645RD QE0.72x reducer (FSQ-106EDX4)

The focal reducer for medium-format imaging with the FSQ-106EDX4 refractor

The 645RD QE0.72x focal reducer is a high-performance reducer that further enhances the attractiveness of the FSQ-106EDX4 refractor, an astrograph already highly acclaimed by astrophotographers.

It consists of four large-diameter lenses, divided into four groups. The resulting focal length is 380 mm (F/3.6) and the corrected image circle is 60 mm in diameter. The off-axis illumination has been improved. The amount of light at the periphery of the field is more than 70%. The amount of light over the 44mm covering the diagonal of a full-format sensor is more than 90%, which is about 50% more than the QE0.73X reducer. Vignetting is almost imperceptible on the cooled CCD and DSLR cameras available on the market today. Finally, the stars are punctual up to the edge of the field.

The QE0.73X reducer showed a slight violet chromatic aberration at wavelengths around 420 nm; this has been almost completely corrected. The diffraction spot was reduced to about 2/3 of the spot size at an image circle diameter of about 44 mm. To increase light transmission, the lenses are coated with a broad-spectrum multilayer coating, which reduces reflections to less than half that of a conventional multilayer coating centred on wavelengths close to 500 nm. This reduces ghost images and increases transmission over the entire visible light range.

The mounting of the reducer unit on the telescope is special. Indeed, unlike other accessories, it is not screwed to the focus. The main body of the reducer is screwed directly to the rear end of the G4 lens, inside the optical tube of the FSQ-106EDX4, in order to reduce the load on the eyepiece holder; the set of adaptor rings on the camera body or camera therefore includes the reducer, which remains fixed. On the classic ED version, the Camera Angle Adjuster (TKP86200, sold separately) can be added to the mount, replacing the TKA36582 extension ring, to facilitate image framing.

With this 645 QE0.72X reducer, the FSQ-106EDX4 now has an image circle for medium format imaging, perfectly corrected for aberrations, at two different focal lengths: 530 mm (F/5) and 380 mm (F/3.6).

Compatibility: FSQ-106ED, FSQ-106EDX4

Spot diagrams

Spot-diagram of the FSQ-106EDX4 refractor with 645RD QE0.72x focal reducer

Spot-diagram of the FSQ-106EDX4 refractor with 645RD QE0.72x focal reducer


Optical and mechanical characteristics
Takahashi 645RD QE0.72x focal reducer
Item number TKA36580L
Corrected image circle 60 mm
Metal backfocus length 56.2 mm
Mechanical interface (IN) M72x0.75 thread, male
Mechanical interface (OUT) None
Total length 74.2 mm
Weight 200 g


Section and mounting of the 645RD QE0.72x focal reducer on FSQ-106ED refractor Download