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Star Testing Orion 150 Mak CaS

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#1 Asbytec

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Posted 02 January 2011 - 08:07 AM

Please, anyone jump in here with some ideas or correcting my logic, but I think I got a good scope. I am no expert in star testing, but a quick skimming of Suiter's (rather detailed) book and using Aberrator were a big help.

The magnitude 5 sky was nicely calm, about a 7 out or 10 and the scope was completely cooled down to ambient temperature. In fact, it was almost ice cold to the touch. One nice thing about the tropics, inside and outside temperatures vary very little even as the sun sets. Seems tube currents are not a problem for a Mak here.

Using Capella for the initial test at 180x with the supplied 10mm Sirius Plossl, defocussing to a donut showed collimation was probably dead on. If there was any mis-collimation, it was small enough to swear alignment was spot on. And this is how it arrived from the factory! Also, the high speed wavering across the donut image of Capella confirmed seeing was indeed pretty good. Most notable was the complete lack of a thermal plume and absolutely no detectable slow moving currents.

The donut itself was as perfectly circular as I could distinguish, on both sides of focus. I feel safe to illuminate astigmatism as a major aberration in the optics. The same argument applies for ruling out any pinched optics. So, onto the other aberrations.

I racked the focus so about 4 diffraction rings were showing inside of focus and back to outside focus. This may be a 'tight' test, as I read explained somewhere later. Aberrator seems to suggest I was working at about 6 waves out of focus. Still, the initial look at the diffraction pattern showed it to be clean, bright, and crisp on both sides of focus. All the rings were clearly defined and sharp, best one can tell with the slight amount of turbulence present. This seems to be a good initial, first impression.

Checking both sides of focus, with the star centered of course, I could see no hint of misalignment or coma. But, I did see one strange effect. Outside of focus, the bright outer ring did appear to be a tad bit smaller in diameter than inside of focus. The only way I noticed this was there appeared to be a bit more light scattered outside the outer bright ring when outside of focus.

So, there may be some spherical aberration present. How much? Couldn't say, but Aberrator did confirm this when I selected 3rd LSA setting of -0.1. It represents pretty accurately what I observed. Adjusting the 5th HSA settings simply showed diffraction effects that simply were not observed. But, since this is the lowest setting Aberrator offers for 3rd LSA, I have to assume any spherical aberration must be tiny. But, as was mentioned in some literature, Mak OTAs are susceptible to spherical aberration. Fine.

According to Suiter's book and some internet research, a turned edge also produces aberrations that were not visible. It seems a turned edge should show a more diffuse diffraction patters either inside or outside focus. Again, the Orion Mak Cas displayed sharp and distinct patterns on both sides. So, if there is any turned edge, it must be minimal enough to escape detection. Apparently, spherical aberration has the same asymmetric diffusing effect on either side of focus. But, I could not see any diffusing on either side. Also, surface roughness seems to be minimal, too, though it's hard (I understand) to test for this with a little turbulence.

Closer inspection of the rings seemed to show they were uniform and circular. I did not notice any brighter portions or any asymmetries within the limits of the seeing that night. I understand this is a zone test, please correct me if I am wrong. Until I hear otherwise, or know otherwise, I am confident there are no significant zonal errors. (More reading is needed, I guess.)

Okay, at focus on Capella, the airy disc was crisp and round. Of course, the brightness of the star caused some random flaring, but the first diffraction ring was clearly visible, round, and not nearly as bright as the airy disc. It seemed to hold it's shape fairly well given the slight amount of turbulence. There were several other diffraction rings visible, but all of them were very thin and dim. They were less steady given the flaring effect. They were more difficult to observe, but seemed fine in that they weren't very bright or 'thick.'

Off to the Orion nebula still at 180x. I pause to first explain Orion showed up nicely with good contrast. I could easily get the central region in the field of view at 180x and noted distinct bright and dark areas. The Trapezium stars were my target, however, and later it would be the Eskimo nebula. Each of the Trapezium stars displayed a compact airy disc and one faint diffraction ring.

