Takahashi CCA-250 Tri-Focal Astrograph

The CCA-250 is Takahashi’s first corrected Cassegrain Astrograph. It was developed as a replacement for the BRC-250 corrected Ritchey. This instrument was developed to take advantage of the digital age and provide image circles to cover large format imaging chips. The large ø88mm can cover these large chip with a flat field filled with pin point stars.

This astrograph offers versatility. It is a tri-focal instrument that can be used at its native f/5, at f/3.6 with the small and large 645 reducer or at f/7.5.  It will support wide field to narrow field imaging.

The design of this instrument includes a carbon fiber tube and a temperature compensated holder to maintain focus during long exposures.

During long exposures temperatures can change and cause the optics to change focus. The CCA-250 is equipped with three computer controlled cooling fans that are operated by three sensors: one on the primary, a second in the tube and a third in the air to measure ambient temperature to help prevent image shift working in concert with the carbon fiber tube and temperature compensated holder. This cooling system can also be operated manually.

The manual controls and computer interface are conveniently located in a control box attached to the rear of the mirror cell.

The control box also includes an input for the supplied 12v power source and a USB input for computer control of the focusing system.

The CCA-250 features a computer controlled secondary ASCOM compliant Active Focus system. This enables computer control by programs such as FocusMax or Maxim DL.

Takahashi’s original secondary focusing system was included with the  Mewlon-250 and 300 beginning in 1989, this latest refinement uses a pulse motor which allows for the ASCOM compliant focus programs to operate the Active Focus system.

The CCA-250 employs a corrected cassegrain optical system with a large three element corrector to produce a flat ø88mm circle with Stars 6 microns or less out to 50mm and 10 microns or less out to 88mm.

Balancing is very important to insure the best possible tracking. The CCa is supplied with a heavy duty 18” long dove tail which is an integral part of the tube assembly and a wide 15” bridge plate to allow attaching a variety of guide scope if needed. An extra heavy duty saddle plate is also supplied to couple with Takahashi mounts.

The CCA-250 is a very versatile optical system. It can be used at three different focal lengths 910mm [f/3.6] with the small or large 645 Reducer QE  which produces a 60mm flat circle to support large format cameras, 1250mm [ f/5] prime focus and 1880mm [f/7.5] with Extender CR 1.5x.

BACK FOCUS RANGE (28Kb pdf)

CR 0.73x Small Reducer
   
Focal length            910mm
Focal ratio                f/3.6
Image circle            ø38mm

CR 1.5x Extender

Focal length            1880mm
Focal ratio                f/7.5
Image circle            ø40mm

CCA-250

Mechanical　Specs

Diameter of main tube:             306mm

Diameter of dew shield:            321mm

Length of main tube assy:          855mm

Weight of main tube:               Approx: 20kgs

Material of tube:             Carbon fiber    

Electric focus secondary computer controlled     

Optical Specs

Aperture:                         250mm

Central Obstruction:              58%

Focal length:                     1247mm

Focal ratio:                      f/5.0

Optical performance:              2 microns RMS-spot radius on axis

                    :             3 microns RMS-spot radius 10mm off-axis

                    :             3 microns RMS-spot radius 20mm off-axis

                    :             4 microns RMS-spot radius 30mm off-axis

                    :             6 microns RMS-spot radius 40mm off-axis

Image circle:                     88mm diameter (60% illuminated)

Image angular:                    4 degrees diameter

Back focus/metal back              155.4mm

                              193.9mm with CAA removed                           

Primary Mirror

Diameter:                         260mm

Aperture:                         250mm

Focal ratio:                      f/2.75

Shape:                            Parabola

Material:                         Pyrex

Coating:                          Enhanced multi-layer aluminum

Secondary Mirror

Diameter:                         125mm

Shape:                            Sphere

Material:                         Pyrex

Coating:                          Enhanced multi-layer aluminum

Conversion Lens Group

Diameter:                         80mm

Number of lenses:                 3

Coating:                          Broadband multi-layer anti-reflection coatings

BRC-250P Conversion Lens Group

645 Reducer 0.72X

Focal length:                       898mm

Focal ratio:                        f/3.6

Optical performance:                4 microns RMS-spot Radius On axis

                    :               4 microns RMS-spot Radius 10mm off-axis

                    :               5 microns RMS-spot Radius 20mm off-axis

                    :               6 microns RMS-spot Radius 30mm off-axis

Image circle:                       60mm diameter (60% illuminated)

Image angular:                      3.8 degrees diameter

Reducer-QE 0.73X

Focal length:                       910mm

Focal ratio:                        f/3.6

Optical performance:                4 microns RMS-spot radius on axis

                     :              7 microns RMS-spot Radius 10mm off-axis

                     :              10 microns RMS-spot Radius 20mm off-axis

Image circle:                       40mm diameter (60% illuminated)

Image angular:                      2.5 degrees diameter

Extender-ED 1.5X

Focal length:                       1885mm

Focal ratio:                        f/7.5

Optical performance:                4 microns RMS-spot radius on axis

                    :               6 microns RMS-spot radius 10mm off-axis

                    :               20 microns RMS-spot radius 20mm off-axis

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