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Equipment Discussions >> Cats & Casses

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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Mewlons, Refractors and Seeing
      #6142247 - 10/17/13 03:24 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


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melinda fry
super member


Reged: 04/28/10

Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6142299 - 10/17/13 05:37 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.


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Mark9473
Postmaster
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Reged: 07/21/05

Loc: 51N 4E
Re: Mewlons, Refractors and Seeing new [Re: melinda fry]
      #6142473 - 10/17/13 09:02 AM

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

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Footbag
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Reged: 04/13/09

Loc: Scranton, PA
Re: Mewlons, Refractors and Seeing new [Re: Mark9473]
      #6142484 - 10/17/13 09:08 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.

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Ed Wiley
scholastic sledgehammer


Reged: 05/18/05

Loc: Kansas, USA
Re: Mewlons, Refractors and Seeing new [Re: Footbag]
      #6142613 - 10/17/13 10:22 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


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Ed Wiley]
      #6142716 - 10/17/13 11:12 AM

Quote:

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


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Eddgie
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Reged: 02/01/06

Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6142787 - 10/17/13 11:54 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.damianpeach.com/pickering.htm


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Eddgie
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Reged: 02/01/06

Re: Mewlons, Refractors and Seeing new [Re: Eddgie]
      #6142801 - 10/17/13 11:59 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.


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Eddgie]
      #6142852 - 10/17/13 12:27 PM

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


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Mark9473
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Reged: 07/21/05

Loc: 51N 4E
Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6142976 - 10/17/13 01:38 PM

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

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Cotts
Just Wondering
*****

Reged: 10/10/05

Loc: Toronto, Ontario
Re: Mewlons, Refractors and Seeing new [Re: Mark9473]
      #6143224 - 10/17/13 03:53 PM

Quote:

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


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Cotts
Just Wondering
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Reged: 10/10/05

Loc: Toronto, Ontario
Re: Mewlons, Refractors and Seeing new [Re: Cotts]
      #6143235 - 10/17/13 04:01 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


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Mark9473
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Reged: 07/21/05

Loc: 51N 4E
Re: Mewlons, Refractors and Seeing new [Re: Cotts]
      #6143246 - 10/17/13 04:07 PM

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

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Full Sun
super member
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Reged: 11/19/06

Loc: Ontario
Re: Mewlons, Refractors and Seeing new [Re: Mark9473]
      #6143304 - 10/17/13 04:42 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.

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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Full Sun]
      #6143449 - 10/17/13 06:17 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


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: melinda fry]
      #6143467 - 10/17/13 06:26 PM

Quote:

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


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roadi
Pooh-Bah
*****

Reged: 08/18/07

Loc: Great Grey Spot "Denmark"
Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6143542 - 10/17/13 07:08 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.


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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6143546 - 10/17/13 07:09 PM

Quote:

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


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: TG]
      #6143705 - 10/17/13 08:46 PM Attachment (20 downloads)

"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.

Quote:

Quote:

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



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


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Eddgie
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Reged: 02/01/06

Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6143754 - 10/17/13 09:11 PM

Quote:

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.


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Eddgie]
      #6143776 - 10/17/13 09:23 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


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Eddgie
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Reged: 02/01/06

Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6143876 - 10/17/13 10:23 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.


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Eddgie]
      #6143981 - 10/17/13 11:35 PM Attachment (28 downloads)

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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Heavens Above
sage


Reged: 01/25/09

Loc: Bristol, UK
Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6144277 - 10/18/13 05:41 AM

Read my review of a Mewlon 210 http://www.cloudynights.com/item.php?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.

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beanerds
sage


Reged: 07/15/08

Loc: Darwin Australia
Re: Mewlons, Refractors and Seeing new [Re: Heavens Above]
      #6144302 - 10/18/13 06:30 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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Asbytec
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Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6144364 - 10/18/13 08:12 AM

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?




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-optics.net/induced.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.