I had to watch the diffraction rings over a period of time as they were not completely steady. At times, they were an arc, then maybe a few arcs, and so forth. But, in those brief moments the when the seeing settled down to almost perfect, the rings did settle into tiny circles, if only briefly. It took many such moments of steady seeing to actually confirm them to be thin dim circles. I do not remember seeing more than one ring on each star. I performed this test at other stars, all pretty much on the zenith by the way...Capella was not far off the zenith, and found similar results.

A quick star hop over to the Eskimo nebula showed it as I always remember it. Of course, it was not as bright or detailed as in my old C11, but the detail (a dark area just inside the circumference) could be discerned with patients. The central star was easy, and another field star nearby was simply a pin point. So, I spent half an hour observing this beautiful object.

A quick test on Jupiter seemed to show a crisp limb at focus, using a blue filter. I am not much of a planetary observer, so inexperience with Jupiter could not reveal much. I only managed to make out several polar belts and didn't really probe the main equatorial belt or surrounding atmosphere for festoons and such. I'll save that for another night, cuz...with a Mak Cas, I guess I'll spend more time on the planets.

I wish I had a higher power eyepiece to perform this test, but I had to work with what I have. Got one on order, the TMB 6mm. I also understand no optic is perfect, but being within tolerance is what we expect. Several hours testing, observing, focusing, defocussing, and repeating the process seems to show a good set of optics, at least to the best I can detect.

Attached is a pretty good Aberrator simulation (turbulence included) of what I observed. Please, if I misinterpreted anything or can improve technique, chime in...thanks in advance.

Attached Thumbnails

  • 4286304-150makcas1.jpg


#2 Asbytec

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Posted 02 January 2011 - 10:09 AM

I went to double check my observations and gaze at the Eskimo, again. As it turns out, tonight was a 9 as far as seeing goes. This time, with 4 diffraction rings both inside and outside focus, there was a slight irregularity. Inside of focus, all 4 rings were distinct and even for the full circumference. However, outside of focus the two middle rings nearly blended into one on the southern edge of the pattern. They were still discernible, however not as distinct as the northern edge. Seems odd inside of focus would not display this irregularity, too. Also, if I moved the star a bit north in the FOV, both rings were well defined around the entire circumference. However, defocussing to 5 rings seemed to lessen the error to the point of swearing it wasn't there. The three inner rings were distinct around the entire circumference.

#3 Starhawk

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Posted 02 January 2011 - 01:08 PM

Have you considered looking at objects in-focus?

In my experience, that markedly improves the image.

-Rich

#4 Eddgie

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Posted 02 January 2011 - 06:38 PM

The amount of defocus you are using is too little.

These scopes can have a little bit of higher order spherical abberation (normal for the design) and too little defocus it is just TOOO sensitive to get a good reading on Spherical abberation.

The check for zones and turned edge is done with considerable more defocus than shown in the image here.

A better way to check for spherical abberation is to use about an 8mm or 10mm eyepiece out from the diagonal about 5mm or 6mm.

Now, focus on a star using the focuser as exactly as possible.

Do NOT move the focuser for this next part. Release the eyepiece set screw and slide the eyepeice in.

Note the point at which the bright inner ring emarges from the shadow of the secondary. You should see a bright ring, one or two dim rings, and another bright ring, then the shadow of the secondary.

Measure the exact amount of the movement of the eyepiece. Let's say that it took 4mm for the inner ring to emerge.

Now, go back to best focus by sliding the eyepiece back out.

Get the focus exact as possible once again only moving the eyepiece.

Now, slide the eyepiece OUT until you once again see the inner ring break out of the shadow of the secondary.

Once again, measure the distance.

If the distance between the breakouts is IDENTICAL, then the system has perfect correction.

If the shadow breaks out at a closer or further distance on one side than the other, this ususally indicates spherical abberation is present.

