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Mewlons, Refractors and Seeing

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#26 Asbytec

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Posted 18 October 2013 - 07:12 AM

"Under conditions of poor seeing, large instruments are relatively disadvantageous because the influence of air currents increases with the square of the entrance pupil diameter."

However, this is just a statement. I am missing an explanation why this should be so. Do you or someone else have an idea?

Why are larger scopes more affected by bad seeing?


It has to do with the coherence of the atmosphere above the objective. Generally, the coherence length is fairly small on the order of a few inches or more. When this coherence length is the same or larger and a given aperture, then that aperture defines resolution - not the atmosphere. In this case, the small aperture is providing diffraction limited images. This equates to about 8/10 Pickering or better.

Under those same seeing conditions, a larger aperture captures more of the varying coherence in the atmosphere - incorrectly called "seeing cells". This begins to disrupt the image (along with capturing more atmospheric tilt.) However, with a larger aperture this image is also smaller. So, the effect is seeing is a little worse in a larger aperture, but resolution is not entirely reduced. Up to a point, it will still outperform a smaller aperture provided a full blown speckle pattern has not erupted.

For example, say a 5" scope is operating in 8/10 Pickering. It is delivering diffraction limited images produced by the aperture most of the time. Here, the coherence length (R0) and the aperture are pretty much the same where the ratio D/R0 ~ 1. A twice larger aperture of 10" would experience something on the order of 6/10 seeing. The ratio of the aperture (D) to atmospheric coherence (R0) is D/R0 ~ 2. Conceptually, aperture is twice the diameter of the "seeing cell." In this realm, seeing begins to influence resolution but the smaller Airy pattern is still basically intact and very close to Lambda/D resolution is still possible. By 3 times the aperture, around 15", the speckle pattern begins to form grossly exaggerating the size of the image well into 2" to 5" arc, depending on the severity of the speckle pattern.

Larger scopes loose much of their resolution when the speckle pattern forms, otherwise they keep most of their advantage over small apertures during short exposures. With a given amount of tilt, smaller apertures will cause the star image to jump around pretty much in tact. In larger scopes beyond 3x the smaller aperture, it forms the ugly speckle pattern.

For example, a 5" scope in 8/10 Pickering will be diffraction limited and provide FWHM of Lambda/Dmm ~0.9" arc. A 10" scope, now with resolution influenced by those same seeing conditions, will have a (short exposure) resolution influenced by the coherence (R0) and FWHM of approximately 0.7 Lambda/R0 ~ 80/127mm ~ 0.63" arc. (Note: seeing FWHM is Lambda/R0, not Lambda/D.) This 0.63" arc is not as good as 10" FWHM Lambda/D ~ 0.45" arc found in 8/10 Pickering or better. However, it's still better than the more consistent and "pleasing" 0.9" arc 127mm can deliver in 8/10 Pickering. This is what Eddgie was saying.

I tried to keep the empirical math out of it but failed.

http://www.telescope...d.htm#surfaces.

Edit: eyeballing your chart above, it looks like my 150mm aperture can do 50x/inch when seeing is better than 0.5" arc. This makes some sense because the diffraction limited FWHM is larger than that at about 0.7" arc in radius. However, it's limited to about 190x in 1" arc seeing. That's just beyond diffraction limited seeing putting it in about Pickering 7/10. That would put the seeing influenced FWHM at about 0.83" arc, maybe a bit larger. Since visual acuity is involved here, have to think about what they mean by that. I can easily see the Airy disc at 150x, thereabouts, so I should be able to see something enlarged by seeing with a bit less magnification.

#27 t.r.

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Posted 18 October 2013 - 10:06 AM

The only variable the above leaves out is the wave front error of the objective. If the above 150mm refractor is made to 1/10 wave or better (totally realistic in high end apos) and the 300mm reflector is only made to 1/8 wave or slightly worse, the reflector will certainly be affected more by the seeing conditions than the refractor and this degrades what information can be delivered to the eyepiece. The 1/8 wave reflector goes to barely 1/4 wave diffraction limited, while the refractor maintains 1/5 wave or greater, staying ahead of the diffraction limit and producing a better image. This is why many say that having just a diffraction limited, 1/4 wave scope is not enough.

A local CNer has both a 250 Mewlon and an AP 160 and TEC 160FL. Here, in the NE, on average the refractors are the better planetary scopes. The OP's location, fortunate for him, should favor the Mewlon (reputed to have good 1/8+ wave optics) and in fact, MR. Yoshida of Japanese amateur fame, agrees. Here in the NE USA however, the refractors almost always prevail. ;)

In addition, check out Daniel Mounsey's article linked and his comments on CN here about the Mewlon 250 specifically...

