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Anyone have the CO measurements for Meade SCTs?

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#1 jrbarnett

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Posted 10 August 2014 - 06:10 PM

I'm looking for the CO sizes for the 6", 8" 10" and 12" Meade SCT OTAs.

 

Lemme know if you have 'em.

 

Thanks,

 

Jim



#2 bratislav

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Posted 10 August 2014 - 07:17 PM

Just a note - don't use a ruler - measuring CO by dividing size of secondary assembly and entrance aperture is not precise. In most cases largest obstruction is actually introduced by the edge of primary baffle (which cuts into a converging cone). A proper way to do it is to take an out of focus image of a star (WAY out of focus, so spherical aberration does not affect it) and then measure size of the obstruction directly. This is the most precise way to do it, by far.

And be prepared to be surprised! Manufacturers tend to be -er- enthusiastic about their obstruction ratios. 


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#3 Dom543

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Posted 10 August 2014 - 07:39 PM

I've just used a caliper to measure the maximum external diameter of the CO and the open diameter of the corrector..

 

Here are the numbers for the Meade f/6.3 SCT OTA's

 

Meade  8" f/6.3 OTA  - CO diameter = 87.6 mm  -  Aperture diameter = 203 mm.

Meade 10" f/6.3 OTA - CO diameter = 97 mm     -  Aperture diameter = 254 mm.

 

--Dom



#4 jrbarnett

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Posted 10 August 2014 - 07:40 PM

Thanks.  That is sensible and very interesting, though not entirely surprising.  Still I'd like to see how the secondary diameters of the Meade's compare to similar aperture Celestrons.

 

- Jim



#5 jrbarnett

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Posted 10 August 2014 - 07:41 PM

Thanks Dom.  I should have specified that I am interested primarily in the f/10 units.  I believe they have smaller secondaries.

 

Regards,

 

Jim



#6 PowellAstro

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Posted 10 August 2014 - 08:32 PM

My 8 inch has a 2.85" secondary holder which is right at 35 percent.

 

My 14 inch has a 4.6 inch holder which is just over 32 percent.



#7 jrbarnett

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Posted 10 August 2014 - 08:47 PM

Thank you, sir!

 

- Jim



#8 Bill Barlow

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Posted 11 August 2014 - 08:45 AM

I owned  a Meade 12 f/10 and measured the CO at 33.3%

 

Bill



#9 jrbarnett

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Posted 11 August 2014 - 12:42 PM

Anyone have the secondary diameter of the 6" Meade SCT?

 

Thanks!

 

- Jim



#10 hudson_yak

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Posted 11 August 2014 - 08:03 PM

Mine, of classic vintage, measures 75mm for the outer holder. It looks like the secondary baffle inside might be more like 76mm. Either way, that's right around 37%.

 

Mike


Edited by hudson_yak, 11 August 2014 - 08:34 PM.


#11 bratislav

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Posted 11 August 2014 - 08:59 PM

I owned  a Meade 12 f/10 and measured the CO at 33.3%

 

Bill

 

Well, here's a perfect example of what I was talking about. If one visits Rohr's test pages on astro-foren, you can see for yourself that based on null-test images he supplied, a 12" f/10 Meade has 38% obstruction, not 33% as measured by a ruler. 

My own C11 fares the same (37.5%, rather than as measured by secondary alone which suggests 34%). As it did C925 before it (38%). And pretty much every SCT I've tested. 

Rule of thumb for f/10 systems - larger SCTs (9" - 12") CO will be around 37-38%. Small ones will be around 40% and often more. F/8 and f/6.3 will be much worse again. Lowest true CO I've measured is in an old orange C14, right on 33%. 

 

Again, measuring secondary in a system with baffles is NOT telling you what central obstruction really is!



#12 hudson_yak

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Posted 11 August 2014 - 09:25 PM

Where the CO% concern is for an object in the center of the field, focused at infinity, and with the assumption that the scope is not being used in a configuration that reduces its aperture in the center of the field (long visual accessory path, R/C, etc.) the simple relative-diameter measurement method seems valid to me.

 

Mike



#13 jrbarnett

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Posted 11 August 2014 - 09:49 PM

 

I owned  a Meade 12 f/10 and measured the CO at 33.3%

 

Bill

 

Well, here's a perfect example of what I was talking about. If one visits Rohr's test pages on astro-foren, you can see for yourself that based on null-test images he supplied, a 12" f/10 Meade has 38% obstruction, not 33% as measured by a ruler. 

My own C11 fares the same (37.5%, rather than as measured by secondary alone which suggests 34%). As it did C925 before it (38%). And pretty much every SCT I've tested. 

Rule of thumb for f/10 systems - larger SCTs (9" - 12") CO will be around 37-38%. Small ones will be around 40% and often more. F/8 and f/6.3 will be much worse again. Lowest true CO I've measured is in an old orange C14, right on 33%. 

 

Again, measuring secondary in a system with baffles is NOT telling you what central obstruction really is!