As long as the breakout ratio is less than twice the distance one once side than the other, the amount of SA present is likely not severe. For example, if the shadow breaks out at 4mm on one side of focus and 6mm on the other, the SA is minor. If it is 4mm on one side of focus and 8mm on the other, then there is more than a little SA, but even this isn't serious.

If the ratio is three to one, then there might be a more serious problem with SA.

For zones, defocus until maybe 8 to 10 rings are visible. A zone will show as some of the interrior rings either being dimmer or brighter (Check on both sides of focus) and may appear to be squeezed together slightly.

Zones are rarely present in all spherical systems unless they are polished with sub aperture polishers as with the case with larger SCTs like the C11 and C14. It is my understanding that these scopes are polished with a sub aperture polisher that polishes the center, then the mirror is rotated while the polisher does the outside circumference of the mirror. I have seen several interferometer tests that showed slight zones on C11s and C14s, and the zones tend to support the report that I have about the use of sub-aperture polishers. My own C14 shows a very minor zone at about where you would expect to see if if a polisher for a C8 were used at the center, then to do the outside edge. In other words, if I had to estimate it, I would guess that the zone is a bit less than 8" in diameter.

So, my own advice would be to use more defocus because at very small amounts of defocus, the test is to sensitive and the higher order sperical abberation in the system may be unfairly influencing your result (though .1 wave is really excellent if this is all there is).

Regards.

#5 KerryR

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Posted 02 January 2011 - 06:55 PM

Nice summary, Edgie.

This is the first time I've seen reference to 'the bright inner ring' (then shadow) being used to help identify the breakout. Where'd you get that? Sounds useful, and may help me be more consistent in my identification of the shadow breakout. I've been using the point at which it first becomes obvious, but this is somewhat tenuous...

#6 Asbytec

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Posted 02 January 2011 - 07:31 PM

Eddgie, okay, sounds interesting. I was mistaken, I was not 6 waves in or out as the image shows. Suiter showed an image striking similar to one I get and he noted it to be about 15 waves. I guess my unfamiliarity with Aberrator led me astray. No wonder Aberrator did not show the pattern exactly as observed, just approximate.

"You should see a bright ring, one or two dim rings, and another bright ring, then the shadow of the secondary." Yes, got it...this was actually where I was working. The image looked textbook, but the shadow break out seems interesting and makes sense.

Thanks for the advice and the clarification on working too close to focus. My scope just arrived and just want to check it over to see if I got a good one. I think so, but a little piece of mind is worth the work.

Anyway, no work tomorrow so I'll be spending a lot of time looking at objects in focus. Apparently Saturn is (or did recently) show some disturbance in it's northern hemisphere. :)

#7 Eddgie

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Posted 03 January 2011 - 10:02 AM

Got that from Suiter's book on Star Testing. It is an excellent reference work.

Yes, the emergence is not sudden. I continue out past the point where the ring forms and JUST before another interrior ring (between the bright rings) forms, then back off a little until the inner ring starts to once again become indistinct. This should approximate the same condition on either side of focus.

Each ring that forms essentially indicates another wavelength of defocus.

Some of the wavenlenth rings are of course shaded by the secondary, so when the pattern shows the bright inner ring, a dim interrior ring, and a bright outer ring, you are seeing the "Difference" of 3 wavelenthts of defocus, but there are perhaps 4 or 5 rings under the shadow of the secondary so that the TOTAL defocus is maybe 8 to 10 wavelengths of defocus.

Using much less than this and the test becomes too sensitive.

Using more than this and the test LOOSES sensistivity.

Suiter gives EXACT amounts of defocus in his book for various apertures and focal ratios, so the info I gave above is more or less roughly approximate, and that is why it is hard to say that 2:1 indicates much more than a minor problem. Unless you have the tables to get the exact measurments of defocus, the best you can do is use this method as a rough idea. If the ring breakout is very balanced, then SA correction is good enough.

