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AP Mak 10" what the next best thing

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#51 bratislav

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Posted 11 July 2012 - 07:30 PM

Here is what Roland says about testing horizontally:

"Another thing to consider: when you test a large mirror in a horizontal
plane, the weight of the mirror at the edge tends to produce astigmatism where
the
weight bears down. It is virtually impossible to test a large mirror in a
vertical configuration.


I'll repeat the question - how come that many of big (and presumably heavy) mirrors Rohr still tested at 0.98+ Strehl ? Apparently, large mirrors can be held in vertical (or nearly vertical) position and not show astigmatism. After all, that is what most Dobs do, day after day (in fact they can't even be used with mirror in horizontal position. It's called Dobson's hole).

I have a lot of respect for Roland, but in this case he's just plain wrong.

#52 Erik Bakker

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Posted 11 July 2012 - 07:36 PM

Mewlons are stunning high-end scopes, the ones I saw had stunning optics. Never heard of a lemon, just out of alignment/collimation or not properly cooled down yet. They should not be mistaken for mass produced scopes.

#53 bratislav

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Posted 11 July 2012 - 07:51 PM

http://www.astrosurf...taz/astatic.htm

http://www.astrosurf...eralsupport.htm

http://www.scientific.net/AMM.79.146

http://www.cruxis.co...ecalculator.htm

#54 TPMack

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Posted 11 July 2012 - 07:53 PM



I'll repeat the question - how come that many of big (and presumably heavy) mirrors Rohr still tested at 0.98+ Strehl ? Apparently, large mirrors can be held in vertical (or nearly vertical) position and not show astigmatism. After all, that is what most Dobs do, day after day (in fact they can't even be used with mirror in horizontal position. It's called Dobson's hole).

I have a lot of respect for Roland, but in this case he's just plain wrong. [/quote]

Here is what has to say about Rohr's testing:
"I would be leery of tests done by Wolfgand Rohr. His test setup is not
ideal for testing telescope optics since he tests everything in a horizontal
position. There is also a question in my mind about the components of his
interferometer, since they seem to be home made and lack any kind of
certification. I know that for my own interferometer I initially used a
commercial beam
splitter which introduced mild astigmatism into the measurements, and I had
to replace it with a certified 1/30 wave hand crafted beam splitter from a
company in NY that specializes in interferometer optics.

Personally I have not seen any astigmatism caused by storing oiled optics
horizontally.

Roland

Since he does not post on CN, I would recommend that if you disagree with him, you give him a chance to defend himself on his AP Yahoo group.

Tom Mack

#55 Ennis

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Posted 11 July 2012 - 07:53 PM

"I've used a 10" Newt optimized for planetary observing and it was a fine instrument. But my 10" Mak has better thermal characteristics, is coma free, is permanently collimated, and rides comfortably on a Mach1 mount. Every scope has its niche."

Yes, but one cannot purchase a new AP 10" Mak-Cass and would have an extremely difficult time finding a used one to purchase.

It is unfortuate that for almost all of the past twelve years, Roland Christen has not made the scopes that most of his potential customers actually want, to wit: 8" or 10" Mak-Casses and 6" apo's. Instead, he makes 130mm apo's (how many companies already make those???) and some other scopes that only a wealthy few can enjoy.

E.

#56 TG

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Posted 11 July 2012 - 07:55 PM


Here is what Roland says about testing horizontally:

"Another thing to consider: when you test a large mirror in a horizontal
plane, the weight of the mirror at the edge tends to produce astigmatism where
the
weight bears down. It is virtually impossible to test a large mirror in a
vertical configuration.


I'll repeat the question - how come that many of big (and presumably heavy) mirrors Rohr still tested at 0.98+ Strehl ? Apparently, large mirrors can be held in vertical (or nearly vertical) position and not show astigmatism. After all, that is what most Dobs do, day after day (in fact they can't even be used with mirror in horizontal position. It's called Dobson's hole).

