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Refractors vs SCTs and Maks for uneven doubles

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#1 fred1871

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Posted 04 January 2013 - 08:41 PM

I'd intended to post comments on refractors vs SCTs on the thread that was running in the Refractor Forum, but it's been locked. Needs to move to Double Stars. Mumble, mutter.

So I'll comment here, though Gord's more detailed remarks on his observing of 90 Her with refractor and SCT need to be read from there in the thread about the double near Jupiter. And then the thread here in Double Stars entitled "Need help: 90 Her" that Gord started a while back (september 2012).

I find it interesting that the refractor did less well than a (slightly larger) SCT in this case (90 Her). Usually refractors do better than scopes with large CO when it comes to uneven close pairs. But Gord obviously found the C8 more consistent on this pair than his 6-inch refractor, and less power needed as well.

I've not found my C9.25 better for this kind of pair than my 5.5-inch refractor, despite a bigger aperture difference than Gord commented on. And, looking back through my observing notes, there are doubles I separated with a 7-inch apo refractor some years ago that my C9.25 does not show (standard version not HD, XLT coatings, good optics).

My first wondering was whether Gord's refractor was slightly less good than some other refractors (good but not great?); or if it's just that an optimised SCT will do better than the large CO might suggest? Gord's suggestion that the extra light of the C8 over the 6-inch appears to help could be a factor.

Apart from trying to apply this information to the ongoing "rule of thumb" discussion, I also have a personal reason - I'm thinking about whether to upgrade my standard C9.25 to a larger SCT or Mak, and double star performance is an important issue for deciding that. Essentially I want a scope that *clearly* outperforms my 140mm refractor for doubles - and I can't afford even a 7-inch apo let alone bigger than that, and doublet refractors in large sizes would require a major observatory upgrade so they're not practical (for me). :(

So - useful thoughts would be from folk who've compared refractors and SCT/Mak observations on uneven doubles. For even doubles, aperture rules if the optics are good; very simple. For uneven it's plainly more complicated.

Optical quality matters, of course - I'm assuming high grade optics. To give one example, of optical quality showing how important it is - years ago I went to a demonstration night at OPT in California, lots of scopes on show, and Saturn with near-edge-on rings was the favourite object. An 8-inch SCT showed a moon sitting just above the ring line, a tiny bead barely above the wire. Visible at medium power (somewhere near 170x from memory) very clearly. A nearby 10-inch SCT did not show the moon at all, despite changes of eyepiece - it was lost in a haze. Collimation appeared good, so poor optics were the likely culprit. I'd expect from what I saw that the 10-inch would be useless for doubles as well; that 8-inch would likely be pretty good.

So - assuming good optics etc, who has compared refractor and SCT or Mak on the same uneven doubles? And what was your experience? The scopes need to be reasonably comparable in aperture - at least similar to the "planetary rule", where you subtract the CO from the aperture to get a rough equivalent. My C9.25 vs 140mm refractor nearly fits that one - 36% CO gives equivalent 5.9-inch for the SCT against 5.5-inch for the refractor. I expected similar results for uneven pairs and that's the case; no surprises. So my experience is different from Gord's. With his telescopes the C8 (32% CO, 5.44-inch equivalent) outperformed a 6-inch refractor.

Another data point. Someone recently told me of their observations with a 260mm Mak (with very good optics) on two particular doubles, one of them usefully uneven. Their notes were a good match for what I saw on the uneven pair with a 7-inch apo refractor. The Aperture minus CO for the Mak (32% CO) gives 7-inches as the equivalent for unobstructed aperture.

What have others found? I think this is an interesting general issue for observers. Refractors rule? Maybe. Aperture rules? Yes, but to what extent?

#2 Asbytec

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Posted 04 January 2013 - 10:33 PM

You know, Fred, this is a great question and yea, too bad the thread was locked. It's something that interests me, too, the interplay between resolution and ring brightness on small scales. I think if anyone wants to do a rule of thumb, there is enough there to account for those differences on the scales where two spurious discs are very close together.

I want to comment further, throw out some ideas based on some limited experience with doubles and some knowledge of CO affects. And, of course, test some of those ideas in the real world, too. Below is sort of the thrust of the argument based on MTF, that really does not take scatter into account. Not, yet. It also assumes reasonably good seeing, optical correction, cooling, and collimation. If we change any of these variables to suit one or the other aperture, the comparison is not valid. So, lets assume very good seeing, etc., for both.