Years ago, there was a web-based urban ledgend that the star test did not yeild reliable results when used with "Complex Telescopes." like SCTs, MCTs, and APOs. This is of course not really the case. The physics of light and wave theory simply are concrete.

What DOES happen though is that if these kinds of telescopes are tested with too little defocus, the test becomes SOOOOO sensitive that even harmless higher order spherical abberation will start to show in the test (which can make a practically perfect telescope somehow look like there is a problem).

This is why you have to use a bit more defocus. But not TOOO much.

Also, the test is based on a 30% obstruction. A bit bigger or smaller won't hurt, but for a refractor, or reflector with a very small secondary mirror, it is best to add the obstruction.

ANYTHING to do with Star Testing can be learned with Suiter's book (Second Edition). It is a masterpiece of explaining how to test and how to evaluate.

He also addresses higher order spherical abberation in all spherical MCTs, and he presents diagrams and strehl for a 6" f/12 MCT design. Basically, for an all spherical aluminized spot MCT at f/12, there is going to be some higher order spherical abberation (not enough to severely damage the image, but enough to limit the Strehl to something like .95 as the HIGHEST possible for the design).

In other words, if the MCT is all spherical at f/12, it will have higher order spherical abberation that is equivilent to about 1/8th wave of lower order spherical abberation.
This means that even though the central obstruction might be a bit smaller in the f/12 MCT than a similar sized SCT, it can in theory never perform as well as a perfectly made SCT. Only when the secondary is recurved by grinding the rear of the corrector or seperating the secondary and aspherizing it can the HSA be eliminated.

Suiter says that unless one of these steps is taken, the focal ratio should be kept to about f/15 before to keep the HSA to levels that are meaningless.


To be fair, if the quality is otherwise perfect, most observers would struggle to see the difference between an all spherical f/12 MCT and one that has an aspherized secondary. A scope with a Strehl of .95 will still present a good image.

#8 atelierbks

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Posted 03 January 2011 - 10:46 AM

What's the ultimate origin of the optics on the Orion MCT 150s? Intes? Kunming? United?

#9 KerryR

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Posted 03 January 2011 - 11:15 AM

I think they're Synta.

#10 wh48gs

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Posted 03 January 2011 - 11:22 AM

Couldn't say, but Aberrator did confirm this when I selected 3rd LSA setting of -0.1. It represents pretty accurately what I observed. Adjusting the 5th HSA settings simply showed diffraction effects that simply were not observed. But, since this is the lowest setting Aberrator offers for 3rd LSA, I have to assume any spherical aberration must be tiny.



The higher order spherical in the Aberrator is actually the balanced higher order spherical, which is the pure higher order (5th order transverse, 6th order wavefront, meaning that it changes with the fifth and sixth power of the aperture radius, respectively) minimized by introducing appropriate amount of lower order spherical (3rd order transverse, 4th order wavefront).

There is no pure higher order spherical in Aberrator, and it is about as irrelevant for a Mak-Cass as the lower order form. The bulk of pure higher order spherical is generated by the meniscus corrector, and it is reduced to less than 1/5 by mixing it with similar amount of lower order spherical of opposite sign. The resulting aberration is balanced higher order spherical, referred to simply as "5th HSA" in Aberrator. Suiter, in his first edition, also uses this inaccurate, potentially confusing terminology.

The HSA patterns given in Suiter's book are for the balanced higher order spherical. Important difference between the lower and balanced higher order form is that the latter has significantly smaller RMS wavefront error for given p-v error. The patterns given in the book are for 1/5 wave p-v balanced HSA, which corresponds to 0.038 wave RMS, the level of 1/8 wave p-v of lower order spherical. Even at such small error level, the intra and extrafocal patterns are noticeably different, as the simulation shows (generated by Suiter's Aperture). At this small error level, balanced HSA is fairly similar to LSA in that pattern's intensity on one side of focus decreases from the center out, and from outside in on the other (the effect is partly obscured by the effect of c.obstruction, but still noticeable).