I have a lot of respect for Roland, but in this case he's just plain wrong.


IIRC, he rotates and "cancels out" astigmatism that doesn't rotate as well. It's only a vague remembrance, though.

Tanveer.

#57 TG

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Posted 11 July 2012 - 08:12 PM

[quote name="TPMack"]

I'll repeat the question - how come that many of big (and presumably heavy) mirrors Rohr still tested at 0.98+ Strehl ? Apparently, large mirrors can be held in vertical (or nearly vertical) position and not show astigmatism. After all, that is what most Dobs do, day after day (in fact they can't even be used with mirror in horizontal position. It's called Dobson's hole).

I have a lot of respect for Roland, but in this case he's just plain wrong. [/quote]

Here is what has to say about Rohr's testing:
"I would be leery of tests done by Wolfgand Rohr. His test setup is not
ideal for testing telescope optics since he tests everything in a horizontal
position. There is also a question in my mind about the components of his
interferometer, since they seem to be home made and lack any kind of
certification. I know that for my own interferometer I initially used a
commercial beam
splitter which introduced mild astigmatism into the measurements, and I had
to replace it with a certified 1/30 wave hand crafted beam splitter from a
company in NY that specializes in interferometer optics.

Personally I have not seen any astigmatism caused by storing oiled optics
horizontally.

Roland

Since he does not post on CN, I would recommend that if you disagree with him, you give him a chance to defend himself on his AP Yahoo group.

Tom Mack [/quote]

Since Roland didn't post it here either, it would be totally wrong to jump over to ap_ug and hit him over the head with it. But look at Rohr's test of an AP StarFire 155 EDT: 1/10.4 wave and 1/55 wave (0.987 Strehl). For comparison here's what A-P promises:

[quote]A lens is finished when it displays a wavefront accuracy of l/50 RMS (Strehl Ratio of 98.4%).[/quote]

So do you still think Rohr is a mere amateur?

He doesn't only provide interferometry results but Foucault-grams, Ronchi grams, star test images and Lyot-tests (to measure surface roughness). Hats off to him for dispelling a lot of hype and smoke.


Tanveer.

#58 JJK

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Posted 11 July 2012 - 08:37 PM

"While the Mewlons seem to enjoy a high reputation, the ones that I have seen test reports on suggest that there are many Celestron SCTs out there with better optical quality."

This statement does not equal "A C8 or a C11 will perform better than an MCT or Mewlon". Nowhere did I say that the OP should buy a Celestron 8" or 11" SCT. My own recommendation was an Intes Micro 9", or a high quality 10" Newtonian with optimized secondary mirror size.

My post simply states that from a quality perspective, the Mewlons that have published tests on the internet don't seem to have quality that is serially in the excellent catagory, and that there are many Celestron telesopes that have been tested that have been shown to have better optical quality.

You inferred something that I in NO way said, that the performance of these scopes would be better than others.

SNIP

So, you think I said something that I did not say. Above is what I said, and if read it and don't attempt to read anything into it, you will see that it is all I said.


Eddgie, there's no need to be defensive. I just don't understand the purpose of your statement. If it was to suggest that not all Taks are up to spec, I wouldn't be surprised (things happen). However, your report wasn't statistically significant, given the number of scopes tested, and the additional qualitative statement that many Celestrons have better optical quality suggests you're driving home a rather different message (which doesn't correlate with my experience).

Are you suggesting the average Celestron would outperform the average Tak? If not the C8 or C11, to which Celestron models were you referring?

Frankly, if I bought a Tak, and it performed like an average Celestron, I'd demand a properly functioning replacement or my money back.

I have nothing against Celestron SCTs. They are quite versatile and represent a very good value. I used to have a C8 (regretfully sold it to buy my first house) and can see getting one someday (I'm thinking of a cherry-picked C14).

#59 JJK

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Posted 11 July 2012 - 08:42 PM

Eddgie, I think amateur/hobbyist testing on Yahoo is not a place to look for consistent test data.).



