First light with the 8in f/3.52 hyperbolic telescope

On Wednesday, March 1, after hosting a mirror-making workshop at Neptunion31, the sky was clear when I returned, after ten days fully covered. I took advantage of this break to test the telescope, painted enough to resist moisture, on some open clusters and bright stars.

After a few battles with the backfocus, I got round stars on the entire (very generous) field of 2.79° given by the 20mm/100° APM eyepiece. Without the corrector, the spherical aberration/over-correction induced by the mirror corrected to k = -1.33 is quite visible, and totally disappears with the corrector around 57.5mm from the ocular focal plane. I should make this adjustment easier by making a body similar to the Paracorr or this type of corrector incorporating an adjustment.

With the corrector, the ronchi shows very straight bands, and the defocused stars with a 10mm eyepiece that I had around are well symmetrical. I can’t wait for high-magnification tests !

To the question “is it possible to get a well corrected large field, with a fast mirror, for a financial investment lower than that buying a Paracorr II”, I am tempted to answer “yes, provided that the primary is hyperbolized” !