Article Link


CN Link





#28 Erik Bakker

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Posted 18 October 2013 - 10:50 AM

The only variable the above leaves out is the wave front error of the objective. If the above 150mm refractor is made to 1/10 wave or better (totally realistic in high end apos) and the 300mm reflector is only made to 1/8 wave or slightly worse, the reflector will certainly be affected more by the seeing conditions than the refractor and this degrades what information can be delivered to the eyepiece. The 1/8 wave reflector goes to barely 1/4 wave diffraction limited, while the refractor maintains 1/5 wave or greater, staying ahead of the diffraction limit and producing a better image. This is why many say that having just a diffraction limited, 1/4 wave scope is not enough.


In addition to the above, consider that 34% CO introduces roughly 1/4 wave abberation to the wavefront entering the telescope. That is why refractors start with an unequal advantage.

This is further enhanced by the lightpath being deflected away from the scope-tube in a refractor, minimizing internal disturbance of the wavefront while travelling through the OTA.

These 2 factors combined make that reflectors with a significant CO will perform best under very steady skies combined with stable outside temperatures (as close to room-temperature as possible if a scope is stored inside the house). Under those conditions, a bigger quality reflector will start to pull away from a smaller refractor. Under different conditions, refractors are hard to beat by scopes with a biggish CO.

When the CO is under 20%, quality reflectors have much better chances to perform well, especially if they have a thermally sound design and good optics.


#29 Asbytec

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Posted 18 October 2013 - 11:38 AM

The only variable the above leaves out is the wave front error of the objective.

This seems reasonable, that the induced wavefront would quickly pull a diffraction limited scope to below the Raleigh criterion while a better corrected optic could stand a little more seeing. I have not seen anything on that.

In addition to the above, consider that 34% CO introduces roughly 1/4 wave abberation to the wavefront entering the telescope. That is why refractors start with an unequal advantage.

This is sort of true, a 0.3D CO can reduce contrast to an equivalent of something close to 1/4 P-V SA, however the CO alone has somewhat better contrast. The light loss is somewhat recovered by the smaller Airy disc leaving peak intensity less affected. SA reduces peak intensity by a bit more leaving the base of the PSF essentially unchanged. The CO does not alter the wavefront RMS itself. It reduces contrast by decreasing the peak intensity of the central disc and adding light to the rings in a way that is similar to 1/4 P-V SA, so the result is nearly the same and visually.

http://www.telescope...obstruction.htm

#30 t.r.

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Posted 18 October 2013 - 12:40 PM

And as I understand it, that CO is beneficial for seeing tight double stars with faint magnitudes by spreading the energy away from the primaries airy disk, reducing its brightness, thus making the secondary star pop into view, but the effect is still a negative when it comes to producing a planetary image.

#31 brianb11213

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Posted 18 October 2013 - 04:49 PM

And as I understand it, that CO is beneficial for seeing tight double stars with faint magnitudes by spreading the energy away from the primaries airy disk, reducing its brightness, thus making the secondary star pop into view, but the effect is still a negative when it comes to producing a planetary image.

What a central obstruction does - in the case of a perfect point object, which single stars are a good approximation to - is to redistribute some of the energy from the central Airy disk to the diffraction rings. This makes the Airy disk appear to shrink slightly as the brightness of the disc falls away towards the first "gap" immediately surrounding it. The bigger the CO, the bigger the effect.

Central obstruction always increases the resolving power, though the effect is in practice quite small.

The downside is that the increased energy going into the diffraction rings reduces the image contrast. This has a significant effect when observing extended images (planetary discs, for example) and also doubble stars when the companion is fainter than the primary and close to it - the secondary can be blotted out by the primary's diffraction pattern. Double stars with components of very similar brightness are actually more easily resolved with a scope with a large (40%) central obstruction than they are with an obstruction free scope of the same optical quality and aperture - though I stress that the effect is modest rather than overwhelming.

#32 Ed Wiley

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Posted 18 October 2013 - 05:01 PM

Would a stopped down 250mm Mewlon would be any worse than a 6" apo? Might be an interesting experiment.

Ed

#33 Asbytec

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Posted 18 October 2013 - 09:47 PM

And as I understand it, that CO is beneficial for seeing tight double stars with faint magnitudes by spreading the energy away from the primaries airy disk, reducing its brightness, thus making the secondary star pop into view, but the effect is still a negative when it comes to producing a planetary image.

