area due to thunderstorm activity (downdrafts bringing cooler air down from altitude). 

 

http://www.brockmann...ell-pattern.jpg  


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#71 Alan French

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Posted 29 July 2019 - 01:12 PM

Perhaps of some relevance here. When you're talking about refractors or Newtonians, the longer focal length scopes have the entrance pupil higher above the ground. With refractors it's also farther away from the heat radiating observer.

 

Clear skies, Alan


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#72 Starman1

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Posted 29 July 2019 - 01:19 PM

Yes, I'm thinking it is low altitude conditions that cause the refocusing necessity.

As to why I see it more in short f/ratio scopes, it is probably related to depth of focus.

But, I have seen it in longer f/ratio scopes.


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#73 MartinPond

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Posted 29 July 2019 - 10:08 PM

The parking lot would make a more stable bubble that breaks off...

   blobs of heat, each lasting seconds to maybe a minute,

That would die down a lot very late at night.

It would go away and be replaced with fast variations on a windy night..

 

In the daytime seeing horizontally can be as much as ~40 arcseconds.

That's the branch on a pine tree 10 miles away looking like it's

  waving up and down 10 ft. 


Edited by MartinPond, 29 July 2019 - 10:08 PM.


#74 Asbytec

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Posted 30 July 2019 - 12:59 AM

Yes, I'm thinking it is low altitude conditions that cause the refocusing necessity.
As to why I see it more in short f/ratio scopes, it is probably related to depth of focus.
But, I have seen it in longer f/ratio scopes.

Vla's illustrate a page or two back showing waveform deformity as it relates to depth of focus. It strikes a cord, seems right, except that he models standard higher altitude seeing.

Edited by Asbytec, 30 July 2019 - 01:00 AM.


#75 Asbytec

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Posted 30 July 2019 - 08:49 PM

So, kind of back on topic, one silly question about field curvature. 

 

I know the field curvature of the objective is Rp = R/2, in other words the same radius as the focal length. I guess this is the Petzval curve Rp. But, we're really concerned about the best focus surface for astigmatism. Is it different form the Petzval curve? If so, then it will have a slightly different radius. No biggie, I guess, just want to know which field curve we actually see when we talk about seeing it (which includes the eyepiece, I guess). But, does the surface of best focus for astigmatism actually form the field curvature of the objective and is it called the Petzval curve? Since we talk about defocus with field curvature, I presume we're talking about the surface of best focus for astigmatism. That when that surface becomes defocused at the edge (presuming a well corrected eyepiece), we see less astigmatism and mostly defocus. Is that surface the Petzval curve of the objective? Maybe we cannot separate the field curvature of the objective from that of the eyepiece, and what we talk about is the sum of the respective field curvatures and not of that of the primary alone. But what, exactly, defines the filed curvature of the objective. Where astigmatism is minimized?


Edited by Asbytec, 30 July 2019 - 08:55 PM.



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