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Orion 127 Maksutov - Back focus and aperture

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#1 Ed Holland

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Posted 27 February 2012 - 12:17 PM

I thought it might be sensible to post this study in its own thread, but it is inspired by: Aperture in C6 & C8 for various configurations elsewhere in Cats and Casses

The OTA under examination was an Orion Skyview Pro 127mm Maksutov. Specified aperture is 127mm (clear diameter at the meniscus), focal length is nominally 1540mm (f/12). The basic design has been produced under a number of guises & brands and has proven popular with owners and reviewers.

I chose 3 configurations for the "flashlight" test. Back focus in each case was measured from the edge of the threaded flange to the eyepiece field stop (+/-2mm). The eyepiece used was a 10mm Sirius Plossl. Refocussing to infinity was performed between each change in back focus distance. The disc of light from the OTA was projected onto white paper approximately 300mm (12") from the front of the scope to permit measurement (+/-1mm).

Accessory configurations:

a) 2" adapter tube and 2" mirror diagonal, approx back focus 155mm

b) 2" adapter tube without diagonal, approx back focus 60mm

c) 1.25" SCT mirror diagonal and 1.25" SCT visual back, approx back focus 126mm

The three cases represent the most sensible use of the accessories I have to hand. A 90° erecting prism I own was not included, but would give similar results to the other 1.25" setup.

Measured data:

Setup - Bck Fcs --------- Measured Aperture ----- CO shadow

A ----- 155 -------------- 117 ------------------- 45
B ----- 60 --------------- 118 ------------------- 45
C ----- 126 -------------- 118 ------------------- 45

(all data in mm)

The results are very interesting on two accounts. Setup A, longest backfocus, was measured first and I thought "Aha there's the aperture reduction that is warned about". However, I was surprised to see that the measurement changed little, if at all for ANY of the setups.

The measurements lead me to two conclusions:

1) This telescope is really operating at 118mm effective aperture, some 9mm less than the manufacturer's specification.

2) It is seemingly tolerant of the additional back focus present when employing 2" accessories.

On point 2 I MUST add further comment. A test suggested by Eddgie some while ago revealed that there is vignetting as one moves to the edge of field with long focal length EPs. This can be detected by observing the ring pattern of a defocussed star, then moving the scope to place this image at different positions in the apparent field. Vignetting can be detected where the concentric ring pattern is disturbed, and this has been confirmed in previous testing with a 35mm EP in the 2" visual back + 2"diagonal setup.


I hope this is interesting, and welcome comments & observations :)

Cheers,

Ed

#2 PHampson

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Posted 27 February 2012 - 01:56 PM

The 127mm probably refers to the size of the mirror. As the Maksutov corrector is, in effect, a negative lens, the effective aperture might be expected to be smaller. 9mm seems a little much but it would be interesting to see what the measured aperture of your corrector really is.

I think this came up some time ago when some noticed that their corrector apertures were less than 'advertised' but I may be mistaken.


Paul

#3 Ed Holland

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Posted 27 February 2012 - 04:20 PM

That makes sense, and I remember a similar discussion topic from a while back, now that you mention it. I think I commented in the first post that 127mm was the corrector aperture, but this was assumed (by me).

I'll make a measurement of the aperture at the corrector this evening and post the result here.

Cheers

Ed

#4 Ed Holland

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Posted 28 February 2012 - 12:56 AM

Clear aperture at the mensicus is 127.5 mm, determined with Vernier calipers. I don't have a way to measure the diameter of the primary mirror without disturbing the OTA, but this would be a useful piece of data. Given the arrangement of the components and the tube dimensions, it is hard to imagine the mirror is any more than 127mm in diameter, so might well be undersized, and is placed behind a divergent lens corrector. This adds up to an effective aperture that is smaller than the physical opening at the front of the OTA.

Notwithstanding these findings, the scope is a good performer, well built and compact in its design. Nothing I have rased here should be regarded as a negative criticism, rather it is an attempt to better understand the instrument and the performance obtained from it.

