From time to time, discussions spring up over this issue. The new triplet APOs usually come out on top with flawless color reproduction and great contrast. Several months ago the last new Takahashi FS-152 APO doublet showed up on Astromart. After a few Emails to Herb York (owner of Astromart), the FS-152 was on the way to the Lake Murray Observatory. Then another stroke of luck. A friend received a new Astro Physics 140 APO. Trouble was he just purchased a new Takahashi TOA-150 Triplet APO. A few more Emails and the APO triplet was on the way to too. While individual samples of an instrument will vary, these two OTAs seem good representative samples. No major problems, good star test and solid construction from a reputable manufacturer. Also, both are nearly new.
Keeping everything on target, an AP-1200 mount was used for this evaluation Robin Casady's Triad bar with Losmandy and Casady clamping plates held the scopes. This allows both scopes to view and image the same object at the same time through the same air column. Between the two is a Tak FS-60c holding the CCD camera for the STV autoguider. On top of the FS-152 is a Tak FS-78 for some widefield imaging.
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Here are the specifications from various sources. Seems some specifications. for the TOA are hard to find or are outdated. I measured the weights of the TOA. It’s certainly a heavier scope. Especially with the 10 pound counterweight attached.
SPECIFICATIONS for the FS-152 (Tube assembly only)
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SPECIFICATIONS for the TOA-150 (Tube assembly only)
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Goals:
I’ve heard a lot about false color in doublet APO designs so when I mention color I'm looking for a purple tint around the Moon's limb and white stars. It's very easy to see in an achromatic or even a run of the mill ED scope. Another item for comparison is sharpness. Some say a triplet APO is sharper than a doublet since the three primary colors come to a more precise focus. And then there is contrast. Fine APO doublets (especially fluorite doublets) have a reputation for very good contrast. .
Some issues on the scopes:
With any new products there is a chance for "issues"…. even with Takahashi APOs.
TOA 150:
The focuser would not work at all with a camera and MaxBright diagonal. I took the cover off and shifted it slightly. This seems to work. It may have been binding the tension collar on the 10 to 1 side.
The camera adjuster didn't work. You move it to match the position of the camera on the FS-152 and it wouldn’t tighten. Turns out the whole assembly was loose on the draw tube. Clamping the adjuster bolt down and tightening it was all it took to fix.
When setup, this is a heavy scope. With the 10 pound counterweight, rings and a dovetail it’s nudging 50 pounds. With its sliding dew shield and smaller tube length it is still an easy scope to handle.
FS-152:
Focuser drags to beat the band. I finally got it adjusted to "pull uphill" with a Tak diagonal and Canon DSL but the tube is very hard to move. This is with the focus knobs on either side. Makes me think I'm about to strip the drive gear. A lot of tinkering has it moving along well now.
With rings the FS-152 is less than 30 pounds. But is it ever long. 54 inches before diagonal and extending the focus draw tube any length. The fixed dew shield also hurts portability. It’s not for an overhead bin in most airplanes.
Cool down:
The TOA has a 10 pound counterweight attached to the rear of the scope. With the counterweight and the weight of the triplet lens cell cool down time would seem slower. The TOA did take longer to cool over several nights testing at various temps. drops. In order to quantify the rate, I used a RadTech thermometer and measured the surface temp of various points on the scope bodies in a three hour period. The lens temperature is from direct measurements of 4 points on the glass surface at the given time and averaged. The body temps are also four readings that are averaged. The FS-78 was easy to include and may help in comparing. For the TOA, the massive 10 pound counterweight circling the base of the scope also is included. I thought it would lag and hold the area of the scope it was in contact with at a higher temperature but it seemed to follow the temp drops in step with the other portions of the OTA. Larger scopes with more mass do indeed cool down at a slower rate. Even refractors.
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Visual:
Visually both perform well with the usual APO pinpoint stars and contrasty backgrounds. No problems with either on the Trapezium E and F stars, Alnitak's double (no flaring problems covering the double like I've seen on some other OTAs) and Mars looked great in both. (I know all this is easy stuff but if you can't do the easy stuff the hard stuff won’t be possible.) The triplet APO has the edge in fine color accuracy. Mars showed distinct markings and great color in the triplet while the doublet did not bring out the same detail and had a warm cast. Side by side with the same EPs (a Nagler 3-6 zoom really shows the detail here) comparisons were easy to make. The APO also seemed easier to focus or less critical finding precise focus even with the lower Focal ratio. Without them side by side over a long period I don't think I could rate one better than the other.
Even the legendary doublet contrast is not detectable using them side by side. However, the doublet FS-152 has a longer focal length. I’ve noticed longer focal length scopes don’t show as much contrast when compared to other shorter focal length scopes of the same construction/quality. The magnification dims contrast a bit under some seeing conditions and this may have affected the Mars observations.
