Askar FRA500 90mm f/5.6 Quintuplet APO

The Askar FRA500: A Portable 90mm f/5.6 Astrograph

The FRA500 fits deep-sky imaging better than the FRA400 and is lighter and more compact than the FRA600, making it a good choice for imaging trips. The 90mm aperture gathers light for faint nebulae and galaxies, and the 500mm focal length suits many popular deep-sky objects.

It suits APS-C and full-frame sensors, and Askar's back-focus design means no back focus calculation or hunting for spacers: you only need to bring the target into focus.

Quintuplet APO Optics with 2 ED Elements

The optical system of the FRA500 is a "3+2" Petzval-style design, a quintuplet (5-element) air-spaced apochromat. It needs no separate field flattener. The front group has three lenses.

Within this front group are two pieces of extra-low dispersion (ED) glass, which effectively correct chromatic aberration and enhance image sharpness. The rear two-element group, closely spaced, improves field flatness; you may barely detect any blurry star at the edge of the image.

90mm Aperture & 500mm Focal Length at f/5.6

The combination of a 90mm aperture and 500mm focal length defines the FRA500's role in astrophotography. The 90mm objective gathers light for fainter detail while keeping the telescope's size and weight manageable.

The f/5.6 focal ratio strikes a balance between speed and image scale. At 500mm a full-frame sensor covers about 4.1 x 2.7 degrees, enough to frame the Andromeda Galaxy (M31) or the North American Nebula (NGC 7000).

A 55mm Image Circle for Full-Frame Sensors

The FRA500's 55mm image circle is amazing: much larger than the standard 44mm full-frame image circle. You do not need a separate field flattener.

An image circle of this size offers several key advantages:

• Full-Frame Coverage: It covers the 43.3mm diagonal of a full-frame sensor with margin.
• Margin for Framing: The circle extends past the corners of a full-frame sensor.

158mm and 85mm Connection Distances

There are two maximum connection distances to the focal plane. From the base of the M86x1 thread on the rotator, it is 158mm; your imaging train, such as an off-axis guider, filter wheel and camera, sits within it.

For more straightforward setups, the FRA500 includes a conical adapter that terminates in a standard M48x0.75 thread. From the base of that M48×0.75 thread to the focal plane, the maximum connection is 85mm.

3.5-inch Rack-and-Pinion Focuser with 1:10 Fine Control

The FRA500 has a massively built 3.5-inch rack-and-pinion focuser with dual-speed knobs (twin coarse knobs and one 1:10 fine knob). It is tough and solid, with a clearly readable graduation inside.

The fine knob gives small focus steps, useful with a Bahtinov mask. The rear of the focuser carries a rotator with an M86x1 thread for framing.

The Optional 0.7x Reducer for f/3.9 Imaging

For imagers seeking an even wider field of view and faster imaging speeds, the optional Askar F3.9 Full Frame Reducer for the FRA400 and FRA500 is available. It is a 0.7x reducer that brings f/5.6 down to f/3.9.

At f/3.9 the same exposure takes about 49% of the time needed at f/5.6. The reducer has a standard 55mm back focus and a built-in M48×0.75 thread for 2-inch filters.

Imaging Train Configuration: M86 and M48 Camera Connections

The Askar FRA500 is designed for straightforward camera connection. The connection point on the rotator is an M86x1 thread, from which Askar measures 158mm to the focal plane.

Askar includes a conical extender with an M48x0.75 thread; from its base the maximum connection to the focal plane is 85mm, for a camera with an M48 T-ring or an astronomy camera with a filter drawer.

Mechanical Build: A 4.1kg OTA with Retractable Dew Shield

The optical tube assembly (OTA) itself weighs only 4.1kg, The total length is 452mm with the dew shield stretched out and 407mm with it contracted.

The main tube and dew shield are milk-white, the focuser is high-gloss piano black, and the dew shield bezel ring and tube rings are bright red. A standard finder base with a silver thumb screw sits on the left side of the focuser for finders, guiders or other accessories.

Included Components

The total weight with tube rings and dovetail plate is 5.2kg. The package includes:

• Tube Rings: A pair of tube rings.
• Carry Handle: A multi-functional handle attached to the top of the rings.
• Askar Losmandy Dovetail Plate: A Losmandy-style dovetail plate at the bottom of the rings.
• Conical Extender: Steps the M86x1 thread down to M48x0.75 for camera connection.

Getting Started: Your First Night with the FRA500

Setting up the FRA500 for its first imaging session is straightforward. First, attach the Losmandy dovetail plate to your equatorial mount and securely lock it down. Next, assemble your imaging train (camera, filter holder and guider) within the 85mm or 158mm connection distance. Attach the completed train to the focuser.

Balance the telescope carefully in both Right Ascension and Declination axes on your mount. This is crucial for accurate tracking. After polar aligning your mount, point the telescope to a bright star and use a Bahtinov mask to achieve critical focus with the 1:10 fine focus knob. Once focused, slew to your desired deep-sky target, start your guiding, and begin your imaging sequence. Remember to never look at the Sun through the telescope, as it will cause irreversible damage to your eyes.