Askar 131F 131mm f/7.5 Flat-Field Quadruplet APO Refractor

A 5-inch flat-field refractor for small galaxies, planetary nebulae and the planets

The Askar 131F Flat-Field is the largest model in Askar's F series: a 131mm f/7.5 air-spaced quadruplet APO refractor with one ED element, three lenses up front and one at the rear. It covers a full-frame sensor with no extra flattener, and its retractable dew shield, 3" dual-speed rack-and-pinion focuser with a 360° rotator, two finder bases and handle make it as useful for visual observing as for imaging. It's for imagers ready to move up from a small refractor to longer focal length, and for observers who want a large, color-free refractor that can also take pictures.

Key specs: 131mm aperture · 983mm focal length · f/7.5 · quadruplet air-spaced APO with 1 ED element · 44mm image circle, no flattener needed · 3" rack-and-pinion focuser, 1:10 fine focus, 30mm travel · 9.4kg (20.7lb) with rings, handle and 300mm Losmandy-style plate

What you can image and see

• Galaxies: the Whirlpool Galaxy (M51), Bode's and the Cigar Galaxies (M81 and M82) together, the Leo Triplet, the Sunflower Galaxy (M63) and NGC 891.
• Planetary nebulae and small nebulae: the Dumbbell (M27), Ring (M57), Owl (M97) and Helix Nebulae, the Crescent Nebula, the Iris Nebula and the Pillars of Creation in the Eagle Nebula.
• Globular clusters: the Hercules Cluster (M13) and M3 resolved into stars across the frame.
• At the eyepiece: Jupiter's cloud belts and the Great Red Spot, Saturn's rings and the Cassini Division, lunar craters at high power, and wide views of the Pleiades and the Double Cluster at low power.

Fields of view at 983mm: 2.10° × 1.40° with a full-frame sensor, 1.37° × 0.92° with an APS-C camera such as the ASI2600MC Pro (0.79″ per pixel), and 0.66° × 0.66° with the square ASI533MC Pro.

How it works

Imaging: thread the included photographic extension tubes onto the focuser. They end in either an M54 × 0.75 or M48 × 0.75 male thread, and the tube has a built-in 2" filter thread. Because the 131F is self-flattened, you don't have to hit an exact back-focus distance. With the focuser's 30mm of travel, the sensor can sit anywhere from 43 to 73mm behind the M48 thread (61 to 91mm behind the M54 thread), so a camera at the usual 55mm, a filter drawer or an off-axis guider all reach focus.

Visual: swap the camera adapters for the included 2" and 1.25" visual adapters, add a 2" diagonal and an eyepiece. A 35mm eyepiece gives about 28x and a 10mm about 98x.

Focusing and framing: the dual-speed focuser has a 1:10 fine knob, and the 360° rotator with scale lets you frame a target without loosening the camera. To add an electronic focuser, remove the coarse knob and mount a ZWO EAF on the focuser's bracket holes with a 5 to 6mm flexible coupling, as Askar's assembly diagram shows.

Mounting: the 300mm Losmandy-style (wide) dovetail plate needs a mount saddle that accepts D-style plates. A handle on top of the rings carries a finder base, and there are two more finder bases at the focuser for a guide scope or finder.

What to add for the best results

• A mount (required): the ZWO AM5N with TC40 tripod carries 15kg (33lb) without a counterweight, enough for the 131F with a camera, focuser and guider.
• A camera: the ZWO ASI2600MC Pro or the ASI2600MC Air with built-in ASIAIR and guider for an APS-C field; the ASI6200MC Pro to use the full 44mm image circle; or the ASI533MC Pro for smaller targets.
• Guiding: at 983mm an off-axis guider such as the ZWO OAG with an ASI220MM Mini guide camera avoids flexure between a separate guide scope and the main tube.
• Autofocus: the ZWO EAFN or EAF Pro keeps focus as the temperature drops through the night.
• Control and filters: the ASIAIR Plus runs the camera, mount and focuser from your phone, and a 2" Askar Colour Magic C1 (Hα + OIII) filter threads straight into the extension tube for nebulae from light-polluted skies.
• For visual use: a 2" diagonal such as the Celestron 2" dielectric star diagonal and eyepieces from our eyepiece collection.

Askar F-series comparison

All five share the same 44mm full-frame image circle and self-flattened quadruplet design. See all Askar telescopes.

What's in the box

• Askar 131F Flat-Field optical tube with retractable dew shield and front cap
• Pair of tube rings with handle bar
• 300mm Losmandy-style dovetail plate
• Set of photographic adapters (extension tubes with M54 and M48 threads and a 2" filter thread)
• 1.25" and 2" visual adapters

A mount, camera, diagonal, eyepieces and finder are sold separately.

Askar 131F FAQ

Is the 131F a good first astrophotography telescope?

It can be, but it asks more of your mount and guiding than a small refractor. At 983mm each pixel covers less sky, so tracking errors show sooner. If this is your first imaging scope, the 71F or 91F is more forgiving. If you already image with a short refractor and want more detail on galaxies, the 131F is a natural next step.

Can I see Saturn and Jupiter with it?

Yes. A 131mm color-free refractor shows Saturn's rings and Cassini Division and Jupiter's belts and moons clearly. With a 2" diagonal and a short eyepiece you can work up to about 200x on steady nights.

131F or 111F?

The 131F has 39% more light grasp and about 28% more focal length, for a closer view of small targets. The 111F is 2.6kg lighter, 213mm shorter and uses a standard Vixen plate, so it is easier to carry and fits more mounts.

What mount do I need?

For imaging, a mount rated well above 10kg, with a saddle that takes Losmandy-style plates. The ZWO AM5N is a good match. For visual use only, a sturdy equatorial or alt-azimuth mount in the same class works.

Do I need a field flattener or reducer?

No flattener is needed. Askar also makes a 0.74x reducer for the 131F that shortens it to about 727mm for a wider, faster field; ask us about availability.

Specifications