Named a Sky & Telescope Hot Product of the Year for 2010, the Astro-Tech photographic field flattener reduces the normal field curvature inherent in all refractors to provide you with images that are sharply focused out to the very corners of your DSLR or large format CCD imager frame. Stars on your images look sharp and point-like all the way across the field. You don't have to worry about photos that are sharp in the center but out of focus at the edges. The field flattener also works surprisingly well with the 6”, 8”, and 10” Astro-Tech Ritchey-Chrétien astrographic reflectors.
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 ofSky & 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. The image shows the corner stars across the entire APS-C sized sensor and 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.
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.