ASTAP only solves when FOV set to 1.6, but reports FOV=1.39 and will fail...
Stacking program, astrometric (plate) solver, fits file viewer
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Using windows10, ASTAP2023.05.09, 64bit with D80 database
Here is portion of log (I start with FOV set to 1.6, and it works but ASTAP then sets FOV to 1.39 which subsequently fails.
0:21:21 Using star database D80
0:21:22 33 stars, 26 quads selected in the image. 22 database stars, 17 database quads required for the square search field of 1.6°. Search window at 196% based on the number of quads. Step size at 100% of image height.
0:21:22 3 of 3 quads selected matching within 0.007 tolerance. Solution["] x:=-1.203915x+ -2.072021y+ 4016.925424, y:=-2.061801x+ 1.212243y+ 7692.007704
0:21:22 Solution found: 11: 05 08.7 +27° 34 41 Solved in 0.5 sec. Δ was 0.0". Used stars down to magnitude: 10.4
0:21:22 Warning inexact scale! Set FOV=1.39d or scale=2.4"/pix or FL=206mm
0:24:27 Using star database D80
0:24:28 33 stars, 26 quads selected in the image. 22 database stars, 17 database quads required for the square search field of 1.4°. Search window at 196% based on the number of quads. Step size at 100% of image height.
0:24:39 No solution found! :(
The 3 quads in the log indicate that the image barely solves. For these difficult images it is not abnormal that a little tweaking can be just enough to get it solved. Forcing binning x2 also works. Best is both binning x 2 and select triplets.
The image is 8 bit. It will likely work better if you can set it to 16 bit in SharpCap.
The reason it solves with 1.6 is that is selects more comparison stars in the database. Normally it tries to match the number of stars in the image with the database for a square area. This area is increased if there are not many image stars detected resulting in some overlap. The image is under an angle of 60 degrees. The program goes through the database in steps of Ra and Dec so with orientation angle zero.. Maybe by a larger area it locks on an extra star in one of the corners normally missed which makes the solving possible.