Askar FRA500 90mm f/5.6 Quintuplet Petzval Flat-Field Astrograph

Askar FRA500 Overview

Like its siblings, the 72mm FRA400 and 108mm FRA600, the FRA500 “Flatfield Refractive Astrograph” incorporates a five-element design to deliver flat fields and sharp, color-free images from edge-to-edge. With a 90mm objective, two ED elements, and Petzval optical layout, this f/5.6 scope has a focal length of 500mm and it’s ideal for capturing stunning wide-field images of the deep sky over a huge 55mm full-frame image circle, much larger than a standard full-frame image circle.

This premium astrograph includes a large 3.5” rack-and-pinion dual-speed (1:10) focuser that’s fully rotatable through 360 degrees. The focuser tube terminates with an M86x1 thread, and it comes with a four-piece conical adapter that includes elements with M68x1, M54x0.75, M52x0.75, and M48x0.75 threads. The latter has a slot to thread in filters mounted in standard 2” cells.

The attractive optical tube of the FRA500 is complemented with a highly functional set of red anodized aluminum rings, "D" Losmandy-style dovetail plate, and a carrying handle. The Losmandy dovetail plate can be removed and replaced with a Vixen-style plate (not included). A finder dovetail mount is attached to the focuser to accommodate a finder or guidescope.

The rings include separate mounting holes for additional accessories. The entire tube has a diameter of 105mm and has a length of just 410mm with the dew shield retracted. It weighs 5.2kg. This astrograph has a back focus of 158mm (from the M86 thread on the rear) which leaves ample room for accessories. Note that 85mm of available backfocus past the M48 male thread on the rear will not be sufficient to accommodate star diagonals for visual use.

While the fast f/5.6 focal ratio is ideal for deep-sky imaging, ASKAR also makes an optional focal reducer (not included) for the FRA500 scope to bring the focal ratio to f/3.9 and focal length to 350mm while still supporting full-frame imaging. The adapter required to attach the f/3.9 reducer to the FRA500 scope is included with this scope as a standard accessory.

For the price, this fast apochromatic telescope offers excellent value and can approach the performance of telescopes that cost three or four times as much. This is especially true when matched with astronomy cameras with smaller 1" and 4/3" sensors. Cameras with larger APS-C and full-frame sensors may reveal slight aberrations at the edge of the field, especially when using the focal reducer.

The telescope is shipped in a foam-fitted hardcase (19.25" x 12.25" x 10").

The ASKAR FRA500 offers excellent value in a premium larger-aperture astrograph for serious astronomical imaging!

Specifications:

• Aperture size：90mm
• Focal length：500mm
• Focal ratio：f/5.6
• Objective lens：Quintuplet air-spaced APO (including two ED glass)
• Image circle：55mm
• Back focus: 158mm（from base of M86×1 thread on the rotator to focal plane）
• Back focus：105mm（from base of M548×0.75 thread on the conical extender to focal plane）
• Back focus：85mm（from base of M48×0.75 thread on the conical extender to focal plane）
• Total length：452mm（410mm when the dew shield is contracted）
• Dew shield OD: 115mm / 4.52"
• Net weight：4.1kg
• Gross weight：5.2kg
• Rear-end thread type：M86×1
• Conical adapters：M68x1, M54x0.75, M52x0.75, and M48×0.75
• Package items：an FRA500 OTA, a pair of tube rings, a handle, an Askar Losmandy dovetail plate, a conical extender, a manual, an M86-to-M68 adapter required only if the optional 0.7x reducer (sold separately) is to be attached to this telescope