Cheers,

Ed

#5 PHampson

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Posted 28 February 2012 - 01:53 AM

That is interesting. I had read somewhere, sometime of certain Mak designs where the corrector was slightly smaller than the mirror but, as I recall, it was only 3 percent or so which wouldn't add up to 9mm. I agree with Eddgie in the other thread, the next place to look would probably be the central baffle but, then, that would mean you'd have to disassemble the OTA. If you're happy with the scope, I certainly wouldn't bother with that. Too much risk for little or probably no gain. It is odd, though.

Maybe someone else can think of another non-destructive test that could explain it.


Paul

#6 Ed Holland

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Posted 28 February 2012 - 02:27 AM

Well, one could roll up a sheet of paper, place this in the baffle tube and so extend the baffle to see if the effective aperture were reduced further.

I wish now I had measured parts when I dissassembled the scope for cleaning, and modified the focus mechanism. It is working so nicely at present that it would be a shame to upset the collimation again.

#7 EdZ

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Posted 28 February 2012 - 07:20 AM

It doesn't necessarily need to be the corrector, the mirror diameter or a baffel. It could be that the light cone coming off of the primary is slightly too wide to be completely encompassed by (seen by) the secondary. So the curvature of the mirror could be the culprit.

edz

#8 Binojunky

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Posted 28 February 2012 - 08:51 AM

I have the Apex 127, nice performer and at the modest price I wasn,t expecting a Questar, to be honest like most things in life sold a lower price level it tends to be a compromise,Dave.

#9 Eddgie

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Posted 28 February 2012 - 12:01 PM

Do you mean for example that if the mirror is supposed to be say f/2.5 (and I don't know what it is supposed to be), but was in fact more like f/2.6 or f/2.7, that the cone would not converge and perhaps is hitting the outside edge of the secondary baffle or not falling completly on to the secondary mirror?

That would be a fabrication error I would think (if this were the case) because I am sure a designer would not have made such a critical mistake.

This system is working with light collection that is scarcely better than a 110mm refractor.

If it were me, I would sure want to know what was going on...

#10 Ed Holland

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Posted 28 February 2012 - 12:22 PM

We have a good optical bench here at work. It might be informative to setup a test configuration whereby the OTA could be probed with a carefully aligned laser pointer to investigate ray paths parallel to the optical axis. The only issue is time...

Any thoughts?

Ed

#11 GlennLeDrew

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Posted 28 February 2012 - 09:11 PM

The fact of the corrector being a negative meniscus is immaterial. The diameter of the emerging light bundle is what determines the true working aperture, no matter what the corrector does to the light.

Imagine a more extreme example (however ridiculous). Suppose the light, after passing through a 5" corrector, diverged to the extent of covering a primary mirror of 10" diameter. Does that mean the effective aperture is 10". No. The photon flux is determined by the 5" corrector aperture, and is being made to spread out over 4 times the area at the larger mirror.

In your case, my suspicion is that the mirror is a bit undersized.

#12 Ed Holland

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Posted 28 February 2012 - 11:12 PM

I think we're saying (roughly) the same thing.

If the meniscus diverges incoming light (by whatever factor), then rays that are on axis, but just inside the aperture stop. If the mirror is the same size as the corrector aperture, light will be lost for all rays intercepted that are beyond a given distance from the optical axis. If the mirror is undersized, the condition will be more pronounced.

I'd like to know how much this effect contributes to the effective aperture data recordedin the first post, and whether there are other factors, such as baffle interference that come in to play. Unfortunately I don't want to take my telescope apart again :)

Cheers,

Ed

#13 GlennLeDrew

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Posted 28 February 2012 - 11:40 PM

Ed,
Here's how you should be able to determine whether it's the primary mirror edge or some other baffle causing the aperture loss. While the light is shining into the eyepiece, peer into the corrector and slowly move your eye laterally toward the corrector's edge. Observe the light and note the point at which it just begins to disappear. Is it exactly at the mirror edge? Or is the light clearly inside the mirror's edge, and something like the primary baffle's edge is 'contacting' the light?

This is the basis of the method I used for years to assess binocular aperture restriction.

