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Mewlons, Refractors and Seeing

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#1 Ryuno

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Posted 17 October 2013 - 02:24 AM

Coming Sunday I am expecting my new Mewlon 250 CRS - directly from Takahashi, so to speak, as I live just a few miles from the factory. It is the first time for me to own this type of scope and I have some questions which occupy me from time to time.

Again and again I have read on the Internet that refractors are such easy going scopes, and in particular that they are supposedly very tolerant to adverse seeing conditions. In stark contrast, the performance of Takahashi Mewlons is described to be very much dependent on seeing. Therefore, people say, a Mewlon should not be your only scope, meaning, when seeing is subpar, leave it at home and take a good refractor or other scope instead.

This is what I read, but I have so far never encountered an explanation for the Mewlons' alleged susceptibility to seeing. Who can enlighten me?

And also: Is this seeing-dependence a Mewlon or Dall-Kirkham specific characteric, or does it affect other telescope designs too, and if yes, what are these and why?

I am very much looking forward to your ideas about these questions, so I can learn to better understand my future scope. Do I really have to buy a big refractor too, if I have a Mewlon?

Best regards
Heinz

#2 melinda fry

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Posted 17 October 2013 - 04:37 AM

Hi Heinz

I live south of Sydney Nsw, And used to own the Tak 210 mewlon. I found the seeing conditions would always be a problem for my location. I ended up selling and hunted down a Tak FS 128 so happy with that scope.
I find refractors better for me in my area , I do have a very good carbon fiber 9.25 sct for when the seeing is good.

#3 Mark9473

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Posted 17 October 2013 - 08:02 AM

In my opinion it's purely a matter of the larger aperture being more seeing-sensitive.

#4 Footbag

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Posted 17 October 2013 - 08:08 AM

I would also think it has to do with the long focal length of the system. I don't think you would hear differently about an SCT.

#5 Ed Wiley

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Posted 17 October 2013 - 09:22 AM

The larger the aperture the more you can "see" the seeing. Last Okie-Tex star party Cotts and I constructed masks for our dobs (both F/5; 17" = 6" mask; 1.2" = 4" mask). (Cotts design, I helped cut them out of cardboard.) In essence we constructed 6" and 4" apos. I think mine was about F/16ish. The apparent seeing dramatically "increased." That is, the star images were sharper and the esthetic experience was better. The actual seeing had not improved at all, but the airy disk at the smaller aperture was larger, "masking" the seeing (as I understand it). So if you have a reflector and want an apo in the 100mm range, just cut up a piece of cardboard. I think you would end up with something like a F/25 apo (if I did the math right). Might be interesting to give it a try.

Ed

#6 Ryuno

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Posted 17 October 2013 - 10:12 AM

So if you have a reflector and want an apo in the 100mm range, just cut up a piece of cardboard. I think you would end up with something like a F/25 apo (if I did the math right).


Interesting observation and theory, Ed. This would of course apply to all telescopes with larger aperture, including large refractors. It would not be a question of design, but purely of aperture. This would in particular mean the talk about "the Mewlon can't be your only scope because of its sensitivity towards bad seeing" is just hot air. You just make a cardboard mask and, woosh, you have a smaller aperture refractor less influences by seeing. I am certainly going to try that. The theory behind this is not so clear to me though. Why would a larger aperture see the seeing more, and what does "see" the seeing exactly mean in this connection? I'll have to think about that. Right now I cannot think of a good explanation.

Heinz

#7 Eddgie

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Posted 17 October 2013 - 10:54 AM

Here is the explanation.

A distant star is for all practical purposes, a perfect geometric point.

Seeing as expressed in arc seconds expresses the distance that the light that would normally be focused on that geometric point are scattered away from that point.

If for example, seeing is 5 arc seconds, the light will be scattered out to that size rather than a perfect geometric point.

The telescope though is incapable of diplaying the light from a distant star as a perfect geometric point.

