Pentax 75mm SDHF apo refractor review
Introduction
About a year ago I purchased a Pentax 75mm SDHF apo refractor from a german
dealer, Teleskop Service. It's primary function was to be an astrophotography
telescope and also an occasional visual and portable telescope. Previously
I owned a 120mm chinese achromat and my "big" telescope is currently an Intes
MN-71 mak-newt. I must say the Pentax is the first apo refractor I've ever
used - there's not many of those here in Croatia! Now that I've used it a
lot, both visual and for photogaphy, I feel confident enought to write this
review.
Image 1: Pentax 75 SDHF on HEQ-5 with Baader solar filter and Pentax eyepieces
Build & purpose
It's obvious the Pentax' main scope (pardon the pun) is astrophotography.
It's a three lens design with a front doublet and a field flattener lens further
back. Pentax guarantees small, pinpoint stars on the film/CCD plane (about
10 microns I think). I have no possibility to verify this, but I can say the
stars are as advertised - pinpoint acros the field of 35mm film, and DSLR
chips too.
Image 2: 100% crop of a starfield near M8 and M20, taken with Canon 20;
several images stacked; no sharpening
The little Pentax is well built, has great coatings and a nice, smooth focuser.
The focus can be locked with a sort of clamp (not just a screw), very useful
to keep the focus from slipping while taking photos. In short, you won't be
dissapointed with it's looks and build.
Adapters galore
You may have heard the Pentax' have some strange focuser/adapter sizes. This
is quite true, but fortunately I got a great adapter from Teleskop Service.
The adapter fits the Pentax' 2-something inch focuser. The adapter has two
threads - 2" on one side and M42 on the other. This is excellent - I can fit
a (D)SLR camera directly on the adapter or use a M42 to 1.25" adapter for
visual use. I use the 2" thread to screw my Astronomik CLS deep sky filter
in for photography. You should check with you dealer for availability of various
adapters before purchasing.
There's also an issue with various eyepieces - the Pentax 75 SDHF may not
have enough in-focus for some eyepieces for visual use. With my adapter racked
all the way in, I can reach focus with a standard TV 1.25" diagonal and my
Pentax 14mm XL and 10mm XW eyepieces. I can barely reach focus with a 2x Celestron
Ultima barlow and those eyepieces. The barlow can be put before the diaginal
and can reach focus without any problems. I also used a 5x powermate (works
fine), but a 6mm ortho (for which I think is the same as the University Optics
ones) cannot reach focus (without a barlow).
Visual performance
Since this is still the first apo refractor I used visually, you may take
my words with a pinch of salt - my only other apo experience was a brief look
through a TV 76 (before I got my Pentax) and a side by side comparison with
a Celestron 80mm ED refractor while looking at the Sun (with a filter, of
course!). Anyway, I feel I had to write this review since there's very few
first hand experience availible on the web on this great little scope.
Deep sky
In short - visual performance is stunning! Yes, the telescope has "only" 75mm,
but on many occasions I find the view more pleasing than other, bigger but
less contrasty telescopes. The stars are really tiny and snap into focus -
no "hunting" for the right spot needed, you'll know exactly when you're in
focus! The sky backgorund is pitch black, the sign of excellent contrast.
Indeed, as I said, often some extended objects (galaxies, nebulas) look more
pleasing in the Pentax because the excellent contrast makes them more visible
against the background sky. For example, the Messier galaxies in Virgo were,
although faint, all easily seen on this year's (2005.) Messer marathon (mag.
<6 skies). I was surprised to see the Crab nebula, brighter than I thought
it would be. I could also detect it's slightly irregular shape. Open clusters
are especially beautiful in a wide field eyepiece as my 14mm and 10mm Pentax.
Still, don't expect miracles, 75mm is not a lot of aperture but it performs
surprisingly well. It is a great telescope for travelling.
Planets
Planetary images are also very good for this aperture, but you won't see a
lot of detail. You'll easily see the two main belts of Jupiter and even some
mottling. Shadow transits can be seen but not very easily. On Saturn, the
Cassini division can be seen without a problem and also the different hues
on the planet itself. I was very surprised to see the detail on Mars while
it was still below 10" in size. The polar cap was seen easily and some dark
markings which I was able to identify as Syrtis Major. I borrowed a 5x Powermate
from a friend to try some really high magnification - amazingly, the contrast
was very good even at 250x!
The Sun and the Moon are also beautiful - I found no obvious signs of chromatic
abberation on either. I can see the Sun's granulation all over it's surface
without a problem. Comparing the views of the Sun with Celestron's 80mm ED
(both with my TV diagonal and Pentax eyepieces), it's obvious the Pentax has
greater contrast. The sky surrounding the Sun is much darker in the Pentax,
despite the somewhat higher magnification when using the same eyepiece (80ED
has a bit longer FL, 600mm vs. 500mm for the Pentax).