Basically, a given unobstructed aperture has a specific peak intensity (brightness distribution) and diffraction radii. An obstruction both changes the peak intensity (brighter rings) and reduces the radii (added diffraction.) That's what the CO does and why resolution is higher near max spacial frequencies (even exceeding a perfect aperture, if conditions permit - as it should in those fleeting moments of near perfect seeing.)

So, given the same aperture, an obstructed scope will have a slightly smaller diffraction pattern and slightly better brute resolution. It will also have a brighter ring structure, especially the first ring, that could (does) interfere in some instances. A slightly smaller obstructed aperture will loose some of that brute resolution by increasing the angular size of this pattern and keep the brighter rings. Of course, a larger aperture will also reduce the angular diffraction pattern, and a larger obstructed scope more so.

You've asked a very big question that deals with both sides of the MTF (planetary contrast vs high frequency resolution), peak intensity of the PSF, and nominal working Strehl ratios (assuming reasonably good optics, which do matter.)

Suggested reading (two pages:)http://www.telescope...obstruction.htm

Not to scale, just to illustrate the idea why a 200mm obstructed aperture might have an easier time than a 150mm obstructed one. If the companion is not sitting on the first ring of the larger aperture, of course.

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#3 Gord

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Posted 04 January 2013 - 10:50 PM

Very interesting subject Fred! I've expressed a few comments on this already as you mentioned, but for people's reading convenience, I'll throw in a few links here to the related threads:

Locked thread in Refractors on min aperture to split the double near Jupiter

My old thread on needing help with 90 Her

Dave's thread from around same time as mine including a report for 90 Her wit...

Clear skies,

#4 Gord

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Posted 04 January 2013 - 10:52 PM

Fred,

I also wanted to ask, do you have a suggestion for a good list of un-even doubles to test with?

I have done some of the ones on Sissy Haas' list, but 90 Her was one of the harder ones on the list (for us northerner's) that would still be possible. Anything really far south just isn't (usually) possible.

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#5 Darren Drake

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Posted 04 January 2013 - 11:00 PM

I can suggest Burnham 1. Its not really all that difficult but what's really cool about is its a quadruple star. Its in Cass and about the brightest star in NGC 281 the Pacman nebula. One of the stars is a bit of a challenge and a beautiful site at high power.

#6 Asbytec

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Posted 04 January 2013 - 11:01 PM

Gord, try 42 Ori. It's very difficult, a 1.1" unequal double well placed and easy to find. It's the toughest one I've observed to date (with hard earned success.) In fact, I still go after it in better seeing just to be sure.

#7 Gord

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Posted 04 January 2013 - 11:01 PM

Norme,

Is there a way to generate those kind of diagrams that would be to scale (ex. airy disk size relative to various apertures)?

I would agree with the way you are showing the intensity of the first ring in obstructed vs. un-obstructed. And as I mentioned before, when the seeing goes out, I find the disk and rings kind of blend together. The other thing I see (I believe) is a bit of roughness of the optics in the glare or flaring. I see this vary with the seeing, but I'm guessing there is an optical component to it. One of the C8's was ever so slightly better in that regard.

I'd have to go back and try with the 10" newt, but that one was a monster for doubles. Very smooth optics in that one. Has split 0.5" so clean you could drive a bus through the black void between the pair! They were reasonably close in brightness though in that case.

Clear skies,

#8 Asbytec

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Posted 04 January 2013 - 11:28 PM

Gord, I wish I could, Maybe. Powerpoint doesn't really allow much sensitivity in scale, so just threw those together. I am not sure what the scale would be.

Yes, seeing matters a tremendous amount, it's a blessing to be in the tropics where it is consistently favorable during our dry season. It all matters, optics, seeing, collimation...the lot.

Really, in the link above, induced and inherent aberrations all affect the intensity distribution which seems so important for close unequal pairs. In fact, it seems best to have a higher peak intensity in the central disc (where refractors rule) and dimmer rings. But, there is a tiny bit of advantage with an obstruction (slightly reduced pattern) to a point where the peak falls off at the expense of the rings. That appears to be somewhere near 30%, or less, while maintaining a reasonably bright and small central disc (~80% peak) in a reasonably good scope (Strehl of 0.95.) Once the CO gets bigger and/or aberrations increase, the intensity distribution makes unequal pairs increasingly more difficult. I am so interested in how that works, too.