The best it can do is to focus light into a small circle which I am sure you are aware is called the Airy Disk.

Now the problem with a larger scope is that the same energy is concentrated into a smaller circle.

Suppose you have 2 arc second seeing and a telescope that can only concentrate the light into a 1.5 arc second circle (like a small refractor). Most of the light that would be deflected away from the perfect geometric point (which of course the telescope cannot produce) will fall inside the diameter of the Airy Disk with only a very small percentage falling outside of its circumference. Because of this, the star will still look like a small disk at high power, and the seeing will show maybe some flaring in the first ring (which is where a meaningful percentage of the energy that does not go into the Airy Disk in a small refractor ends up).

Now, lets take an aperture three times as large. Now, the energy from the geometric point is concentrated much more into an Airy Disk that in perfect conditions would be far smaller in diameter.

But if you still have that 2 arc second seeing, once again, light that would be concentrated into a geometric point is now being spread across 2 arc seconds though the peak distribution is still at the center) and in tis case, is extending out considerably further than the size of the far smaller Airy Disk.

So rather than being covered by the larger Airy Disk, it is scattered into the "Open" area where we can see it.

But this is not a terminal condition.

Lets say that seeing improves to 1.5 arc seconds.

In this case, the smaller scope will present an almost perfect Diffraction ring because all of the energy that normall would fall into the Airy Disk still falls into the circle.

In the bigger scope though, once again, it is falling outside of the area covered by the Airy disk, but once again, the highet concentration is still in a smaller circle than in the smaller telescope.

What you see is that when you look a the Airy Disk, it will not be stable and perfect, but because the energy is still concentrasted in a smaller circle than in the smaller instrument, even though the Airy Disk does not look as clean and as perfect, the energy from the point is still more concentrated in a tighet circle.

At this point, the larger aperture starts to transfer contrast better than the smaller aperture even though the Airy Disk is not perfect.

And the better the seeing gets, even if you go to a larger and larger aperture where the Airy Disk gets smaller and smaller, the fact that the energy is being encircled into a smaller and smaller area means that the larger the scope the better the contrast transfer can be.

So, even when not producing a perfect Airy disk when seeing is not perfect, the larger instrument is still concentrating the energy far more powerfully and can (and will) still outperform the smaller instrument in terms of contrast transfer.

Just seeing a more disturbed Airy Pattern does not mean that the larger scope is incapable of outperforming the smaller scope on extended targets. In fact, even on nights of much less than great seeing, I would almost always see more detail in my C14 than in my 6" APO.
The 6" APO could be showing a fairly good diffraction pattern at the same time that the C14 was showing a very distrurbed pattern, but since most of the energy was still being concentrated in a circle with a circumference smaller than the Airy Disk of the 6" APO, the C14 sill performed better.

Stars just looked ugly.

But performance is performance. I could spit closer doubles, but they looked horrible. A split is a split so that means the performance advantage is still present, but many people don't consider the split desirable unless it is cosmetically pleasing.

Bottom line.. Seeing disrupts the energy In the Airy disk making it hard to allow larger aperture scopes to show perfect Airy disks on all but nights of excellent seeing.

But just because the Airy Disk is not perfectly defined, don't think that this means that it cannot perform better than a much smaller aperture. It simply means that it is not concentrating the energy as well as it could, but it still may be concentrating the energy far more than a much smaller aperture could ever do. And once it encerciles the same amount of energy into a smaller diameter circle, it performs better.

Don't let the inability to see perfect Airy Disks delude you to think that the bigger aperture has no advantage. That is a total mistake.

Once seeing gets better than about 2 arc seconds, your 250mm aperture will easily pull ahead of a 150mm aperture, and once seeing gets to one arc second, a 300mm apetuure is useful.

Suggest the book "Telescope Optics." It covers magnification and aperture for different seeing conditions.


