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Pentax 105SDP Apochromatic Refractor

Posted by Pollux Chung   08/15/2008 07:42PM

Pentax 105SDP Apochromatic Refractor
Pentax is no stranger to everyone in North America for their sport optics and camera products. Telescopes? Probably not so.

Why I go Pentax?

But why did I chose this route anyway? Probably due to my love of "made-in-Japan" as I have been a huge fan of Japanese products since I was a kid. I watch Japanese anime, collect Japanese action figures & models, all the cars I have owned are made in Japan. Furthermore my hi-fi system at home is made in Japan, my piano is made in Japan, my camera is made in Japan, the stationery I use is made in Japan. Even my first telescope that my father bought me when I was 7 was made in Japan. I guess you have picked up my obsession here :-)

I started astronomy as a hobby when I was 7 and stopped completely when I was 13. My enthusiasm in astronomy returned in full force in 2003 inspired by the Mars opposition. I bought myself a Skywatcher 5.1-inch reflector as a start, just to see if I would stick with the hobby again or not. I definitely was sticking, going through over 20 low to mid-range telescopes in two years.

In the summer of 2005, whenever the sky was clear I went up the local mountain with some friends. At that time I was still debating if I should upgrade to higher end equipment when a friend of mine lent me his Pentax XW20 eyepiece. As soon as I looked through this high quality eyepiece my "Japanese Optics Saga" began.

In the spring of 2006 I started to collect Pentax eyepieces. I looked at classifieds and auctions sites almost everyday hoping to find good deals. It has taken me about 14 months to complete my eyepiece collection. I now have a full set of Pentax XW series eyepieces and a few extras from other Pentax series like the XF's and XL's. Because of my Pentax craze friends have given me the nick-name 'Pentux'.

I take this as compliment ;-)

Moving Forward...

After upgrading my eyepieces I started the same process with my telescopes: I have sold all my Chinese made telescopes and now only own Japanese.

In the year of 2007 I acquired my first Japanese imported refractor, an ED refractor by Vixen, which is a very nice scope and definitely a step up from the Chinese ED refractors. At the same time my friend's Televue NP127 showed me an impressive flat- field view. Since then I wanted all my future telescopes to have a built in corrector. In pursuing upgrading my telescopes I was looking at what options were available and Pentax came to my mind.

Risk, risk, risk.................

I have to admit, I was taking a huge risk when I decided to acquire this Pentax telescope. At that time Pentax Canada still had not finalized any retail prices for their telescopes when I made the purchase. That meant the price could be much higher than I bargained for. Fortunately due to the strong Canadian dollar when I placed order (January 2008), I was able to acquire it at the price in line with the US.

Another risk and complication arose as there is no official Pentax telescope repair centre in North America at all (as far as I know). Therefore if goes wrong with the telescope it will have to be returned to Japan.

Pentax telescopes are not cheap, and they are the most expensive refractors I have ever bought. Having paid a considerable amount of money my expectation was set very high, I had many sleepless nights while waiting for the delivery. I wondered if I had spent my money wisely, or I was going to be very disappointed.

Wait finally over!

After three months of waiting the local telescope store finally phoned me; the telescope had finally arrived. I got to the store and was surprised that the box just looked very clean and simple. It was smooth and thick cardboard with no frills, just a very classy package. As a graphic and visual guy I am very picky on first impression; the packaging to me is important.

Un-boxing the scope

When I un-boxed the scope I was extremely pleased with the appearance of the scope. Very "high class."

The scope is very well made and heavy - about 14 lbs just for the tube itself. I managed to get the scope reaching 20 lbs with the (Pentax) 50mm finder, 2" diagonal, 2-inch 30mm Pentax XW eyepiece, dual-dovetail, etc.

The scope is painted in metallic pearl color with a glossy overcoating. One thing that surprised me is that the dewshield of this scope is NOT retractable (the 75SDHF dewshield is). However, this scope is still short - only a bit longer than a Takahashi TSA102 (with its dewshield retracted). The PENTAX logo on the dewshield is painted on, unlike other brands which use stickers.

I really love the quality of this telescopes baffling and blackening which, in my opinion, is very well done, ensuring it is perfectly dark and free of reflections. All lenses are coated with Pentax' proprietary SMC coating and the glass is almost invisible when looking at the objective lens from the front. With a telescope built to this high level I was expecting the contrast to be very high.

The scope comes with no finderscope: this is a $320 option. Yes, this is painfully expensive, but since I care about appearance so much I had to get one for the telescope. The finder is just like the telescope, overbuilt and with great optic quality. The finder does provide a cross hair which is a nice touch; however it does not illuminate, which is disappointing.

The oversize rack-and-pinion focuser (with a whopping 90mm wide drawtube) is robust and heavy. It certainly can handle heavy weight without any complaints. The focus knobs are huge and the tactile grip is excellent. Focus action of this focuser is smooth and has absolutely zero play or shift.

One thing I have to mention: all Pentax refractors have an unusual visual back as they are designed to be used straight through. If you plan to use a diagonal with the visual back that came with the scope, good luck....... Fortunately there is a workaround. Baader Planetarium offers special low-profile adapters to allow Pentax refractors (except the 75SDHF) to adapt 2-inch accessories without inducing any focus problems. You will need 1) Pentax Adapter (M84/M68) *PLUS* 2) Zeiss/Vixen compression clamp adapter (T2-32) or Four In One Clamp adapter. I got them from Alpine Astro.

