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Atmospheric limitations (1 Viewer)

Thanks, Henry! That sounds pretty good. The ED/APO connection is made in my mind now.
Certainly works great at f/8. Am I going to have spherical aberration issues, though?

I can tolerate some compared to an f/12....
That is to say, I don't have to go to 120x or even over 100x. My job is ball-parked now.
One thing I learned with my 'tin-man' setup is that 70-85x works really well for the eagles.
As they move I pick up more details about the coloration. They bank and dive when they are tacking.
If I could make 80x and sharp (chromatic and spherical), I'd be pretty happy.
Add brighter due to 80mm and due to better glass, and it looks like f/8-ED-APO is a good play.

Note that I'm not going for 'crystal clear'. Even on a good day, 500-1000m has its way with things.
I can ask for power...I can't ask for weather.
 
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Aha....others in the game.
It's strange, though. It comes with 24x, and they give maximum power as the diffraction limit,
but I can't find much on using it at an actual high power. I am also realizing the shorter length
drives the eyepiece size down or cost up. Just something to ponder. I need to rummage for
some experiential or photo proof. The data confuses me about the genre. I need to beat
both forms of aberration to get the power.

Update: I got a raft of user review information on using the Orion at ~100--200x.
OK....that one and maybe both are up to the job..
Not many at that type, shortness, and price...most are really pricey.
 
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Thanks, Henry! That sounds pretty good. The ED/APO connection is made in my mind now.
Certainly works great at f/8. Am I going to have spherical aberration issues, though?

I can tolerate some compared to an f/12....
That is to say, I don't have to go to 120x or even over 100x. My job is ball-parked now.
One thing I learned with my 'tin-man' setup is that 70-85x works really well for the eagles.
As they move I pick up more details about the coloration. They bank and dive when they are tacking.
If I could make 80x and sharp (chromatic and spherical), I'd be pretty happy.
Add brighter due to 80mm and due to better glass, and it looks like f/8-ED-APO is a good play.

Note that I'm not going for 'crystal clear'. Even on a good day, 500-1000m has its way with things.
I can ask for power...I can't ask for weather.

Spherical correction can be excellent in a f/7.5 doublet. I have two Fluorite refractors with f/8 doublets corrected to around 1/8 wave. Unfortunately that's something that is subject to a lot of sample variation, particularly in inexpensive mass produced scopes. One trick for improving under-corrected short focal length scopes is to use a prism diagonal rather than a mirror. The prism glass provides some compensating over-correction. However, the 45º Schmidt roof prisms that sometimes come with these will ruin the image in even the best scope.

Are you sure your current scope is well corrected? Have you star-tested it?
 
My wife showed me to a number of star-pairs that split very easily the other night,
but then the mosquitoes drove us in. That was with a Kellner 11mm, though (78x).
They seperated very crisply with big dark gaps, but she probably chose easy targets.

You're probably referring to this testing though:
http://www.backyardastronomy.com/Backyard_Astronomy/Downloads_files/Appendix A-Testing.pdf

I have a 5mm Ortho in .96" I could shim. They recommend 100x to 150x.
There is no clock-drive, of course. Maybe a white LED at 200 yds?
Must be careful....might blind me.
I have a keychain light that is a lot like this:
http://www.hubbleoptics.com/artificial-stars.html


I'm at f/12. Even the clunky store-kit Huygens are fairly sharp at f/12...it's shocking.
Small fields, dim, but sharp. The binocular EPs are very bright and very sharp.
There is grounds to worry about the diagonal. It seems good at 78x for now, though.
Would star-testing prove a level beyond what I need?


There's only so much I will worry when the 78x view exceeds expectations.
I'm watching ants work the pine tips at 200 ft. and the
chromatics on the pine spills aren't big until ~150x. Just a slight tinge at 78x.
CA comes in large about the same time as Spherical....or diffraction, not sure.

So eagles at 78x look pretty good. They would be horrible without my glare treatment.
I pan at about 1-2 degrees per second....its a moving scene.
The worst defect in the cheapo telescope was contrast/glare, by far. Fix that
and it's a nice objective. Increasing the field with the bino EPs enables the panning (at long range).