#14 freestar8n

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Posted 29 February 2012 - 05:24 AM

I'm afraid it does look like the primary is acting as the aperture stop when it should be the corrector itself, which should be smaller. My concerns about the flashlight test in the other thread don't apply here because the entrance pupil is very near the front corrector. It corresponds to a slightly smaller image of the primary formed by the (negative) corrector, and it would be in between the corrector and the primary in object space. 115mm sounds about right for the entrance pupil diameter.

Two things are happening: 1) The corrector should be undersized or have a mask over it so it is smaller than the primary and acts as the entrance pupil 2) The stated aperture of the 'scope should be the actual entrance pupil diameter - not the size of the front lens, which may not be related.

Camera lenses wouldn't normally make this mistake. A 28mm f/2.8 lens might have a 70mm diameter lens in front, but the actual entrance pupil diameter is 10mm.

I have a Meade 7" Mak and the corrector is clearly undersized as it should be. Questars appear to have a full aperture corrector, so the question is - when they say 3.5" diameter - is that the corrector diameter or the actual entrance pupil? I don't know.

In your case there is actually good news in that you may be able to improve contrast by putting a mask around the edge of the corrector, without any loss of light on-axis. This might have a tangible effect on bright things like the moon, and might reduce glare.

If you have photo equipment you can compare a before and after shot to see if there is any loss of light - but there shouldn't be as long as the mask is the right size.

Frank

#15 Vondragonnoggin

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Posted 29 February 2012 - 09:31 AM

I have a Meade 7" Mak and the corrector is clearly undersized as it should be. Questars appear to have a full aperture corrector, so the question is - when they say 3.5" diameter - is that the corrector diameter or the actual entrance pupil? I don't know.

Frank


Frank, you are the first person I have seen to use the term "undersized corrector" rather than "oversized primary". Interesting, but you have the Chinese produced mak they did right. I believe Meade used to advertise it as an oversized primary, but not a lot of consumers know that this was one of the best designed mass produced maksutovs for this reason.

Why they discontinued is really a mystery to me. A really well made scope.

I don't believe any other mass produced uses this design, but maybe the Intes scopes use it. Anyone have an Intes Micro scope they can measure out to see if this was the case?

JOC has the ability to produce these again. Maybe in the future ES will put one out.

I love my Bosma 6", but if I had known better at the time, I would have saved and picked up a used Meade 7".

Now if someone would pick up that design, add factory installed fan, and dual speed focuser, twist on dew shield, I bet they would sell a bunch. Add some knife edge baffles in the baffle tube like my Bosma has, and you have one winner of an MCT that could compete with the Russian scopes.

#16 freestar8n

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Posted 29 February 2012 - 09:47 AM

Well - I don't like the term "oversized primary" because it isn't oversized - it is *correctly* sized in that it is larger than the corrector - as it should be.

When I said "undersized corrector" I really meant it is smaller than the tube, and/or masked off so it acts as the entrance pupil. The corrector on my 7" mak appears to be "correctly" sized.

The thing that is "over" for some maks is that the aperture is "overstated" because they are going by the corrector diameter rather than the pupil diameter. You can have the corrector the same size as the primary mirror and it isn't such a bad thing - but you should give the actual entrance pupil diameter as the scope aperture. I'm still not sure if Questar does this.

My 7" mak does have a fan but it also has the big weight inside. At some point I intend to overhaul it and sell it. I also have a C11, which has an almost identical focal length since it is f/10 vs. f/15. I don't see any dramatic advantage of the mak over the sct due to its intrinsic design - I mainly just see a loss of aperture compared to the C11. But it's nice that it doesn't need collimation.

Frank

#17 Vondragonnoggin

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Posted 29 February 2012 - 09:58 AM

That is one selling point the mak had for me - collimating less frequently. I haven't had to touch mine. Also the corrected flatter field, but the Edge series and ACF's also have this. No one makes a 7" though. I like that size. The 8" MCT's start getting a little heavy for me.

I do like the reports I hear on the Orion/Celestron 127mm maks. Many good reviews. Haven't looked into whether th ETX 125 designs differ much.

