Impression of the Takahashi Mewlon 210 for Astro-imaging
I begin this short impression write-up with some major caveats. This setup is not a typical imaging rig, and I do not encourage first-timers to do this and the targets are usually very tiny around 2 to 3 arc min size. Second, there is debate on the f-ratio vs aperture importance with regards to imaging, an issue which is beyond this article's scope. Third, the mechanics of the rig are extremely important to get it just so...the long focal length requires special care in set up. Fourth, there is again debate over whether the Ritchie-Chretien or Dall-Kirkham system is better etc...another area beyond this article.
It is important to note that the Mewlon is renowned as a VISUAL tour de force and not as an astrograph!
The Mewlon is a Dall-Kirkham design with a spherical secondary, in contrast to the RC. This supposedly makes it easier to make and easier to keep collimated - an area of telescope design I leave to the experts at Takahashi to decide, since they also make the RC based design scopes. The focal length of this particular model M210 is 2415mm (F11.5) native and a simple reducer/corrector brings it to 1961mm. It flattens the field somewhat and makes decent imaging possible. For a 45mm diagonal full-frame chip, this produces a 60 by 40 arc-min FOV.
Typical targets are 1/10 of the field, so the image circle of about 40mm works. Smaller size chips are probably more than adequate. Pricewise this is about the cost of a bare FSQ85 OTA, so aperture per $ is decent (for Takahashis anyway).
While I had used the M180C previously, it tends to produce uncertain results as the primary mirror shift makes precision focusing difficult. The Takahashi rack and pinion focuser is only suitable for the M210 and up, so I traded up, as I was encouraged by those initial attempts with my M180C. With this rack and pinion focuser, there is no movement of the primary, so the mechanics of focusing are surer and more secure. It even has a built-in camera angle adjuster so orientating the image is straightforward. Critical focusing with a Bahtinov mask is a must and the stars are pinpoint with the 4- or 8-point diffraction spikes seen on brighter stars.
There is some field distortion at the edges and severe vignetting on my full-frame chip, but these are expected and within the specifications. The target objects, as mentioned, are typically within the centre 50% of the FOV, so perfectly acceptable.
The long focal length is extremely demanding on the mechanics and attention to detail in the setup is critical, especially mobile setups. To get a star in the FOV requires it to be positioned almost exactly in the centre of the finderscope, which must be perfectly aligned to the main scope. Precise polar alignment is a given, as is a rigid imaging train. The tracking should ideally not have more than 4arc sec peak to peak for decently round stars and guiding must be smooth. Any guiding star lost to clouds for more than a few seconds may result in a trail. However, the trade-off is the spatial resolution is much increased in these small targets and the fine details begin to show up. If and when all goes well, the resulting image makes all that hard work an extremely satisfying effort!