Regulus! The Takahashi Mewlon 210 Telescope

The Takahashi Mewlon 210 Telescope

John A. Blackwell
johnb@regulusastro.com

Introduction:

For years amateur astronomers have flocked to the venerable 8" SCT telescope. Many purchased due to the maximization of portability versus utility. It offers fine deep sky and planetary views and is also usually quite compact allowing for ease of transport and setup. For years amateurs have also suffered all the issues that come along with SCTs, especially those by the mass manufacturers that we all know and maybe even love. I have certainly had my share of telescopes. Many were made by these mass manufacturers. Some were superb, some were extremely good after some touch up work, and other were just not worth the money paid out for them. Now let us take a look at an often overlooked member of the telescope menagerie, the Takahashi Mewlon 210.

The Takahashi Mewlon 210 is not a Schmidt-Cassegrain. It is a Dall-Kirkham design which sports a prolate ellipsoid primary and a spherical secondary mirror. It is a Cassegrain focus. See ATM-Page Dall-Kirkham Information. This design makes for mirrors that are easier to grind, but it results in fields that are usually more curved than that of a SCT and showing more coma. At least, this is what the various books on telescope optics have to say about it. This scope is an f/11 with 210mm of useful aperture. The mirror is really a 220mm diameter but is masked, presumably to reduce aberrations. It would thus be good for planetary and Lunar studies. With an optional focal reducer, one could use it for deep sky visual and CCD imaging projects (my goal).

Arrival

The unit arrived after two days shipping. I had ordered it from Anacortes Telescope and Wild Bird and was given fine assistance (as always) from Ray York about the scope selection and the various options. I purchased the Mewlon 210 OTA along with an adapter for attaching it to a Losmandy G-11 DUP and the focal reducer. The scope came with the reducer and adapter plate all tripple boxed and in fine condition. The hex-bolts provided with the DUP were too short, but a quick trip to Home Depot fixed that problem in no time at all. I was anxious to obtain first light, but no. What happens whenever fine optics are purchased and delivered? Rain. Of course. It was a week later before first light.

The Mewlon 210 is painted Takahashi off-white with the familiar Takahashi green for the mirror cell and hardware. The visual back and 1.25" adapter are black anodized aluminum with matte black interior. The visual back accepts standard 2" equipment, while the 1.25" adapter is the Takahashi compression ring device which accepts all standard 1.25" equipment without marring the finish with set screws. The dust cover is a leatherette material which fits nicely over the tube's front end. One will want to keep this on whenever not observing, since this is an open-tube design. Looking down the tube will reveal the 4-vane spider holding the secondary mirror. The interior is a rough matte black as is the spider and all other exposed hardware. I can see that I will not have to go in with a paint brush to touch up any missed spots of shiny metal. Through the center of the primary mirror is a long light baffle to enhance contrast. The secondary mirror has a shorter light baffle as well. Along the bottom of the outside of the tube is the dovetail wedge to which the Takahashi mount clamp attaches. There is some slide travel available for fine balancing. Most of the weight is in the rear with the primary mirror. One of the nice features of the Mewlon is the attached 7x50 finder and bracket. Their finders are the best I have ever used. The finder bracket is so firmly attached that Takahashi actually tells the user to hoist the scope around by it! Now, that is a finder!

Now, for some images:

This is an image of the Takahashi scopes mounted side-by-side in the dome. The smaller scope is the FSQ-106n and the larger is the Mewlon-210. Both have 7x50 finders attached. They both fit nicely on the dual saddle plate of the G-11 mount, and they do not overburden the mount by their weight.
The Takahashi side by side: Mewlon-210 and FSQ-106n.

The Takahashi side by side: Mewlon-210 and FSQ-106n.

Here is a look down the tube at the primary mirror. The primary mirror is really 220mm in diameter but only 210mm is clear aperture. The edge is masked off to reduce the coma common in this scope design. Behind the name-plate are three collimation screws for the secodary mirror. The name plate just unscrews from the housing.
Looking down the tube at the primary mirror.

A closer look down the tube shows the primary mirror light baffle. This adds to the spectacular contrast that this scope delivers. Note that the interior of the baffle is also baffled with ridges to prevent internal reflections. To the right of this image is the secondary mirror and its light baffle.
Detail of the primary light baffle.

Here is the business end of the telescope. A standard Takahashi visual back is brought to a standard 2" holder. Inserted into that is a 1.25" compression ring eyepiece holder. The zenith prism and eyepiece are not provided.
The business end of the Mewlon-210.

Here is a closeup image of the famed heavy-duty finder. This is a 7x50mm finder with superb optics. The finderscope is rigidly attached to the telescope tube. The manual actually states that one should life the tube by the finder! Yes - it is true, and it works.
The heavy-duty finder.

First Light:

First light took place moments after a weak cold front passed through, leaving us with damp and humid conditions, but clear skies and still winds. The thermal conditions were poor. I took a good look at the collimation, and it seemed very very well aligned visually. I then took two CCD images of Arcturus, each a 0.5 second integration with an SBIG ST-7 at prime focus. The results are below.

Inside focus Outside focus

The star seems to show a bit of misalignment with the optics, but remember that the CCD chip itself is not centered on the optical axis. Still, I think that there is a bit of collimating to do. The optics are very very good. There is no problem there. All the stars in M-13 were resolved to shimmering diamonds sparkling on a velvety black background. No problems there at all. A 40mm Plossl makes a fine wide field view, while I was using a 10mm and 3mm Radian on the Moon without any difficulty. The Moon showed marvelous contrast and no ghosting whatsoever. Of course, there is no false color to be seen here. Having seen the Moon so well visually, I decided to take the following CCD image series to show the capabilities of these optics:

This is Rupes Recta and the surrounding area. My best image of this taken with a C-8 can be seen Here. There is no comparison:

This is the area of Aristoteles, Eudoxus, and Mare Frigoris:

This is the Eratosthenes and Montes Apenninus region:

Other Thoughts and Conclusions:

I spent quite some time observing deep sky objects as well as the Moon. No planets are currently visible from my location, so I couldn't test on them. Deep sky objects are readily enjoyable with this telescope, but imaging might be a challenge. The focal length should be reduced to optimize the field of view to pixel size of CCDs, and the field would be too narrow as well without reduction. A reducer is available from Takahashi, but it gets one down to f/9.3, which could be better. The Optec MAXField reducer will get one down to f/3.3, but with the caveate that the backfocus will not support a filter wheel. So, given those choices, one has to stay monochrome or find/build a thin filter wheel. Needless to say, there is not enough room for a flip wheel device.

Many will worry about the focus mechanism: It uses the same method as the ubiquitous commercially available SCTs. In other words, with the Mewlon 210, the primary mirror is moved to reach focus. Is there image shift? Yes, but only a slight amount. There is so little shift, that objects did not fall off of an ST-7's CCD when drastic focus moves were made in both directions. It is a great improvement over the other telescopes which use the same method. Those looking to buy a larger diameter Mewlon should note that all the larger models (250+) focus by moving the secondary mirror via a small electric motor with a hand control. This is nice, especially for imagers, but there is no manual focus mode available when the batteries run dry.

Final thoughts? The Mewlon 210 is an extraordinary telescope. I would recommend it to anyone, especially visual observers. It has enough light gathering ability to keep someone busy for a lifetime. The optics are crisp and clean. The telescope is light and easily carried. Maintenance might be higher with an exposed mirror. If you are an imaging nut, then be prepared to jump through some hoops and do some research to get the correct equipment. Enjoy! Telescopes are great fun!

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