Askar 91F Flat-Field Astrograph

Askar 91F: A 91mm f/6.7 Astrograph for Full-Frame Imaging

The defining feature of the Askar 91F is its self-flattened design. It lets you avoid calculating the back focus: after attaching the imaging accessories, you can shoot directly once the OTA is in focus.

With its 91mm aperture, and f/6.7 focal ratio, it balances image scale and exposure time, in a package weighing 4.52 kg with its included mounting hardware.

A 4-Element APO Design with 1 ED Glass Element

At the heart of the 91F is its quadruplet air-spaced apochromatic (APO) objective lens. It uses four lens elements, including one ED (Extra-low Dispersion) glass element, which can effectively reduce chromatic aberration.

The four-element layout consists of three lenses at the front and one at the rear of the optical tube. The 91F supports 44mm full-frame imaging with great performance of the surrounding star points.

The 609mm Focal Length and f/6.7 Speed

On a full-frame sensor, the 609mm focal length gives a field of about 3.4 x 2.3 degrees, enough to frame the Andromeda Galaxy (M31) or most of the North American Nebula (NGC 7000) in one shot.

The native f/6.7 focal ratio is forgiving of focus, compared with faster systems.

A Flat 44mm Image Circle, No Flattener Needed

The standout feature of the Askar 91F is its self-flattened optical design, which produces a 44mm corrected image circle. This is large enough to cover a full-frame sensor. You do not need to buy a separate field flattener or calculate its back focus distance.

This integrated correction simplifies your imaging train significantly. You attach your camera to the included adapters, reach focus and shoot.

M48, M54, and M64 Adapters for Your Imaging Train

The Askar 91F comes standard with a photographic extension tube that includes M54×0.75 and M48×0.75 adapters and a built-in 2-inch filter thread. The adapter set runs M64×1 to M54×0.75, M54×0.75 to M54×0.75, and M54×0.75 to M48×0.75.

• M54x0.75 Adapter: The maximum accessory connection is 93mm from the base of the M54×0.75 male thread.
• M48x0.75 Adapter: The M54×0.75 to M48×0.75 adapter has an M48×0.75 filter thread for 2-inch filters.

The manual specifies a maximum accessory connection distance of 75mm from the base of the M48x0.75 male thread with the focuser fully retracted.

3-inch Rack & Pinion Focuser with 30mm of Travel

The Askar 91F is equipped with a dual-speed rack-and-pinion 3-inch rigid focuser with a 1:10 fine adjustment.

The focuser offers 30mm of travel. A 360° scale field rotator at the rear end allows precise angle adjustment without rotating the optical tube in its rings. The manual marks the screw positions that can be used for electric focusing bracket installation.

The Optional 0.75x Reducer for an f/5 System

For imagers seeking an even wider field of view and faster imaging times, Askar offers a dedicated 0.75x reducer for the 91F. This optional accessory turns the telescope into a 455mm f/5 system; at f/5 the same exposure takes about 56% of the time needed at f/6.7.

The reducer is a triplet with built-in field-flattening correction that supports a 44mm full-frame image circle. Its back focus from the base of the M48 male thread is a standard 55mm, both end threads are M59×1, it has a built-in M48×0.75 2-inch filter thread, and it weighs 0.3kg. The 91F photographic extension tube connects directly to it.

Covering Sensors up to the 44mm Image Circle

The Askar 91F suits cameras with sensors up to the full-frame (36mm x 24mm) format, whose 43mm diagonal sits inside the 44mm image circle.

The figure of 75mm from the base of the M48×0.75 male thread is the maximum with the focuser fully retracted; plan the optical length of an off-axis guider (OAG) or filter drawer within it.

2 Finder Bases and a 230mm Vixen Plate

Two finder bases sit on both sides of the focuser; this removes the need for disassembling when changing the finder position and gives extra mounting positions for other accessories.

The telescope is supplied with tube rings and a 230mm standard Vixen dovetail plate with slotted 1/4 and 3/8 screw holes for direct tripod attachment. The handle bar has a finder base slot, adding another mounting point.

A 3.92 kg OTA Designed for Portability

The Askar 91F is designed for portability. The optical tube assembly alone weighs just 3.92 kg. Even with the included tube rings, handle, and dovetail plate, the total weight is a manageable 4.52 kg.

A retractable dew shield reduces overall length: 550mm with the photographic adapter and the dew shield contracted, or 513mm with the 1.25-inch adapter. The inside of the tube is pasted with matting felt against stray light.

Adapting the 91F for Eyepiece Observation

Askar designs the 91F for visual observation as well as astrophotography. The rear interface includes both 2-inch and 1.25-inch adapters for diagonals and eyepieces. With a low-power, wide-field 2-inch eyepiece, you can sweep through the Milky Way's star fields and take in large open clusters like the Pleiades (M45).

Using higher-power 1.25-inch eyepieces, the 609mm focal length provides enough magnification to resolve details on the Moon, view Jupiter's cloud bands and Galilean moons, and see Saturn's rings.