As Sky & Telescope pointed out in their Hot Product citation, “With only a few noteworthy (and expensive) exceptions, refractors need optional field flatteners in order to deliver acceptable star images across the field of today's DSLR cameras. Custom-designed flatteners usually cost hundreds of dollars, but the Astro-Tech 2” Field Flattener is only $150. Furthermore, it's designed for any refractor with the focal ratio between f/6 and f/8. And there are reports that helps flatten the field of Astro-Tech Ritchey-Chrétien astrographs. The flattener was reviewed in the September 2009 issue of Sky & Telescope, page 38.”

The Astro-Tech 2” field flattener has been designed to optimize the flat-field imaging performance of Astro-Tech and TMB apochromatic refractors, even the very fast f/5.5 focal ratio TMB-92. However, the field flattener will also work well with any other brand and type of f/6 to f/8 focal ratio refractor, and can even improve the imaging performance of refractors with focal ratios somewhat outside these limits.

Designed to work purely as a field flattener, and not simultaneously as a focal reducer or telecompressor, the 2” Astro-Tech field flattener does not change the focal length or focal ratio of your telescope.

A review in Sky & Telescope magazine of the Astro-Tech field flattener and the Astro-Tech AT106 refractor said the field flattener was “extremely easy to use with any SLR camera. You simply screw on the appropriate T-ring, connect your camera, and slip the flattener into the AT106’s 2-inch focuser. Daylight shots made two things immediately apparent – the field flattener does not change the telescope’s focal length (it’s a true zero-power flattener), and it sharpens images at the edge of the field . . . At night, star images in the corners of my Nikon D300’s APS-format frame, while vastly better than those recorded without the flattener, still showed a slight elongation . . . By focusing at a point slightly off center, thus balancing the imaging performance between the center and edge of the field, I could fill the frame with nice, round star images. For photographers shooting day or night, the flattener is well worth its current $150 price.”

The small image to the right, of the North American Nebula, was taken by AT2FF owner David Rosenthal with a self-modified Canon 400D DSLR. Click on this link or on the small image to see a full-sized uncropped image showing the corner stars across the entire APS-C sized sensor. The image consists of 27 360 second exposures using an Orion Atlas EQ-G guided by a Meade DSI Pro on a Takahashi FS-60C.

David says, “I am quite impressed with the performance and the mechanics of the unit. I have used it on my Borg 76ED with 0.250 inches of spacers and the corners on my Canon 400D (APS-C Sensor) show better stars than I'd hoped for. Well worth the $150.00 you are charging for it.” You can see the results for yourself by taking a close look at the stars in the corners of David’s full-size image. If you want to see more of David’s fine astrophotography, a click on this link will take you to his website.

While not specifically designed to work with Ritchey-Chrétien astrographs, field tests by adventurous Astro-Tech R-C owners have shown that the Astro-Tech 2” field flattener works remarkably well with these advanced coma-free reflectors as well as with refractors.

The 2” barrel of the fully multicoated dual-element Astro-Tech field flattener inserts directly into your scope’s 2” focuser drawtube. A groove machined into the field flattener barrel engages the thumbscrew or compression ring of your focuser to act as a safety stop. This prevents the field flattener and camera from falling to the ground should the thumbscrew or compression ring accidentally loosen during use.

Standard 42mm x 0.75mm pitch T-2 photographic T-threads on the observer/camera side of the black anodized field flattener accept a standard T-ring for connecting a 35mm or DSLR camera to the field flattener. They also allow direct connection to many CCD camera/color filter wheel combinations. Optimal spacing from the field flattener lens to the imaging sensor or film plane is 60mm (57mm from the shoulder of the field flattener body to the imaging sensor) +/- 4mm. Some experimentation with spacers may be necessary to achieve optimum performance.