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William Optics FLT110mm f7 TMB designed triplet and matching field flattener

Posted by dominique dierick   01/04/2007 12:00AM

William Optics FLT110mm f7 TMB designed triplet and matching field flattener
First impressions

After nearly 16 years of owning and using apochromatic refractor telescope from Astro-Physics (128 Starfire, 105 Traveller, 130EDFS, 155EDFS), TMB (80mm, 105mm) and Takahashi (FS60-C, TOA130, FSQ106), I thought it was time to give a newbie on the market of APO refractors a try. William Optics has built itself quite a good reputation the last few years, and the only scope I tried from them was a Megrez with triplet Lomo TMB lens of 80mm, a couple of years ago (I still use a TMB 80mm triplet as my all time favourite grab and go scope). For me testing equipment is as big a part of the hobby as taking pictures with them.

So I decided to get a FLT110mm triplet design refractor from them, together with the new matching field flattener with adapters for DSLR and CCD. The price seemed to be really right for this size of an apochromatic refractor. Lenses are made in Taiwan, not Russia like the well known TMB triplets on the market, which made me even more curious.

The scope arrived a month ago in a big cardboard box, well packaged for airfreight transport. No issues there. When I opened up the box, I found the refractor sitting cosy in a nicely fitted lightweight black alu flight case. That’s already a great start for any telescope. When you get a TOA130F or a TMB 105, you have to go look for a case. I thought these scopes could have had a case included in the premium price.

The image below shows the refractor as it arrived last month. Beautiful to look at (although the gold accents may not be for everybody’s taste)…

The finish of the optical tube is white pebble, giving the impression of long lasting durability and stability. Mechanics of the focuser are impressive and one gets the impression it is possible to tune it to the very last bit. The focuser has two rotating mechanisms, one for the complete focuser, and one at the end of the drawtube for an extra camera rotation. Perhaps a bit of overkill, but still, if you want to rotate the camera independent from the focuser knobs, it will be possible.

A little warning should be made here. Loosening the focuser rotation mechanism setscrew too much will cause the entire assembly to tilt suddenly in the rotation holder. I haven’t tried to unlock the setscrew completely and see whether there is a risk of dropping the entire focuser, but ok, if you know that care is needed, I don’t see a major issue here. Unlock the setscrew just enough so you can rotate the focuser. If you use heavy accessories you may want to screw in the plastic setscrews a bit with a flat screwdriver as WO suggested to me. By default it ships quite loose. This does help in fixing the issue, but it’s not the same quality feeling as the Takahashi TOA rotator or the 3.5” Feathertouch focuser rotation mechanism though. So, rotate first, fine focus then is the message here. Anyway this is a policy I am using with all rotation mechanisms on focusers.

The focusing mechanism itself is buttery smooth and stable for visual and DSLR use. I could not detect imaging shift while focusing. The dual speed mechanism is a blessing for every telescope and in this case very well done. It certainly feels more like the well known Feathertouch quality than the one of the Takahashi TOA dual speed knob (which I think could be made better). The focuser locking screw does what it has to do: lock the focuser in place when needed.

But again a little warning… by default the tension on the focuser is NOT sufficient for an SBIG STL camera. You need to lock the hex screw underneath the focuser to avoid a potential heart attack when you see your SBIG slipping … fast! Fortunately there is an internal mechanism of the focuser which prevents the drawtube from falling out, but you don’t want to take any risk with your expensive camera. It is a bit difficult to find the right balance point between adding friction for a heavy STL camera and still keeping smoothness with the 10-1 reduction. Too much friction and the focusing knob will start slipping; too little friction and your camera will slip. When the hex screw is tightened for the STL, the standard locking thumbscrew became quite useless, it does not lock anything anymore and I was relying on the hex screw friction alone. William Optics suggested replacing the thumbscrew with a longer one so that it locks better under heavy weight. They also mentioned using not only the large hex screw to put additional friction, but also the two other (smaller) ones, again with the drawtube fully racked out when performing this operation! Basically when doing all this, one should find a good combination. At least it works for my STL. On the image below you can see the camera sitting cozy held by friction on a vertically placed tube. Focusing is still possible. I can’t show this so you will need to believe me on that one.