You gotta try 42 Ori...one tough cookie. Rutilus split it with difficulty with a 6" refractor. I did it but with extreme difficulty and patients over nearly an hour for several, intermittent glimpses of the companion. Just an occasional knot on the first ring.

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#9 azure1961p

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Posted 04 January 2013 - 11:43 PM

This reminds me of my Trapezium experience. I had a 10" f/5 PARKS reflector that had ruler straight ronchi, and a mirror even the optician SWORE (tho didn't swear) had first rate optics. At the time my 5mm exit pupil induced low power astigmatism. I knew nothing of such things at the time so I mailed the mirror back had them retest, George and the owner attested that the mirror was again excellent.

This leads up to doubles so hang on...

They send it back and like their retest the intra and extra focal tests were good and in focus showed classic patterns. I studied the trapezium a lot and Jupiter and realized I just didn't like the views regardless. I sold the 10" f/5 and moved on to ordering the 8" f/9 custom. I also profusely apologized to PARKS when I realized the source of my astigmatism.

Upshot here I'd this: despite the good optics , even great optics and half a dozen trapezium studies at all powers to 400x I just never saw the E and F stars. I shrugged and forgot about it as I never considered bad just the way it is.

Well first time I swung the 8" F/9 and put in 200x I was amazed. First the E star then F and these clean diffraction patterns (seeing was average 6/10. I was beside myself, thinking huh... Just like that ... And there they are.

To this day it floors me that the 8" can handle out perform that ten on those two stars. A real part of md wants to make excuses as to some exception in the seeing but there wasn't any. The 8" co is 16% and Im guessing the 10" was a hefty 37%. I'm still floored by this. There's a size able jump in angular res but the stars in the 10" always tended to be a mess while the stars in the 8" had the text book look.

Just passing it on. I'm guessing in better seeing where the ten could shine maybe it'd beat the 8" on these unequal doubles of the trap or would it ever due to magnitude CO and light scatter differences.

George Clements the optician stressed to Chris the customer service guy over the phone who was my contact thru all this that I seriously ought to do doubles with this scope. No one had ever suggested that about the 10".

Anyway my differing magnitude scope experience.

Pete

#10 7331Peg

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Posted 05 January 2013 - 01:35 AM

I'll add my experience here.

I've had one C11, one C9.25, two C8s, a C6, a six inch AT Mak, and an ETX 90 (none of those Celestrons were the Edge version)and I NEVER found it easier to split uneven doubles in those scopes in comparison to a refractor.

I did have an eight inch Edge recently, and it was a huge improvement in image sharpness in comparison to the conventional Celestron SCT's. I was very satisfied with it on Jupiter and deep sky objects. But when it came to matching a refractor on uneven double stars, although the view was certainly improved, it wasn't quite there.

I wouldn't say it's absolutely impossible for an isolated example of a perfectly figured SCT to match the view in an average refractor, but it would be a real rare specimen for sure.

John :refractor:

#11 fred1871

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Posted 05 January 2013 - 01:46 AM

A few replies - Gord, I'll put together a list of possible doubles for testing. Some of the Haas pairs (such as 90 Her) are suitable. I'll choose from equatorial and northern pairs as there are not a lot of us way south of the equator. 90 Her is too far north for me as a test object - except for testing air steadiness.

BU 1 in Cassiopeia is a nice multiple - I observed it from California back in the 1990s, and could see the tight AB pair split nicely at 200x with a C8. It's not super-difficult, though the stars are not bright (mags 8.6 and 9.3 for AB).

42 Ori is a definite testing one. Since mentioning it in the 32 Ori thread, I found I had an observation of it from 3 years ago - companion clearly seen, 140mm refractor, 320x. So I looked at it again this week - hints of it at 285x, definite visibility at 400x. Seeing this week was good-plus; 3 years ago very good.

Pete - interesting story. Does it mean the optician misread the Ronchi test for the 10-inch? Easy with Ronchi... More seriously, I think there are several factors here. One is that an f/5 10-inch needs more precise collimation (much more precise) than an f/9 8-inch. Second, the diffraction limited area of the short scope is much smaller than for the long one. Third, suppositional - the polish on the short mirror might have been less good, and perhaps zones occurred - more likely with short than long mirrors. Of course the difference in CO has to be significant as well - that's a BIG difference, 37% to 16%. Finally, eyepieces generally work slightly better at f/9 than at f/5. Add up the factors and it's not surprising the long-8 is much better.