If your Mak shows similar patterns, it has an excellent correction.

Vla

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#11 Danno2006

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Posted 03 January 2011 - 11:25 AM

Would you recommend an 8 - 10mm eyepiece for a C14 test?

Thanks

#12 KerryR

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Posted 03 January 2011 - 11:36 AM

Vla's images show exactly why I consider Gregory Maks tough to star test-- the apparently significantly different intra and extra focal images are misleading-- you wouldn't want to see this in a Newt, for example. But, the secondary shadows remain identically sized at 10 waves in and out, suggesting the 'breakout' test is still valid, and probably the better way for most users (like me, who don't understand much about high and low order aberrations) to assess one of these scopes...

Interestingly, Vla's images are close to what I see in my Orion 127mm Mak. My ota passes the breakout test, but, owing the the image difference on either side of focus, I had assumed either a)the star test was impacted by the 5th order SA or b) was showing 'normal' SA.

Good post.

#13 Eddgie

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Posted 03 January 2011 - 12:43 PM

Yes, this is the important point. The breakout test is the most valid way to check for SA, and as the diagrams presented show that the secondary shadow is the same size on either side of focus (though this is harder to judge at the eyepiece due to the change in brigtness of the rings, and this is why Suiter suggests using de-focus as a more reliable way than relying on the human eye).

Just looking at the expanded ring pattern itself is sketchy. The eye doesn't do a very good job of discriminating between the brightess of the rings on one size vs the other side of focus, and camera images tend to over-expose so even with photots, it is not so easy to tell.

Measureing the defocus of the inside ring breakout is extremly reliable as a pass/fail, and with experience, it is possible to make a very decent estimate of severity using this method.

Refractor people that star test without an obstruction are getting almost totally unreliable results. Simply impossible to judge amounts of spherical abberation even up to maybe 1/4th wave in a refractor. Using an obstruction though, and it is very quickly see if a refrator has any meaningful SA.

#14 Asbytec

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Posted 03 January 2011 - 01:36 PM

Learning a lot from your comments, thank you. Again, just picked up my scope and want to test drive it. I suspect the optics are pretty good...a bit more testing to confirm. I'll try the break out test tonight for a bit. (I already suspect it's gonna pass, actually seen it many times...but with no idea of how to measure it.)

Saturn will rise on about an hour, can't wait to see it, again. There is an outbreak occurring and I will push the scope as hard as I can. Scope should be cooled and ready to rock.

Vla, thank you for the pics and the confidence boost. Wish I could speak to these things better, I get lost when someone days 6th order wave...RMS...this or that. Pictures work. :)

#15 Eddgie

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Posted 03 January 2011 - 04:13 PM

An 11mm eyepiece would be optimal for the spherical abberation test. The eyepiece should be the same focal length in mm as the telescope focal ratio. An f/10 telescope should you about 10mm and an f/11 telescope (C14) would be 11mm.

When checking for Astigmatism, much higher power can be used because small amount of astigmatism are best seen very close to best focus.

But for checking SA, it is better to use an eyepeice with a focal length in mm that is about the same as the focal ratio of the telescope being tested. For the OP, that would be a 12mm eyepiece.

It is not exact though. Just a rule of thumb.

#16 Asbytec

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Posted 03 January 2011 - 04:13 PM

The eyepiece slip method seemed to produce good results. It took about 6mm of in-focus to get the inner ring to break out and about 1.5x outside focus (9mm). I did several measurements to the best of my ability, got a few that seemed longer (closer to 2x.) But most measurements ranged very near 1.5x.

I worked it for about an hour. They western sky was dead calm, seeing in the 8 range or better. I simply could not get used to how clean the patterns looked on both sides of focus. I gotta call it a good scope.