Rohr uses an optical test bench with an interferometer. That is far from typical amateur equipment.

But believe as you wish. It is pointless to try to reference emperical data on these forums. People seem to prefer to believe other people more than very objective testing. I trust my own star testing and optical bench testing by Rohr far more than most of the postings here on CN. I encourage others to learn to test properly as well. Once you learn to properly perform optical testing, the vail of confusion becomes more transparent.

Rohr is a great tester. Very methodical, and proven to be able to diagonose errors with excellent consistency.


Objective optical testing (pardon the pun) done correctly is indeed useful.

#60 JJK

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Posted 11 July 2012 - 08:50 PM

Guy's:

I have no horse in this race. I say this:

Eddgie provided some data from some interferometer testing. Roland would like that. Certainly it is not a sufficient sample to determine the average optical quality of the Takahashi Mewtons but it is real data and for that it deserves respect.

It ought to raise your eyebrows and make you think rather than attack Eddgie for posting it.

Well, I take that back.. I do have a horse in this race, my favorite, the high quality Newtonian... the issue there is the small diffraction limited field of view.

Jon Isaacs


I don't see any one here attacking Eddgie. Besides, he's certainly entitled to his own opinion, no matter how wrong it is. :)

Kidding aside, I think Eddgie needs to clarify what he meant. I doubt he's suggesting that the average Celestron would outperform the average Tak Mewlon. If he's simply suggesting that not every scope performs to spec, and that we'd benefit by understanding how well our instruments operate, I would of course agree.

#61 JJK

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Posted 11 July 2012 - 09:01 PM

I had my Mewlon 300 parked next to a C14 at Black Forest several years back. On Jupiter, there was nothing on Jupiter's orb in the Mewlon that could not be seen in the Celestron; however, saying that, the background sky in the Mewlon was noticeably blacker while the Celestron's was charcoal.


Tom, you're the devil incarnate who forced me to buy a Tak Mewlon 300CR. Keep the suggestions coming!

#62 bratislav

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Posted 11 July 2012 - 09:36 PM

Here is what has to say about Rohr's testing:
"I would be leery of tests done by Wolfgand Rohr. His test setup is not
ideal for testing telescope optics since he tests everything in a horizontal
position. There is also a question in my mind about the components of his
interferometer, since they seem to be home made and lack any kind of
certification. I know that for my own interferometer I initially used a
commercial beam
splitter which introduced mild astigmatism into the measurements, and I had
to replace it with a certified 1/30 wave hand crafted beam splitter from a
company in NY that specializes in interferometer optics.

Personally I have not seen any astigmatism caused by storing oiled optics
horizontally.

Roland

Since he does not post on CN, I would recommend that if you disagree with him, you give him a chance to defend himself on his AP Yahoo group.

Tom Mack


This is getting way off topic, but one thing remains - if there was anything wrong with Rohr's setup, how come that there is a lot of optics that does test out at very high numbers ? Stuff from Lomo, Aluna, Zambuto, Zeiss and like (including AP)? How would you ever get to a 0.98+ Strehl if your interferometer setup was astigmatic ?

I know Roland needs to defend his company, but he is not infallible. Noone is (including Rohr of course).
Check out some results of Roddier testing of an early Traveler on AM. There was a lot of blaming of the tester followed by quick step retracing from Roland after some hard data was shown ...

#63 Mike Harvey

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Posted 12 July 2012 - 02:57 AM

JJK -You stated:<<Are you suggesting the average Celestron would outperform the average Tak? If not the C8 or C11, to which Celestron models were you referring?
Frankly, if I bought a Tak, and it performed like an average Celestron, I'd demand a properly functioning replacement or my money back.>>

If you are referring to the Celestrons of the past, I agree! I wouldn't own one. Same for Meade.
However (with respect) that is NOT was I was referring to. If you have not used a new 10" Meade ACF, you simply do not know what you are talking about (and I don't mean that to be insulting).
These ACF scopes are simply "game changers"!
While I still love the tactile "feel" and solid physical qualities of Taks...they no longer hold the monopoly they once had when it comes down to optical excellence. Times have changed.