Norme, I see the point of the diagrams you've produced. Gord's question about the scale of them is mine as well.

And, Gord, your 10" Newt - what f-ratio and CO size? Smooth optics is a great start so what else can you tell us about it?

#12 Rutilus

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Posted 05 January 2013 - 04:28 AM

My Skywatcher 6 inch f/8 Achromat constantly splits/shows
un-equal doubles much better than my C9.25 SCT.

#13 Asbytec

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Posted 05 January 2013 - 05:46 AM

I NEVER found it easier to split uneven doubles in those scopes in comparison to a refractor.

That's a fair statement based on matching observations of 42 Ori above. It's a classic, uneven pair sitting on the first ring that certainly appeared to be easier in a refractor. It's curious the 8" Edge just, "wasn't there" even if it gave a smaller, better corrected pattern. Especially if there as an aperture difference.

You know, isn't Pete's E and F experience really just another example of picking a dim star from a brighter background and all that is involved with that? Or is something more sinister afoot as a companion tries to form an image amidst the reinforcing and destructive interference of the primary? (Pure speculation.)

Does Sissy Haas research include variations in ring brightness with CO and the intensity of the central disc of the companion (related to it's magnitude as altered by added CO diffraction?) It seems neither 150mm scope should have been able to resolve 42 Ori. I wondered what was so hard about it. If the magnitude difference between the companion relative to the first ring is just large enough, resolution can occur regardless of angular separation alone. Of course, it can get no better than with a zero obstruction (and good optics.)

But, detection will vary with CO size, too - 42 Ori seemed much harder in my scope than the refractor. But it could be done if less light were evacuated from the central intensity (better correction and smaller CO approaching that of a refractor), and not entirely dependent on angular radius alone.

Very interesting discussion striking at the heart of a topic that interests me, too. The idea the central disc shrinks with obstruction and actually brightens - diffracted light loss concentrated in a smaller angular area. So, an obscured companion might actually brighten a tiny bit (or not dim as much) with a CO. Not sure I got my head around it. Not yet.

I would love to actually do a scale of the diagram above to see how it might play out with observation. Just not sure I have the tools to do it right. Maybe. Anyway, scope is cooling and observation is important. Tonight's targets, 42 Ori, again, 7 and 31 Tau (equal pairs), again. Really curious about 90 Herc, too.

"How hard can it be?" That silly question got my attention.

#14 Darren Drake

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Posted 05 January 2013 - 07:27 AM

I propose an experiment here. We should consider someone (maybe I will)make an off axis aperture stop for their SCT which has about the same aperture as their high end apo. Now it would be an apples to apples comparison. It would be the same aperture and same zero obstruction. If the sct provides the same level of contrast and resolution and same general viewing experience as the apo then as long as there is no thermal issues then the quality of the optics of the sct are on par with the apo.

If that is the case then it would be reasonable to use aberrator to simulate various doubles and see how the results compare. This would provide reliable results as the program has many true real doubles and their seperations and mags in the database. It is possible to change all the settings such as obstruction % and aperture as well as level of SA and so on. I believe this to be a powerful tool in this discussion any way. Just I thought and I believe I may try this to see if my C8 can perform like a little apo.

#15 WRAK

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Posted 05 January 2013 - 08:49 AM

I am still struggling with my RoT model for small refractors of 1-6" aperture and have so far not considered the impact of CO (assuming that without CO optical quality is ident). But I have the impression that reflectors (good kollimination and thermal conditions given) do slightly better near the Dawes limit for equal bright doubles up to +6mag because the spurious disk is smaller and do less good for unequal and fainter doubles with a relation between the CO and the difference of the required aperture for a refractor to the Dawes limit.
I am confident that I will soon be able to provide lists of challenging doubles for any given aperture (up to 6") for a given field of view in terms of constellations with little effort based on a beta version of my RoT model. If these lists are then applied to both reflector and refractor of the same aperture we may then find some conclusive results - always keeping in mind that there is an average error to consider covering differences in factors not specified in the model like seeing or quality of optics or acuity of the observer ...
Wilfried

#16 Asbytec

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Posted 05 January 2013 - 08:57 AM

You know, maybe Aberrator can show something. Need to re-install it and maybe give it a look. I wonder what Aberrator will show for 42 Ori and BU 87, the latter being fairly easy and well within Haas limits for 150mm (1.4" Haas and 1.9" actual.)