If I go to f/7.5 I'll have more challenges for a diagonal and an eyepiece.
I definitely wouldn't use the diagonal from this scope on a finer, shorter one.
All your parts need to pull up their socks as the barrel gets shorter.
 
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After the fussing, I still couldn't resist a star-test.
My artificial star was a 3-white-led keychain light wire-bound to a street sign 60m away.
My power: 144x , with both ortho and Huygens.
Nothing fun happens at 78x.

The leds are .125" apart and resolved easily...no big challenge anyway.
I could see (with both EPs) the edge of the little metal shroud around each led as well.
That's a much higher resolution.

Looking at the 'star-test patterns:
http://www.backyardastronomy.com/Backyard_Astronomy/Downloads_files/Appendix A-Testing.pdf

---defocusing forward brought a smooth fuzz, no rings

---defocusing backward made a nifty rippled-pyramid (likely due to the 3 leds) like the
'on-axis astigmatism' illustration, but quieter, smaller. That was the Huygens. It went down
with the ortho. Huygens is surprisingly effective at f/12, but it has quirks.

----there was a 'blue halo', quieter than the .6-wave one in the f/6 example. There was a
'strange violet ghost veil' outside that for the Huygens but not for the ortho.
Leds have a very powerful violet emission line: it powers the phosphors that make
the white light mix of a white led. For the ortho, results were bteer overall than the
example.

All in all, it was pretty cool. I'm pondering making a two-slit gadget.
High power is very interesting...you can see diffractive things happening.
I repeated a peek with the final tube turned glossy again....the glare got pretty bad.
An off-axis light would be good for testing that (at night) glare.

(I do realize the plain f/8 in the test example doesn't have spherical aberration compensation,
and the f/8 ED/APOs almost certainly do)....
 
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ON,

This set-up won't work for a star-test. First, you need to use a single light source, not three closely spaced sources, and the source needs to be much smaller than an LED at 60m. It will probably work if you stretch aluminum foil with a single pinhole over the reflector so that only light from one tiny source reaches the the scope. Or, since you're doing this at night why not just use a real 2nd magnitude star near the zenith?

There are other good suggestions for daylight artificial stars in the Nikon thread you posted to this morning.

Henry
 
I have to stick with artificial star because I have no clock drive.
At 150x the movement is fast.
I do have a light with one led....so I can keep rolling...
Maybe the LED points at a pinhole with magic-trans tape.
 
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You could try Polaris. It doesn't move much and it's 42º above the horizon in Boston, probably high enough in the steady air of a warm night. You don't need to use 144x for a 70mm scope. Suiter (the guy who wrote the book) suggests magnification equal to the aperture in millimeters.
 
Hey, that's right, Polaris. The center of the swirl, mostly.
I did see it the other night. Anything araound 70X is good? I have two excellent
pocket-binoc-salvage oculars right around that power...bright and sharp. By comparison with knowns,
about 70x and about 90x. (eyepieces from damaged 7x21 and 10x25).
I was going to say, Arcturus looked very small and intense, but the tests are done defocused
and it will be more intense at 70x.

Now it's just the skeeters to worry about, and weather.
One thing to worry about through the window at the artificial source....two layers of float glass,
obliquely angle. If I go outside I can place an artificial at 200 yds. In a pinch.

Meanwhile, an interesting bit on why long refractors are surprisingly good with resolution:
http://neilenglish.net/the-mysterious-achromat/
Including some pretty amazing historical achievements.
I'm just getting lucky with daylight, (so 70mm works), the slew rate and field width at 500m,
and the way hi-f-ratio kills two birds with one stone. An ED APO really shortens a good barrel:
that's the main purpose in the end, to have a short, wider view AND not die on the long reach.
 
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I've had some rough nights, what with the murky night weather pattern and the hordes of mosquitoes.
A 1-LED artificial star at 125 yards told me ~80x is kind of hopeless with the image and pattern size.
I see many recommendations for 200-300x. The relevance is not clear anyway.