2 Arc seconds though, and you are going to outperform anything smaller than about 200mm just from a seeing standpoint.

Here is a good page. Remember that the Airy disk for the 5" scope being modeled is twice the size of the Airy Disk for a 10" aperture, so most of the energy being shown for better seeing would still be falling into a circle smaller than the Airy Disk for the 5"..

http://www.damianpea...m/pickering.htm

#8 Eddgie

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Posted 17 October 2013 - 10:59 AM

Just a quick followup. Pickering 7 in a 5" aperture shows a nice Airy Disk. At this point, in a 10" aperture, the Airy Disk would be half the size.

As you can see though, most of the energy is being compacted into the Airy Disk of the 5" scope, and in the larger scope the energy that is focused into the smaller Airy Disk is likely still more concentrated than the diameter of the 5" Airy Disk.

And once this happens (there is more energy in a slightly smaller circle) the larger aperture starts to pull away. The Airy Disk and first ring may be flaring or wavering, but if the first ring of the bigger apture is smaller in diameter than in the smaller apeture's Airy disk alone, this is when the bigger aperture starts to pull away.

#9 Ryuno

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Posted 17 October 2013 - 11:27 AM

Hi Eddgie

Thank you very much for your detailed answer. I think I understand what you want to say. The result is basically that the star images are under all circumstances more concentrated in larger apertures than in smaller ones plus also the contrast is better. The images are just aesthetically less pleasing. This can only mean that people (under bad seeing) let themselves somehow "fool" by the aesthetically more pleasing (i.e. rounder) star images of smaller apertures. For extended (non star) objects, such as the moon, the planets or galaxies, the ugliness of the star images should not play any role, and the larger aperture should under all seeing conditions show a more detailed, more contrasty and even aesthetically more pleasing image. Objectively, a larger aperture will always, even with object that consists only of stars, show more with superior contrast and resolution regardless of how good or bad seeing is. Please correct me if I got it wrong.
This would mean that it is a myth and illusion to believe a small refractor has (apart from aesthetics) a better image than a larger reflector, all other parameters equal.

Heinz

#10 Mark9473

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Posted 17 October 2013 - 12:38 PM

The smaller refractor does have a better image but less information in it.

#11 Cotts

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Posted 17 October 2013 - 02:53 PM

The smaller refractor does have a better image but less information in it.


Yes. We have to distinguish between aesthetics and raw information. The smaller scope gives a more classic, 'pleasing' view because it cannot resolve the seeing disturbances resolved by the larger scope. The word 'pleasing' is often tossed out by $10 000 6" APO refractor owners when chatting with owners of big dobs, cats etc. The refractophiles rarely bring up the topic of 'information' which Eddgie so rightly calls 'contrast transfer'.......

The other thing to keep in mind is that the seeing is a very dynamic process and varies on both short (seconds), medium (hours) and long (seasonal) time scales. It also varies significantly due to geography - just look at where Damian Peach takes those incredible Planetary Pics - it ain't in England!!!

On many typical evenings the larger scope will show more detail in moments where the seeing settles down. If you are either using an aperture mask or have put away your Mewlon in favour of a 100mm refractor at the magic moment you will have missed it! Every skilled planetary sketcher will tell you - Patience at the eyepiece and the detail will come!

Dave

#12 Cotts

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Posted 17 October 2013 - 03:01 PM

And another thought. 8-inches aperture seems to be a turning point for being able to get clean diffraction patterns on any sort of regular basis. 0.5 arc sec seeing does happen a fair amount even here in the great lakes area. I can see the diffraction pattern in my 8" TEC on many nights of the year. I have only ever seen the diffraction pattern of my 16" Zambuto/Teeter once in three years (why, yes, it was beautifully perfect....) and I had to be in the Florida Keys for that magical evening, 1600 miles from home.

So, "size matters" is a very complicated, multiple-edged sword.....