What's interesting is how refractors seem to be dominating in the realm of higher frequency resolution where aperture and CO should be dominating. It's understandable, though, in terms of peak intensity where they easily dominate. And it's "new" to me as a budding double star observer how unequal pairs act like planetary contrast. Shouldn't they act like hi res point sources? If not, why not?

Lately, been studying the very limits of resolution where the MTF curve exceeds "perfect" aperture. That's why I chose 7 Tau as a target, because it's reported 0.74" sep is just below 0.77" theoretical Dawes limit of a perfect aperture. And 7 Tau does indeed have a faint contrast difference between the touching discs. It actually looks like a textbook Dawes split (if one were to imagine one.) Conversely, 31 Tau at a reported 0.8" sep has a more distinct dark space (not black, best I can tell, in the way 52 Ori is black.) So, it does indeed seem to be exceeding it's theoretical resolution (preliminary) in the real world of aberrations, obstructions, and seeing.

Again, this makes sense because the Airy disc is smaller in an obstructed scope by a factor equal to the diffraction and obscuration effects. (At least close to the math approximations of those effects used to approximate 0.72" Dawes limit [1-(coD^2)]^2.) Observation with a CO seems to best Haas' estimate by half an arc second, but I doubt I can reach the "perfect aperture" at that 0.72" arc.

http://www.telescope...obstruction.htm

But, I'll be darned if 42 Ori (Edit: not seen) looked easy on paper and certainly was not under the skies. Still, two reports of a 150mm detecting the companion exceeds Haas by half arc second, again. A "perfect" refractor can do it, and a moderately obstructed scope did it with some difficulty. Still, what makes it so difficult?

So, is this due to some smooth gradient of decreasing brightness from the primary star of 6.5 mag or brighter? Seems to me, it should be more lumpy due to the rings, especially the first one. Anything outside that offending first ring, unequal or not, seems fair game. (Just a guess.)

Still, in the realm of the very small, it seems the CO only has a minor benefit on equal pairs. Hmmm...

#17 Asbytec

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Posted 05 January 2013 - 09:10 AM

...do slightly better near the Dawes limit for equal bright doubles up to +6mag because the spurious disk is smaller...


Well, someone is going to kick me, but there is one reference that will show the actual Airy disc itself, as I read it and contrary to everything we know, is smaller. This is due to the added diffraction effects of the CO to that of the clear aperture (in the Raleigh case.) In fact, at .3D, the first minimum is 1.11 Lambda/D, not 1.22 in an unobstructed aperture. This is what set me on a mental journey I have not returned from, yet.

"The reason is the effect unique to CO (at least in its extent), namely, the reduction in size of the Airy disc caused by it. The linear disc reduction is closely approximated by a factor (1-ο2) for obstructions of ~D/3 and smaller, and by a factor (1-ο2+ο4) for larger obstructions, up to ~0.7D. Good approximation for the 1st minima reduction ratio for any obstruction size is 1-οn, with n=2+[ο2/(1-ο)] or, alternately, n=a+(1/a), with a=1-ο. Apparently, the overall smaller diffraction pattern and brighter central disc give to the obstructed aperture an edge in contrast transfer efficiency with respect to spherical aberration error of near identical nominal energy loss from the Airy disc." (And then he goes on to explain that edge over the equivalent form of SA.)

In other words, it appears an obstructed aperture exceeds the maximum spacial frequency of an unobstructed one by a factor of the CO and a smaller and brighter "Airy disc."

Study this closely...
http://www.telescope...obstruction.htm

#18 Gord

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Posted 05 January 2013 - 01:24 PM

Fred, you asked for a little more info about my 10" newt.

It started life as just a common Synta 10" (Celestron in my case) F4.7. Spec wise they have a 63mm MA secondary for about a 25% CO by diameter. Fairly small compared to any SCT.

Mine has had many things changed, most importantly the primary (which was decent to start) was redone by Normand Fullum. His work is as good as the best out there. I won't quote numbers as they become an academic exercise after a point. But I will say that I can easily see the difference. There are a number of other improvements, some around contrast enhancements. The scope performs very well, although on planetary for example, the C14 does better.

On the 10" F5 having a 37% CO, that would seem unusual to me. Possible for sure, but not an ideal setup, and not necessary as you can see from mine.

