A quick test of the Astro-Tech 0.8x reducer/flattener

Refracting telescopes generally have curved focal surfaces.   When projected onto a flat sensor, star images become increasingly out-of-focus and smeared with distance from the center of the field.  This curvature can be mitigated with an additional optical element called a field flattener.  Some flatteners also incorporate a small amount of convergence which shortens the effective focal length and creates a brighter image.  These are called reducer/flatteners.

I did a quick test of a newly-acquired Astro-Tech 0.8x reducer/flattener (model ATR8) attached to an Astro-Tech AT72EDII refractor.  The reducer/flattener shortens the focal length from 432 mm to 346mm and the focal ratio is reduced to f/4.8 (from f/6).  The scope was mounted on a Celestron CG-4 motorized equatorial mount.

The camera attached to the flattener was a micro-four-thirds (17.3 x 13 mm sensor) Olympus E-M5. The back-focus spacing between the shoulder of the flattener and the image sensor was set to 55 mm.  A Bahtinov mask was used to ensure proper focus.

The 16-January sky was bright with normal city sky glow fortified with light from a 6.3-d waxing-crescent moon. There is a whole series of tests and comparisons that I would like to do, but it's winter and therefore cold and uncomfortable, so only three images were obtained.  More comprehensive tests will have to wait for better weather and darker skies.

Orion Nebula (M42).  1:1 crop from a larger image. E-M5, ISO 800, 30 s.

The full-size image (2x binning) with the cropped area outlined.

There are some power lines that cross the field of view in the above image and introduce diffraction spikes in the stars near the top.  I can also detect the trails of at least six satellites (probably Starlink).  It is becoming increasingly difficult to take astrophotos without being photo-bombed by satellites.

Sirius.  E-M5, ISO 800, 10 s, 2x binning.

 

A star field in Gemini.  The reddish star near the bottom is mag-3.3 Propus (Eta Geminorum). E-M5, ISO 800, 30 s, 2x binning.

 In the above image I was aiming the telescope with a laser pointer (difficult with a bright sky) and trying to get near the star cluster M35.  The aim was a bit off.  The chart below shows the region covered in this image.

credit: SkySafariAstronomy.com

The test was mostly successful.  The star images are acceptable out to the edges of the frame, with only some minor distortions.  An even more pleasing result is that the mount tracks well with no guiding for 30-s exposures (at least).