Dave

#13 Mark9473

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Posted 17 October 2013 - 03:07 PM

The turning point is closer to 5" where I live. :(

#14 Full Sun

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Posted 17 October 2013 - 03:42 PM

I found that I really couldn't push the Mewlon 250 to higher (planetary) powers untill the temperature stabilized well after midnight. The 5.5 inch Apo was not so restricted due to it's closed tube nature and one light pass. IMO the Mewlon has to be well understood before general everyday (night) field use. in my experience this is also holds true for larger SCT's as well -but DSO hunting passes the cooling time required, quite nicely.

#15 Ryuno

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Posted 17 October 2013 - 05:17 PM

This already has become a very exciting thread. Thanks to everybody for your input.

I also find it important to distinguish between how smooth and pleasing an image is on the one hand, and how much information and detail it contains on the other. I cannot imagine that a refractor of say 130mm aperture could ever beat a 250mm aperture reflector in the detail/information category under any seeing condition.
This means, as I indicated above, that the only advantage a refractor of smaller aperture could have over a scope of larger aperture would purely be an aesthetically more pleasing image, and this only in star-like, non-extended objects.
Put in a different way: Whereas a larger aperture shows all the wrinkles and imperfections on the faces of the stars, i.e. all the information that is there, a smaller aperture is more forgiving and flattering in that it introduces a merciful blur or unsharp mask making the faces of the stars appear more beautiful, but loses out on something that a lover might not want to know or see, but a doctor would be interested in.

Is this what it basically amounts to, or am I missing something?

I am looking forward to doing some hands-on for myself. Besides the Mewlon 250, due to arrive within two days from now (can hardly sleep any more) I have a Sky90 refractor, a CN-212 Cassegrain-Newtonian (actually two of them) and a decent 12"5 Dobsonian (Ninja320 from Kasai Trading, Tokyo). After the Mewlon is here, I will still need a good, if possible portable, mount (any suggestions? I am going back and forth between an iOptron iEQ45 and a T-Rex). Then I am going to do some comparisons. In this part of Japan, we have beautiful weather in winter (hardly any rain), but often the good transparency is combined with bad seeing due to the jet-stream above (very annoying). Interestingly transparency and seeing seem to have the tendency to relate inversely to each other. I that your observation too?

Best regards and clear skies
Heinz, at the end of the world

#16 Ryuno

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Posted 17 October 2013 - 05:26 PM

Hi Heinz

I live south of Sydney Nsw, And used to own the Tak 210 mewlon. I found the seeing conditions would always be a problem for my location. I ended up selling and hunted down a Tak FS 128 so happy with that scope.
I find refractors better for me in my area , I do have a very good carbon fiber 9.25 sct for when the seeing is good.


Melinda, have you ever used the Mewlon 210 and the FS128 side by side? In the light of your experiences with these two scopes, what do you think about the thoughts brought forward so far in this thread? Were the images your FS128 delivered just aesthetically more pleasing than those produced by the Mewlon? Or did the refractor also show more detail, contrast and information than the larger reflector? And how was this with star-like objects such as double stars or clusters on the one hand and extended objects such as the moon, planets, nebulae or galaxies on the other hand?

If you haven't done this comparison when you still had the Mewlon, do you think you could do it now with the 9.25 SCT?

Best regards
Heinz

#17 roadi

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Posted 17 October 2013 - 06:08 PM

Can only comment on the mewlon 210 and I found it very seeing sensitive.
The small clearence between tubewall and mirror, "not more than an inch" is a little suspecious.
From what I've heard,read and also experienced in one newtonian, the clearence between the primary and tube wall has to be at least 1" preferable 2 inches to avoid thermal disturbances or constantly tubecurrents!! The mewlon 210 has 1" or lesser if I remember correctly.

#18 TG

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Posted 17 October 2013 - 06:09 PM

Coming Sunday I am expecting my new Mewlon 250 CRS - directly from Takahashi, so to speak, as I live just a few miles from the factory.