I can see the difference in appearance of the various CO sizes. The stars *look* nicer as the CO gets smaller. At the point of the 25% in my newt, it's starting to look pretty good, and I can easily see the general rule of anything being below 20% being in-consequential.

The other thing to keep in mind is about comparing a closely as possible in terms of magnification and exit pupil. If there are differences, it does affect how things appear.

On a point John made, I think you need to qualify "better". I would agree that my 6" refractor delivered a better view in terms of aesthetics of the split (cleanness of the whole image), but it was much harder to see and seen less often. The C8's by comparison showed it more easily, more often, but with lots of fuzz, and at the best moments more noticeable first rings.

Which is "better"? Both are better in their own ways, but I guess it comes down to what you value more. I myself like having all of the above and enjoying them all at the same time is "best"! It just get's harder and harder to convince the CFO that this is "need" for additional options!! :lol:

Clear skies!

#19 7331Peg

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Posted 05 January 2013 - 02:21 PM

Gord, I didn't use the word "better" in my post. And I wasn't referring to the aesthetic aspect of the view, which I've always found to be much more satisfying in a refractor.

What I said was I never found it to be "easier" to split uneven doubles in the SCT's I owned as opposed to a refractor. And I do mean NEVER. I could generally manage a split with the SCT on the uneven pairs, but only with considerably higher magnification -- which is what I was referring to when I said I had never found it be easier with an SCT.

There could well be cases where it would be easier to see a secondary in an SCT vs. a refractor because of where the secondary falls in relation to a diffraction ring. I never ran across a case like that, though. I'm currently without an SCT and don't have plans on picking one up again in the near future, although an Edge model would be my preference if and when I do.


John :refractor:

#20 Gord

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Posted 05 January 2013 - 04:08 PM

Hi John,

Sorry, I didn't mean to put words in you mouth or imply you said something specific. I guess what I was getting at was the message I was taking away from your post, i.e. you found refractor images to be better.

There's no doubt I can see the effect of the CO. But in some cases it comes to light grasp too and that's where the SCT's have the advantage.

Actually one question about your previous use experience with the SCT's. Where did you store them? Were you having to deal with any kind of cool down other than relative drops through the night?

Clear skies,

#21 WRAK

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Posted 05 January 2013 - 06:11 PM

...the actual Airy disc itself ... is smaller ...

The slightly smaller size of the diffraction pattern due to CO alone seems to have no real impact - the additional reduction of the size of the spurious disk due to the loss of energy seems to have more effect I think.

Here the list I promised (all from Taurus with a required refractor aperture of 150mm +/-10% error range and for an assumed NEML of 4.5):
WDS ID Name RA Dec Sep M1 M2 Req. Ap
WDS04348+2242 STF562 4,5793 22,692 1,9 6,82 9,94 135
WDS03598+2848 A465 3,996 28,806 1,9 9,53 10,89 136
WDS04160+3002 STT78 4,2673 30,036 2 7,82 10,49 137
WDS05175+2250 HO334 5,2923 22,829 2,2 9,16 11,07 138
WDS04391+1024 A2036 4,6511 10,399 2,8 9,76 11,42 138
WDS03355+0625 A1933 3,5918 6,4106 1,2 8,91 9,99 140
WDS05055+1948 STT95 5,0922 19,807 0,9 7,02 7,56 140
WDS05305+2223 J590 5,5097 22,361 4,8 9,4 11,80 141
WDS04197+2750 LEI12 4,3281 27,83 1 8,84 9,47 141
WDS05416+1913 STF770 5,693 19,22 1,1 8,76 9,77 142
WDS04294+2433 LEI4 4,4901 24,551 1,3 7,55 9,77 145
WDS03463+2411 BU536 3,7711 24,19 1 8,13 9,39 146
WDS05340+2225 A2106 5,5668 22,424 1,5 9,7 10,80 146
WDS03578+2255 COU364 3,9631 22,924 2,6 8,99 11,39 146
WDS03520+0632 KUI15 3,8667 6,5349 0,8 6,26 6,56 152
WDS03374+2632 COU690 3,6233 26,539 4,2 9,7 12,00 152
WDS03362+2959 BU1040 3,6032 29,983 3,5 7,8 11,50 154
WDS03544+1601 BOV28 3,9066 16,017 0,9 7,2 8,60 154
WDS05293+2816 MLB519 5,4892 28,267 3,7 9,82 12,00 155
WDS05428+1806 CHR213 5,7131 18,097 1,7 9,5 11,20 156
WDS03576+1130 HU24 3,9599 11,497 1,4 8,65 10,66 159
WDS04594+2012 A2427 4,9905 20,194 4,3 8,49 11,98 159
WDS03552+0417 A2349 3,9208 4,2887 1,8 9,33 11,30 159
WDS03372+0121 A2419 3,6206 1,3494 0,8 8,73 8,93 160
WDS05476+2056 BU91 5,794 20,941 1,5 8,1 10,60 161
WDS05436+1300 A117 5,7267 12,996 0,8 8,81 9,24 163
WDS04458+2840 COU706 4,7639 28,661 5,9 7,1 12,00 164
WDS05013+2632 BU1238 5,0219 26,534 1,6 7,56 10,52 164
WDS03588+0230 HEI637 3,9796 2,5079 2 9,17 11,49 164
WDS04285+1059 HEI457 4,4757 10,988 1,4 9,13 10,95 165
With a 150mm refractor the split probability for the 135mm req. Ap. should be 85% and for the 165mm req. Ap about 15%.
Norme, may be you can check this list with your 150mm MCT as you are anyway interested in difficult pairs for your scope and may be somebody else with an 6" refractor.
Wilfried
PS: You have already covered from this list KUI15 (31 Tau) with a req. Ap of 152mm and outside of this list STF412 (7 Tau) with a req. Ap of 173 - it seems you have a tendency to overperform

