TheSkySearchers

Hi All. I've been extending and updating my list of OTA options (the non-updated list is at viewtopic.php?f=71&t=9584 ) for visual astronomy, and have begun to seriously consider a Takahashi Mewlon 210 ($3k) despite its smaller aperture compared with all the others in the list. This follows my gradual evolution toward splitting my "dream home observatory" project into two stages: stage 1 being to acquire a telescope & modern goto mount that is appreciably superior to the best I already have (a 6" SCT on a manual Twilight 1 mount), but still small and light enough to move it from place to place and to setup/tear-down with minimal time and effort. Step 2 (in a few years) would then be to build a fixed-location small home observatory and put a larger-aperture (12-14") telescope in it.

Anyway, if you were seeking a general-purpose, portable, high-quality visual-only instrument for planets and DSOs, would you rather own a Takahashi Mewlon 210, a Vixen VMC260L, a Celestron 9.25 Inch Edge HD, or ?? In terms of weight, they are almost interchangeable (18-22 lbs). The above OTA's are also priced roughly comparably (~ $2.4k to $3.8k). So let's exclude price and weight from consideration, at least for those three. The Mewlon 210 is a non-corrected Dall-Kirkham (unlike its far more expensive big brothers, the 250CRS and 300CRS) so it will have some coma and field curvature, yet many people say its optics are splendid to look through and the coma and FC are tolerable. The Vixen VMC260L has a bigger aperture and gets mostly good reviews, but not nearly as uniformly-enthusiastic reviews as the Mewlon. Celestron's 9.25 EdgeHD, which should perform very nicely in theory, will (at-best) be much more prone to dew than either of the others; dew is a rather big deal where I live (e.g., my eyeglasses can fog over when I go outside), but would applying a dew heater really be that bothersome?

Ideally, the OTA should be lightweight, exhibit superb error-free optics, be mechanically excellent, offer high contrast, require minimal or no cool-down, stay collimated and be easy to keep collimated, never dew-over, provide a large aperture, and be inexpensive. Heh! Decisions, decisions.

In the other thread you've linked you identify yourself as preferring an aplanat. 'm in the process of evaluating a VMC200L in comparison with an Edge 9.25 to decide what larger Cat to acquire to complete my ensemble. Preview is that it is close and not final but I think there might be an edge to the Edge.

So I'll focus on the theoretical merits of the Takahashi Mewlon 210 taking into account the excellent reviews it gets.

The Tak μ-210 is advertised and specified as a Dall Kirkham unmodified and uncorrected. If you compare spot diagrams as calculated in Rutten and Venrooij one gets a mixed view of the situation.An unmodified Dall Kirkham should have infertior spot diagrams compared with a Classical Cassegrain of comparable f ratio. This holds true whether the magnification by the secondary is M = 2 or M = 4.

We know that the primary of the Tak μ-210 is f=2.9 with an overall f ratio of 11.5 we get M = 3.96. So the encircled light should be better in a classical Cassegrain. The CC has a better sharper spot diagram than the Tak μ-210.

(I'm getting my numbers from the Tak Europe site: https://www.takahashi-europe.com/en/mew ... ations.php)

However the Tak μ-210 has an advantage in that the radius of the curvature of field at M = 4 is 50% larger than the classical Cassegrain. So if you are sensitive to curvature of field issues the Tak μ-210 might win.

I strongly suspect that the tremendous reviews the Tak μ-210 gets might be a result of Takahashi ever so slightly fiddling with the design. Are there higher order deviations from conic sections? Does the secondary deviate slightly from a sphere? Does the primary deviate slightly from a prolate ellipsoid?

I wouldn't ignore the possibility that Takahashi could incorporate higher order than quadratic (conic section) curves into its designs without advertising that fact. We know that Vixen can do that. Vixen's VISAC is an acronym for "Vixen's Six-Order Aspherical Cassegrain".

I don't have an answer. Just some thoughts.

I would buy the Edge and what you see in the image for trouble free dew control. The Edge is the least expensive, but may give the same quality views. The money saved can be used for some high quality eyepieces. Right click the image.
http://www.tech2000astronomy.com/zap/

I can't comment on other designs. But Edge 9.25" is a very nice scope.

