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

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#1 Dave Ponder

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Posted 15 March 2017 - 12:23 PM

I have seen some posts questioning this.  Is the mirror undersized or is it a baffle tube problem or what? I have a new SW 127 Mak and would like to know what is the real scoop?

Thanks!



#2 Abhat

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Posted 15 March 2017 - 12:27 PM

Based on some posts I have read in the past regarding Orion's 127 Maks, the true aperture is 118mm. 127mm was the size of the corrector.


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#3 Peter Ceravolo

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Posted 15 March 2017 - 12:33 PM

I found that the few Chinese Mak-Cass I tested were roughly 1/2" smaller in true aperture than advertised. They are probably the same optical system with different branding/packaging. The culprits are the undersized 2nd surface on the corrector and the undersized primary mirror.

 

The only as advertised scopes I've tested are the Meade 7" and 3.5" Questar.



#4 Eddgie

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Posted 15 March 2017 - 12:57 PM

I have seen some posts questioning this.  Is the mirror undersized or is it a baffle tube problem or what? I have a new SW 127 Mak and would like to know what is the real scoop?

Thanks!

The real scoop is that direct measurements done by CN members did show that several version of 127mm MCT were working at less than stated aperture.

 

The test method was debated and dismissed by a few people as being flawed but I have used the method (sending a laser beam though the system to project the pupil on to a flat surface) and my own findings that were that based on ray traces for SCTs at different back focus spacing, the laser projection resulted in seeing aperture reduction very much in line with the ray trace suggestions, so while some may question the method used, I found it to give a result consistent with theory so it appeared to me to be reliable.

 

Absolutely has been measured in a few cases though, and from the discussion, it very much seemed to be a design factor and not sample specific problem.  You should be able to find some threads on this my using the search function in CN.  I would search on "118mm" and "MCT"


Edited by Eddgie, 15 March 2017 - 12:59 PM.


#5 GlennLeDrew

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Posted 15 March 2017 - 01:54 PM

For the Orion 127 MCT I measured several years ago, the *ahem* 'primary' culprit was the primary mirror; it was undersized for the corrector's 127mm aperture. The beam-expanded laser test unequivocally indicated a working aperture of just about 119mm. Others have got results around 118mm.


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#6 Dave Ponder

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Posted 15 March 2017 - 02:20 PM

Thanks guys for the info.  So its a 4.65 inch Mak....LOL...5 sure would sound better.



#7 freestar8n

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Posted 15 March 2017 - 03:26 PM

The mirror isn't really "undersized" - it's just that the true aperture will be less than the corrector aperture if the mirror and corrector have the same diameter.

 

In a mak like this the aperture stop can be at the corrector or at the mirror - either way.  If it's at the corrector, like in the Meade 7", then the primary will need to be larger than the corrector in order for the primary itself not to be the limiting aperture of the system.  And it needs to be even larger than that to avoid vignetting off axis.

 

But you can also have both be the same size - and then the primary is acting as the limiting aperture or aperture stop - and the true aperture will be less than its physical size because the view through the front meniscus makes the primary mirror seem smaller than it is.  In terms of true collecting aperture - or entrance pupil - it's that apparent size that matters when viewed from the front.

 

As long as the corrector and primary in a maksutov are the same size, you know right away that the true aperture will be much less than the diameter of the meniscus.  And 118mm is about right.

 

But it doesn't mean the primary is undersized - it just means the stated aperture of 127mm is incorrect and doesn't represent the true aperture of the system based on the entrance pupil.

 

You also need to be careful estimating the secondary obstruction - because it could be much larger than the secondary mirror spot size if there is a baffle flaring out and attached to it.  But just like with the primary - it isn't the physical size of the baffle that matters - but how big it appears when viewed through the meniscus.  Since the meniscus makes it appear smaller - the true obstruction will be a bit less than the physical size of the baffle.

 

But since the true aperture is less than expected - the corresponding sec. obstruction percentage will be higher than expected.

 

As for the ability to measure the true aperture accurately - as has been explained and discussed at length - as long as the limiting stop is at the corrector or the primary you can measure the true aperture with confidence.  But if the limiting aperture is at the secondary baffle or some obstruction near focus - it becomes much harder to measure the true aperture accurately - or even to tell what the limiting aperture really is.

 

Frank


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#8 Dave Ponder

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Posted 15 March 2017 - 03:53 PM

Frank,

So, if the meniscus is 127, then the primary would need to be larger than 127mm ("oversized") for the scope to really have 127mm of light gathering, right? 

Dave



#9 freestar8n

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Posted 15 March 2017 - 04:01 PM

Yes - that's right.  And for larger maks, like the 7", the meniscus is more expensive so they have a big motivation to make the meniscus smaller and the primary larger.  You can tell those right away because the view from the front shows a much "undersized" meniscus relative to the tube.  Of course - you also need to have a primary that is larger than that - but you would hope the primary is much larger otherwise there is no need for a big tube.

