If you are interested in the pinnacle of portability and imaging performance, the FRA600 deserves a hard look. The field flattener is a second set of lenses just forward of the focuser that corrects the incoming light rays such that all stars remain pinpoints, even out to the edge of the field where many APOs show distorted stars. Astrographs like this are a must for excellent wide field shots of galaxies and star clusters.
The FRA600 Flatfield Refractive Astrograph is built on a Petzval design (as are all astrographs in the FRA- series). This design comprises a front triplet (the objective) and rear doublet lenses (the field flattener), made of ED glass. This combination provides a high level of apochromatic performance for astroimaging.
What is an Astrograph?
The second set of lenses forward of the focuser is important because it ‘flattens the field’ meaning all rays coming through the lens are not distorted or curved. This effect is especially noticeable in stars at the edge of a field of view viewed through a short focal length refractor: without a flattener, stars appear deformed and stretched towards the edges of an image. A field flattener takes the bending rays of light at the edge of the lens and straightens them out, leaving nice pinpoints of stars at the edge. These elements are mounted just ahead of the focuser in the optical tube and have a diameter of 80mm.
The 108mm objective triplet offers amazing light collection and color correction and, at f/5.6, gives a field of view allowing for most large deep sky objects. Coupled with the optional focal reducer and the telescope transforms to a f/3.9 wide field imager that gives breathtaking images of large swaths of sky. At f/5.6 the image circle is 66mm, larger than full-frame cameras!
The FRA600 comes with a large 4.2" rack-and-gear-based focuser which includes a standard 1:10 focal adjustment knob. This focuser is stout enough to carry heavier payloads (cameras and other accessories), offering smooth operation without slipping.
Askar offers a standard conical extender with threads of M54×0.75 and M48×0.85 to connect various cameras an accessories. The optical tube of the FRA600 has a diameter of 120mm and a retractable dew-shield. With the dew-shield retracted, the total length is only 493mm, making it a great portable 4" astrograph.
A set of multifunctional red anodized tube rings includes mounting points on three sides, making itself easy for attachment of accessories such as guide scopes for backyard viewing.
In addition to supporting wide field astrophotography, the FRA600 is also perfect for high quality observation, providing exquisite chromatic aberration correction and creating images with impressive contrast comparable to a visual experience.
If the FRA600 were only an APO refractor, this would be a good value, but with the added field-flattener it becomes an amazing value. This 4-inch astrograph will provide decades of high-performance imaging and capability.
Corrected Image Circle
Askar 4.3" 108mm F/5.6 Quintuplet Petzval Flat Field Astrograph
Pair of tube rings
Sectional conical tube adapter (M54x0.75 and M48x0.75 male thread)
1.25" eyepiece adapter
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