The Astro Tech Field Flattener for f/6 through f/8 Refractors is a great accessory.
- Astro Tech Field Flattener - Sky & Telescope Hot Product 2010
- Astro Tech Field Flattener reduces the normal field of curvature generated in all refractors to deliver images that are sharply focused at the very corners of the 35mm large format CCD Imagers.
- By adding the Astro Tech Field Flattener stars will look sharp accross the field.
- The Astro Tech Field Flattener weighs 8 ounces and comes with a full two year warranty.
As Sky and and 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 this product helps to flatten the field of Astro-Tech Ritchey-Chretien astrographs. The flattener was reviewed in the September 2009 issue of Sky and and 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 and and 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."
While not specifically designed to work with Ritchey-Chretien 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.
