All Askar Flatfield Refractive Astrographs (FRA), incorporates an innovative five-element optical design that yields flat fields and color-free images across the image plane. Compared with its siblings, the 500mm-focal-length refractor has an aperture size of 90mm (f/5.6), so Askar FRA500 provides a larger aperture for brighter imaging of deep-sky imaging than Askar FRA400.
What is an Astrograph?
Askar FRA500 has inherited the legacies of superior FRA- series designing. It boasts a quintuplet air-spaced APO optical structure, a “three plus two” Petzval lens combination. The front lens group consists of three lenses (including two pieces of ED glass) for excellent color correction. The posterior group - just forward of the focuser - is a field flattener which is composed of two lenses separated by small space which is able to provide fine sharpness across the whole image. The stars at the edges of the field are amazing in this design.
The second set of lenses 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.
The FRA500 has an image circle that is an amazing 55mm, much larger than the standard 44mm full-frame image plane. Its special back-focus design frees users from calculating confusing back-focus distance or rummaging around for a proper spacer. Once your target is in focus, that’s it, just start taking images.
Askar’s FRA500 optical is a wonderful mix of pragmatism and aesthetics. It has smooth lines and a sound layout of components. From the front, the Askar FRA500 has an impressive dew shield decorated with a classic bright-red bezel ring. The total length of FRA500 is 452mm when the dew shield is stretched out and 410mm when contracted back. At the tail end of the astrograph is a 3.5" massively-built focuser with dual-speed hand knobs (twin knobs for coarse adjustment and one 1:10 fine adjustment knob), which is of great use in real observation or astrophotography.
The f/5.6 focal ratio is ideal for deep-sky imaging but ASKAR also makes an optional FRA72RD focal reducer which is not included but changes the scope to a an f/3.9 wide field astrograph that still supports full-frame imaging.
Askar FRA500 comes with a pair of solid tube rings. There is a handle on the top of the rings and an Askar Losmandy-style dovetail plate on the bottom.
The Askar FRA500 is 410mm long and weighs about 4kg, so it’s quite convenient for users to carry around or mount on an EQ mount. It’s particularly suitable for astronomy enthusiasts who frequently engage in long-distance trips or out-door astrophotography.
This astrograph is an amazing performer for the price. The fast apochromat offers very wide fields with pinpoint stars throughout the field of view and approaches performance seen in telescopes costing three times as much. If using large format cameras, this telescope won’t disappoint!
Corrected Image Circle
Pair of tube rings
An Askar Losmandy dovetail plate
A conical extender
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