Intruments: Pentax 75mm f/6.7

The tube of Pentax75 mounted onto a Gemini G41 mount together with a Konus Vista 80 guiding scope.

It's a wonderful astrograph, each detail has taken care, but above all the optical quality is impeccable with the 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 Moon 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 focusing 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.

Hints for autoconstruction of accessories:
  • 1"1/4 visual adapter: this accessory allow to use standard star diagonals, while the original adaptor is too long and don't permit to reach focus using the standard star diagonal, except the expensive original Pentax one.

Images of the 1"1/4 visual adapter
  • No vignetting photographic adapter for generic T rings: allow to get a fully illuminated 24x36mm field, without any hint of vignetting. Normally vignetting is unavoidable because of the internal diameter of the T-adapter that have a front thread of 42x0.75mm, so the photo adapter cannot have an internal diameter bigger than about 38mm. And considering that the 24x36 diagonal is 43mm, vignetting is unavoidable! This adapter allow to use only a part of the T-adapter, and to overcome the limit of the front thread. I suggest to verify your T-adaper before realizing this adapter.

Images of the no-vignetting photo adapter (on the left with the Nikon adapter)
  • No vignetting photographic adapter for Canon EOS: the Canon EOS adapter has a free aperture of 47.4mm, bigger than the Nikon adapter and nealy all other brands. To get the maximum from this, a modification to the T-ring is necessary, removing part of the material. I've tested this adapter on my Canon EOS 5D with full frame sensor and the field is really perfectly illuminated.
  • Finder adapter: allow to use all the standard finders. The original Pentax finder is very expensive.

Images of the finder adapter

HTML Editing and Publishing by Lorenzo Comolli. Email me at
Back to Main Page