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What is the true aperture of the Sky-Watcher 127 Mak?

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#26 Wildetelescope

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Posted 17 March 2017 - 02:20 PM

Being a former commercial kinda guy, I am amazed that no one is expressing shock and dismay at the misleading advertising. In my day, if I were found out advertising my HD145 scopes a 1/2" smaller than real, I would have been skewered.... Having said that, the scopes are a great value for the money, if they work well, and most do.

In my experience, if you measure the diameter of the corrector of your mass produced Mak or SCT, and then divide that into the width of the outside portion of your secondary, you will get a CO percentage that agrees with what advertisements publish.   This is a simple (simplistic?) definition that is likely legally (if not scientifically) defensible should a customer try to call them on this.  The point made by others here and in other threads, is that there is MORE stuff that can happen to the light  once it enters the telescope that can further reduce the "true" or I prefer the term "Effective" aperture and central obstruction.   This include Primary and secondary Mirror size, Central baffle diameter, etc...  

 

Is what the ads say WRONG?  Well, not really, if one define aperture as the diameter of the front of your telescope.  Do the Ads provide a COMPLETE STORY?  Well, not really when one considers that other things impinge on the light path AFTER it enters the Scope.  Welcome to the fuzzy world of marketing:-)  In the end, Caveat Emptor, is sage advise to the buyer, as always.  It always help for one to understand what they are buying before they buy it, and then not lose any sleep worrying about things.  Things like collimation and optics aberrations are going to have a bigger effect on what you see than a few percent different in "effective" aperture, as is being discussed in another current thread. 

 

My two Cents.  Remember what you paid for it:-)

 

JMD

 

P.S. I would terribly remiss if I did not add, that manufacturers of premium optical systems, (like yours:-), usually take ALL the math into account when they design them:-)  and report accurate numbers based on what reaches the eyepiece, rather than what the light first encounters upon entering the scope.  Your customers are likely VERY knowledgeable and as such will see through any marketing haze.  Which is why they are coming to you rather than some of the other options discussed here.


Edited by Wildetelescope, 17 March 2017 - 02:34 PM.

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#27 PXR-5

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Posted 17 March 2017 - 04:00 PM

Being a former commercial kinda guy, I am amazed that no one is expressing shock and dismay at the misleading advertising. In my day, if I were found out advertising my HD145 scopes a 1/2" smaller than real, I would have been skewered.... Having said that, the scopes are a great value for the money, if they work well, and most do.

Aperture means opening, so that being said, mine measures exactly 127mm as advertised.

Us nerds, LOL, like to use terms like "effective​ aperture* so the manufacturers aren't false advertising.

Edited by PXR-5, 17 March 2017 - 04:01 PM.


#28 Peter Besenbruch

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Posted 17 March 2017 - 05:19 PM

Being a former commercial kinda guy, I am amazed that no one is expressing shock and dismay at the misleading advertising. In my day, ...

One expresses shock and dismay if such behavior were out of character for the company. I bought the SW180 when they corrected those claims by redesigning the telescope. Celestron, now a part of Synta used to sell "tenth wave optics," though they never produced a single scope that came close to that level. I heard very few complaints about that. Eventually, the infamous Sky and Telescope SCT review gave those false claims too much exposure.

 

"In my day," every Celestron sold came with a model. Every Celestron "telephoto lens" was aimed squarely at peeping Toms. As for "your day." I wish you many more of them, and am sorry that no-one in our club owns one of your telescopes. I still hope, some day, to check one of them out to see for myself what they can show.



#29 GlennLeDrew

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Posted 17 March 2017 - 05:23 PM

 

Being a former commercial kinda guy, I am amazed that no one is expressing shock and dismay at the misleading advertising. In my day, if I were found out advertising my HD145 scopes a 1/2" smaller than real, I would have been skewered.... Having said that, the scopes are a great value for the money, if they work well, and most do.

Aperture means opening, so that being said, mine measures exactly 127mm as advertised.

Us nerds, LOL, like to use terms like "effective​ aperture* so the manufacturers aren't false advertising.

