Askar 91F 91mm f/6.7 Flat-Field APO Refractor
The Askar 91F combines a 91mm aperture with a 609mm focal length for a useful middle ground between wide-field nebula imaging and smaller deep-sky targets. Its four-element, self-flattening optical design produces a corrected field without a separate field flattener. Connect a suitable camera and adapters, then focus; there is no flattener spacing to tune at the telescope's native focal length.
With a 44mm image circle, the 91F supports APS-C cameras such as the ZWO ASI2600MC Pro and can cover full-frame sensors. The 609mm focal length gives a narrower field of view than a 500mm astrograph, so check the framing of your favourite targets before choosing between them.
Why choose the 91F?
- Flat-field optics: four air-spaced elements, including one ED element, with a corrected 44mm image circle.
- Flexible imaging train: supplied photographic adapters provide M54 and M48 connections; the M48 adapter includes a 2-inch filter thread.
- Precise focusing and framing: 3-inch dual-speed rack-and-pinion focuser with 1:10 fine adjustment and a 360-degree camera rotator.
- Practical mounting: tube rings, a handle, two finder bases and a 230mm Vixen-style dovetail plate. The retractable dew shield makes storage easier.
- Electronic focusing: Askar publishes an EAF installation diagram for this model. Check the motor, coupling and brackets supplied with your chosen EAF before assembly.
Specifications
| Aperture | 91mm |
|---|---|
| Focal length | 609mm |
| Focal ratio | f/6.7 |
| Optical design | Four-element air-spaced APO, including one ED element |
| Image circle | 44mm |
| Focuser | 3-inch dual-speed rack-and-pinion; 30mm travel |
| Weight | 3.92kg OTA; 4.52kg including rings and dovetail |
| Photographic connections | M54 x 0.75 and M48 x 0.75 adapters; M48 filter thread |
| Included | 91F OTA, pair of tube rings, handle, 230mm Vixen dovetail plate and photographic adapter set |
Connection note: Askar specifies a permissible accessory connection range rather than a fixed 55mm flattener back-focus. With the focuser fully retracted, the maximum camera-side connection length is 75mm from the M48 male thread or 93mm from the M54 male thread. Check the full imaging train length and focus travel before ordering unusual adapters.
Product diagrams
Example images
Technical specifications and diagrams: Askar / Sharpstar official 91F product information.