I think a practical comparison might involve various instruments at a decent distance, using the
Optometrist's rules for resolution (20/20 vision is an acuity of 60 arc-seconds, and the Snelling rule for
acuity/font-height is 1/5). I can compare binoculars, the Swift spotter at 40x60, which has a 3-element
objective (so it is no slouch for 12 yrs ago), and the plain 70mm telescope with a nice Plossl. One thing
becomes clear right away: the same eyepiece has much more trouble in the binoculars at the field edges
than in the 70mm/f-12 (no surprise). The 40x ortho shows more stress in the spotter than a 35x Plossl
in the 70mm/f-12. I could be headed for a no-brainer with these instruments: that things slip in binocs at
10-20x, 40-60x in a spotter, and over 100x in a long-barrel scope. What I wish I had, of course, is an
f-7/ED scope, which would be free to devote most of its spare curvature to compensating for spherical
aberration.

We have very strong hints already, in terms of claims, packaged eyepieces, and personal reviews
that mention the powers used. In terms of best power and highest usable power, I do see talk
of high numbers..

For the Orion f/7.5 80mm ED APO:
http://www.amazon.com/Orion-9895-Ap...s/B0000XMTCY/ref=dpx_acr_txt?showViewpoints=1

There is a comment on "slight softening at 200x", which is awesome indeed.
Keeping it out at f/7.5 gives it a fighting chance.
I was surprised to see 'slight softening' in my starter 70mm at ~150x.
Sharper and brighter and still fitting the trunk is nice.

Now, if my typical use will be at 80x / 1/2-mile, One thing I'm wondering is
if the wider field a faster barrel offers is actually usable at that power.
I'm getting a decent apparent field now. My biggest issue is the smoothness
of the mount. One option would be a Vixen 80mm f/11.4, where over half the money
seems to be in their smooth solid tripod. That's starting to make a lot of sense now.
It would be bigger than an f/8, to be sure.
 
Given this:

--------
Reference: http://neilenglish.net/the-mysterious-achromat/

Minimal or no CA happens beyond:
60mm : f/7
70mm : f/8 (560mm-plus long)
80mm : f/10 (800 mm long)
90mm : f/11 (990 mm)
100mm : f/12 (1200 mm)
120mm : f/15
152mm : f/18
---------------------------

Very Interesting:

Chromatic aberration is minimal beyond a certain f/ratio,
but that actually varies by actual aperature.
Thus, as you go to 80mm or 100mm, your length goes up by two factors..

I'm not pushing my luck....f-10 is fine, f-8 seems risky in an achro.
At 70mm/f-10 I see a bit of fuzzing at 140x that's not chromatic...maybe spherical?
Not bad: I want 70-90x. It gets dim after that, and hard to slew with the target.
I think the depth of field indirectly reduces the effect of the aberrations.

--------------------

I took the chance and shifted down slightly to an f/10 achromat, at 70mm.
Looks good (after doing a little darkening/flocking inside the final tube and the hood).
This adds a little field of view and shortens the tube. I had to add washers to
smooth the jerky alt-az height screw issue. A Vixen mount might be a lot better,
but for now, it's washers and taping on the hacksaw blades at the site. Weight is important.

I need the time and weather to get to the "eagle elevator" in Methuen, to test in action.
These sites are more focused than I thought, the soaring bird 'travel lanes' and 'elevators'.
Maybe the rocky shallows of the Merrimack near the elevator makes for good fishing.


I might work my way up to an APO, but this rig is light enough to hike a small mountain with.
An 80mm APO is a little heavier, and a finer tripod would add much more weight (not without
panning benefits though!). Maybe one of the nice Vixen 'Porta' tripods Still, driving it
around and using the 80mm APO at 70-100x would be a piece of cake. Fields,
parking lots, or short hills. You get also some nice rich-field action at night, of course.
 
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Minimal or no CA happens beyond:
60mm : f/7
70mm : f/8 (560mm-plus long)
80mm : f/10 (800 mm long)
90mm : f/11 (990 mm)
100mm : f/12 (1200 mm)
120mm : f/15
152mm : f/18
---------------------------

These apertures and focal ratios represent the minimum standard for "achromatic" correction using ordinary glass types, which means a red and blue blur size equal to 3 times the diameter of an Airy disk of yellow-green light (555 nm). That's a far cry from "minimal or no CA".

There's a simple calculation for determining the minimum focal ratio required for a given aperture:

0.122D, where D is the objective lens diameter in millimeters.
 
