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# Sky-Watcher SkyMax-127; true Aperture, and Diagonal size...

69 replies to this topic

### #51 dakinemaui

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Posted 25 April 2019 - 10:44 AM

It's impossible to give a general answer. Some people have shown evidence the primary is the limiting element in their particular scope. Others have shown the primary baffle is clipping in their particular scope. Others may be clipping at the rear port in their particular scope. It also seems quite likely the design may have changed over the years, so keep that in mind.

Not trying to dodge the question, though. The math when the near/far measurements are equal -- i.e., tilt estimated at 0 -- is this:

uncertainty_pupil^2 = uncertainty_shadow^2 + uncertainty_tilt^2 * f^2 * (f/z-1)^2

uncertainty_shadow = 0.29 when the measurements are made to +- 0.5 mm

uncertainty_tilt = 133 urad (0.5 amin), when near/far measurements are made to +- 0.5 mm, and near/far separated by a focal length (1540 mm)

The hard part is, what value to use for z (distance from aperture stop to focus)? Assuming the rear baffle, perhaps z ~ 160 mm? (Just spitballing here, if you know more accurately, please advise.)

This gives the uncertainty in the pupil estimate when linear measurements are +- 0.5 mm and identical near/far measurements separated by one focal length (f=1540 mm) and assuming 160 mm from limiting element to focus:

uncertainty_pupil = +- 6 mm

To know any more precisely, you're going to have to do more work to eliminate elements from consideration. If you've verified -- somehow -- that the limiting element is not the primary baffle or anything at the secondary, you can perhaps consider the primary to be the limiting factor. (Reasonable when meniscus and primary are the same size.) Spitballing the primary location from focus at 920 mm from focus (again, please advise if you know a more accurate value), gives a pupil uncertainty of +- 1 mm.

I think much of the discussion over the years has been useful in a pedagogical sense. (I especially enjoyed reading the circa 2012/2013 threads as people's understanding of optical principles seemed to solidify. :-)  I hope it is clear that while the pupil location (or even limiting element) does NOT figure into the BELT-estimate of pupil size (as Glenn has maintained), it most definitely does impact the uncertainty thereof (as Frank has maintained).

Edited by dakinemaui, 25 April 2019 - 11:07 AM.

### #52 dakinemaui

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Posted 25 April 2019 - 11:05 AM

So, is the uncertainty of the estimate so large as to render the flashlight and BEL tests useless? Or do they still have utility for the casual observer who'd like to know if advertising claims are at least reasonably accurate?

This not terribly different than assessing the utility of the star test-- it's pretty easy to see if something fairly large is wrong, but it's far more difficult to assign accurate numbers. It still has significant utility.

It certainly has utility, with the caution being don't think it's more accurate than it really is. Especially when you have no idea what the limiting element is.

That said, I think if you measure the near/far over a longer distance, you can reduce the uncertainty fairly well (even assuming the worst case element is limiting, say 160 mm from focus). If you want the pupil uncertainty not to exceed +- 1 mm, you need a near/far baseline on the shadow measurement of about 5.5 meters. (Still assuming identical near/far beam diameter measurements to +-0.5 mm precision.) As mentioned previously, I'm just guessing as to that z distance -- I think it's equivalent to the primary baffle.

Bear in mind, however, this guidance is specific to this focal length. A near/far baseline of 5.5 m for a C11 (2800 mm focal length) BELT estimate would only pin down the uncertainty to +- 6 [EDIT: +-3] mm. If you want more precision, go longer or eliminate elements from consideration via alternate means and recompute.

Edited by dakinemaui, 25 April 2019 - 11:22 AM.

### #53 Eric63

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Posted 25 April 2019 - 11:39 AM

I think we are splitting hairs here. Let's just keep in mind that owners who have done the flashlight test on their new Synta 150Maks have found them to be at or near 150mm. This is after it was announced on CN that the 150Mak aperture issue had been resolved. These Maks used to test at about 140mm. The 127Mak though is still testing at around 120mm (give or take a few mm).