The drawtube itself is engraved with a scale. Handy to more or less raw focus cameras after you have determined the needed distance once. It will not be good enough for repetitive CCD or DSLR work, but you have a starting point for fine focus very fast. I remember that with my AP focusers, I marked the drawtube with a thin hairline using a razorblade. No need to get the razor out with this focuser.

The 2” adapter only has one setscrew. I don’t like that kind of system, especially since the 2” WO adapter is a bit on the wide side and definitely not the best part of the focuser. My Herschel wedge for solar observations did not sit very comfortable in the focuser, was difficult to lock in place and hard to keep it square. I do have a Baader multifunction 2” adapter which happened to fit the 4 inch William Optics focuser, so I replaced the original 2” adapter with the Baader one (which is more robust and has two setscrews to lock accessories) to get a much better fit. You can see the Baader adapter on the STL setup picture. OK, I am being picky on these things. A scope may be optically excellent, but if you cannot mount accessories square, you reduce the potential. It doesn’t cost an arm and a leg to make a good adapter like the clamp ring style of the 3.5 inch Feathertouch or the 3 setscrews of an Astro-Physics focuser.

There is one thing I regret with the WO 4 inch focuser. It has the same end threads as the Takahashi focusers, but female, whereas the Takahashi has male threads. Too bad, as it would have opened the door for use of all Takahashi accessories like their fabulous reducer-correctors! Anyway, I had an adapter made locally by Eric Moerman to convert to Takahashi threads, so I can use their 2.7” reducer-corrector for photographic testing of the FLT110. It should be easy for William Optics to add such an adapter to their product line. As a matter of fact, standardizing connections in telescope land would be a great thing in general for consumers…

Perhaps this all sounds a bit negative on the focuser. For many purposes it will be excellent, but one should know it does have its little shortcomings which you may need to look after with heavy accessories. A few of these shortcomings could have been factory solved easily.

The tube itself is well baffled and dark inside for high contrast. Looking from the back through the telescope, I do not have the impression any of the baffles would vignette the light cone, although the first baffle on the front end is pretty close.

By now it is time to mount the telescope on a mount. The image below shows the FLT110 on my AP600 GTO mount, with the dedicated field flattener and DSLR adapter in place.

The FLT110 is light enough a telescope (6 to 7 kgs depending on accessories) to not require a heavy equatorial mount. It will fit easily on a Vixen Sphinx or GP-DX mount, or on Losmandy’s GM8 equatorial system. So portability will hardly be an issue, not even for people who need to drive to an observing site.

Visual observations, in depth comparisons

Even though I am a photographer more than a visual observer, I decided to first have a good visual observation run with the telescope before star testing it. This avoids getting biased upfront. The telescope certainly does not disappoint visually.

Compared with TMB 80mm and Takahashi TOA130F

I observed some deep sky objects with Nagler eyepieces and the stars to me are sharp as needle points. My TOA130 still does a better job in rendering the stars perfect, but this scope is not in the same price league! The moon is quite spectacular in the 110mm telescope and views are gorgeous with plenty of contrast and resolution. Saturn pulled up a nice show, even at 277x (5mm TMB eyepiece and 1.8x TMB ED Barlow lens) it was still very good. Higher magnifications could be possible, but some slight image softness started to become noticeable at 277x, so I did not push it beyond that limit that night.

I compared the FLT110 with my 1/10th wave TMB 80mm (made in Russia) and found the in and out focus images of the 80mm to be in a somewhat higher league than the FLT110. While the in and out focus images of the little TMB are virtually textbook perfect, much like the TOA130F I have (which again is a bit better than the TMB - how perfect perfect can be I wonder), I do see slight residual spherical aberration left in the FLT110 telescope and a tiny bit of a zone. Not much, but it is not a perfect star tester. The patterns are not as well defined as with the 80mm TMB or the TOA130.

Now, how serious is this slight residual spherical aberration and little zone? Not that serious I would be worried about it. It will take superb seeing conditions, excellent eyepieces and a keen visual eye to detect real differences on a planet I thought that night. Unfortunately William Optics does not give absolute quality numbers for these refractors, so it is difficult to judge if the one I received is representative. If judged by the simple criterion “Better than diffraction limited”, it most certainly passes the visual test.