#22 azure1961p

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Posted 05 January 2013 - 06:36 PM

Fred,

No this Ronchi wasn't misread, it was actually a good mirror. It just wasn't my cup o joe though. I must confess though I never really had it collimated much better than you can get out of the collimating aperture or peep that came with the scope . Perhaps that was the reason? I never liked the beast and sold it after a couple months. Funny I don't mind that same size CO in a sct though. Maybe lack of coma, I don't know. The eight was a revelation though.

Pete

#23 Asbytec

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Posted 05 January 2013 - 06:59 PM

...the actual Airy disc itself ... is smaller ...

The slightly smaller size of the diffraction pattern due to CO alone seems to have no real impact - the additional reduction of the size of the spurious disk due to the loss of energy seems to have more effect I think.

...outside of this list STF412 (7 Tau) with a req. Ap of 173 - it seems you have a tendency to overperform


I am starting to agree with you on the CO affects. The most improvement tends to be on a very small scale of the central disc, while the degradation occurs on the scale of the first ring. Maybe further out. So, the whole right side of the MTF is trapped in that small confine and creates so much discourse. To me, that's interesting - putting a face on the argument.

On over performing, I took that as a compliment. Thank you. I do struggle to report and interpret what I see. hopefully such interpretations are not violations of the laws of physics. For example, is the apparent limb shading of 7 Tau Airy discs considered a drop off in contrast? And does that constitute resolution? In my interpretation, it does.

Taurus is well placed, thank you for the list. I will observe a few tonight.

#24 Asbytec

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Posted 05 January 2013 - 09:14 PM

Forgive me posting profusely, again this is just fascinating for me. Also, a eureka moment occurred when one realizes the scale we're talking about. If you already know this, excuse the redundancy. But, it seems to explain why unequal doubles behave like planetary contrast - because they are on the same scale (1x to 4x Raleigh.)

The illustration below is only approximate. But it seems to show something close to 2x the Raleigh limit might be very hard. Further out to about 4x the Raleigh limit might be hard, depending on the CO and the light scattered in that distance (CO, aberration, or otherwise.)

It illustrates how a CO can improve very fine resolution very near the Airy disc itself. It suggests a brighter primary will flood that scale (2x to 4x Raleigh) with additional rings. If the magnitude difference is not sufficient, the companion can struggle to be seen. So, the smaller or cleaner you can make that range the "better."

Edit: Zeta Ori fits within that scale at 2.4" arc and at 2 delta M was an easy split.

Credit for the MTF goes to the link below, I added the diffraction pattern to give some sense of scale.
http://www.handprint...e3.html#centobs

Attached Files



#25 azure1961p

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Posted 05 January 2013 - 11:46 PM

Over perform...

I don't know what that entails, what's performing when its on the mark? He's for a well trained eye and seeing conditions that have t seen Connecticut since ... August?

Pete






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