The FRA400 (FRA stands for Flat-field Refractive Astrograph), is a member of a new line of instruments from Sharpstar/Askar. It is a Quintuplet refractor with 72mm aperture and f/5.6 focal ratio (400mm focal length), and is ideal for wide-field imaging of the deep sky. Three elements make up the APO objective while the remaining two are in the field-flattener just forward of the focuser.
What is an Astrograph?
As with other members of the FRA line, this astrograph has a 44mm image circle, ideal for smaller cameras with sensors of 1” or less, with pinpoint stars all the way to the edge of the image plane.
The FRA400 astrograph has a retractable dew shield, a 3" rack-and-gear-based high-precision reducer containing a 360-degree rotator, M68×1 thread, a 2" adapter ring and a 1.25" visual adapter. The reducer is equipped with a fine focal adjustment function for more precise calibration in practical observation or sky imaging.
The back focus of FRA600 is 140mm, starting from the base of the mounting thread on the housing to the focal plane. And users don't need to worry about the calculation of back focus when using FRA600, as long as the object in the objective lens is focused.
F/5.6 is a relatively fast focal ratio for astrophotography use, but for even shorter focal lengths, an ultra focal reducer is included to satisfy the special needs of experienced imagers, yielding f/3.9 when inserted. This combination supports full-frame imaging and provides even wider fields and dramatically reduces exposure times.
This fast, extremely portable astrograph is a great value, in many cases approaching performance levels seen in telescopes costing much more.