For curiosity sake, on another night I pushed the magnification on a distant spider web (illuminated by a streetlight) to 385x and the image remained sharp. I perform this spider test with all my telescopes, and the WO obviously did very well. Comparison with the TOA130 on the same spider web target definitely gave a resolution and brightness edge to the TOA (with 2 cm more aperture) which was to be expected. The image and contrast quality was very close, with the TOA still giving an advantage in sharpness and contrast (which does not surprise me as the Takahashi TOA is a superb piece of optic).

The image below shows the test setup with the FLT110 and TMB 80 on my AP1200 GTO mount. Please note there is a blue sky each time I take an image of the equipment. Usually this blue sky disappears as soon as I want to do observing with the instruments.

Compared with TMB 105mm

We (Philippe Vercoutter, Jorg Versluys and myself) also compared the TMB 105mm f6 with the FLT110 under the dark skies of Grandpre, France. Given the star test reveals even tiny imperfections, we started with that test this time.

We tried to evaluate the in and out focus patterns as good as seeing would allow us and the results were very good visually, but not as symmetrical as the patterns in the TMB 105. We also tried recording the impressions on video. Ideally, such video tests should be done in-house in lab conditions we soon realized. Seeing was not really good enough to allow the desired highly magnified star patterns on video and still keep enough resolution in the resulting image stack. But ok, the low magnification test does give an idea on what to expect. The human eye still has an edge on adaptive optics it seems wink Perhaps I will try to do the test with video later on to show a higher magnified version. Given the fact it is cloudy most of the time, that could be in a very distant future though!

The left side of the image below shows the infocus pattern, the right side the outfocus pattern, while the middle is a star in focus with the first diffraction ring visible. The outfocus image as usual suffers more from seeing turbulence.

One can see a few “nicks” in the outer ring of the pattern. This may have been caused by the producer clamping the lens on 3 points during production, or even from lens cell effects. Given the effect is so minor and does not really give the impression of pinched optics, I did not bother to dig deeper into possible causes. For the perfectionists amongst us, this may lead (being careful here) to bumps or tiny spikes in the halo’s around stars when using image histogram stretching with DDP processing on long exposure images. But if you would use a DSLR or one of the blooming CCD’s most likely they will give more artefacts around stars on their own already.

All images are the result of a stack of a few AVI movies in prime focus of the FLT110, made with a DMK 21AF04 firewire camera.

For the visual observations, we used similar eyepieces from TMB, and both equipped with a 1.8x TMB ED Barlow to obtain higher magnifications. The three of us had a very hard time to detect differences between the Orion Nebula trapezium stars and immediate vicinity.

Both scopes show clean sharp stars with nice hints of real colour. One observer noticed a slight contrast advantage for the TMB 105, but not by all that much. I did not detect significant contrast differences on the nebula, nor any difference in the sharpness and clarity of the Trapezium stars.

Saturn under good seeing was very nice in both scopes, with no differences to be noticed on the Cassini division, nor in the belts on the planet. We knew about the slight optical imperfections upfront, so we really looked very carefully to spot effects from it, but without success. Saturn was slightly brighter in the FLT110, showing 0.5cm more aperture. I did have the impression the equatorial belt on Saturn may have been slightly darker and give the idea of a bit more contrast in the TMB 105, but I wouldn’t place a bet on it, as it can be related to the slightly brighter overall image. Both telescopes showed the subtle colour hues on Saturn.

All this really makes evident how critical a star test can be. Everything shows, even marks that have in practice little influence on the end image.

The clever design of the TMB 105mm dew shield (seen on the picture with Jorg) did prove its use in the field. Thanks to the tiny front baffle in the dew shield, the TMB 105mm remained dew free during the night with lots of moisture in the air. The FLT110 has no such front baffle and needed a Kendrick dew heater after a few hours.

Jorg Versluys is seen carrying both the TMB 105mm and the WO FLT110. Notice the heavier looks of the TMB 105, but it is not really that much heavier than the FLT110.

While I was doing the star tests, another thing to look for was the centring of the lens. Since the lens cell cannot be collimated (there are no push-pull screws, just like on the smaller TMB 80 or AP’s Traveller), there is no room for error here. William Optics did a good job here: the lens is well centre collimated (as can be seen on the star in focus on the star test image too).

