Telescopes and Telescope Accessories

More Information Price $150.00 Weight 8 oz. Warranty 2 years

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 a 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 for imaging, 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. It is not designed for visual use, having photographic T-threads only and no provision for using a star diagonal or eyepieces.

A separate 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, images taken with the field flattener by 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. Comments like “Works great on my AT8RC. Won’t image without it," and “Works well with my AT6RC," show up regularly in on-line imaging forums. An optical analysis and ray tracing of the field flattener was done in ZEMAX and applied to the R-Cs by Roger Ceragioli, the noted optical designer who did the final optimization of the design of the Astro-Tech Ritchey-Chrétiens. Here is what he had to say about the #AT2FF, “My conclusion, which seems supported by what users are saying, is that you don't need any other field flattener. This one performs well over a 40mm image circle in all three small RCs (6", 8", and 10")."

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.

Spacing from field flattener to film plane: Optimal spacing from the shoulder of the field flattener (where a T-ring threads on) to a camera’s imaging sensor or film plane is 57mm +/- 4mm. The following DSLR camera body/T-ring combinations typically all fall within this optimum spacing range and generally should require no additional spacers.

A Canon EOS DSLR/T-ring combination typically has a T-ring front flange to camera sensor spacing of approximately 55.5mm, although this number is dependent on the thickness of the T-ring you use and cannot be guaranteed. (Without T-ring, the distance from the front flange of the camera body to the sensor is approximately 44mm by itself.)

A Minolta/Sony DSLR and T-ring combo typically has an approximately 54mm T-ring front flange to sensor spacing. The exact number is again dependent on the thickness of your T-ring. (The camera front flange to sensor distance by itself is approximately 43.5mm.)

A Nikon DSLR and T-ring combo typically has a total spacing of approximately 56mm. This number is again dependent on the T-ring thickness. (The camera front flange to sensor spacing is typically 46.5mm by itself.)

With CCD and DSI-type cameras, some experimentation with spacers will be necessary to achieve optimum performance, as these typically have much shorter camera front plate to sensor spacing than DSLR cameras.