Images 3 & 4: Sun and Moon taken with Canon 20d and a 2x barlow; several
images stacked in Registax
Astrophotography
If you're looking for a small telescope for astrophotography, look no further!
I'm sure most of the telescope dealers will tell you the same. With a built
in field flattener and a large focuser, the Pentax 75 SDHF is specially made
for astrophotography. It's perfect for 35mm or DSLR astrophotography, but
I think you'd get some slight vignetting on a larger format, although it's
advertised for up to 6x4.5 medium format photography. It's true, the stars
are pinpoint from edge to edge and you'll get no vignetting on 35mm film or
DSLR.. The field of view is excellent for prime focus photography of galactic
nebulae and clusters - you can just fit Lagoon and Trifid nebula in the same
FOV when taking photos with a Canon DSLR (300D or 20D). You can see some examples
in this article.
For 35mm photography on 400 ASA film, you'll need exposure times of 30-40
minutes. A good, sturdy mount and an autoguider (or a lot of patience for
manual guiding!) is advised. If you made your preparations well (mount alignment,
focusing), you'll get some great photos! If you find the stars elongated on
the edge(s) ? it's not the telescope's fault and you should check your mount's
tracking instead. Some day I'll try the same technique as with DSLR's ? taking
multiple exposures of the same subjects and stacking the scanned negatives/diapositives
to get an improved image. I think it would work great ? no dark frames needed
and no amp glow from the DSLR. The main problem is finding good focus on film
? you can't check the focus on the LCD screen!
I must repeat this ? the little Pentax is optimized for astrophotography.
I don't think you'd find a similar telescope in this price range with a flatfield
corrector included. Other apo's may be OK for smaller CCD chips, but you'll
get elongated stars in the corners of 35mm or DSLR photographs.
Image 5: M45, Pleiades, single20min photo on Fuji Sensia 400; full frame
My experience has mostly been photography with DSLR's ? Canon 300d and 20d.
I usually take photos with an Astronomik CLS deep sky filter. Exposure times
are around 10 minutes @ ISO 800 with the filter and 5-6 minutes without the
filter. There's a light blue halo around brighter stars that cannot be seen
on film photos. Still, I think it's not that bad ? the cause may also be the
deep sky filter. Sorry, I did not make any tests to verify this. Here's another
image of Pleiades to see the effect:
Image 6: M45, Pleiades, 6 photos stacked, Canon 20d; slightly cropped.
The little Pentax is also great for eclipse photography or a full Sun/Moon
photo with a barlow. I'm guessing it would be great if you'd like to capture
the solar corona in prime focus during the total solar eclipse. With a barlow,
the Sun and the Moon almost fill the FOV of my Canon 20d. I also tried a few
terrestrial shots - it's doable even without the tripod, but I reccomend you
use some kind of mount for it. When shooting with a barlow lens, I get uneven
illumination on the image. This is very obvious when taking images of terrestrial
subjects during daytime, but presents no problem on the Sun and the Moon.
A dedicated focal lenght extender is availible from Pentax, and a focal reducer,
too.
Conclusion
I love this telescope! Perfect for astrophotography, with a few issues for
visual observing. Again, if you'd like to take high quality photos and don't
want spend a fortune on big apo's, this is it - a no compromise telescope
for astrophotography which can also be used for some pleasing visual observing.
Pro's
- pinpoint stars and flat field
- excellent for high quality astrophotography
- very good, smooth focuser (smooth even at -15 degrees Celsius) with
a locking mechanism for astrophotography.
- no chromatic abberation in focus, visually
- small, great for travelling
- excellent build quality
Con's
- you need special adapters to use western standard (non-japanese)
2" and 1.25" eyepieces
- even with adapters, some eyepieces may not reach focus - consult
you dealer first
- expensive accesories (finder, focal reducer and extender)
- you'll get used to Pentax image quality - if you get aperture fever,
you'll find out the bigger refractors are quite expensive :)
More astrophotos
Image7 (left): M42, 6 images stacked, Canon 300d, ISO 800, Astronomik
CLS deep sky filter, 6 images stacked, exposure times 12 min for the faint
parts
Image 8 (right): Comet Machholz, 6 images stacked, Canon 300d, ISO 800,
7 images stacked, exposure times 4-5 min
Image 9: M31, Andromeda galaxy, Canon 300d, ISO 800, 4 images stacked,
no filter
Image 10: M8 & M20, mosaic of 13 images, Canon 20d, ISO 800, Astronomik
CLS deep sky filter
