The tube of Pentax75 mounted onto a Gemini G41 mount together with a Konus Vista 80 guiding scope.
It's a wonderful astrograph, each
has taken care, but above all the optical quality is impeccable with
stars that are perfect points in the center and in the extreme edges of
the 24x36 film, thanks to the correcting lenses integrated in the half
of the tube (black ring in the middle). Even if some purist object, I
think that the apocromaticity is great: visual observing the full
show no hints of residual color. Moreover photographic tests performed
from very dark skies don't show any trace of halos of chromatic
aberration on any star, bright or dim.
Internal baffling with many diaphragms is excellent, and this allows to
image Deneb in an angle of
the field without showing any sign of reflections in other zones. The
system is very reliable, with a blocking ring that allow a definitive
clamping of the focuser, preventing any type of bending and focus shift.
The focal of 500mm is ideal for imaging a big number of deepsky objects, i.e. the Andromeda galaxy, California, North America nebula and so on.
The focal ratio (f/6.7) is not extremely fast and an exposure of 45 min is necessary with 400 ISO analog films and dark skies, but in this price range no best alternative is available: in example a Takahashi FSQ-106ED will cost 4 times more (diameter 106 mm, focal lenght 530 mm, f/5.0).
The accessories I got are very few: no adapter for direct focus photos with a reflex, and the provided adapter for 31.8 mm eyepieces is useless with standard diagonals. The solution is not buy the (very) expensive Pentax diagonal but simply lathe a shorter adapter: mine requires only 24 mm of backfocus and allows any eyepiece or diagonal to reach focus. Also the finder is optional, and very expensive! The solution was to build an adapter for mounting the common and economic 6x30 finders (hint: you can also find a cheap mountin base for dovetail finders).
The only provided useful accessory is the mounting ring for attaching on a photographic tripod or on a plate, like in my case.
Unfortunately Pentax stopped production of this wonderful scope in 2008.
Intra-extra focal images of Pentax75, and the image at focus: note the perfect image at focus, with round Airy disks. Philips Vesta Pro webcam with 5x barlow.
autoconstruction of accessories:
Images of the 1"1/4 visual adapter
Images of the no-vignetting photo adapter (on the left with the Nikon adapter)
Images of the finder adapter