So, off to Saturn. Saturn was having a hard time behaving, seems some heat must have been coming from my neighbor's roof. It was almost like looking at it through a jet blast. Still, there were fleeting moments of steady seeing. Best I could tell, Cassini's division was easy on the edge of the rings. Couldn't swear to have seen it across the front. However, there was some evidence of the break out in in the northern hemisphere. Various filters confirmed this.

Focusing on Saturn was no chore. It took only a light tweak of the focuser to get good focus, much more and it was obviously out. The limb seemed pretty sharp at 180x, and it should be able to take much more magnification. I'll wait for Saturn to reach opposition and when it's better pieced in the sky.

M3 was simply stunning. A fairly bright glow with tiny speckles across it. The Sombrero was easy, the dust lane was very prominent...and my favorite galaxy, by the way. Beautiful. M51 is as how I remembered it, almost. I have seen spiral structure in my 6" reflector from the high desert. Tonight, it appeared to have a slight molting.

The seeing was fine tonight, but transparency seemed to limit to about mag 4.5 naked eye. Other nights can easily get to mag 5 or better...the neighborhood. We are very rural with almost no sky glow, mostly direct lighting is the problem. Hate to bang on the door at 3AM and ask the neighbors to shut off his living room light.

Thank everyone for walking me through this. I am pleased with the scope...very pleased. Sun is coming up, time for bed. :)

#17 Oz Alfert

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Posted 03 January 2011 - 06:11 PM

Wow... great thread.

#18 wh48gs

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Posted 03 January 2011 - 06:19 PM

As far as we can trust simulations, balanced higher order spherical is more pronounced in extrafocal patterns at larger defocus values, while lower order spherical is more pronounced at smaller defocus values. This means that the former requires less of magnification (for 10-12 waves of defocus), while the latter needs significantly more (for 4-5 waves defocus).

The large simulation patterns are deceiving, since actual patterns are much smaller, and less clearly defined due to seeing/thermals. The ones shown on top, when looked at from about 12 inch have angular size corresponding to that in a 6" aperture at about 1500 magnification. More realistic apparent size is shown at the bottom; in the actual conditions, the 4 wave defocus patterns for higher order spherical, and 10 wave defocus patterns for the lower order, would appear nearly identical.

It should be noted that since pure higher order spherical, just like its lower order cousin, can be either overcorrected or undercorrected, the balanced higher order can have the extrafocal patterns switch sides from what is shown above (overcorrected).

Vla

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#19 Asbytec

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Posted 03 January 2011 - 09:49 PM

A correction, on re-figuring the math from my previous post, the inner ring outside focus broke out at about twice (not 1.5x) the distance from inside focus. I screwed the simple math. Had so much fun last night at 3AM, will probably hit it again tonight, weather holding, and be more precise. A few more hours of testing might be enough waiting for Saturn to rise.

A weird thing about the weather here, it can look like the night is ruined by clouds, but by morning...every morning...the skies are clear and blue. And, really, the best seeing conditions of any skies I've had the pleasure to experience. Guess it has to do with the large expanse of pacific ocean not disturbing the air. It's a pleasing thing to see stable star images, focused and otherwise. One might notice a single twinkle with the naked eye every few seconds or so.

Kerry, as Val mentioned "If your Mak shows similar patterns, it has an excellent correction." This is very similar to what I see, too. My inner ring might be a bit brighter. But, like you, this stuff is difficult to grasp. Not SA per se, but all the variations. I've yet to see a second diffraction ring in focus, maybe because I've used dim stars.

In his set of images above, his rendition of lower order SA simply do not resemble anything I observed at least while defocussing through 4 waves where both sides are more similar than not. At 10 waves out LSA, okay...maybe. Outside of focus, there is a bit of a halo outside the outer ring. So, maybe there are some similarities. The balanced high order images are more familiar, especially when looking at the 'realistic apparent' sized ones, except the outer ring is a bit brighter inside of focus. If memory serves, the outer ring is prominent on both sides.