 

If "aperture means opening", then a 12" Newt with a 13" window (instead of spider) should be spec'ed as a 13" n'est pas? wink.gif

 

Getting serious: Just because the first surface of a hunk of glass has a certain diameter *does not* mean it defines the actual working aperture. A good example is a fisheye camera lens. A 16mm f/2.8 by definition has an aperture of 16 / 2.8 = 5.7mm. Yet the front element might be 55mm in diameter.


Edited by GlennLeDrew, 17 March 2017 - 05:27 PM.

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#30 Cajundaddy

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Posted 17 March 2017 - 06:02 PM

It's a lot like buying an audio amplifier based on rated watts, with no mention of the speaker load, whether it was taken at RMS or peak output, whether rated output was above 1%THD, or what the response curve looked like.  We depend a great deal on spec sheets and so often they only really give us a very rough indicator of performance.  Objective testing,  direct performance comparisons, and trusting our eyes and ears is often far more meaningful than raw specs.


Edited by Cajundaddy, 17 March 2017 - 11:39 PM.


#31 Retired Dave

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Posted 17 March 2017 - 06:56 PM

Being a former commercial kinda guy, I am amazed that no one is expressing shock and dismay at the misleading advertising. In my day, if I were found out advertising my HD145 scopes a 1/2" smaller than real, I would have been skewered.... Having said that, the scopes are a great value for the money, if they work well, and most do.

I wish I had one of your scopes for what it's worth!  Advertising the 127s as such is not right in my opinion. Why can't Synta just call it a 120? At least 118 is closer to 120, lol!  Now if I had paid $3k for a 102mm refractor and found out after the purchase it was only 95mm, I would go nuclear.  It's a little easier with the 127 in that it is not very expensive.  Even at 118mm it's still worth the money, but we all would respect honest advertising.


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#32 Retired Dave

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Posted 17 March 2017 - 07:29 PM

Why ask? Measure it yourself. Shine a bright light (flashlight) from about 12" from the back into the visual back and project it onto a screen out front. Measure the light beam width about a foot out front.

Its not that hard. Tell me what you got.

Clear skies,
Peter

Peter,

i just did the test as you described, got 119mm...so I guess 118-119mm must be about right. Thanks!



#33 Wildetelescope

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Posted 17 March 2017 - 09:00 PM

 

 

Being a former commercial kinda guy, I am amazed that no one is expressing shock and dismay at the misleading advertising. In my day, if I were found out advertising my HD145 scopes a 1/2" smaller than real, I would have been skewered.... Having said that, the scopes are a great value for the money, if they work well, and most do.

Aperture means opening, so that being said, mine measures exactly 127mm as advertised.

Us nerds, LOL, like to use terms like "effective​ aperture* so the manufacturers aren't false advertising.

 

If "aperture means opening", then a 12" Newt with a 13" window (instead of spider) should be spec'ed as a 13" n'est pas? wink.gif

 

Getting serious: Just because the first surface of a hunk of glass has a certain diameter *does not* mean it defines the actual working aperture. A good example is a fisheye camera lens. A 16mm f/2.8 by definition has an aperture of 16 / 2.8 = 5.7mm. Yet the front element might be 55mm in diameter.

 

No arguments here!  And a thread like this is great for helping less experienced people understand what it is they are truly buying.  

 

JMD



#34 PXR-5

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Posted 17 March 2017 - 09:07 PM

Refractors are probably the worse offenders, my 102gt was about 98, my ST80 appears to be about 77, and the focuser cuts into the light cone :(

#35 Retired Dave

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Posted 17 March 2017 - 09:48 PM

I just did the flashlight projection test on my C5....got exactly 127mm.  I rechecked the 127mak after focusing at infinity, got 120mm.  


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#36 Asbytec

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Posted 26 March 2017 - 07:25 PM

One final trick that I'm still not sure about is whether the secondary baffle itself is limiting the aperture in some scopes.

 

This was the case in my Orion 150 MCT, the primary is "properly" sized at 162mm about 8% larger than the meniscus.