Colour correction of the FLT110 is excellent. I could not detect false colour on the moon or bright stars. Defocusing the telescope did pop up ever so slight colour at the lunar limb when seeing degraded the image temporarily, but since the moon was only 40 degrees up in the sky it could very well have been diffraction colour. Nothing to worry about in practice again, as nobody looks at an image out of focus anyway.

Compared to the 80mm TMB, the moon colour in the FLT110 was a tad warmer in tint. It reminds me of the tint of the moon in the Astro-Physics 130EDFS and Traveller I once used. The coating of the lens itself, when inspected with a flashlight, looks good. I have no means to measure transmission etc., but it did not show visible defects to me.

Perhaps the best way to categorize overall visual optical performance of the FLT110 is to look at a lunar image I made. It is constituted out of 8 AVI movies with a DMK 41AF02 megapixel firewire camera for the luminosity part. I combined it with a few images made with Canon 20D for colour. A Takahashi Extender Q 1.6x Barlow system was used to magnify the image.

For a full resolution and size image, see on my website at:

The image below is a reduced crop of this image to show resolution and sharpness with the high resolution DMK camera on an evening of mediocre seeing conditions (unfortunately late fall is not the best season for stable seeing over here)

Photographic performance

As mentioned, I am much more into astrophotography than into visual observing, so for me the big question always is: how does the telescope behave with a CCD mounted wink. Weather so far was not good enough to be able to do real deep imaging with the telescope, but I will try to update the review as soon as I get those kind of images.

With the FLT110, I got the brand new TMB designed matching 68mm field flattener. This one comes with two adapters, one for DSLR (T2 thread) and one ring that ends in SC thread. The latter is a relatively thin ring, so it does not by itself provide the right spacing needed for best quality results! One should use a 2 inch nose piece with it and try to aim at the same metal back distance as the DSLR adapter, or replace it with a dedicated adapter for large format sensors. William Optics told me they will be making an adapter dedicated for the STL11000.

The DSLR adapter will not introduce vignetting with a 1.5x APS sensor size, but at full frame it vignettes for sure. Have a look at the below image comparing the hole sizes of STL (left), a dedicated STL adapter (middle) and the DSLR adapter of William Optics…

I think William Optics should still make a better DSLR adapter, aiming at the EOS 5D and similar cameras. Quite a few mm’s can be won back by using a wide T mount system like the well known Takahashi or Astro-Physics wide rings. The standard T2 screw threads on the current DSLR adapter will do fine for smaller sensors, but leave full frame DSLR users with unnecessary light fall off (agreed, with current T2 adapter style for full frame DSLR’s even a wide ring will introduce some problems, but definitely less).

Now, using a Canon 20D and STL4020 CCD myself, I don’t have any of the mentioned problems of full frame, testimony this daylight sky image made with the FLT110, field flattener and 20D.

The only noticeable vignetting in this histogram stretched image is from the EOS reflex mirror itself. Till recently, I also had a Takashashi FSQ106n for photography, which really was a pain to use from my severely light polluted location. Razor-sharp, but the bright centre “hotspot” in the FSQ was strongly pronounced by light pollution, making processing a difficult task and precise flatfields absolutely mandatory. No such thing here (nor in my 80mm TMB or TOA telescopes fwiw).

Next step in testing is of course the image quality. For this I aimed the telescope towards a distant tower of Ghent University. The tower is about 2km away from my apartment and imaging over this distance does cause some seeing turbulences. With this in mind, I made a few images at 100 ISO taking care to use mirror lockup to avoid vibrations and using a remote control in order not to introduce fuzziness above seeing influences. The result is excellent. At least with APS size sensor the image is virtually perfect to the very edges as you can see on the image with an inset of one of the corners.

And this is the corner itself at 100% size straight out of the camera....

The excellent performance of the William Optics field flattener itself is also obvious on tests performed at night. I have analysed a few images made with my Canon 20D with CCD Inspector 1.2. The 3D surface below shows the good quality of the optical combination.

With a bit more care in picking the right spot to focus in the field of view, I suspect even better results will be possible.