Mike Harvey

#64 chboss

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Posted 12 July 2012 - 03:03 AM

I have the highest respect for Wolfgang Rohr since he is offering a huge collection of optical tests for amateurs that would otherwise not be available. If you are interested in optics you can learn a lot from his reports that show foucault, interferometer, ronchi and star tests of the same optics.

But please keep in mind that his collection is mostly dictated by the scopes people send or lend him.
Many times (as with my own telescope) scopes are sent to him because the owner suspects or has found a problem doing a start test. Mostly Wolfgang will confirm this and try to correct misalignment or miscolimation where possible.

So it is quite save to say that the "lesser" optics will find the way to him, while realy good examples will not land on his optical bench.

Is there something specific about the Tak Mewlon 210? - I do not know from own experience...
Still all of them are around Strehl 0.9 or higher is that so bad in reality? ;)

Our club owns a rare Tak TSC-225 SCT which is excellent, I am the owner of a Carbon C9.25 comparing them side by side did not show any big difference. I obviously got lucky with my C9. - Given the choice between the two I would take the Tak in a heartbeat - The chance of getting a lemon is just much smaller!

Just my 2 cents.
Chris

#65 TPMack

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Posted 12 July 2012 - 05:42 AM

[quote name="bratislav"][quote]



I know Roland needs to defend his company, but he is not infallible. Noone is (including Rohr of course).
Check out some results of Roddier testing of an early Traveler on AM. There was a lot of blaming of the tester followed by quick step retracing from Roland after some hard data was shown ... [/quote]

Yes we are way way off topic. A friend is well acquainted with the owner of that Traveler and he tells me that Roland has tried several times to get that scope back for refiguring. The owner continues to keep it as is. A little like owning a coin that was defective during minting, ironically it's now a collectable. I cannot disagree, everyone makes mistakes including AP.

Tom Mack

#66 jmiele

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Posted 12 July 2012 - 08:17 AM

Agreed, so getting back on track...In the 10" class as an alternative to the AP Mak 10 I submit the Takahashi "250" Mewlon. First it's available new from the manufacturer. Second it has a fixed mirror like the AP Mak. And it is the same aperture. They are tremendous instruments. If anyone would like to discount the 250 Mewlon from contention, please do so with data regarding the 250 Mewlon.......NOT the 210. It is a different design and has a movable primary. It's also 2" smaller.

Eddgie, try making a suggestion without having to knock someone else. Also, I'd like to hear a recommendation from you based on personal experience with the instrument. If we had more of that, we'd have less problems. Because if there is anything that bothers me more than someone that doesn't use their real name when posting, it's folks that make recommendations concerning instruments they have never owned.

Sorry if that sounds harsh, however you know what they say about opinions... :)

Joe

#67 JJK

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Posted 12 July 2012 - 08:19 AM

JJK -You stated:<<Are you suggesting the average Celestron would outperform the average Tak? If not the C8 or C11, to which Celestron models were you referring?
Frankly, if I bought a Tak, and it performed like an average Celestron, I'd demand a properly functioning replacement or my money back.>>

If you are referring to the Celestrons of the past, I agree! I wouldn't own one. Same for Meade.
However (with respect) that is NOT was I was referring to. If you have not used a new 10" Meade ACF, you simply do not know what you are talking about (and I don't mean that to be insulting).
These ACF scopes are simply "game changers"!
While I still love the tactile "feel" and solid physical qualities of Taks...they no longer hold the monopoly they once had when it comes down to optical excellence. Times have changed.

Mike Harvey


Mike, I'm actually a fairly experienced observer, have owned a decent number of top-notch OTAs, and know enough math and optics to understand the contents of Suiter's book & others.