In either case, shouldn't 0.037 RMS produce a 'good' image? Focusing on Saturn seemed very easy. Only a slight tweak (1/10 turn or less, guessing) in or out would defocus it enough to be noticeable. This is important, too, right?

Again, I was working with a 10mm plossl in an f/12 scope. I am sure, as Eddgie pointed out, might be close enough to the rule of thumb for my purposes. It'll have to do for now.

Danno, seems the 10mm is closer to the f/11 ratio. Try that one.

It's humbling to meet folks with so much knowledge who are willing to help. Thank you. I just hope what I say is consistent, it's tough to describe such observations.

#20 Asbytec

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Posted 04 January 2011 - 03:14 AM

Eddgie, was thinking about the eyepiece 'slip' technique. I imagine it's important to know how far inside of focus the inner ring breaks out. I mean, something would be odd if it broke out wayyy tooo far inside (or out), say several inches, eh?

Suiter's chart says for an f/12 we should defocus 5mm (8 waves) and 7.6mm (12 waves.) For an obstructed OTA, one would imagine the inner ring would break out somewhere in this vicinity. So, if slipping the eyepiece shows about 5mm inside of focus to break out, that should be pretty normal? (I'm probably operating under an assumption.)

Now, after achieving break out inside of focus, then refocusing the star (eyepiece locked) would give you some idea if how far the focuser must move the primary. Say for example 5mm is about 1/4 turn which refocuses the star. Then one could use the focuser knob to get an idea of how much rotation is required to break the ring outside of focus. If it broke out again at 1/4 turn outside focus, then this would mean 5mm just like the inside focus test. Right? In other words, nearly perfect correction.

Truthfully, under the stars, I forgot how to perform the second part of the test you explained. You warned about moving the focuser, but when I dropped the eyepiece by the required amount...I forgot where to go from there. So, I improvised and refocused the star with the eyepiece locked in it's 'inner break out' position. I noted how much focuser travel I needed.

Then, I backed the focuser out until I got the inner ring to break outside of focus. Then I compared the ratio of how far I had to turn each way. For example, (if memory serves) it took 1/4 turn to refocus the star from the inside break out. Then it took less than half a turn to break out outside of focus. So, the ratio is less than 2:1, but greater than 1:1.

If this is really a way to 'rough it' with some accuracy, I'll confirm it tonight. But, it did seem, using this method, from inside break out was 1/4 turn to focus and back about 3/8 turn for outside focus break out. And I was able to repeat the observations readily and easily.

So, how does Suiters wave/distance chart play with this? With 31% obstruction, should the break out occur somewhere between 8 and 12 waves as (may be) illustrated above? I imagine if break out occurred at 30 waves, then something is wrong; your focal point is either very 'long' or your obstruction is huge. So, it seems a reasonable break out distance should be expected between 8 and 12 waves. If so, one can use the ratio of focuser turns inside to outside of focus to determine the ratio of relative break out distances on either side, provided one could measure the focuser travel accurately enough.

#21 CowHampshire

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Posted 04 January 2011 - 04:58 AM

Great optics lesson here - thanks Asbytec! You're description of Saturn was too much to bear. I get great images with my little refractor - but nothing like the detail you describe.

Eddgie - the "eyepiece slip method" - I've got an Intes MK67 coming (from a forum member here) and I'm definitely going to use that man.

#22 Asbytec

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Posted 04 January 2011 - 05:03 AM

"thanks Asbytec!" I just ask the dumb questions...experts chime in.

Please post your MK67 experience. I am actually a bit envious. :)

#23 Asbytec

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Posted 04 January 2011 - 07:23 AM

Now, reading Suiter's book, it appears my scope is (probably all Orion's are?) under corrected(?), meaning the inner diffraction ring breaks out a little farther from focus than it did inside focus. Apparently, it's because the shadow of the secondary is a bit larger outside of focus. It's interesting how the light cone 'throws' the shadow in that way.