For visual aperture is the king. If you have a choice between several high quality instruments, go with largest aperture you can handle.

I had 11" SCT briefly, but it was too large and heavy for my taste. 9.25" is the largest I can handle comfortably.

Bigzmey wrote: ↑Mon Jul 13, 2020 6:04 pm ...For visual aperture is the king.

Hmm. Interestingly, the Vixen VMC260L boasts the largest aperture area/weight ratio among any of the OTAs I've considered:
(and yes, sorry, I know the Mewlon 210 is actually a non-corrected Dall-Kirkham).

AntennaGuy wrote: ↑Tue Jul 14, 2020 10:59 pm

Bigzmey wrote: ↑Mon Jul 13, 2020 6:04 pm ...For visual aperture is the king.

Hmm. Interestingly, the Vixen VMC260L boasts the largest aperture area/weight ratio among any of the OTAs I've considered:
(and yes, sorry, I know the Mewlon 210 is actually a non-corrected Dall-Kirkham).

I don't think that anyone was suggesting that you didn't know that the μ-210 was advertised as a non-corrected Dall-Kirkham. What I was suggesting is that one explanation for the great reviews it gets is that Takahashi is tweaking the mirror surfaces slightly with a higher order curve. That's the only way it could be better than a classical Cassegrain of equal aperture and secondary magnification factor.

It's certainly understood to have no refractive corrector elements. But the Dall-Kirkham design has only quadratic (conic section) curves and if there is a higher order curve on a mirror surface then there is a semantic quibble about whether it is still uncorrected.

notFritzArgelander wrote: ↑Tue Jul 14, 2020 11:20 pm

AntennaGuy wrote: ↑Tue Jul 14, 2020 10:59 pm

Bigzmey wrote: ↑Mon Jul 13, 2020 6:04 pm ...For visual aperture is the king.

Hmm. Interestingly, the Vixen VMC260L boasts the largest aperture area/weight ratio among any of the OTAs I've considered:
(and yes, sorry, I know the Mewlon 210 is actually a non-corrected Dall-Kirkham).

I don't think that anyone was suggesting that you didn't know that the μ-210 was advertised as a non-corrected Dall-Kirkham. What I was suggesting is that one explanation for the great reviews it gets is that Takahashi is tweaking the mirror surfaces slightly with a higher order curve. That's the only way it could be better than a classical Cassegrain of equal aperture and secondary magnification factor.

It's certainly understood to have no refractive corrector elements. But the Dall-Kirkham design has only quadratic (conic section) curves and if there is a higher order curve on a mirror surface then there is a semantic quibble about whether it is still uncorrected.

Oops, my comment here was evidently misleading! I was actually referring to my own error in the table that I just posted! I was not complaining or referring to your remarks (which, by the way, have been very educational)

AntennaGuy wrote: ↑Tue Jul 14, 2020 11:27 pm

notFritzArgelander wrote: ↑Tue Jul 14, 2020 11:20 pm

AntennaGuy wrote: ↑Tue Jul 14, 2020 10:59 pm
Hmm. Interestingly, the Vixen VMC260L boasts the largest aperture area/weight ratio among any of the OTAs I've considered:
(and yes, sorry, I know the Mewlon 210 is actually a non-corrected Dall-Kirkham).

I don't think that anyone was suggesting that you didn't know that the μ-210 was advertised as a non-corrected Dall-Kirkham. What I was suggesting is that one explanation for the great reviews it gets is that Takahashi is tweaking the mirror surfaces slightly with a higher order curve. That's the only way it could be better than a classical Cassegrain of equal aperture and secondary magnification factor.

It's certainly understood to have no refractive corrector elements. But the Dall-Kirkham design has only quadratic (conic section) curves and if there is a higher order curve on a mirror surface then there is a semantic quibble about whether it is still uncorrected.

Oops, my comment here was evidently misleading! I was actually referring to my own error in the table that I just posted! I was not complaining or referring to your remarks (which, by the way, have been very educational)

Ah, it's a relief to know I've not offended. Thanks.