Please get it straight that I said nothing about Meades for two reasons. First, I haven't looked through a large Meade OTA in about 20 years (I own a relatively small Meade/Wegat 125, which of course does not qualify). Second, Eddgie referred to Celestrons in comparison to Tak Mewlons, not Meades.

Lastly, I make a living designing precision (and sometimes innovative) measurements in the biotechnology field. I understand the desire and need to properly design and execute measurements and tests of devices. I therefore applaud folks like Rohr. However, I don't know him, and I don't know if he really has the right equipment and true capability/experience to properly test telescope optics (I'm not saying he doesn't, I'm just saying I don't know).

It wouldn't surprise me if a properly executed optical test of a subset of Tak OTAs showed the scopes were below specification. A friend of mine bought a used Tak 130, and the optics look ever so slightly miscollimated. We then noticed the ScopeGuard case had evidence of being dropped very hard on the objective side. The scope will eventually take a trip to TNR for collimation.

I would be surprised if any mass-produced compound telescope with optics that are ground, polished, figured, and coated quickly appeared smooth. That said, what is important is what you see at the EP in focus. Scopes with decent (not necessarily first-rate) optics can provide highly pleasing views. I used to have an Obsession 20" f/5 Newt with Galaxy pick of the litter optics (but it was "only" 1/5.6 wave). The views of DSOs in that scope were absolutely stunning (I've seen pastel colors in the Trifid, extraordinary detail in the NA and Pelican nebulae, etc.) and when the thermals were managed correctly, the stars were tack sharp. One hasn't lived until they view the many little pinprick, ruby-appearing stars in the heart of M42 (three are visible in a 155 mm refractor, about a dozen in a competent 10" scope, and even more in a larger OTA; folks, I'm not talking about the Trapezium). The better the optics (and seeing), the more pleasing that view appears.

I wouldn't rule out me looking at a large SCT (Meade or Celestron), and buying it if it provided good views. By saying "large", I'm not dissing smaller M or C SCTs. I personally have no interest in them because I have immediate access to first-rate 12" and smaller objective OTAs.

#68 maknewtnut

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Posted 12 July 2012 - 09:10 AM

To expand on the differences Joe stated, the 250 also has provisions for cooldown the 210 (or 180) doesn't.

#69 ahlberto

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Posted 12 July 2012 - 01:58 PM

Today`s SCTs are good,really good.At the eyepiçe they provide awesome views.My C8 sample shows beautifull jupos,tempest clouds in Saturn and is great at deep sky,-it shows detail in the Cat Eye nebula and M64 fo ex.By any stantards,its a very good 8" scope.Theres more than numbers and in the end,what counts is what you see at the eyepieçe ;)

#70 TG

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Posted 12 July 2012 - 02:48 PM

With all kidding aside, I think it's unfair to brand the Mewlon line as an equal to a mass produced line of telescopes. Sure there are a few examples of mediocre Mewlons out there but I'm sure if we dig deep enough we will find that all the top shelf scope lines have suffered a bad apple or two as well, but I wouldn't automatically say well if two are bad then so are the rest.


It would be interesting to know what makes you assert that the Mewlons are not mass-produced scopes. They seem to be available readily but I don't know how the mirrors are produced. I really don't care about whether the optics are mass produced or not as long as they meet some spec in the end. Even A-P uses machines to polish their lenses (they claim 1/10 wave accuracy just using the machine and IIRC Roland said that Zeiss machines exist that can put down 1/20 wave surfaces and the Chinese have been buying them up). A-P lenses do get hand-figured in the end. The same is true for Carl Zambuto's mirrors. Do you know whether Takahashi does this with their mirrors? Rohr's reports of sub-par Mewlons leads me to believe that this is actually not the case. It's hard to believe they would turn out sub-par optics if they hand-figured every single one to spec. If the quality of the mirrors shows a statistical distribution allowing for poor specimens as well, it's highly like that there is a largely automated production process involved (though it hardly needs mentioning that the Japanese are experts at making the distribution narrow).