 

But it is valid to have the meniscus the same size as the primary - and in that case nothing is "undersized."  But the stated aperture should be less than the corrector size in that case.  The stated aperture and secondary obstruction should always be based on the actual size of the entrance pupil.  But the entrance pupil is somewhat abstract and - in general - hard to measure.  But in this case 118 sounds about right - if the primary really is the limiting aperture of the system - which I think it probably is.

 

Frank



#10 Wildetelescope

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Posted 15 March 2017 - 04:17 PM

I have seen some posts questioning this.  Is the mirror undersized or is it a baffle tube problem or what? I have a new SW 127 Mak and would like to know what is the real scoop?

Thanks!

If you have interest in this as an academic exercise, then that is fine, and one can certainly check things out with a laser as others have suggested.  It requires that you make certain that you have everything square and aligned, but it is certainly possible.  I have read a number of these threads arguing about cited aperture numbers for a variety of Maks and SCTS, and have come to the following conclusion, I would rather spend time looking through the scope than shining lasers down the pipe.    If one is really worried about valid aperture measurements, then buy a refractor, or a Newtonian.   In my experience at the eyepiece, the Synta Maks are excellent scopes that provide very sharp and contrast filled views of most objects. Same is true for my C8 Edge. The exact magnitude of the Effective aperture does not change that.  Life is too short and so are clear nights:-) 

 

Cheers!

 

JMD


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#11 freestar8n

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Posted 15 March 2017 - 04:22 PM

Many people make purchase decision based on a comparison of metrics - like aperture and secondary obstruction.  So getting the right numbers is important in that case.

 

And with maks - there is often a claim that they have advantages because the sec. obstruction is smaller than other scopes.  Again the real numbers may differ and make that a false claim.

 

But for anyone who already has one of these scopes and enjoys it - sure there is no real need to worry about the true aperture if they are happy.

 

But if they start considering larger aperture - and going over the numbers - it would be good to know their current true numbers and the true numbers of the larger scope - to have a good idea of what the benefit really would be.

 

Frank


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#12 Dave Ponder

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Posted 15 March 2017 - 04:32 PM

Well, it seems like you take a hit twice on the 127 Mak for example.  You get less true aperture than a Mak with the primary the same size as the meniscus and you certainly get less true aperture than a Mak with the so called "oversized" mirror.  So is the true aperture really less than 118mm?

Dave



#13 freestar8n

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Posted 15 March 2017 - 05:05 PM

Just one hit. 118.

Frank

#14 Eric63

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Posted 15 March 2017 - 05:40 PM

Mine is 118mm, but still a great little scope.
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#15 Dave Ponder

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Posted 15 March 2017 - 08:15 PM

Mine is 118mm, but still a great little scope.

It's is a great little scope, never said anything negative, I just wanted to understand what the true "size" of the 127 really is. I can't wait til I have another night out with my almost 127!

Thanks all.


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#16 GlennLeDrew

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Posted 15 March 2017 - 09:10 PM

To assess the secondary obstruction ratio by diameter, using a 7-10X loupe with reticle graduated to 0.1mm will permit measurements of the exit pupil and secondary shadow to a confidence of about 0.05mm. Use a longer f.l. eyepiece so as to obtain a larger exit pupil and thereby reduce the relative error.

 

Or you can take a photo of the exit pupil and measure the width of it and the secondary shadow in pixels, or by using a ruler held against your monitor.

 

While the optical quality of these MCTs is generally quite good, with decent correction and smoothness of figure, the effectively increased secondary obstruction will bring contrast transfer down to about the level of an equally good SCT. The advantage of the nominally smaller CO is defeated, or at least largely negated.

 

But fret not! SCTs are not generally quite so well figured, and a better wavefront beats a little extra diffraction.


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#17 Eric63

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Posted 15 March 2017 - 09:49 PM

Another thing to note about these scopes is the longer focal length than advertised. With my 2" diagonal (SW Version) my actual measured focal length is 1750mm, making it an F14.8 scope (when using the 118mm Dia aperture).
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#18 Ptkacik

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Posted 15 March 2017 - 11:34 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

#19 GlennLeDrew

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Posted 16 March 2017 - 06:30 AM

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,

Perhaps the long f/ratio would help to keep the error small in your suggested eyepiece-less test, but for a reliable indication it's best to install an eyepiece at least near to infinity focus. Then shine the flashlight straight into the eyepiece, from a distance of no less than about 10 eyepiece focal lengths.

 

An at-focus eyepiece assures that the light path is like that during use, except for being in reverse.

 

To verify that the geometry is correct, measure the cylindrical bundle of light emerging out the front immediately in front of the scope and a few feet distant. If the same, the setup is correct. If different, tweak the focus until these two measures are the same. Then will you have a reliable measurement; especially if the edges of the disk of light and the central shadow are nicely sharp.

 

A green laser whose beam at least fills the exit pupil is excellent because it assures a sharp-edged annulus of light. This might require to use an exit pupil of about 1mm, possibly a bit smaller. Or else a beam expander is used to widen the laser beam so that it fills a larger exit pupil. A good beam expander is a simple 6X30 or 8X50 finder; the laser shines into the eyepiece and a collimated, wider beam enlarged by factor the magnification emerges out of the objective. For example, a 1mm laser beam aimed into the eyepiece of a 6X finder provides a 6mm beam that can fill exit pupils up to 6mm.