As Brian said, this is true. The CO has two affects. The first is on peak intensity making the central disc dimmer. This is a diffraction effect as well as loss of clear aperture, light is diverted to the rings. This is why refractors rule, their peak intensity is equal to their Strehl without the additional diffraction effects or light loss due to a CO. Obstructed apertures suffer light redistribution to the rings with reduced peak intensity due to the CO and their Strehl. For example, at Strehl 0.98 and CO of 0%, the nominal operating intensity of a refractor is 98%. With a 0.3 CO and the same 0.98 Strehl, an obstructed scope would have a normalized peak intensity of (1-co^2)^2 or about 0.81 * 0.98 Strehl ~ 79% - compared to 98% for the refractor. (I hope the math is correct.)

So, while a refractor will have 98% of the available light (83.8% real * .98 Strehl, or about 82% of the 83.8% available) in the central disc improving contrast to nearly perfect for that aperture, obstructed scopes are hard pressed to keep a nominal 80% of the light in the disc. To do so requires both a smallish CO and a very good Strehl to be refractor-like. With A Strehl of 0.95 or so, the largest CO is about 0.3D for maintaining the Raleigh criterion of 80% light in the central disc.

The CO does more that dim the central disc, it is an aperture and it affects the pattern of diffraction interference as well. The effect is to actually reduce the diameter of the Airy disc from the standard 1.22 Lambda/D (138.4/Dmm, the Raleigh criterion we all know) to something less by a factor of (1 - co^2). For example, a 0.3D obstructed scope actually has a reduced Airy disc diameter of 1.11 Lambda/D (126/Dmm.) This is what causes obstructed scopes to actually exceed contrast transfer of perfect apertures above spacial frequencies around 0.6 to 1. We've all seen the MTF curve peak above the perfect curve and return to 1, this is because the graph is normalized. If it were not normalized, the obstructed curve would hit zero contrast past 1 out to about 1.1 spacial frequency. This is pure high frequency resolution (in good seeing.)

Yes, very close equal doubles are high frequency resolution and obstructed scopes dominate this tiny realm. Again, as Brian said, this difference can be glimpsed at extreme high spacial frequencies at Raleigh and beyond. It's small, but it can mean an increase of about 0.1 in high resolution, Raleigh from 0.92" arc to 0.83" arc, for a 150mm example, and Dawes improved as much from 0.77" arc to about 0.70" arc. While, refractors of equal aperture will put up better mid to low range contrast on planetary scales due to the dimmer ring structure. Unequal doubles behave more like planetary contrast since they can be on the same scale and equal aperture refractors are better in this realm (left hand side of the MTF.)

Thus the battle rages as to which is better. The answer seems to be, at equal aperture refractors dominate the mid to low frequency range and obstructed scopes dominate at higher frequencies. This is provided enough magnification is used to see the Airy disc around exit pupils of 1mm. At lower frequencies and when the scales are very large, both types tend to equalize as contrast improves toward 0.1 (about 5x the Airy disc diameter) spacial frequency even in obstructed apertures especially at lower magnifications where diffraction effects are hardly noticeable. On this scale it seems stray light control is importsnt.

http://www.telescope...obstruction.htm

#34 Ed Wiley

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Posted 19 October 2013 - 11:27 AM

Another thing to think about. For point sources (I am thinking doubles), larger apertures sample more of the "atmospheric cells" that contribute to seeing. Informally, each of the "cells" contained a defraction-limited view of a pair of stars. Small telescopes do not sample as many of these cells as large telescopes, thus the more esthetically pleasing visual images. But, of course, they are limited in resolution.

Now, if you have a large-enough scope that can sample many cells, and a fast camera (talking millisceonds), you can sample each of these cells. The sum is a "speckle image" and this can be reduced by autocorrelation to produce a defraction-limited picture of the pair. Such interferometric techniques are used by professionals to resolve really close binaries right down to the limits of resolution and in average seeing conditions. Similar techniques can also be used by amateurs quite successfully:

http://www.astrosurf...k/speckle10.htm

Smaller scopes that do not sample many cells can use similar techniques or lucky imaging. Of course this is not of much use to visual observers, but us measurers find such techniques quite useful as they allow us to image and measure close doubles in average (or worse) seeing conditions.

Ed

#35 Alph

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Posted 19 October 2013 - 01:38 PM

Here is the explanation.