 

Testing showed the vignetting occurred on the secondary baffle. Among other tests, a laser square to the edge of the meniscus did hit the edge of the primary and the outside of the secondary baffle. (I removed it to no ill effect with veiling glare.) 

 

Some other models, as I recall during Ed Holland's monster thread a few years ago, had "undersized" primaries. Many of us decided, regardless, we love our Maks. 


Edited by Asbytec, 26 March 2017 - 07:29 PM.

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#37 BaysidePhotonics

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Posted 19 April 2020 - 04:56 PM

Sorry to post to such an old thread, but I just made some measurements that someone may find useful.

 

I have a large optical flat that I use for autocollimation testing of telescopes and objectives. I use a modified eyepiece that has a singlemode optical fiber coupled to a HeNe laser to provide a star test in double pass that I use to check for correction and astigmatism. When this setup is used, there is a collimated beam between the telescope and the flat that corresponds to a diffraction limited object placed at the focal plane of the objective.

 

I just tested three Maksutov-Cassegrain telescopes with this outfit, and increased the power coupled into the fiber until I could see the illumination from the collimated beam on a machinist's scale (precision ruler) placed there.

 

If a flashlight is used at the focal plane, it corresponds (in reverse) to the image of an extended object, and so creates a diverging cone of light exiting the objective. A measurement of this beam would yield different results depending on the distance between the telescope and the measuring scale. With the optical fiber as a point source, the beam is collimated, so this dependence is largely removed. The size of the beam and the diameter of the central obscuration can be measured accurately this way.

 

In making these measurements, I removed the diagonal and placed the eyepiece directly into the visual back.

 

The three telescopes were:

 

1. Orion 102mm Mak-Cass.

2. Orion 127mm Mak-Cass

3. Explore Scientific Explore First Light 127 Mak-Cass

 

The dimensions I measured were:

 

1. Orion 102mm: Aperture 96.5mm, Central obstruction 35.0mm, giving a 36.3% ratio to aperture

2. Orion 127mm: Aperture 119.4mm, Central obstruction 48.3mm, giving a 40.4% ratio to aperture

3. Explore Scientific 127mm: Aperture 125.7mm, Central obstruction 51.3mm, giving a 40.8% ratio to aperture.

 

I estimate the error of measurement here to be approximately +/- 0.5mm.

 

I did not try to determine the surface that was acting as the stop, or whether baffles were responsible for the aperture limitations. For what it's worth, though, when I replaced the diagonal on the Orion Maks there was no measurable difference in the size of the aperture or the obstruction measured this way. This may imply that the eyepiece baffle (as opposed to the secondary baffle) was not the limiting surface, since the refocus and resulting change of the optical spacing should probably have caused a change. I have not raytraced this to see if my intuition is correct here, though.

 

I am not sure why it is necessary to make these kinds of measurements to get accurate numbers for the basic optical parameters of a commercial telescope, but here we are :)

 

Warren


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#38 eblanken

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Posted 26 April 2020 - 04:16 AM

So now I'm curious about the ETX-125 Meade as contrasted the SW127 aka Orion (Synta) MCT 127: Is the Meade suffering from the same issue or does it have an oversized primary ? I get that the C5 being an SCT doesn't have the same issue because the Schmidt corrector doesn't diverge the light like the MCT's corrector. So is the ETX-125 processing more light than the Orion (Synta) MCTs that this thread has been discussing ?

 

Ed


Edited by eblanken, 26 April 2020 - 04:26 AM.


#39 Auburn80

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Posted 26 April 2020 - 10:23 AM

So now I'm curious about the ETX-125 Meade as contrasted the SW127 aka Orion (Synta) MCT 127: Is the Meade suffering from the same issue or does it have an oversized primary ? I get that the C5 being an SCT doesn't have the same issue because the Schmidt corrector doesn't diverge the light like the MCT's corrector. So is the ETX-125 processing more light than the Orion (Synta) MCTs that this thread has been discussing ?

Ed


IMHO, I'd say technically yes. The differences would be tiny though and sample variations would play a bigger role. Eg; a good SW would perform better than a mediocre ETX.