I was at first freaked out by this adapter problem. This is an ultra-expensive scope; now I have to shell out more money for a visual back? You gotta be kidding me! However in the cool light of day I realized I always needed to spend extra money on things like compression ring visual backs for even my previous (non-Pentax) scopes...........

The design

The optics of this scope are based on the classic Petzval design, with a doublet objective in front and a 2-element corrector before the focuser. The front lens of the doublet objective is made of SD (super-low dispersion) glass, and the front lens of the corrector is made of ED (extra-low dispersion) glass. The aperture is 105mm and the focal length is a short 670mm (F/6.38).

With the heavy-duty focuser and 4-element optical layout this scope was designed especially for imaging. According to the specifications provided by Pentax this telescope is capable of providing an 86 mm image circle with the ability to support heavy equipment like large format film cameras or big CCD cameras. One thing to consider, because of the corrector lenses and the big focuser, is that balancing this scope can be tricky as it is very BACK HEAVY.

Now, let's do some stress tests

Since I don't have access to any of those large expensive imaging devices I will focus on the visual performance. I also will show a few photos (can't really show too much as my skill isn't that great).

First, I used an artificial star to check for spherical aberration. The diffraction rings of the star intra and extra focus are identical even at high power. I immediately told myself this is a very good sign and planets are going to look great through this scope.

Next, false color. This scope, I have to tell you, doesn't know what false color is! In general, good ED doublet refractors show no false color in focus but will show red or blue when out of focus. Perhaps, because this Pentax is not a simple doublet design it shows amazing color correction. You will never see false color whether you are in or out of focus. To my eyes, it is just as "color-free" as a reflector. Not only that, the image sharpness and amount of detail are still amazing even at insanely high powers.

At night...

Now lets move on to night-sky objects. The eyepieces that I used were from the Pentax XW series.

Due to the completely sealed design this scope does take a noticeably longer time to cool down or warm up compared to other doublet refractors I have used. In extreme conditions (like 15-20 Celsius of difference in temperature) it can take almost 45 minutes to reach thermal equilibrium.

First on the moon. Pretty much what you would expect from an apo - 1) Sharp, contrasty image 2) Black shadows 3) Zero false color even at high power.

I am not too crazy about looking at the moon so I moved on to the next target.

Saturn was still visible in May and June (though close to the horizon). Under medium and high powers the image was very 3-D. The planet displayed a distinct yellow with darker belts highly visible (I saw three or four belts). The Rings showed a contrasting white. Even with the Ring's narrow opening I still managed to see the Cassini Division clearly although it took a little effort.

Jupiter was absolutely breathtaking. I did a side-by-side comparison between the 105SDP and various popular apos (doublets, triplets) from Japan and China. I looked through the "other apos" first. Jupiter looked great. Sharp, contrasty and no obvious false color. The planet showed as a bright (slightly off-white) disc with medium brown color belts. Then I moved to the 105SDP. I was shocked! The planet showed as a pure white disc with dark brown belts. The image was so sharp and contrasty that I could see the texture of the belts right away even at medium power. Then cranked up the power to around 335X and the planet STAYED sharp, contrasty, and zero false color. Then I moved back to the "other scopes" again.......oh boy! the Pentax had spoiled me: through some of the other scopes Jupiter was actually displaying a purple tint

Now on to some deep sky....keep in mind this scope is only a 4.1-inch instrument. Faint objects won't be visually spectacular. But brighter clusters aren't bad at all.

Since the scope has a very short focal length there really is no need to use a 2-inch eyepiece and diagonal. A 70-deg XW20 will give over two degrees AFOV (which covers many many objects). Of course, I could still use my 2-inch eyepieces and diagonal but they would make the scope needlessly heavy (as I mentioned earlier).

On to the Double cluster. Absolutely unforgettable! Background is absolutely black. Stars are all pin point across the entire field of view. If you see distorted stars in the field you definitely need some help ;-)

On M13, I managed to "resolve" the faint core of M13 despite the scope's limited aperture. Some other brighter nebulae were spotted without much difficulty due to the scope's high contrast view.

How about imaging? Well, I don't have a fancy large format CCD camera. All I have is a Canon Digital Rebel XT, and my so-so skill in astrophotography doesn't help either. I'm including a few shots here anyway just to give you some idea on what I have achieved so far with this scope. Promise me. DON'T LAUGH!

In conclusion

So, what's my verdict? If you are talking about image quality this scope is VERY tough to beat. Sharpness, contrast, color correction, field flatness are absolutely top notch. To my eyes this is the best 4-inch refractor I have ever looked through. Of course, YMMV. It would be impossible for me to let go of it, unless I decide to move up in aperture ($$$$$). I really hope Pentax will get a bit more aggressive on marketing their astronomy products in North America.

What I like:
• Build quality - Everything is overbuilt, amazing baffling and blackening, "high class" feel
• Outstanding image quality - Ultra high contrast and sharpness. False color - where is it?
• Can take insanely high power
• Relatively compact despite the lack of retractable dewshield.

What I dislike:
• Price - Expensive, expensive, EXPENSIVE!!!!!!!!
• Unusual visual back forces users to acquire third party adapter to allow standard 1.25" and 2" accessories. Yes I can use 1.25" eyepiece with the scope's straight through adapter, but I don't want to break my neck.
• Takes a while to cool down
• Repair can be a bit of a problem to some (have to send back to Japan unless Pentax ever decides to have an official repair shop in North America)

Note: I have no affiliation with Pentax or their affiliates in any way or description.