False color was checked on both OTAs using a variety of stars and EPs. At moderate power the results were the same in both with neither scope showing noticeable false color. I then put in an AP x2 barlow with a Nagler 3.5mm T6. This raised the magnification to 694 on the FS-152 and 628 on the TOA. The TOA going from outside focus to inside focus and back showed no false color. The FS-152 going from outside to inside and back showed minor greenish yellow on outside and minor purple false color on inside focus. As you approached focus on the doublet the color slowly diminished and when crossing exact focus the color seemed to melt away. At one very small sweet spot the doublet showed almost no false color. The triplet would be much easier to work with and less forgiving at these powers. However, seeing conditions would rarely permit this much magnification.
At high power on the Moon, I went all around the limb moving through focus with both scopes. The TOA was rock solid with no visible color. The FS-152 doublet again showed a minimal green or purple tint as you traversed focus but again, right at focus, the color was very hard to detect. Using the Nagler 3.5mm I could just barely make out the faintest tinge of brownish green shading almost within the edge of the Moon at exact focus……. in both OTAs. The shading stayed with the EP and was not visible in a 7mm Nagler T6. Both showed remarkable detail and contrast with both EPs.
The Double Double was also low in the sky and testing here would duplicate bad seeing conditions. Refractors are often said to beat seeing so this seemed a good object to have a look. Both scopes made the split/splits easily with the two Naglers. I then put in a 35mm Panoptic and couldn’t make the split in either scope. They were oblong but no clean split. Then a 10mm Pentax XW was tried on both scopes. With the target low in the sky the 152 doublet looked the cleaner split to me. Over and over I swapped the EP and moved through focus. The TOA was viewing at 110 power while the FS was at 120 with the 10mm Pentax. This might have had some influence but there was still less than 10% difference in magnification. Each time the close double just jumped out at you with both scopes but the 152 view was a tad clearer with just a bit darker background. A very pretty sight.
I repeated similar test on both scopes over several clear nights. A week later under Moonless skies I did another star test for both. On Sirius they both looked great. Just a touch of color at 500 plus power in the doublet that seemed to go away at one critical focus spot. At a very small focus zone it would lose all color with a 10mm Pentax XW and all magnification positions of a Nagler 3-5 zoom. On progressively dimmer stars the diffraction patterns of both scopes were textbook. On Mars the FS doublet had just a hint of warm color while the triplet had lighter tints and was very natural looking. To my eye the triplet showed more detail and clarity. The legend of the doublet is well deserved and the best for it's time. But when Takahashi brought out the new and improved triplet.... it is an improvement. You pay for it in weight and cool down time but the benefits are there. Either one just beats anything I've tried yet for ease of use and results. The 6" APO has got to be the sweet spot of the market.
Imaging:
A few sample images are included below. Both scopes are easy to image with. The TOA has a much better camera angle adapter. The FS-152’s camera angle adapter has three small knurled knobs screwing into a “V” grove on the draw tube. The TOA’s angle adapter has one knob and a very positive feel. The FS-152 has slop when you loosen the knobs. I can’t help worrying about loss of critical focus when adjusting the FS-152. It seems to hold but doesn’t “feel” right when adjusting. For imaging, focusers are adequate on both after some adjustment. Nothing special and no special feel. A FeatherTouch or Moonlight upgrade would help both in the “Look and Feel” department.
On a Canon DSLR size chip, the TOA needs a field flattener while the FS-152 stays sharp to the edge. While not as obvious visually with a widefield EP, you can see the oblong stars below.
Cameras and focus:
A Canon 20Da DSLR (Digital Single Lens Reflex) camera and a Happ Griffin modified Canon 350D DSLR are used on most objects. Where the extra IR response of the modified Canon 350D isn’t needed a stock Canon 350D may be used. Focus is set on both with a Stiletto focus aid and confirmed with the 20Da “live focus” feature at the FC-2 setting (most magnification). Focus is rechecked after every image run if the temperature is dropping.
FS-152 image of the Double Cluster. Lower left magnification is of upper left portion of image.
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TOA 150 image of the Double Cluster.
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A side by side Moon Image:
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Moon limb:
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FS-152 Deerlick Group image with TOA-150 central galaxy superimposed.
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Conclusion:
Overall, these things are so close it's scary. Just the smallest bit less color in the TOA. The camera adjuster is worlds better on the TOA. Functions like it should. The FS-152 camera adjuster is less secure and flops the diagonal when loose. The TOA is a nice size and has a weight similar to a Celestron 14” SCT. The FS-152 is huge and weighs similar to a Meade ETX. The FS-152 doublet is no longer in production so the used market is the only source. The TOAs are off the shelf available. A real plus when you want to get into the 6” APO world without a substantial wait. Which is my choice? I’d say either one. They are close enough to continue long term testing and comparing to other scopes before deciding. Maybe a slight edge to the TOA right now. But that FS-152 is just plain classy and cool.
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