Takahashi CCA-250 corrected Cassegrain astrograph review

Cassegrain telescopes come in a rangeof designs – beyond the classic, the variants include the Schmidt-Cassegrain, Maksutov-Cassegrain, Ritchey-Chrétien and Dall-Kirkham.

But what these designs all have in common is that their internal reflecting surfaces direct light out of the rear of the telescope through a hole in the primary mirror.

Primary mirrors like this are known as Cassegrains after their 17th-century inventor, Laurent Cassegrain.

This design produces a folded light path that results in a long focal length contained within a short tube length.

The Takahashi CCA-250 is based on the classic Cassegrain, although the CCA in its name stands for ‘corrected Cassegrain astrograph’, which means there’s a correcting lens in the final stage of the light path.

Describing the telescope as an astrograph indicates that it is designed with imaging in mind.

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This unusual and substantial telescope is beautifully built in typical Takahashi fashion.

The main tube assembly is manufactured from carbon fibre with a contrasting green-grey aluminium back plate anda front ring that supports the rolled-lip aluminium dewshield.

The interior of the telescope has an exceptionally good matte black, non-reflective finish.

A silver-finished top bar for support and a matching base support with a built-in Losmandy-style dovetail bar complete the ensemble.

Turning to the rear of the telescope, there is no external focuser as an electric motor moves the secondary mirror towards or away from the primary mirror to achieve focus.

There is, however, a rotatable connection port in the centre of the back-plate for mounting accessories, such as imaging cameras and star diagonals, using a wide range of optional adaptors.

This connection port can be spun through 360º.

Our review telescope came already set up with the correct spacing adaptors needed to attach a DSLR, but a simple 1.25-inch eyepiece adaptor was also supplied.

Completing the back plate are three miniature cooling fans and a small control module that houses two buttons for focus adjustment, a single button to turn on the fan, a USB connector and a 12V input socket, which powers the focuser and fans from an external power supply.

After a one-hour cooling down interval withthe automatic cooling system in operation, a startest confirmed that the CCA-250 was perfectly collimated.

When we powered it up the focusing system completed an alignment cycle, but we hadto gently pull a small shroud surrounding the secondary mirror into place before the focus system would operate using the push buttons.

After this, focusing was quick and smooth although we would have preferred a small, wired hand controller to adjust focus rather than having to make adjustments by pressing the two buttons on the rear of the telescope.

The Takahashi CCA-250 ships with an ASCOM driver for full auto focus, but this was not compatible with our version of MaxIm DL sowe were unable to test it.

Eyes on the skies

Although designed for astro imaging, we wanted to try the CCA-250 as an observing tool and it doesn’t disappoint when used in this capacity.

An early morning opening in the clouds gave us the opportunity for some fine observing.

The telescope showed the Orion Nebula’s Trapezium region beautifully in our 5mm eyepiece and a huge swathe of nebulosity in our 24mm eyepiece.

Open clusters M34, M36, M37 and M38 were stunning; turningto Jupiter, the seeing wasn’t good enough to handle the 250x magnification of our 5mm eyepiece, butthe planet was a fine sight in our 8mm one, especially so with an unplanned straight-line alignment of the Galilean moons.

But, this scope is an astrograph so we happily replaced our eyepieces with our QSI 683WSGCCD camera.

We captured images of theGreat Globular Cluster in Hercules in LRGBand the Dumbbell Nebula in narrowband using hydrogen-alpha and Oxygen III filters, to assess colour correction and field flatness.

Althoughsky conditions were not ideal during the review period, we were delighted with the star shapesand colour that we captured.

The CCA-250 is aimed at advanced deep-sky imagers with a permanent observatory and stargazers in this bracket won’t be disappointed with this telescope.

A flat lot of good

Whereas the ever popular Schmidt- and Maksutov-Cassegrain telescopes have a correcting lens element at the front of the telescope which also holds the secondary mirror, Ritchey-Chrétien, Dall-Kirkham and classic Cassegrains like the CCA-250 are open at the front and have a spider vane that supports the secondary mirror.

On the CCA-250, light enters from the front of the telescope and passes down the tube where it is reflected back by the 10-inch wide, f/2.75 parabolic primary mirror.

This light is then reflected back down the tube oncemore by the 5-inch spherical secondary mirror, where it is corrected by an 3-inch, three-element correcting lens.

This lens is designed to produce a very flat field of focus, which is important if you want to avoid the distorted stars at the edges ofthe field of view – a common feature of reflecting telescopes.

In our tests, star shapes were excellent right to the edge of the field of our Kodak KAF-8300’s 22.5mm diagonal sensor with no false colour around bright stars.

Computerised cooling

There are three computer-controlled fans on the rear plate.

Temperature is monitored by three sensors: one monitoring the ambient air, another the primary mirror and the last the tube.

These work in unison with the fans to maintain a stable overall temperature.

Electronic focusing

The unusual focusing system uses a micro-stepping motor to move the secondary mirror.

Focus can be adjusted using two push buttons on the rear panel of the telescope, or by clicking on the relevant direction button in software using the supplied ASCOM driver.

Achieving accurate focus manually was also simple using this system.

Dew Shield

The permanently attached, 260mm-long aluminium dew shield does an excellent job of protecting the secondary mirror from dewing over and helps to keep stray light at bay.

Like the carbon fibre tube itself, the interior is finished in a very good matte black to further counter stray light and increase contrast.

Finderscope

The CCA-250 is supplied with a 7x50 illuminated reticule finderscope that can be attached to one of two locations on the rear of the telescope with a single large bolt.

This attachment allows you to choose a wide range of positions to suit your requirements.

Making adjustments to align the finder with the telescope is also simple.

This review originally appeared in the December 2013 issue of BBC Sky at Night Magazine.