So let me turn your assertion on its head and claim that perhaps it is you who is being unfair to "mass produced" scopes when there's a lot of evidence of consistent high quality for SCTs of recent vintage, especially for Celestron's Edge HD line.

BTW, on a tangent, one example of a non-mass produced D-K is the Gladius and here I would direct folks to the interesting case of a Gladius that was not performing as expected. There was denial by the maker at first but they eventually relented and the problem was exactly as Rohr found. So, even with a boutique scope, you have no guarantees unless they have a rigourous QA process like A-P/TEC/Zambuto etc. but this case just confirms Rohr's testing is spot-on about which there was doubt raised earlier in the thread.

http://www.astro-for...report-–-part-I


Tanveer.

#71 TG

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Posted 12 July 2012 - 02:50 PM

...if there is anything that bothers me more than someone that doesn't use their real name when posting, it's folks that make recommendations concerning instruments they have never owned.

Joe


I thought everyone here knew Eddgie's real name. He's posted enough reviews under his real name on astromart.com.

But let's not dis people not posting with their real names as there's a legitimate reason for doing this: when you own a lot of expensive optical equipment, it may be prudent not to reveal your location.

Tanveer.

#72 Heavens Above

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Posted 12 July 2012 - 03:07 PM

As reported in my recent review http://www.cloudynig...hp?item_id=2791 my Mewlon 210 'shows considerably more planetary detail' than my friends 8 inch Meade. These were the words of the owner of the Meade, not me!

I also have a copy of the very detailed 5 page review of the Mewlon 210 in the short lived but excellent UK Magazine, Practical Astronomer. In its conclusion it states 'Extremely well corrected and smooth optical surfaces. Well engineered with thoughtful design. If a compact compound telescope is required, one that is highly portable and will be primarily used for visual observing, and perhaps planetary photography, then the Mewlon 210 is probably the best choice available.'

I love mine

#73 TG

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Posted 12 July 2012 - 03:10 PM

Agreed, so getting back on track...In the 10" class as an alternative to the AP Mak 10 I submit the Takahashi "250" Mewlon. First it's available new from the manufacturer. Second it has a fixed mirror like the AP Mak. And it is the same aperture. They are tremendous instruments. If anyone would like to discount the 250 Mewlon from contention, please do so with data regarding the 250 Mewlon.......NOT the 210. It is a different design and has a movable primary. It's also 2" smaller.
Joe


It may indeed be a fine substitute for someone looking for the A-P Mak but there are differences:

* The A-P actually has a moving primary, not fixed mirror as you say (check their website)

* 29% CO vs 23% on the A-P

* Quartz primary on the A-P. Don't know what the Tak uses. The A-P quartz mirror's polished clear on the back to radiate the heat out quickly. Interestingly, my Mak-Newt has the same kind of primary and I can say it's cooled down and ready to go in 15 min.

* Spider vanes vs. no vanes in the A-P.

* The A-P's permanently collimated (though I would take issue with the word "permanent" as there's no such thing :grin:). Not so with the Tak.

* The A-P's coma-free. The D-K, by design, has gobs of coma, far more than even a classic-design SCT.

I would suggest that one of the TEC Maks or even Intes Maks, which don't command the astronomical price of the A-P Mak -- when you can find one -- would be a closer substitute in terms of optical config. I don't know whether this would make a difference at the eyepiece but the vanes vs no vanes would certainly be noticeable.

Tanveer.

#74 saemark30

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Posted 12 July 2012 - 03:13 PM

Remember there was a time when telescope makers did not use an interferometer to test telescopes. So even that old AP or Tak may not be 1/10 wave.

#75 saemark30

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Posted 12 July 2012 - 03:16 PM

Roland can make his own Maks. I don't know why he bought the optics from Aries. He said he had to refigure them anyways.


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