Ed Holland
Pooh-Bah
Reged: 06/16/10
Loc: San Jose, CA and Oxford, UK
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Orion 127 Maksutov - Back focus and aperture
#5093831 - 02/27/12 12:17 PM
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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
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PHampson
sage
Reged: 10/25/05
Loc: Nashville, TN
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: Ed Holland]
#5094059 - 02/27/12 01:56 PM
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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
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Ed Holland
Pooh-Bah
Reged: 06/16/10
Loc: San Jose, CA and Oxford, UK
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: PHampson]
#5094347 - 02/27/12 04:20 PM
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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
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Ed Holland
Pooh-Bah
Reged: 06/16/10
Loc: San Jose, CA and Oxford, UK
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: Ed Holland]
#5095120 - 02/28/12 12:56 AM
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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
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PHampson
sage
Reged: 10/25/05
Loc: Nashville, TN
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: Ed Holland]
#5095169 - 02/28/12 01:53 AM
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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
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Ed Holland
Pooh-Bah
Reged: 06/16/10
Loc: San Jose, CA and Oxford, UK
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: PHampson]
#5095194 - 02/28/12 02:27 AM
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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.
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EdZ
Professor EdZ
Reged: 02/15/02
Loc: Cumberland, R I , USA42N71.4W
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: PHampson]
#5095258 - 02/28/12 07:20 AM
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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
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Binojunky
Carpal Tunnel
Reged: 12/25/10
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: EdZ]
#5095332 - 02/28/12 08:51 AM
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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.
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Eddgie
Postmaster
Reged: 02/01/06
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: EdZ]
#5095628 - 02/28/12 12:01 PM
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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...
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Ed Holland
Pooh-Bah
Reged: 06/16/10
Loc: San Jose, CA and Oxford, UK
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: Eddgie]
#5095662 - 02/28/12 12:22 PM
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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
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GlennLeDrew
Postmaster
Reged: 06/18/08
Loc: Ottawa, Ontario, Canada
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: Ed Holland]
#5096570 - 02/28/12 09:11 PM
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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.
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Ed Holland
Pooh-Bah
Reged: 06/16/10
Loc: San Jose, CA and Oxford, UK
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: GlennLeDrew]
#5096731 - 02/28/12 11:12 PM
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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
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GlennLeDrew
Postmaster
Reged: 06/18/08
Loc: Ottawa, Ontario, Canada
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: Ed Holland]
#5096771 - 02/28/12 11:40 PM
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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.
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: Ed Holland]
#5096942 - 02/29/12 05:24 AM
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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
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Vondragonnoggin
Post Laureate
Reged: 02/21/10
Loc: Southern CA, USA
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: freestar8n]
#5097181 - 02/29/12 09:31 AM
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Quote:
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.
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: Vondragonnoggin]
#5097216 - 02/29/12 09:47 AM
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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
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Vondragonnoggin
Post Laureate
Reged: 02/21/10
Loc: Southern CA, USA
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: freestar8n]
#5097231 - 02/29/12 09:58 AM
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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.
Edited by Vondragonnoggin (02/29/12 10:10 AM)
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GlennLeDrew
Postmaster
Reged: 06/18/08
Loc: Ottawa, Ontario, Canada
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: Vondragonnoggin]
#5097302 - 02/29/12 10:39 AM
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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.
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Ed Holland
Pooh-Bah
Reged: 06/16/10
Loc: San Jose, CA and Oxford, UK
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: GlennLeDrew]
#5097541 - 02/29/12 12:41 PM
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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
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Asbytec
Guy in a furry hat
Reged: 08/08/07
Loc: La Union, PI
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Re: Orion 127 Maksutov - Back focus and aperture
[Re: PHampson]
#5100798 - 03/02/12 10:40 AM
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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?
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