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ACF central obstruction

SCT Meade
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#26 fred1871


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Posted 03 September 2021 - 07:26 AM

Difficult to "prefer" in a meaningful way an ACF if you've not looked through an Edge HD.


The problem remaining for ACF is the field curvature, and that gives increasingly enlarged (but round) images the further off-axis you go. So you have a distinctly smaller field that's diffraction limited, compared to the Edge system, though also better (wider) than the older models from C and M, which C also continues to make as a cheaper alternative.


Further up the page, there seems to be a notion that if a White Paper is used for sales, it must be a tissue of lies. Consider that it could be pretty accurate, it's even better for promoting sales. The Edge scopes have a price disadvantage. They need to do something better to justify that. They can if the quality of fabrication matches their competitors so that buyers get the benefit of the new design. From the examples I've seen that is the case. The buyer can then decide what they can afford, knowing the differences among the three (main) versions of popular SCT.

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#27 555aaa


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Posted 03 September 2021 - 09:27 AM

The OP asked about the central obstruction and here we are talking about a totally unrelated thing which is off axis performance. If the Meade is faster than the Celestron then it will have a larger central obstruction.

Edited by 555aaa, 03 September 2021 - 09:30 AM.

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#28 ihf



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Posted 03 September 2021 - 10:59 AM

The question is "Red or Green?" and not "Blue or Orange?". Well, first a few more words on the latter.


The central obstruction of the 8 inch Meade was given as 74-76mm in a previous discussion vs. the Celestron spec of 64mm and 68.5 mm measured. There are no 9.25 and 11 inch Meade, right? Clever marketing! Can't compare.


The price of the enhanced 8 inch OTAs are mostly a wash. The 8 inch LX65/85 Meade are a bit lighter with Vixen, while the LX200 comes with a Losmandy dovetail, mirror lock system. The 8" EdgeHD is the same weight/price as LX200. There is more differentiation starting from 9.25-10 inches. But things get unwieldy quickly. The whole attraction to me of the 6 and 8 inch size SCTs is that they are simple to handle (transport/mount).


Red for me - green only roasted.

#29 Upstate New Yorker

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Posted 03 September 2021 - 02:16 PM

Please pardon any diversion from the intent of the original poster, but something tells me that the differences between the two brands are likely to be small holding aperture constant.  But (and here's the diversion), I'm wondering if the Vixen VMC200L (or VMC260L) design that replaces the front corrector plate with lenses between the secondary and primary mirrors to correct distortions, isn't an interesting design change?  The tube is open, avoiding dewing on the corrector plate and easing temperature adjustments; and the weight of the OTA is reduced.  Moreover, the Vixen design tends to be lighter because of a lighter OTA housing.  In other words, I'm wondering whether more design innovations to the SCT are not possible.  Just asking.

#30 fred1871


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Posted 06 September 2021 - 09:04 AM

The VMC 260L (which differs optically from both the VMC 200L and the VC 200L) is does not ease temperature adjustments. I had one for a number of years (bought new) and it was a monster to get to ambient temperature and stay there. You're in Floida, so it might be fine there with gradual temperature gradients. Where I am it's not like that, and the combination of what Vixen called "the chimney effect" along with a very long secondary mirror/lenses in a metal housing retained heat and tube currents like no other telescope I've encountered in my 50-plus years of observing. The secondary obstruction was typical in diameter for SCT/Mak designs, but hugely longer down the tube - from memory, about 4-inches.


I eventually gave up and sold it. I now have a Mewlon 210, which has its thermal issues, but nowhere near the Vixen level, even allowing for the smaller size. I also had the impression that the Vixen optics were nothing special, when I could get it thermally stable. I had better images from my average quality C9.25 XLT, and now from the Mewlon.


The Vixen VMC 260L appears to be a variant of a Klevtsov design. It's a Cassegrain, obviously, but no Schmidt plate so we're on a different design expedition.

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