Another interesting thing about SA is it throws light into the rings, more noticeably below 1/4 (if memory serves) and it tends to reduce contrast more than anything else. One reason I want to know this one better is the choice of high power eyepiece: 250x (7mm) or push it to 300x (6mm.)

So, a break out ratio of 2:1 (roughly) corresponds to 1/4 wave aberration and a Strehl ratio of .80, which is the lower boundary of what is considered good, acceptable, or whatever. Anything less than 2:1 approaches excellent, I guess. If I can get even slightly better than 2:1, I'll push for 300x. (Make sense?)

Weird and interesting...still don't understand this aberration well. That'll be the goal tonight, to see if this scope's ratio is less than 2:1. Some testing last night seemed to suggest it was slightly better. Still, no complaints from the in-focus images...

#24 TG

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Posted 04 January 2011 - 05:07 PM

Years ago, there was a web-based urban ledgend that the star test did not yeild reliable results when used with "Complex Telescopes." like SCTs, MCTs, and APOs. This is of course not really the case. The physics of light and wave theory simply are concrete.

What DOES happen though is that if these kinds of telescopes are tested with too little defocus, the test becomes SOOOOO sensitive that even harmless higher order spherical abberation will start to show in the test (which can make a practically perfect telescope somehow look like there is a problem).

This is why you have to use a bit more defocus. But not TOOO much.

Also, the test is based on a 30% obstruction. A bit bigger or smaller won't hurt, but for a refractor, or reflector with a very small secondary mirror, it is best to add the obstruction.


I wouldn't call it an urban legend - it was an assertion made in an article written by none other than Roland Christen and can be found here:

http://geogdata.csun.../startest2.html

I didn't quite understand what he meant till I had the chance to own a Takahashi TSC-225. From its star-test, and what I remember, it was barely 1/3 wave corrected given its shadow breakout distances. And yet, its Ronchi-gram, using a 133lpi grating showed absolutely flat lines and the performance on Mars (at 15") and the Moon was superb (I didn't get a chance to evaluate it on Jupiter as I sold it before Jupiter became visible). It was sharper than my excellent C9.25 which measured 1/6 wave with Roddier.

I now understand the reason that Roland stated: the central zone of higher order spherical aberration present in complex CATS which a 33%+ CO normally hides. The Tak's smaller CO allowed that HSA to come through but ultimately, in the final image, it didn't make any difference. I suspect with a Mak, especially one with a smaller than 33% CO, you will see something similar.

Regards,

Tanveer.

#25 Eddgie

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Posted 04 January 2011 - 07:39 PM

Most of what Mr. Christen wrote was probably based on unhappy customers that did not know how to conduct the test properly. The cause is almost always using insufficient amounts of defocus, which is TOOO sensitive for estimating small amounts of spherical abberation, and is also easily influenced by higher order spherical abberation.

When done at 8 to 10 wavelengths of defocus, and when done with a 30% obstruction, and using carefully measured defocus amounts, the test should show a fairly balanced shadow breakout for any telescope with good spherical abberation correction. There may be a slight difference due to something like higher order spherical abberations that are harmless, but any deviation from a perfectly balanced inside and outside of focus means that SOMETHING is influencing the light cone.

This is why Suiter said in Verson 2 of his book that he was sorry that he emphasized SA so much and now says a 2 to 1 breakout should be fine.

Otherwise, it is possible that either the test was not conducted with sufficient care (especially with respect to placement of the artificial star if one is used) or there is some kind of optical abberation influencing the result. This abberation may not be particularly damaging, but anything that changes the caustic cone from being perfect on either side of focus at the exact same distance from perfect focus indicates that some kind of abberation is present. Either the light cone converges perfectly (and of course this means that it must diverge perfectly after it passes though the focal plane) or it does not. The star test simply doesn't care about the telescope that formed the cone, only how it slices up when propagating though space.

Suiter addresses this issue in detail in Version 2 of his book starting on page 302.


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