:foreheadslap: That's not a correct explanation. Please do some reading on Fried parameter and how it relates to aperture. In a nutshell, a telescope with the aperture smaller than the Fried parameter is affected by seeing differently than a telescope with the aperture larger than the Fried parameter

#36 Alph

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Posted 19 October 2013 - 02:05 PM

Would a stopped down 250mm Mewlon would be any worse than a 6" apo? Might be an interesting experiment.

Ed

How are you going to stop down central obstruction?

#37 Ryuno

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Posted 20 October 2013 - 05:40 AM

Waiting for my Mewlon 250 at Starbase Tokyo. It's in the box!!!

Heinz

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#38 Ryuno

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Posted 20 October 2013 - 05:45 AM

Another perspective.

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#39 Pete-LH

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Posted 20 October 2013 - 05:46 AM

Oh, sooooo many Taks! It is fortunate for my wallet that that store is on the opposite side of the world.

#40 Ryuno

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Posted 20 October 2013 - 07:33 AM

Here it is: his Majesty at last. Whereas the CN212 is female, I feel that the Mewlon is male. I don't know why. But in any case, it's not a dream any more. When I saw this telescope about 15 or 20 years ago, I thought, I should have something like this. It delivered just so beautiful images of Jupiter which I had never seen before. I know, there are other scopes today, that can produce very similar images. But this was the first really good telescope I saw, and I was very impressed. At that time, I never thought I would own this one day.

I am planning to use this with a T-Rex mount from Kokusai Kohki. I am feeling a little bit crazy. On the other hand, without craziness life would be boring.

Clear skies
Heinz

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#41 t.r.

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Posted 20 October 2013 - 09:27 AM

Nothing crazy about that TAK! My dream scope too! :bow:

#42 Erik Bakker

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Posted 20 October 2013 - 09:54 AM

Heinz,

Congratulations. That scope is just too beautiful :love:

#43 Ed Wiley

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Posted 20 October 2013 - 10:11 AM

Would a stopped down 250mm Mewlon would be any worse than a 6" apo? Might be an interesting experiment.

Ed

How are you going to stop down central obstruction?


You go off axis, displace the aperture to the side between the spider vanes. How bid the aperture depends on the unobstructed distance between the edge of the secondary housing and the outer edge of the mirror. With my 12.5" dob the limit is about 5". No CO at all. One resource:

http://www.atmsite.o...oppingDown.html

Ed

#44 Sunspot

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Posted 20 October 2013 - 10:57 AM

Ah yes!! I remember the thrill when I opened my Mewlon 250 back in 2005. It was so beautiful set up in my living room, I almost had to kick myself to take it outside. The first evening I imaged Mars and it absolutely blew the doors off the C925 I owned at the time.

I own a C14 on a CGE Pro mount now, but I will not sell the Mewlon 250...unless I get a Mewlon 300 to replace both the C14 and the Mewlon 250.

I hope you enjoy yours as much as I have enjoyed mine!

Paul

#45 Ryuno

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Posted 20 October 2013 - 01:59 PM

Paul, thank you. it looks as if I really have something to look forward to.
So you have both the C14 and the M250. That's interesting. Others are writing, that the M250 cannot compete with the C14. You don't seem to think the same way, as you are keeping the M250. May I ask, how the two scopes compare in your eyes? Which mount are you using with the M250?

Have a good week. With nice weather. We have just gone through a Taifun, now it is clear outside after the storm at 4 a.m. And when I get up later in the morning I will be able to see Mount Fuji in the distance.
Heinz

#46 payner

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Posted 20 October 2013 - 06:35 PM

Enjoy that beautiful Tak. Please give us a first light report ... soon.

Best,
Randy

#47 Kunama

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Posted 21 October 2013 - 05:30 AM

Why do people rave about Takahashi scopes? I don't see the attraction ........
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.That is a seriously beautiful scope.

#48 Ryuno

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Posted 21 October 2013 - 06:16 AM

Why do people rave about Takahashi scopes? I don't see the attraction ........
.
.
.
.
.
.
.That is a seriously beautiful scope.


Everybody needs something to rave about...

#49 Sunspot

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Posted 21 October 2013 - 09:19 AM

Thanks!

The Mewlon is on the EM200 mount. That setup really is well matched. My only interest are the planets and for the most part the views are similar with both scopes. Seeing in my back yard never gets good enough for the C14 to really out perform the Mewlon. Just more light and I can use my imaging setup at like F/18 instead of pushing to F/25 for the same image size.

Paul






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