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Thanks, Henry!
Although, when does spherical come in, and are most telescopes too dim when
you see the Airy disk? I see some kind of non-chromatic edging at 175x that seems
a little worse than the chromatics (70mm/f10), but it's too dim and narrow for
chasing raptors.

If the red-blue blur is 3 times the size of the Airy disk,
and I can just begin to notice coloration when it gets dim over x100,
I'm confused about why I should strive for Airy-Disk levels at x60-x80.

0.122D, so that's:

50mm f/D > 6.1
60mm > 7.3
70mm > 8.54
80mm > 9.76
100mm > 12.2

The "minimum focal ratio required" looks almost like the same numbers as in Neil's table.
Maybe "minimal" depends on the power you are using. It could be anything
without a practical standard.

---------------
Needs some digging...

Using the calculator,
http://www.calctool.org/CALC/phys/optics/f_NA
70mm f/10 gives an airy disk of 13.4200 microns.
But...that isn't a finished number like 'maximum usable power'.

I need something to give maximum usable power
that takes into account the f/ration and aperature of the scope.
My practical maximum is probably a lot less since I'm watching
a moving eagle and not a star.
 
OK, the common equation for maximum usable power:
http://en.wikipedia.org/wiki/Magnification

They say, 2 times the aperature in mm, and that's diffraction-limited.
So, for 50mm, that would be 100 power. I wouldn't try that power due to dimness.
We also have the problem that they did not mention the f/ratio at all.

Another common formula says the "best view" is 1 times the diameter in mm.
50x for 50mm, 70x for 70mm, 100x for 100mm, etc...
I have seen sharp images at those powers and aperatures, but they were all f/10 or more.

For a spotter, I see chromatic complaints at slighter lower powers than that, but they
tend to run f/5 to f/6 or so. I start to see a tiny bit of color on my Swift 60mm spotter
at 40x, but not on the 70mm f/10. No complaints atpower = twice diam-mm for ED glass
in articles, but you cannot outrun the dimming.


So the Airy would give me 140x max for 70mm, but it ignores f/ratio and it's too dim,
and my real experience is that 70x using 70mm f/10 is nice and crisp, so I am not sure
what these formulas actually are saying. f/10 might be a lot better than f/8, maybe.
The first article mentions that the chromatic
effects are reduced by the depth of field. Perhaps that explains why it looks so good.

If We say the diameter 3 times worse than Airy, that puts me
at a 47 power maximum. That doesn't agree at all with live viewing experience.
50x is a piece of cake for 70mm f/10. I believe anyone could observe that.
I do much better with Plossls, but I can easily beat that with the junk Ramsdens
that came in the package. These are real observations.

Something is not correct,
and it cannot be the image witnessed.
A formula involving diameter, f/ratio, and a 60-arcsecond apparent detail would be nice.
 
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There's a simple formula for the size of the 555 nm Airy disk in arc seconds. It's twice the Rayleigh Criterion or 280/D, where D is the objective diameter in millimeters.

A particular scope's highest useful magnification, or put another way the lowest magnification that will show you the smallest resolved details, will depend on how well the objective is made and on your eyesight acuity. You can't know without first measuring both your acuity and the scope's resolving power.
 
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The highest useful magnification stated for various scopes, always seems too dim
and too narrow for following a raptor at 500 yds. Also, things like contrast and depth
of field are crucial to me, too. The animal is moving across a blue sky or a grey sky.
If I can see lots of detail at 175 power, that means little at 80 power.
Maximum power is a red herring in this case. That's easy to show with a swap of the eyepiece.

At the optometrist, I can go past the 20-20 line with my prescription. That puts me under
60 arc-seconds (the 20-20 std) and over 30, for whatever that's worth.

If I'm reading the equipment recommendation correctly, you are saying I must possess and test
each instrument under special conditions to find it's resolving power before I compare any.
I'm not sure that's realistic. But...as I said, I'm not interested in maximum resolution per se.



This line of inquiry began when you were saying there was a very wide blue-red difference for
some kind of setup relative to the Airy disk. I wanted to know what that had to do with
resolution, and whether it mattered to my case, since I could not see much for chromatics
at 175x and I used the scope at 70-80x. Then you are saying the resoltion cannot be
known without testing. That seems to directly undercut your mention of the aberration.