Below is a sample test shot of M42 (from my city balcony location), 30 second exposure, no extra guiding and made with the FLT110, Canon 20D mounted on my AP1200 mount.

I also tried the FLT110 with a Canon 5D full frame camera. As expected the narrow T2 adapter introduces corner vignetting, which will be less with a wider T2 adapter, and most likely not visible with a custom made STL adapter when using an STL11000.

The image below shows the vignetting of the adapter with an EOS 5D after blurring to get rid of most of the stars except bright Regulus in the centre.

The most important fact however is that the William Optics TMB field flattener is sharp to the very corner of an EOS 5D full frame camera on the FLT110, no modest feat for any telescope to achieve and excellent news for those amateurs wanting to image with e.g. a modified 5D and the FLT110. I have not tried to make images with a dedicated CCD imaging camera so far, but I think the 5D images do reveal the possibilities of the combination.

Below is the CCD inspector curve of 5 images, focused in the centre of the field of view resulting in 19.4% field curvature. By carefully focusing somewhere between the edge and the centre of the view an even better overall flatness number can likely be achieved. So one has to consider the image of the field curvature as the “worst case” scenario. Also, CCD inspector is not perfect and is best subject to an average of many sessions to get real figures, which I didn’t do here.

How it translates to a real life situation is seen on this corner image, histogram stretched, and exposed through the standard DSLR adapter and an EOS 5D at full resolution at 1600 ISO (unguided, 30s). Nicely sharp stars to the edges can be seen with no coma visible. Vignetting has not been corrected.

The same test on the TMB 105mm refractor with TMB field flattener gave a value of around 16% curvature, but the curvature cone was not centred on the optical axis, giving a slight asymmetric result over the 35mm format.

As mentioned, with an adapter ring made by a friend of mine, I could also test the Takahashi TOA/TSA 2.7” reducer-corrector to work at approximately f 5.3. On the image below you can see how the TOA reducer is adapted to the William Optics focuser:

Weather was not cooperative to perform a test outdoors, but indoors I made the following image (not cropped) with a Canon 20D at ISO 800:

This is the small print on the box:

Not bad for a reducer! In fact the image is sharp without any camera of software sharpening in Photoshop, down to the corners. I predict very good results on the stars (unless you start drinking too many of the above test subjects of course!).


So, who will benefit from the FLT110 telescope? Certainly most amateurs who are in the market for a relatively budget friendly APO refractor of moderate size.

I would say that the FLT110, optically and mechanically gives excellent overall quality and value for money, except for the slight star test imperfections and perhaps here and there some things could be made better on the focuser. Usually however you are not star testing a telescope, rather observing with it, and under these conditions these star test imperfections are not really significant as far as I (and others) could judge on multiple occasions. Excellent seeing conditions might reveal them to the experienced eye, but at least where I live, that is very rare.

For deep-sky photography where – by the nature of things – you are working at an image scale which make it next to impossible to detect slight optical imperfections and where a sharp, apochromatic and flat image is the important criterion, from what we have seen above, most likely you can’t go wrong with this telescope and its field flattener. But as mentioned, even for higher resolution lunar or solar imaging you will need superb seeing, a small pixel size camera and a perfect high quality Barlow to start to detect the difference with an optically perfect telescope.

One important thing when buying a new telescope is the support you get after you bought it. In that sense I experienced William Optics as really helpful and responsive whenever a small issue popped up. Good to know.

If on the other hand you only settle for the best in class beyond anything, and are willing/able to pay for it, you may want to move to another brand refractor. My Takahashi TOA130, Lomo TMB 80mm and the tested LZOS TMB 105mm are in terms of optical quality the better scopes, but not by all that much. I guess in that sense William Optics has succeeded in offering a good apo with decent optics at a competitive price. Direct competition I see for the FLT110 are the vendors of Russian made TMB lenses (105 and 115mm) and the Takahashi TSA102 (be it at a smaller diameter and slower focal ratio). Not to mention even cheaper WO “clones” hitting the market just recently. But not much is known about those yet.

Now if William Optics would provide a wider T ring for an EOS 5D, and an STL adapter for the STL11000...

Click here for more about this subject. -Ed.