Edited by Asbytec (10/18/13 08:37 AM)


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t.r.
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Re: Mewlons, Refractors and Seeing new [Re: Asbytec]
      #6144642 - 10/18/13 11:06 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





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Erik Bakker
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Re: Mewlons, Refractors and Seeing new [Re: t.r.]
      #6144721 - 10/18/13 11:50 AM

Quote:

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.


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Asbytec
Guy in a furry hat
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Re: Mewlons, Refractors and Seeing new [Re: Erik Bakker]
      #6144805 - 10/18/13 12:38 PM

Quote:

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.

Quote:

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-optics.net/obstruction.htm


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t.r.
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Re: Mewlons, Refractors and Seeing new [Re: Asbytec]
      #6144927 - 10/18/13 01:40 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.

Edited by t.r. (10/18/13 01:46 PM)


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brianb11213
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Re: Mewlons, Refractors and Seeing new [Re: t.r.]
      #6145313 - 10/18/13 05:49 PM

Quote:

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.


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Ed Wiley
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Reged: 05/18/05

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Re: Mewlons, Refractors and Seeing new [Re: t.r.]
      #6145329 - 10/18/13 06:01 PM

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

Ed


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Asbytec
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Re: Mewlons, Refractors and Seeing new [Re: t.r.]
      #6145774 - 10/18/13 10:47 PM

Quote:

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-optics.net/obstruction.htm

Edited by Asbytec (10/19/13 01:56 AM)


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Ed Wiley
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Reged: 05/18/05

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Re: Mewlons, Refractors and Seeing new [Re: Eddgie]
      #6146616 - 10/19/13 12:27 PM

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.com/hfosaf/uk/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


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Alph
Carpal Tunnel


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Re: Mewlons, Refractors and Seeing new [Re: Eddgie]
      #6146810 - 10/19/13 02:38 PM

Quote:

Here is the explanation.



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

Edited by Alph (10/19/13 03:09 PM)


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Alph
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Re: Mewlons, Refractors and Seeing new [Re: Ed Wiley]
      #6146867 - 10/19/13 03:05 PM

Quote:

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?


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Asbytec]
      #6147930 - 10/20/13 06:40 AM Attachment (12 downloads)

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

Heinz


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6147935 - 10/20/13 06:45 AM Attachment (17 downloads)

Another perspective.

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Pete-LH
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Loc: Wilmington, DE
Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6147936 - 10/20/13 06:46 AM

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

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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Pete-LH]
      #6147995 - 10/20/13 08:33 AM Attachment (17 downloads)

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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t.r.
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Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6148156 - 10/20/13 10:27 AM

Nothing crazy about that TAK! My dream scope too!

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Erik Bakker
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Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6148199 - 10/20/13 10:54 AM

Heinz,

Congratulations. That scope is just too beautiful


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Ed Wiley
scholastic sledgehammer


Reged: 05/18/05

Loc: Kansas, USA
Re: Mewlons, Refractors and Seeing new [Re: Alph]
      #6148222 - 10/20/13 11:11 AM

Quote:

Quote:

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.org/contrib/Harbour/StoppingDown.html

Ed


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Sunspot
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Reged: 03/15/05

Loc: Surprise, AZ
Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6148294 - 10/20/13 11:57 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


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Sunspot]
      #6148550 - 10/20/13 02:59 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


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payner
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Reged: 03/22/07

Loc: Bluegrass Region, Kentucky
Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6148975 - 10/20/13 07:35 PM

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

Best,
Randy


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Kunama
professor emeritus


Reged: 10/22/12

Re: Mewlons, Refractors and Seeing new [Re: payner]
      #6149324 - 10/21/13 06:30 AM

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


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Ryuno
sage


Reged: 05/09/13

Loc: Tokyo
Re: Mewlons, Refractors and Seeing new [Re: Kunama]
      #6149348 - 10/21/13 07:16 AM

Quote:

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...


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Sunspot
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Re: Mewlons, Refractors and Seeing new [Re: Ryuno]
      #6149574 - 10/21/13 10:19 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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