Pentax-65
Achromat Review
Pentax astro’
scopes had a great reputation for Takahashi-like quality and innovation for
imaging, but they were discontinued over a decade ago. Pentax’s 50mm and 60mm
beginners scope can be found on eBay, but their premium scopes and are rarely
available used now; I had never reviewed one. So when this little Pentax-65
achromat came up, with outward appearances like one of their high-end imaging
apochromats, I grabbed it to try.
At
A Glance
|
Telescope |
Pentax-65 |
|
Aperture |
65mm |
|
Focal Length |
700mm |
|
Focal Ratio |
F10.7 |
|
Length |
74.5cm |
|
Weight |
1.7 Kg w/ rings and plate |
Data from Me.
Design
and Build
The Pentax-65 was sold from about 1982
through at least 1987.
Despite being just a little larger of
aperture than the more commonly found Pentax J60 beginners’ scope, the
Pentax-65 is an altogether more expensive and premium telescope (over $1000 on
its mount in the mid-80’s).
Everything about this OTA, from the
quality of the focuser, through the textured tube finish to the beautifully
made accessories, are suggestive of Pentax’s more familiar high-end offerings
and indeed Pentax also sold an ED apochromat that looks very similar.
Also sold on a matching (and otherwise
similar) medium equatorial was the Pentax-85.
Pentax-65 and contemporary Takahashi
FC-65 apochromat.
Optics
The
objective is a simple air-spaced achromat with a fairly short focal ratio for
the type of F10.7 (700mm), instead of the usual F12-15.
There are no
adjustment screws for collimation and just (good quality) single coatings.
Tube
Build
quality is superior to an entry-level Pentax J-60, or even a Vixen achromat
from the mid-Eighties. It is just a 65mm achromat, but clearly wasn’t designed
(or priced) as a kids’ scope.
The 75mm
diameter tube is finely made and finished in a textured pale lime powder coat
that’s honestly more practical than a high-gloss white.
Unusually
for a basic achromatic OTA, both the focuser and dew-shield thread on. The
dew-shield is baffled with threads at the front too and these would originally
have held a threaded dew-cap.
Internally
there are a couple of knife-edge baffles to kill stray light.
Focuser
The focuser
is a well fabricated rack-and-pinion unit with a cast body and black drawtube
and a lock screw on the side. It’s finished in the classic Pentax not-Vixen
dark hammered green enamel and bears a model sticker with a serial number. The
knobs are metal with rubber grip strips.
Quality
build continues out of sight, with three more baffles in the drawtube to help
with stray light and contrast.
Pentax do
things their own way and the visual back on the drawtube is an odd one. Instead
of a metric thread like a Vixen or Takahashi, it ends in what appears to be an
eyepiece holder with a set-screw. Except this is no 1.25” or 2” holder, but
something in between that measures ~38mm aperture. Helpfully though, Pentax
provide standard accessories to fit, see the accessories section below.
There is a
threaded port on the bottom of the focuser to mount a solar projection screen.
High quality
focuser has plenty of travel, but insufficient inside for imaging.
Pentax-65 is
over mounted on this Vixen SX2.
Mounting
This would
originally have been mounted on Pentax’s own unusual small manual German equatorial,
that had a standard polar finder and a some innovative features and was of very
high quality – more evidence this is not your typical small cheap achromat.
Accessories
The
Pentax-65 came with a number of standard accessories to fit in the unusual 38mm
visual back. All are unique in style suggesting they were made in-house and of
high quality:
·
A 0.965”
mirror diagonal
·
A 0.965”
adapter
·
A barrel
with an M36.5 female thread that accepts a standard 1.25” eyepiece holder (but
see comments below)
·
An M36.4
0.965” thread on eyepiece holder to fit the barrel
I
immediately threaded in a Tak’ M36.4 eyepiece holder onto the barrel and pushed
it into the visual back, only to discover there’s not enough in-focus travel
for a 1.25” diagonal plus eyepiece. This seems a puzzling limitation, given
that Pentax surely chose the short focal length to make a compact OTA.
The
Pentax-65 would originally have come with a good quality 6x21 finder, but sadly
this was missing (only the finder mount remained).
A perfect
match for this scope would have been Pentax’s own 0.965” MC Ortho eyepieces,
but these are rare and very costly now. Alternatives include Vixen’s and the Takahashi
MC Orthos I used for this review.
In
Use – Daytime
My standard test
of viewing silhouetted branches at 100x gives only mild false colour – purple
and green either side of focus and a wash of purple in focus – typical of small
achromats.
In
Use – Astrophotography
No. The
Pentax-65 simply doesn’t have enough in-travel to bring my DSLR to focus. Below
is a shot of the Pleiades at the limit of the focuser’s in-travel.
In
Use – Observing the Night Sky
General
Observing Notes
The
Pentax-65’s inability to take 1.25” accessories means I made good use of my
Takahashi MC Orthos.
The standard
Pentax mirror diagonal is of high quality, so I didn’t have to resort to Tak’s
own (a little dim) 0.965” prism, but the push fit feels like it could scratch
my barrels if I’m not careful!
The focuser
is good, smooth and precise, apart from that lack of in-travel.
Cool
Down
As you would
expect from a small doublet, cool down is fast.
Star
Test
Good, with
just a little under-correction as usual.
The
Moon
Long focus
achromats are lunar specialists. At 140x with the Takahashi 5mm MC Ortho, the
Pentax-65 showed lots of detail at last quarter, including the peaks and
slumped walls of Copernicus, Longomontanus to the south, the Aristarchus
plateau and rille.
However, the
Pentax-65’s reduced focal length means a light wash of purple on the Moon at
100x that F15 achromats avoid at this aperture.
Mars
I caught a hint of albedo detail with Mars at just 9.8” across, long
after the 2020 opposition. But I noted some false colour blur either side of
focus and a little in focus too.
Deep
Sky
My deep sky
viewing was mainly done using the standard 0.965” mirror diagonal with a
Takahashi 25mm MC Ortho’ giving 28x. Stars are sharp to the field edge with
this eyepiece and contrast is excellent.
M42 was a
bit dimmer than the best modern refractors with full coatings, but a good view
nonetheless.
The Double Double, a good basic test for a small refractor, was an easy
split at 140x with the 5mm MC Ortho. Trying to pick Rigel’s faint companion out
of the glare is a stiffer test and needs the finest optics at this aperture.
Sure enough, in this case I struggled to see it, but got there in the end.
The Pleiades
looked great – brilliant pinpoints shining in misty blue. But the Pleiades show
up a fundamental limitation of this telescope: the ~1.5° field of view only just fits all seven sisters and
that’s close to the maximum field you can get with 0.965”. Forget extended DSOs
and star fields with the Pentax-65.
Summary
The Pentax-65
is a puzzle. Optically it’s very good, whilst mechanical build quality is at
near-Takahashi levels – much higher than most small achromats. The standard
accessories are the same. I love small Tasco
achromats for their nostalgia value, but this little Pentax is in another
league and would have been an expensive scope new.
The optical
spec’, with its shorter-than-usual focal length seems carefully chosen to give
a compact OTA with good false colour correction.
The main downside
is that it seems designed for 0.965” only, like many small refractors of the period.
This limits maximum field of view to about 1.5°.
The Pentax-65 is a beautifully made
scope, a taste of Japanese high-end for a modest outlay. But lack of in-travel
limits it to visual with 0.965” eyepieces only.
