91F Flat-Field

The Askar 91F is an OTA with an aperture of 91mm, a focal length of 609mm, and a native focal ratio of F6.7.   The optical design of the Askar 91F features a four-element configuration (three elements in the front and one in the rear), including one piece of ED glass， which can effectively reduce chromatic aberration.The Askar 91F supports 44mm full-frame imaging with great performance of the surrounding star points.  The Askar 91F is a self-flattened design that allows users to avoid calculating the back focus and buying additional flatteners. After attaching the imaging accessories, users can directly use it for shooting after the OTA is in focus. This further reduces the cost and improves the pleasure of shooting.  The Askar 91F strikes a balance between practicality and versatility while achieving lightweight portability through its compact design.The OTA weighs just 3.92kg, with a gross weight of 4.52kg (including the tube rings and dovetail plate).   The Askar 91F is versatile in its use, allowing for both visual observation and astrophotography.     For user convenience, it comes standard with three visual accessories: an 8mm eyepiece, a 20mm eyepiece, and a 1.25-inch erecting prism.  The 8mm eyepiece has a magnification of 76x and the 20mm eyepiece has a magnification of 30x.The Askar 91F can be used with different eyepieces depending on the target, and both eyepieces are ideal for viewing birds and landscapes.  Both eyepieces are 1.25 inches and have a soft rubberized top for comfortable use.   The 1.25-inch erecting prism provides correct left-to-right and down-to-up image views, making users easy for you to conduct terrestrial and astronomical observations without the hassle of seeing images reversed up, down, left, and right. The Askar 91F also comes standard with a photographic extension tube with a built-in 2-inch filter thread for easy shooting connections, providing M48×0.75 & M54×0.75 adapters to accommodate various DSLR cameras and professional astronomical cameras, meeting diverse imaging needs.   The mechanical design of the Askar 91F itself adheres to Askar's consistent philosophy of taking into account the user's habits and minimizing the user's invisible costs.  The Askar 91F features a retractable dew shield, further reducing overall length and enhancing portability.The inside of the tube is pasted with matting felt to better eliminate stray light, while the exterior sports a sleek blue-and-white color scheme with high-quality CNC machining and paint finish. The Askar 91F’s handle is 130mm long and has a finder base slot. Besides, there are two finder bases on both sides of the focuser, which not only eliminates the need for disassembling when changing the position of the finder scope but also provides additional mounting positions for other astronomical accessories. In terms of the focuser, the Askar 91F is equipped with a dual-speed R&P 3-inch rigid focuser, which has a 1:10 fine adjustment and a 360° scaled rotator at the rear end for precise angle adjustment.The 230mm Vixen-style dovetail plate includes 1/4" and 3/8" screw holes for direct tripod mounting.   The rear interface of the Askar 91F includes both 2-inch and 1.25-inch adapters, enabling compatibility with a wide range of astronomical accessories.   Whether for deep-sky astrophotography, planetary observation, or birdwatching and travel, the 91F is your ideal companion for exploring the world.  Specifications Parameters Aperture size 91mm Focal length 609mm Focal ratio f/6.7 Objective lens Quadruplet air-spaced APO(including one ED glass) Image circle 44mm Maximum accessory connection (with the focuser fully retreated) Imaging mode:   75mm (from the base of M48×0.75 male thread）  93mm（from the base of M54×0.75 male thread) Observing mode:    112mm（from the end of 1.25" eyepiece holder）  122mm（from the end of 2" eyepiece holder） Minimum accessory connection (with the focuser fully extended) Imaging mode:   45mm (from the base of M48×0.75 male thread）  63mm（from the base of M54×0.75 male thread) Observing mode:    82mm（from the end of 1.25" eyepiece holder）  92mm（from the end of 2" eyepiece holder） Total length 513mm(including 1.25" adapter as dew shield contracted）  599mm(including 1.25" adapter as dew shield fully stretched）  550mm(including the photographic adapter as dew shield contracted）  636mm(including the photographic adapter as dew shield fully stretched） Focuser travel length 30mm Net weight 3.92kg Gross weight (Including tube ring&dovetail plate) 4.52kg Rear-end thread type M64×1-M54×0.75          M54×0.75-M54×0.75          M54×0.75-M48×0.75  (with M48×0.75 filter thread) Standard package items  A 91F OTA，a pair of tube rings, a handle, a 230mm Vixen dovetail plate,a set of photographic adapters, a 1.25-inch erecting prism, an 8mm eyepiece, a 20mm eyepiece   Product Size Diagram 91F Connection Diagram  The Locking Screw Position Diagram Electronic Focuser Assembly Diagram Optical Path Diagram  Spot Diagram  Images NGC281  |  张宇航  |  Askar 91F Flat-Field + ZWO ASI2600MC NGC7293  |  张宇航  |  Askar 91F Flat-Field + ZWO ASI2600MC Gyulbudaghian's Nebula  |  Nico Carver  |  Askar 91F + ZWO ASI2600MC Air