Photos or it didn't happen :grin:

#19 Ryuno

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Posted 17 October 2013 - 07:46 PM

"From what I've heard,read and also experienced in one newtonian, the clearence between the primary and tube wall has to be at least 1" preferable 2 inches to avoid thermal disturbances or constantly tube currents!! The Mewlon 210 has 1" or lesser if I remember correctly."

Sounds interesting, this could indeed contribute to some kind of image degradation. I don't know about the M250, but the newer CRS-version has a built-in fan system to cool down the primary that works automatically using three sensors, one on the mirror, one in the tube and one on the outside, to bring the scope as quickly as possible down to ambient temperature. Obviously Takahashi has learned something. (see picture below from their website) I'll see if it helps.

Coming Sunday I am expecting my new Mewlon 250 CRS - directly from Takahashi, so to speak, as I live just a few miles from the factory.

Photos or it didn't happen :grin:

Ok, ok, I'll send some on Sunday, with pleasure. :D

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#20 Eddgie

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Posted 17 October 2013 - 08:11 PM

under all circumstances


Well, in most circumstances.

Once seeing gets over 3 or 4 arc seconds, so much energy is removed form the Airy disk in the larger aperture that you just get a big blur.

In this event, a smaller aperture can indeed present a better image.

This is documented in a book called "Telescope Optics"

When seeing is 5 arc seconds or worse, a 6" aperture at 100x will outperform any other size aperture at any other magnification.

But once the seeing improves to maybe 3 arc minutes, a 200mm aperture starts to pull ahead, giving best performance at about 150x, and when seeing gets 2 arc seconds, 300mm is optimal.

You have the basics though. It is all about encircled energy.

Bad seeing is bad seeing, and using my 6" APO at 100x on planets in bad seeing is in no way satisfying to me personally, so most of my observing has been done with much larger apetures, and I have seen much more.

But seeing rarely was good enough that I could use more than 350x to good result in the C14, so maybe 1 arc second seeing, but this is rare.

I get 2 arc second pretty often, and have done some great observations with seeing this limited, but usually it requires patience to find moments where seeing improves. This is pretty common in a 2 hour session to have either brief moments where seeing was very stable, or sometimes even periods of 20 to 30 seconds.

Over an hour or two, your brian starts to keep a very detailed map, and when you get a moment of good seeing, your eye immediately resolves all known detail. By the end of a two hour session, I have pretty much resolved all of the small detail seeing will allow and and a little extra..

Many people though do not even get 3 arc second with regularity.

#21 Ryuno

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Posted 17 October 2013 - 08:23 PM

"This is documented in a book called "Telescope Optics""

Eddgie, you are thinking of the book by Rutten? This one I have. I'll try to find the information there.

Heinz

#22 Eddgie

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Posted 17 October 2013 - 09:23 PM

Page 224, Figure 18.10.

This table shows the optimum aperture and optimum magnification for 5, 2, 1, and .5 arc seconds of seeing, and apertures from about 60mm to 600mm.

I would guess that the model is based on encircled energy but they do not specify how they came up with the figures.

Also, "Optimum Magnification" is not defined, but I would assume that it is the point at which all angular detail that can be resolved visually has been magnified to an angular size sufficient for the eye to resolve it.

And once it is big enough to resolve the smallest detail that can be resolved for the conditions, I would guess that the thinking is that more than this is empty magnification. You can make it bigger, but no new detail will be resolved, and the image gets progressively dimmer.

They do not articulate this, but it is consistent with much that I have read on the topic.

Also, it is believed that a comcept similar to what happens with speckle pattern interferometry is at play.

There is a name for this when it is in the context of visual astronomy, but it goes something like this.

Just like electronic speckle pattern interferometry, it is believed that the eye/brain does something somewhat similar. It registers the peak of the speckles and processes the image to enhance the contrast.

In the vast majority of these dialogs on Cloudy Nights, the eye is divorced from the aperture and the topic is treated somewhat clinically.

The eye/brain though is always processing things we see, and does a very good job of filling in gaps in the data.