#40 BaysidePhotonics

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Posted 26 April 2020 - 10:26 AM

It is my understanding that the Explore 127 mak and the ETX 125 are optically the same telescope, made by JOC. If anyone has better information please chime in and correct me.

 

The optical design of this scope is different from the Synta. The Explore has a stated focal length of 1900mm, giving a focal ratio of 15 while the Synta has a stated focal length of 1540mm, which with the corrected aperture gives a focal ratio of 1540/119 ~= 13. I have not measured the EFL of either of these scopes, though, so I suppose we should take these values with a grain of salt. Looking into the corrector, it appears to me (please understand I have not disassembled the scopes and made any measurements) that the corrector looks significantly thicker in the Synta. I don't currently have plans to take them apart, but if I do I will post some actual numbers. The focuser on the Explore is quite tricky,  and I may decide to take it apart to replace the thick grease that must be in there with something that performs better, especially at low temperatures.

 

The primary mirror in the Synta appears to be smaller than that in the Explore, just looking at the gap between the inner wall of the tube and the edge of the mirror. Again, I haven't made any measurements at this point.

 

One other non-quantitative observation: the amount of spherical aberration in the Explore seems much greater than that in the Orion. In autocollimation artificial star testing (double pass), the Explore does not have a well defined focus point while the Orion is very snappy. There is obvious spherical aberration in the Explore, while I can't see any 3rd order SA in the Orion, just a hint of 5th order. On a real star in good seeing, the Explore has very obvious differences inside and outside of focus, while in the Orion there is almost no difference, even very close to focus. If I had to choose, I would pick the Orion in a heartbeat, even with the 5% reduction in aperture.

 

Warren


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#41 Eric63

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Posted 26 April 2020 - 11:19 AM

When I first measured my 127Mak back in 2012  (118mm primary and 48 mm CO),  I was concerned and somewhat upset. But the more I used it the more I realized how great a little scope it is. I also compared it to my 150F5 Newtonian with a 33% CO, which I had optimized (re-figured mirror, flocking, cooling fan etc) and found the little mak to be not far behind the Newt , in fact it was very close. So in the end I just stop worrying about this an enjoyed the scope.  The size and weight of the little guy is why I love it so much.

 

One thing to note though for those with the 2" diagonal version.  The focal length climbs to 1750mm and not 1540 mm. Not that it's a problem, but it will impact EP choice.

 

Eric 


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#42 Sixptelk

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Posted 12 March 2022 - 08:59 PM

I've had a number of the 127 MAKS, all Orion Apex or Starmax, one and the same and only rebranded by Skywatcher, Celestron and the rest of the pack. When I say a number, I'm talking more than three. All gave incredibly highly resolved views on the planets, moon and close double stars. I've also had the 90mms and 102mms in the past. 

 

I happened to have two Orion MAKS out one night, actually had the 102mm Apex and the 127 Starmax aimed at M13 at the same time. The eyepieces were pushing around 140x, a decent mag for the sized scopes and a globular cluster. Seeing was good and from a Bortle 5 location, not that it particularly matters. What mattered to me was that both of these scopes were very clean and both performed extremely well on the moon and with the 102mm it was actually the first scope of many in which I saw the polar ice cap on Mars. However what I did find using both scopes at the same time on the same object was that the image was really no brighter in the 127mm than in the 102mm. This would imply either different seeing conditions on different nights or optics that were dusty or otherwise not up to par, thus reducing light gathering ability.

 

And then I started reading about my 8" GSO CC and light loss and there was a blurb in that thread about the 127s. And here is that thread in detail. Do I want to spend the extra money for a 5" that is more like a 4"...NO! Would I consider advertising by many that sell the 127 fraudulent, false or just plain naive...you be the judge. I sold the scope and I also sold the GSO 8". When I buy aperture by the inch or mm that's what I expect. And even given the 8" CC claimed 96% reflectivity off quartz mirrors, it operated more like a 180mm when compared side by side to a standard Celestron 8" SCT. In the GSO there was definitely a light loss due to the manufacturing process. Does that make them bad scopes...NO, it just means that you didn't get all the aperture you paid for....


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