 

Understood, but the actual obstruction will never be SMALLER than the secondary diameter, so I'm really looking to compare the relative difference in secondary size between the Meades and corresponding Celestrons, not determine the actual CO for each scope.  Would it be fair to say that the larger the secondary in diameter, the worse the actual obstruction diameter, measured accurately, is likely to be?

 

- Jim



#14 Dom543

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Posted 11 August 2014 - 10:01 PM

Bratislav,

If you get light cones involved, then the result will depend on the position of the primary. Similarly, as the focal length also depends on the position of the primary. If you cannot assure that everyone takes the measurements in the same primary position, then you will compare apples to oranges.

--Dom

#15 bratislav

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Posted 12 August 2014 - 12:25 AM

Jim,

 

not quite sure. It is hard to read into designer's mind, and even same type scope from a different era has definitely been built differently (with different baffle lengths and even diameters). The only way to be sure is to measure it using out of focus star images (or interferogram/foucaultgram in double pass, if you happen to have one already).

 

Dom,

 

I'm sure Mr Rohr knows where to measure SCTs when he puts them on his optical bench (most are designed with focal point 4" behind the flange). Moreover, unless you are silly enough to extend your observing/measuring point WAY behind you are not going to change geometry significantly. Have the Zemax handy, it shows this stuff really well.

But if you are happy to continue to calculate CO using purely diameter of secondary assembly, by all means, be my guest. I really have nothing to gain from this. In fact as I'm going to exit the SCT world soon (my 16" is going to replace my C11 as planetary tool so SCT will be sold), I may just exit this forum and leave you guys in peace.

 

Cheers! 



#16 bratislav

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Posted 12 August 2014 - 12:44 AM

Where the CO% concern is for an object in the center of the field, focused at infinity, and with the assumption that the scope is not being used in a configuration that reduces its aperture in the center of the field (long visual accessory path, R/C, etc.) the simple relative-diameter measurement method seems valid to me.

 

Mike

 

Sure, and world is flat :lol: 

If you can't visualize the problem have a read at http://www.telescope.../two-mirror.htm and imagine primary baffle getting longer (the thing marked with 'L' on the diagram). That is more or less how every SCT is designed today, as they strive to have baffle wide enough to have maximal unvignetted field of view (SCTs are used as astrographs too) but still minimize the sky flooding. The end result is front of primary baffle starts to cut into the converging cone, effectively increasing the obstruction.  

But if you can't visualize (or don't want to), it's OK. Differences are usually "only" a few %, so if Celestron/Meade tells you the obstruction is 33% (but in fact it is 38%) it matters very little. MTF has already suffered quite a bit as you crossed the 30% mark ...



#17 Bill Barlow

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Posted 12 August 2014 - 09:09 AM

Then why don't Celestron, Meade and Intes-Micro use this method to calculate the CO?  Even with the numbers you report, the views through these scopes with smooth, high Strehl optics are excellent.

 

Bill



#18 jrbarnett

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Posted 12 August 2014 - 10:15 AM

Then why don't Celestron, Meade and Intes-Micro use this method to calculate the CO?  Even with the numbers you report, the views through these scopes with smooth, high Strehl optics are excellent.

 

Bill

I can think of one reason - it makes their scopes look like low contrast scatter-buckets.  38%-40% CO is HUGE and DAMAGING.  So much for folks who say the CO issue is overrated.  :grin:

 

- Jim



#19 PowellAstro

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Posted 12 August 2014 - 10:41 AM

The bottom line is as Roland has stated many times, what matters is the in focus airy disk image.  The star test is great to give clues of any issues but if your CAT shows a bright airy disk with one or two diffraction rings much dimmer than the disk, enjoy because thats what counts. As Roland has seen, I have also tested SCTs with matching intra and extra focal patterns and in focus was a mess. Then I have seen and own a SCT whos intra and extra patterns are completely different but in focus the unit produces a very bright airy disk and very faint diffraction rings(two).  Only on the best nights can two be seen, most the time just the first but its intensity is more on the order of a 20 percent obstructed scope, even though its obstruction is around 34 percent.


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#20 DesertRat

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Posted 12 August 2014 - 04:00 PM

Roland's statement on the startest, which has been referred to many times here, has to be read carefully.

 

No one argues that a good scope cannot have differing intra/extra focal images.  But for them to be completely different is a sign of aberrations.  The startest is very sensitive for small defocus, so much so it can lead to unwarranted concern. 

 

Of course it depends on how different those images are.  Nothing magic occurs at focus, except for an interesting pi phase shift.  An unaberrated wavefront, that is one with a perfect spherical shape, will show indistinguishable images on either side of focus.  Failing that is an indication of an aberration.  Anything contrary would violate symmetry, in addition to wave theory. 

 

If there is any difference in the images it will result in a less than ideal Airy pattern, the Strehl will be less than 1.

 

A star test of an obstructed scope, showing a pattern suggesting a smaller obstruction, is unusual to say the least.  To verify that would require a photometric validation, an almost impossible thing to perform in even the best seeing.  I choose to be skeptical of that result.  The diffraction rings are quite fragile (more so than the central spot), and for any aperture over ~4" they are often in motion and fragmentary except in very good seeing.