Eric
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### #54 KerryR

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Posted 25 April 2019 - 12:04 PM

So: Check my understanding (or lack of): If we measure the width of the beam, say, 5mm in front of the meniscus, and then again at a minimum 5.5M (for this scope), and find the measurements identical, then we can reduce the uncertainty of the test approximately to +- 1mm.

Regarding effective aperture testing: what of the edge intrusion tests I mentioned previously-- especially the one where you focus for the scope at infinity and examine the exit pupil with a loupe while measuring the point at which an object (or small hole in a full-aperture mask) first becomes visible as it's introduced at the edge of the aperture?

### #55 Magnetic Field

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Posted 25 April 2019 - 12:27 PM

So: Check my understanding (or lack of): If we measure the width of the beam, say, 5mm in front of the meniscus, and then again at a minimum 5.5M (for this scope), and find the measurements identical, then we can reduce the uncertainty of the test approximately to +- 1mm.

Regarding effective aperture testing: what of the edge intrusion tests I mentioned previously-- especially the one where you focus for the scope at infinity and examine the exit pupil with a loupe while measuring the point at which an object (or small hole in a full-aperture mask) first becomes visible as it's introduced at the edge of the aperture?

Do the 127mm Maksutovs all roll out from the same factory in China?

Would be interesting to know if every 127mm Maksutov (no matter the manufacturer) there has an effective diameter of 120mm.

### #56 KerryR

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Posted 25 April 2019 - 01:19 PM

Do the 127mm Maksutovs all roll out from the same factory in China?

Would be interesting to know if every 127mm Maksutov (no matter the manufacturer) there has an effective diameter of 120mm.

Well, the Synta 127 Maks (Orion, Skywatcher, Celestron, others) probably do, but, who really knows?

The Explore Sci. 127 Mak, which is not made by Synta (I can't remember who, specifically), has been said, if memory serves, to flashlight test (yeah, I know*) very close to 127mm.

The Meade ETX Maks, which I've read here on CN are made by the same company that makes the Explore Sci Maks (no idea of the validity of this), also appear to flashlight test (yeah, I know again*) very close to the advertised aperture...

*Obviously, the flashlight test is under very heavy contention, so do with that info what you will.

### #57 freestar8n

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Posted 25 April 2019 - 05:45 PM

I understand your meaning, but to be precise (mostly for the general reader): one can't even compute the error if the limiting element isn't even known. One can, however, compute the uncertainty in the estimate. Like Optics 101, uncertainty/sensitivity analysis is a well known subject, and there are specific ways to do it correctly.

Certainly, the BELT method gives an estimate, but the uncertainty in that estimate is well beyond that of the shadow measurement. It is a function of the tilt uncertainty, the shadow measurement uncertainty, the distance uncertainty from limiting element to focus, and even focal length uncertainty.

In other words, it's fine to say, "my best estimate of pupil diameter is X", but measuring the shadow to +- 0.5 mm and concluding the pupil diameter is also known to +- 0.5 mm is completely unfounded. I think that has been the crux of this disagreement that has been ongoing for years.

Hi-

Well - I completely agree.  The main points I emphasize in these discussions of the flashlight test are 1)  It's a good topic to learn how pupils are defined and what aperture means in the first place  2)  The location of the pupil in object space plays a key role in how reliable the flashlight test will be  3)  It's important to know what is acting as the aperture stop in the system - and therefore where it is in object space - and how reliable the flashlight test would be.

This topic has come up several times over the years - and it's unfortunate if people are looking for a "consensus" result.  Topics like this can take ten years to make progress on - because people tend to think as a herd.

For this particular thread - if you are only concerned about the primary limiting the aperture - then it's all pretty simple and the true aperture will be a bit less than the meniscus size.  But that doesn't mean the flashlight test is reliable in general, or has some expected accuracy independent of other factors.  And it doesn't mean that reading about how the entrance and exit pupils are defined - isn't an interesting and enlightening thing to do by itself.

In these past threads there was often a claim that the exit pupil is defined as the image of the objective.  Or it's the image of the entrance pupil.  Things like that.  Both are incorrect as definitions - and miss the point of how this stuff works.

Frank

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### #58 dakinemaui

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Posted 25 April 2019 - 06:10 PM

@KerryR - with the other assumptions being fairly accurate, yes I think you have got it.