The Bosma maks have had good reports too, although some get concerned about CO size being so large.

Anyway, I guess "correct size" corrector is really the right term.


Sorry to veer off topic here. I find this interesting with Celestron now offering 2" visual backs.

I also think that vignetting concerns should be prominent if blaring out at you and would not hesitate to use one of these 127mm maks visually with whatever eyepiece/diagonal/visual back I wanted, but I suppose it maybe becomes less of a concern if you are changing diagonals for appropriate targets, that is, you use a 2" diagonal with longest focal length, lower power eyepieces and put up with vignetting for the widest view targets and when you want to use the scope high power and need the most contrast it can provide, switch to 1.25" diagonal with high power eyepieces as in the case of getting the best planetary views.

#18 GlennLeDrew

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Posted 29 February 2012 - 10:39 AM

It would be instructive to observe the behavior of the illuminated circle when the light is aimed into the eyepiece from increasing off-axis angles. The circle of light will translate laterally, but will its edge reach the edge of the corrector aperture, or instead always be clipped before that point is reached?

If the former is observed, one could derive some small satisfaction from the fact that off-axis illumination is slightly improved due to the maintenance of a larger entrance pupil when off axis. If the latter is observed, then the outer portion of the corrector does not contribute to image formation whatsoever, and is needlessly large, increasing the chance for non image-forming light to do harm.

#19 Ed Holland

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Posted 29 February 2012 - 12:41 PM

Great contributions everyone, solidly "on topic" IMHO

I'll play some more this evening.... it's raining anyway.

Glen - your last comment is a good one. I suspect we'll see a baffle cut off. With a low power ep (35mm) a star test with defocus to reveal a doughnut shows cut off as one places the star towards the edge of the field (about 75% out in a 50 deg ep)

I'll look into this further (pun intended)

Ed

#20 Asbytec

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Posted 02 March 2012 - 10:40 AM

Ed, this post got me curious. So, performing the test with a 10mm Plossl with a diagonal (90mm back focus added) and straight through made no difference. Both tested at 140mm. The illuminated circle did not change with back focus.

But, I messed up, the LED flashlight was right where my eye would have been - not a foot away. Not sure if that makes a huge difference. Testing with a 12mm Ortho without a diagonal, there was a slight difference, about 141mm. But that could just be error in measuring.

However, using a 25mm Plossl at the proper distance from the flashlight, and without a diagonal, the result was an illuminated aperture of 143mm for the Orion 150. So, I'll accept that as a rough figure fairly consistent with yours. Since the earlier test with the diagonal showed no change, I am confident it is as you say...the scope is pretty tolerant out to at least 90mm.

Kind of sloppy testing, but curious the results show some aperture loss, regardless. Wonder why that seems true for an MCT but not for the C6? Corrector curve? Not? Baffle? Test is valid?

#21 Eddgie

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Posted 02 March 2012 - 02:56 PM

the scope is pretty tolerant out to at least 90mm.



This is kind of a "Catch 22."

The scope may not suffer any effective apeture loss at 90mm, but the problem is that it has already had effective aperture reduction!

Suppose I made the baffle tube 3mm longer and this reduced the effective aperture to 130mm. Now, we could say that the scope "tolerates" maybe 150mm of back focus before additional effective aperture is lost.

So, it is kind of a Catch 22 in that you can't loose effective aperture that has already been lost!

This is a surprising fact (that some of these MCTs are working at reduced effective aperture).

This might explain why some people report that they do not see any loss of brigtness with 2" visual backs.. Their systems never had the additional brightness to lose!!!

We ofthen her that these little MCTs have a darker sky background than similar sized SCTs... I wonder why! :roflmao:

Unbeliveable. If all of these little scopes are working out of the box with a 15% brightness deficit, it would seem that people are being cheated of the performance implied by the supporting literature.

Also, the central obstruction would actually be relatively larger than stated.

But I think that the mirror size is not the coulprit here. The Meniscus does indeed bend the light outward but only bye a tiny amount.