 

The best number for the true obstruction can be measured from a Foucaultgram, also know as a focogram by ATM'ers.  This can be achieved by imaging with a camera focused at the pupil one focal length away,  with a knife edge at the focal point.  To average out seeing one can stack many video frames, larger apertures require a longer integration.  If your tracking is really good a time exposure can also be employed with a still camera.  The focogram is also a great indicator of how smooth your optics are, and whether there are any trenches, zones or other faults.

 

Glenn



#21 PowellAstro

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Posted 12 August 2014 - 04:30 PM

I have a friend who owns the AP 7 inch triplet oil spaced scope. It has been tested more than once by Roland himself and has a Strehl of .99. This scope shows clear rings inside focus and none at extra focal. There is no color on either side but the two are very different with a very high Strehl(tested).

 

My 14 is the scope I was talking about with the different patterns. Mine is from HSA on the order of around 1/10 wave and it causes the images to look completely different at 50x per inch. At 25x per inch they almost look the same. 



#22 hudson_yak

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Posted 12 August 2014 - 05:48 PM

Deleting to do some more thinking first...


Edited by hudson_yak, 12 August 2014 - 06:25 PM.


#23 DesertRat

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Posted 12 August 2014 - 06:00 PM

I have a friend who owns the AP 7 inch triplet oil spaced scope. It has been tested more than once by Roland himself and has a Strehl of .99. This scope shows clear rings inside focus and none at extra focal. There is no color on either side but the two are very different with a very high Strehl(tested).

 

My 14 is the scope I was talking about with the different patterns. Mine is from HSA on the order of around 1/10 wave and it causes the images to look completely different at 50x per inch. At 25x per inch they almost look the same. 

 

I respect greatly Roland and what he builds.  The finest scope in the 6" class I ever used was one of his early f/12 super planetaries.  But if I had a question about optics and aberration theory I would refer to someone else.

 

A SCT scope with only 0.10 wv HSA would be an exceptional, almost unbelievable instrument.

 

I assume such a measurement was from IF?  Surely not a startest.  For a startest investigation in low versus high order balanced spherical (BLSA and BHSA) one is looking for a differentiation in various ring intensities at different defocus positions.  Since that is highly dependent on the actual obstruction, which likely does not correlate with any of Suiters examples, I am a bit skeptical.

 

Even IF would have problems in that measurement.  There is a good bit of scatter with those tests, even done by experts.

 

Now a Roddier test could tell you something.  But even there one would have to collect many trials and do statistics to get a useful measure.  That is to discriminate between low order and high order spherical at such low levels.  In addition the test would have to be photometrically accurate, not an easy exercise requiring good flat fielding etc.

 

A 0.10 wv BHSA equates to a Strehl of 0.986 and if BLSA approx 0.966.  I don't think that is possible to tell visually.  And any real scope has other contributions of coma, even if well collimated, as well as astigmatism, and some other terms.  Finally even if a SCT had only 0.10wv BHSA, say in green light, it would likely have more than that of LSA in blue (overcorrected) and red light (undercorrected).

 

Glenn


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#24 PowellAstro

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Posted 12 August 2014 - 06:35 PM

This is a special SCT and Scott Roberts, when he was still with Meade, had one of their top optictians spend a lot of time with the unit before it left the factory, in fact almost two years. I feel very blessed to own this scope. I do believe the test reports and my years using this scope supports their test data.

 

Almost any SCTs I have ever tested you can see the red and blue flange ring in the defocused pattern but not in this one as all rings show white only. 



#25 bratislav

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Posted 12 August 2014 - 06:46 PM

I have a friend who owns the AP 7 inch triplet oil spaced scope. It has been tested more than once by Roland himself and has a Strehl of .99. This scope shows clear rings inside focus and none at extra focal. There is no color on either side but the two are very different with a very high Strehl(tested).

 

 

Almost any SCTs I have ever tested you can see the red and blue flange ring in the defocused pattern but not in this one as all rings show white only.

 

First, refractors (particularly fast APOs, which suffer a lot from spherochromatism) should be only star tested using green filter. Sharper the cutoff the better. I use 7nm OIII with great results. If you do that on a refractor with 0.99 Strehl you'll find out that rings magically become very uniform.

Second, you are taking Roland's comments out of context. What he was referring to is that in systems with high order spherical components present (as most Maksutov Cassegrains will be) you can have markedly different out of focus Fressnel patterns and still be essentially perfect (have very high Strehl). It doesn't mean that if patterns were the same scope would be worse - in fact quite contrary. A Mak-Cassegrain with perfectly identical in and out of focus patterns would have even higher Strehl. It would just be much harder to make, for no real gain; Roland needed to explain this to potential customers before releasing the AP/Aries Mak.

 

PS SCTs suffer from spherochromatism too. All of them. Scott Roberts' or not. The only way to reduce it is to make the corrector plate achromatic (try UBK7 and LFF6), which I doubt very much Meade was capable of doing. 


Edited by bratislav, 13 August 2014 - 12:41 AM.

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