No real comment on the other methods, other than it's good to consider the uncertainty in them and seek to quantify that.

I will say they seem to assume the primary as the limiting element. If that wasn't the case, I'm not sure they could tell. Again, haven't thought about it in depth, so I could be totally off base with that.

Sent from my ONEPLUS A5010 using Tapatalk
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### #59 DesertRat

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Posted 25 April 2019 - 06:36 PM

How are people measuring the diameter of a light pattern +-0.5mm?  That would seem difficult since its not a material object, and as such will have some fuzziness to it.

Also what role do aberrations in the system play?  Are people measuring this light pattern along different diameters and taking averages?

Sorry if this has been asked before.  I can recall only one time I "contributed" to one of the many long threads on this subject, and I left as it was a bit contentious, and people were talking past each other.

Now there is one thing one can measure with great accuracy using a camera.  Plate solve a star field and calculate the effective focal length. This is a fundamental property of an optic and what telescopes are good for.  Not to diminish an apertures role - so don't get excited.

Glenn

### #60 makeitso

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Posted 25 April 2019 - 07:07 PM

Sounds to me like a good idea to skip on this scope since no one seems to know what the actual aperture is. I haven’t planned on buying this scope but, I wasn’t planning on buying the ones I have now not to long ago either.

Jack

### #61 Eric63

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Posted 25 April 2019 - 07:17 PM

I don't think the aperture issue should discourage anyone from getting this scope. There are many happy owners here on CN (including myself) with many positive reports. The impact of the reduce aperture and larger CO would be difficult to notice for the average observer and most likely very subtle for the well seasoned astronomer.

Eric
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### #62 freestar8n

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Posted 25 April 2019 - 07:17 PM

Sounds to me like a good idea to skip on this scope since no one seems to know what the actual aperture is. I haven’t planned on buying this scope but, I wasn’t planning on buying the ones I have now not to long ago either.

Jack

This would be an unfortunate conclusion since it is likely the aperture is just a bit under what is claimed - and other maks may be similarly a bit under.  As long as there is nothing intentionally clipping inside (other than the primary - which is normal if the primary is the same size as the meniscus) - and as long as user reports suggest it works well - I wouldn't worry too much about what the true aperture is.

Frank

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### #63 Conaxian

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Posted 25 April 2019 - 07:40 PM

It's a marketing malfunction. They should change the primary or change the name.  Otherwise we will never hear the end of it.

I admit I was a tad disappointed to buy a 5" that was a 4 3/4".  That first light on the moon one fine night made me glad I bought the little feller anyway.

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### #64 makeitso

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Posted 26 April 2019 - 08:29 AM

You guys are saying you’re happy with a scope that isn’t what it claims to be? If you go into it knowing that’s one thing. I know if I went a got a pound of hamburger and found out when I got it home it was only 3/4 pound, I’d be upset.

If I ever get one, I’ll make sure I’m getting one that has the full advertised aperture. Maybe they should claim it’s a 120 instead. I’m sure it’s a great scope.

Jack

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### #65 Simon B

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Posted 26 April 2019 - 08:51 AM

I would have to agree - how many people buy it, not knowing it is 7mm undersized?

Imagine if for example the ES127 APO turned out to be 120 instead of 127... it would be a scandal

### #66 Stargazer3236

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Posted 28 April 2019 - 03:26 AM

This would be an unfortunate conclusion since it is likely the aperture is just a bit under what is claimed - and other maks may be similarly a bit under.  As long as there is nothing intentionally clipping inside (other than the primary - which is normal if the primary is the same size as the meniscus) - and as long as user reports suggest it works well - I wouldn't worry too much about what the true aperture is.

Frank

Au contrare! Meade specifically makes their SCT's with a 1/4" overlap (meaning their 8" scopes are actually 8.25"), so you get a fully illuminated image in the eyepiece. Now, if manufacturer's did that to their maks, then there would not be scandal. If they insist of saying their maks are 127, when in fact they are not, then they should stop being creeps and lower the price of their maks to a price point that reflects the aperture size they really are. You wouldn't want to buy a car that is stated to be 6 cylinders when in fact they are giving you just 5 cylinders or 4. It still runs OK and drives like any other car, so what is the problem? Any body would be furious that they spent thousands of dollars more for a 5 or 4 cylinder car than what they expected would be 6. That's what your comment is suggesting and that is insane thinking!

