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Takahashi - Mewlon 250CRS Dall-Kirkham Optical Tube Assembly


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Manufactured by: Takahashi

Takahashi Mewlon 250CRS Dall-Kirkham OTA


Note: The picture shows the Mewlon 250CRS with a mount and tripod, but this package includes the Optical Tube Assembly only.


Takahashi Mewlon 250CRS Dall-Kirkham Optical Tube Assembly



When Takahashi developed the Mewlon Correctors it meant any Mewlon-250 or 300 ever made could be converted into a flat field corrected telescope. This 3-element ED corrector allowed the Mewlon to be used as an imaging instrument or used as a corrected visual instrument capable of producing flat-field corrected views of the Moon, planets and deep space objects. The 250/300 Mewlons use an electric focus controller that moves the secondary mirror. Takahshi developed the secondary focusing system in 1990 and wanted to improve them with computer control.

The CRS Mewlons are the product of this development.

Additionally, to make imaging easier a 3-fan cooling system is included. This system uses three sensors: one on the mirror, a second in the tube and a third outside to monitor and control the low vibration fans to bring the primary to ambient temperature.

The cooling system is controlled manually or automatically with the hand control. When the sensors read a difference in the temperature of the primary and ambient temperature the fans turn on and cool until primary is brought to ambient.

The provided hand control is used for manual focusing and cooling and as a connector for the computer controlled focusing program.

The USB port is used for the connection to the computer for auto focusing.

The auto focusing system is called Active Focus. Active Focus includes a backlash revision function.

The M-250CRS uses a four vane spider to support the pulse motor focusing system. It can be controlled with FocusMax, Maxim DL or similar software.


The M-250CRS is an outstanding medium sized flat field imaging and visual instrument. It weighs 22lbs so it is portable and does not require a large mount to operate, so it can be easily transported to remote sites.

A dedicated f/7.3 reducer and 1.5x extender are available for added flexibility.

A 7x50 illuminated reticle finder and tube holder are standard.








Product Series

The Mewlon series of Dall-Kirkham Cassegrain telescopes from Takahashi offer the experienced observer a level of performance and portability not found in other Cassegrain telescopes mass produced for amateurs. Classical Cassegrain telescopes offer excellent performance but they are extremely expensive to produce at large apertures. By concentrating on the Dall-Kirkham Cassegrains, the Mewlon offers a professional level of performance within reach of most amateurs.

The Mewlons use an ellipsoidal figure on the primary mirror and a spherical figure on the secondary. By focusing on very tight tolerances for these surfaces, Takahashi is able to deliver a compact telescope with reasonable aperture and high resolution. Where fast F/ratios are not required, the F/12 Mewlons provide excellent contrast by utilizing a smaller secondary mirror than comparable Cassegrain designs. Secondary obstruction as a percentage of diameter is 29-31% on the Mewlons. Classical Cassegrain telescopes usually have secondary obstruction of 32% or greater. Commercial Schmidt-Cassegrains have secondary obstructions of approximately 38% for F/10 systems. Ritchey-Chretien Cassegrains have even more secondary obstruction making them less suitable for visual, high contrast applications.

Takahashi uses extensive knife-edge baffling to minimize stray reflections as well as a specially designed tube that also acts as a light baffle. The result is an instrument that rivals an excellent long-focus Newtonian or refractor for contrast and sharpness. Some opticians have criticized the Dall-Kirkham design as having unacceptable coma. With most eyepieces (including Panoptics & Naglers) the coma is negligible and well outside the field of view. Stars are much smaller and sharper than in commercial Schmidt-Cassegrains. Coma may be a problem for wide-field astro-imaging, but these instruments were not designed for such tasks. With Takahashi's field flattener/reducer, these instruments offer superb off-axis images at F/9.

The 250mm and 300mm models have electronic focusing accomplished by moving the secondary mirror. This eliminates the image-shift problem inherent to commercial Schmidt-Cassegrains with moving primary mirrors. Because of the loose tolerances required in moving large and heavy mirrors on a baffle tube, most SCT's will not maintain perfect collimation. Image quality is then compromised in such optical systems. The large Mewlons also have removable covers for their primary mirror cells, this aides in rapid mirror cooling so that the observer can take advantage of favorable seeing conditions more quickly. These features make the large Mewlons ideal for high-resolution CCD imagery.

With the Mewlon series, one doesn't have to sacrifice optical performance for high portability. The 8.3" Mewlon 210 weighs just 18lbs (8.2kg) with a 7x50 finder attached. The Mewlon 250 weighs only 28lbs (12.7kg) and is remarkably compact for a 10" Cassegrain. Both of these instruments are highly portable and offer deep sky views that are exceptional. On the planets, many observers have reported seeing details they thought impossible with telescopes of this aperture. With the exception of a few diffraction spikes around bright stars, on might believe they were observing with a large apochromatic refractor. The advantage, though, is a greater amount of light grasp and resolution than a comparably priced refractor. Indeed, the Mewlons offer an exceptional value in their aperture class.

So if you desire a professional grade instrument in a compact, lightweight package or are tired of compromising light grasp for crisp detail and contrast, check out the Mewlon Cassegrains by Takahashi. You will be pleasantly surprised by their performance. For further analysis of the Takahashi Mewlon Series of Dall-Kirkham Cassegrains, please read Jean Dragesco's review in his book High Resolution Astrophotography, Cambridge University Press, 1995.

Additional Information

Name Takahashi - Mewlon 250CRS Dall-Kirkham Optical Tube Assembly
Manufacturer Takahashi
Model TLK2502
Manufacturer Part No TLK2502
Telescope Aperture 8.1" - 10"
Aperture ø250mm
Finderscope 7 x 50 Optical Finder
Focal Length 2500mm
Focal Ratio f/10
Focuser Driven Secondary with Active Focus
Highest Useful Magnification 600x
Image circle 40mm
Limiting Stellar Magnitude 13.8
Optical Design Corrected Dall-Kirkham
Optical tube diameter ø280mm
Optical Tube Length ø21mm/ø42mm
Optical Tube Weight 26.4 Ibs