If we back up to your statement about chromatics:
I cannot see CA at 80x, and barely at 175x, so I cannot see what you mean.
It is likely true but non-relevant. The person in the original article may have said
the chromatic aberration was at 'little or none' because that it had little or no effect on the actual viewing.
Saying you can't know without testing the maximum resolution has issues with significance as well.


----------
note...
(I think the phrase "minimal or no" traces back to this chart:)
http://www.cityastronomy.com/CA-ratio-chart-achro.jpg
Where "minimal or no visual levels" starts various f/ratios.
whatever the various makers and users of that charts have decided is 'minimal or no'.
I think 'visual levels' may be the key.

The APO has vastly lower levels of CA, but both the Apo and the Achro run out of brightness
at higher powers. I'm simply noticing that, at the power and brightness that suits my
application, at 70mm//f-10//70x, the CA seems to be at 'minimal or no' visual level.
I don't want to go to 200x or use time exposure...the eagle is flying.
 
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The article in the link is confusing you because it addresses the one and only resolution problem, splitting double stars, where very high magnification of 2D is useful and the CA of an ordinary achromat does not do much damage. For low contrast extended objects, like the surface details of planets or plumage details on distant birds the CA of an achromat is damaging.

What you want to know for bright daylight observation is the magnification beyond which no more detail on extended objects like birds can be seen under ideal conditions. If you want a general rule of thumb for that it's a magnification equal to around 20% above the aperture in millimeters for a perfect telescope and an observer with 20/20 vision. BUT, it may be significantly different if the telescope is not perfect and/or the observer's eyesight is particularly good or bad. To determine the potential for a particular telescope and observer requires knowing the resolution of the scope in arc seconds and the observer's acuity in arc seconds. Ophthalmological measurements of acuity are based on lines per millimeter and telescope resolution is usually based on line PAIRS per millimeter, so 20/20 vision in ip/mm is 120 arc seconds, not 60. A perfect 70mm scope resolves about 1.64" (lp/mm), so for a person with 20/20 vision the smallest line pairs resolved by that 70mm scope become just barely visible at 73X, but would be easier to see with a little more magnification, perhaps around 80-90X. Magnifications higher than that offer no advantage and eventually begin to damage the image from loss of contrast and brightness. Obviously, if your vision is better than 20/20 or the telescope is not perfect the maximum magnification would be lower. For a person with exceptional 20/10 vision it would be half as much or even less if the scope is not perfect.
 
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Good point on the plumage versus the stars.
Especially since you may mistake one coloration 'texture' for another.
I think that makes a case for CA trumping Spherical aberration with birds..

I can see the coloration nicely at 70x...so I'm fairly happy. Texture is frankly problematic
with the bird moving at 500 yds. I rely on color, movement, and shape of extremities.
Many days the atmosphere reduces contrast.

The optomologists say it takes 5 seperation units (or a 5x5 block) to distinguish the shapes of their font.
That's how the 20/20 line becomes 1 arc-minute of acuity. (Not 2, but actually 5 pairs, as it were).

Using that rough standard and good lighting, and a 5mm Ortho, I was able to back-figure about
2 arc-seconds for the scope. At that power, my eyes' acuity isn't relevant. So, not the greatest,
but surprisingly good. You can pick a power to cause the telescope to fall short before your eyes do.
(that helps) This is similar to using a monocular 'booster' with binoculars. It's so much easier to see.

The central question in my mind is....does going to f/10 at 70mm and 70x make a sharp enough image
such that an APO would be "keeping my house safe from tigers" (fixing a problem I don't have).
When it comes to general resolution, I really think it's fine and it's not worth beating that horse.

If I buy your 'plumage theory', though,
it means CA even a bit below my eye's acuity would be worth
improving. You have sowed some good seeds of uncertainty there.
So...if I were 'measuring' anything, it would be subtle plumage at great distance. Or textures.
I think your plumage theory and texture testing could be a lot more important than star tests or fonts.

This would be a worthy and interesting venture.
It wouldn't even require a moving target at first.
I just wish I could fiddle with an 80mm APO a little without buying it :)
The answer would come pretty quickly on Holt Hill.
 
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