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### #67 Asbytec

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Posted 28 April 2019 - 07:27 AM

That's what your comment is suggesting and that is insane thinking!

While I agree with you and was equally shocked and modestly disappointed when I realized my 150 MCT was not performing as advertised (best I could test in several ways), I have to say missing a few mm of aperture is hardly - if at all - noticeable in terms of image brightness or resolution. I decided my Mak was performing nicely, anyway, and felt good about it. So, how do I know there is no real difference? I removed the offending aperture stop and opened up full aperture and reduced the obstruction slightly. The most noticeable performance improvement was in my attitude. The scope, however, never skipped a beat and kept on giving nice images. To date, it's my most used scope. Ever. They're nice as is. They are giving us the aperture of the meniscus. At least that part is correct. And, yea, it sucks but it's not really insane thinking on our part to enjoy the scope anyway.

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### #68 dakinemaui

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Posted 28 April 2019 - 10:12 AM

In any other industry, it would be called "deceptive advertising" and litigation would ensue. Then disclaimers would be put on in fine print, similar to weight-loss products: "Entrance pupil may be less than 127 mm. Focal length may be less than 1540 mm with your choice of eyepiece." They might send a \$3 refund as part of a class-action settlement. Nothing meaningful would change. Yeah, I'm cynical sometimes.

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### #69 Conaxian

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Posted 28 April 2019 - 02:08 PM

In the clip on bicycle engine business there was a similar discrepancy.  What was truly a 69cc engine was and is many times marketed as an 80cc engine. Some vendors were including the combustion chamber in the volumetric specifications, others the traditional displacement calculation which ignores it.

The engines were identical, it was a marketing ploy.  Apparently SW measures the meniscus and markets them accordingly.

I have one, and it performs admirably.  Very sharp, long focal length, compact optic.  Orion and Celestron rebrand and sell them, are they calling them 127mm?

If SW is guilty, so are Orion and Celestron.  These companies must have evaluated them before bringing them into their product lines.  They were apparently satisfied with their aperture, price point and overall quality.

### #70 freestar8n

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Posted 28 April 2019 - 05:42 PM

Au contrare! Meade specifically makes their SCT's with a 1/4" overlap (meaning their 8" scopes are actually 8.25"), so you get a fully illuminated image in the eyepiece. Now, if manufacturer's did that to their maks, then there would not be scandal. If they insist of saying their maks are 127, when in fact they are not, then they should stop being creeps and lower the price of their maks to a price point that reflects the aperture size they really are. You wouldn't want to buy a car that is stated to be 6 cylinders when in fact they are giving you just 5 cylinders or 4. It still runs OK and drives like any other car, so what is the problem? Any body would be furious that they spent thousands of dollars more for a 5 or 4 cylinder car than what they expected would be 6. That's what your comment is suggesting and that is insane thinking!

I said, "other maks may similarly be a bit under." I didn't say all were - and I have a Meade Mak7 myself - and often point out that its smaller meniscus in front lets you know that it acts as the pupil stop - and as the entrance pupil - so you can measure it directly.

As it is there is nothing wrong with having the meniscus and primary the same size. Optically it's fine to do - and similarly the secondary in any scope isn't "critically" sized so it only collects full light on axis. There is one element that acts as the choke point for on-axis light - and all the others need to be correspondingly larger to prevent vignetting as you go off axis.

I completely agree that scopes should have their true aperture stated. In this case they are getting away with measuring the "clear aperture" of the front lens - or something. But anyone with a wide angle dslr lens knows that the size of the front element doesn't directly tell you the true operating aperture of a system.

Car analogies are always dangerous - but I would say it's more like getting 19.5 mpg instead of a stated 20 mpg. The car is the same and may be fine - but some of the stats are a bit off. Yes - whether it's a chamfer on a primary or a mak with an internal stop - the true aperture should be stated.

Frank

Edited by freestar8n, 28 April 2019 - 06:05 PM.

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