The eye also likes more illumination and for a given magnification, a bigger aperture will usually have a stronger "Signal." The contrast can be the same, but if you turn up the gain (to use a radio analogy) the eye is better able to process the image.

Take a newspaper out on a night of a half moon and see what the smallest print you can read.

Take the same newspaper out on the night of a full moon, and see how much smaller or how much easier the same exact print is to read.

Same contrast on the target, same magnification, but the second time, the paper has 50% better illumination.


Same with using a telescope with about 40% more aperture. You get about 50% better illumination at the same power.

If the optimum power you can use for given night is 200x, and you apply two apetures, one 40% larger than the other, the larger aperture illuminates the image about twice as bright.

And on a really big instrument it gets even better. As the eye moves from scotopic to mesoptic, the contrast sensitivity increases pretty dramatically because now the rods are resolving subtle shadings and there is a lot of data in a color signal.

At 200x in the C14 Jupiter and Saturn are pretty colorful places. At 200X in my 6" APO, it is more like a black and white movie.

How we can continue to have these dialogs and not include the physiological aspects of human vision seem to be somewhat omissive of the major component in the system...The observer's eye.


Think about it.

#23 Ryuno

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Posted 17 October 2013 - 10:35 PM

Eddgie, You really do know your stuff. Thank you again for your fine explanations. I am beginning to understand. Out of curiosity: May I ask you what you are of profession?

You may have seen it already, but at the beginning of the chapter, there is a kind of definition of optimal magnification, which is basically the same as the one you gave in your last post:

"Telescopic magnification for a given detail is optimum when the visibility of that detail decreases at both lower or higher magnification."

And from there he goes on specifying the exact optimal magnification further. And additionally, as you already mentioned, here comes the physiology of the eye into the picture.

Regarding the influence of seeing, figure 18.10 is very nice. I am attaching it as a jpeg-file. The graph shows the optimal magnification for various seeing conditions and apertures (y-axis). Obvious ly for non-optimal seeing, there is an ideal magnification, which is biggest at the maximum of the curve in the x-direction (magnification-axis). This maximum again defines a specific optimal aperture which is 150mm for 5"-seeing, 300mm for 2"-seeing and 500-600mm for 1"-seeing, whereas for excellent seeing (0.3-0.5") only the sky is the limit.
This indeed indicates, that it is not only a matter of aesthetics, which aperture I choose at which seeing, but that with bad seeing smaller aperture also delivers more detail and more information than larger ones, objectively.
The reason for this, according to the book is (quote):

"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?

With the Mewlons as opposed to other designs, there may be some additional factors involved such as tube currents and the like, but these are unrelated to seeing and need not concern us here.

Best regards
Heinz

The following graph is from:
Telescope optics : evaluation and design / Harrie G.J. Rutten, Martin A.M. van Venrooij ; edited by Richard Berry
Copyright © 1988 Willmann-Bell, Inc.
(I am not violating the copyright here, because in the preface the book explicitly allows the use of short passages without permission.)

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#24 Heavens Above

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Posted 18 October 2013 - 04:41 AM

Read my review of a Mewlon 210 http://www.cloudynig...hp?item_id=2791 to read how I dealt with seeing issues. Give your scope time to cool. Do not believe you can cool it in 40 mins, its a myth.

#25 beanerds

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Posted 18 October 2013 - 05:30 AM

Hi Heinz,I like Melinda live in Australia , and knew the M210 that was sold , it was sweet scope BUT ! like all DK cassegrains the columation has to be spot on before they deliver , and DELIVER !!! they do .
I have a nice M210 and the views it throws up are awsome in every way , even the diffraction spikes are perfect .
I use my Takahashi M210 less than my Istar 127mm f8 , but I also use the Istar less than my beautiful Takahashi SKY90 so it all depends on the seeing , how tired I feel after work ,,, ETC , ETC ,so most week nights its the SKY90 .
Brian.






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