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#20 Jon Isaacs

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Posted 16 March 2017 - 07:18 AM

Another factor with the scopes is the size of the secondary obstruction.  Orion makes the statement that the size of the "secondary mirror obstruction" is 39mm. I measured the size of secondary baffle at 48mm..  Now that is clearly the size that matters but just exactly how does that play with the reduced effective aperture?

 

Does that somehow also reduce the size of the CO as seen at the eyepiece or does one just figure 48mm/118mm = 40.7%?

 

Jon



#21 Dave Ponder

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Posted 16 March 2017 - 10:06 AM

I would say 40.7% based on your numbers...



#22 GlennLeDrew

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Posted 16 March 2017 - 11:09 AM

If I recall correctly, the primary aperture in the sample I had apart was about 123mm. In this Mak, the external axial cylindrical bundle accommodated by the system has a diameter of 118mm. Upon entering the corrector this light is diverged outward fairly notably, in the manner of a negative lens. Upon exiting the rear surface of the corrector the light becomes almost parallel again, but still slightly divervent. The bundle diameter at this rear surface might be 122mm. Upon reaching the primary it has expanded slightly to 123mm (using my potentially faulty memory!).

 

If a *physical* diameter for the widest part of the secondary baffle is obtained, then the obstruction ratio should be based on a primary aperture of just slightly smaller than the physical clear aperture of that mirror. If this latter is 123mm, then the figure to use is the diameter of the axial bundle where it passes the plane of the secondary baffle's rim, which would be nearer to 122mm.

 

If one is using a determination of the secondary baffle diameter as it *appears* through the Mak corrector, the refraction will make the thing look to be smaller than it is.

 

The surest way to measure the obstruction ratio is to measure it directly, via the laser aperture test or the exit pupil. The latter's small size demands more care and precision.


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#23 freestar8n

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Posted 16 March 2017 - 03:18 PM

. I measured the size of secondary baffle at 48mm..  Now that is clearly the size that matters but just exactly how does that play with the reduced effective aperture?

 

Does that somehow also reduce the size of the CO as seen at the eyepiece or does one just figure 48mm/118mm = 40.7%?

 

Jon

Hi Jon-

 

No - the physical size of the obstruction isn't what matters.  It's the apparent size of the obstruction that matters - as seen by the object looking into the telescope.  The meniscus has a demagnifying effect on both the baffle and the primary - so both will appear smaller than they really are.  And those two objects are at different distances from the meniscus - so their sizes will be reduced by different amounts.

 

It's analogous to the field stop inside an eyepiece.  If there are lenses on the telescope side of that field stop, the physical size of that stop won't correspond to the field of the telescope at its focal length.

 

These discussions of pupils and measurement consistently view them as simple things you can just see and measure - but if you stick to how they are actually defined - and what matters in terms of light gathering - everything gets complicated when there is a series of optical elements between you and the aperture stop.

 

Frank

 

[addendum]

 

Oh - but you are definitely correct that the secondary spot itself isn't what matters.  You need to include the obstruction due to the baffle.  It's just that the impact of the baffle will be slightly less than its physical size.

 

The main games played with maks and their specs are:

 

1)  The true aperture is less than the front "clear aperture" if that aperture fills the front of the tube.  There is no way that can be the true aperture of the scope.

 

2)  The secondary obstruction is larger than the mirrored spot due to the baffle

 

3)  The focal length is often longer than expected

 

4)  The combined impact of 1 and 3 greatly relaxes the challenge of canceling out spherical aberration

 

5)  The greater central obstruction reduces the rms wave error contributed by high order spherical aberration

 

One final trick that I'm still not sure about is whether the secondary baffle itself is limiting the aperture in some scopes.  If that's the case it can greatly reduce the aperture and it would be hard to tell it was happening.

 

None of this stuff means the scope is good or bad.  In fact most of these factors will make the overall performance better in terms of Strehl.  But at the expense of reduced aperture, larger secondary obstruction, and slower speed.


Edited by freestar8n, 16 March 2017 - 03:38 PM.

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#24 Peter Ceravolo

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Posted 16 March 2017 - 11:24 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.


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#25 Ptkacik

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Posted 16 March 2017 - 11:26 PM

Thanks for the measurement description as I used it once but didn't recall the details.

When I took my 7" Mak apart, the primary mirror was noticeably bigger than the 180mm description but I don't recall the light throughput measurement. It wasn't 180 but it wasn't as small as I was expecting either, (178mm?). I also remember the light followed the baffle so that obstruction was closer to the mirrored circle than the cone baffle.

I've loaned my 127mm Mak to my son but I'll try your measurement technique to get its numbers this weekend. Now, I'm curious.
 
On your second comment or addendum, light throughput is distractedly valuable to me but the views from my Maks are visually appealing nonetheless, (I really like them regardless of technical numbers).
 
Clear skies,
Peter
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