I suspect that the problem is in the fabrication or the design. If the baffle tube is too long either because it was designed to long or because the scope was improperly assembled, you would get a scope with permanently reduced effective aperture.

And that is what I think we have here. I just can't believe that the Meniscus is so refractive that it would cause so much expansion of the light cone as to miss the edge by an astonishing 5 millimeters. The curves just dont see that strong to me.

But I could be wrong.

Either way, I would be very disappointed to know that my 127mm scope was really a 117mm scope.

Of course the good news is I could use my 117mm scope with a 2" diagonal and never have to worry about inflicting an effective brightness loss.

Crazy. Really crazy. Want to see what happens with this as other people weigh in with measurements.

#22 Ed Holland

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Posted 02 March 2012 - 05:15 PM

Asbytec - it is very interesting to see what a larger member of the family would show. Thanks for adding your measurements. From reading the other thread, I believe that the measurement setup is not hypercritical.

Eddgie - I really want to get on it and find the explanation for the measured aperture. Perhaps this weekend I can set something up. I dont think it is a baffle tube intrusion issue, since I believe this can be observed as one moves off axis when using longer focal length EP's. Only proper examination will tell us though, and I will try to pursue this.

Am I upset that the 127mm scope is working at 117mm? Not really, especially for what it cost me. It still gives excellent views that are favourably comparable to my 5" refractor.

Thanks everyone for your contributions :)

Ed

#23 Eddgie

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Posted 02 March 2012 - 05:59 PM

The problem is not that just the edge of the field is being vignetted.

If the central baffle intrudes too far into the light cone, even the brightness at the center of the field will be dimmed.

The outside will show "Hard" vignetting when a defocused star is moved to the edge of the field, but what that won't show is that even the very center of the field is loosing brightness.

While the outside edges may only be illuminated to 60% under these conditions, the center of the field may only be illuminated to 85% (or less).

For visual observation of extended objects, image brightness is an integral part of the function of the aperture. If you used a 15% neutral density filter over the end of the eyepeice, the image would be dimmed, and your eye would struggle harder to detect the lowest contrast detail present on the target. The detail would still be there when effective aperture is lost, but because it is dimmed, our eye has more difficulty detecting it, or discriminating it from its surroundings.

Now a 15% ND filter is not all that dark. But it is dark enough that it very slightly dims the image, and that is what is occuring when these scopes work at less than their full aperture.

I don't own one of these, so I personally have no reason at all to be upset.

I would think though, that in this hyperciritical community, where people report seeing infinitesmal differences in light transmission between diagonals and eyepeices (and these typically differ by less than 5%) that this kind of defect would be considered fairly serious.

I think it is fairly serious. But again, I don't own one, so for me, it has zero impact.

But now, I know that this possibility exists that many of these scopes may have this condition, I would not buy another one without confirming that it was working at full effective aperture at least in normal configuration (1.25" eyepiece if that was the recommended eyepeice).

For people that have them, then they have no choice to be happy with them.

But as knowledge of this seeps out, expect used prices to reflect this, because now, these scopes will be competing with smaller scopes on the used market.

It is very sad to me that this is being discovered. If it were me, I would have grave concerns.

#24 Mark9473

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Posted 02 March 2012 - 06:32 PM

Now a 15% ND filter is not all that dark. But it is dark enough that it very slightly dims the image


That sounds dramatic, but in actual fact it will be hardly noticeable. A #82A very light blue filter, for example, only has about 70% transmission. I'm not saying you can't detect it, but only just. Personally I would be more concerned about these scopes giving up some resolving power compared to the full aperture.

#25 Asbytec

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Posted 02 March 2012 - 08:22 PM

While I am not disappointed with the views either, it's disheartening to think it's operating at a reduced aperture.

I am not expert in optics, ray tracing, or the test. But, I question using a flashlight for any testing of the optical train. It is not coherent light. Maybe it can be simulated and close, but it's still not offering parallel light rays.

Yes, I agree, if anything it's probably the baffle. My baffle offers a clear view of the secondary and what looks to be a full reflection of the primary.


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