Taka FSQ106 test report

The Takahashi FSQ-106EDX-IV is a 106mm (4.2") aperture f/5 (530mm focal length) four-lens photo/visual system using two ED (Extra-low Dispersion) glass elements. The FSQ is a two-doublet Petzval design that uses a "Highly air-spaced quadruplet of 4 lenses in 2 groups using 2 x S-FPL53 ED glass" to correct for: coma, astigmatism, field curvature, secondary spectrum, spherical aberration and spherochromatism.

Takahashi have discontinued the use of fluorite crystal seen in the earlier FSQ106N series refractors. Users of both variants report the geometric aberrations are unchanged in the ED model but, the chromatic aberration is improved.

The FSQ EDX-IV is optically identical to the previous FSQ EDX-III model (I contacted Takahashi Japan, they confimed the lens design is unchanged). The main difference being the "captains-wheel" camera rotator in the MK-III being replaced by Takahashi's large aperture camera angle rotator at the eyepiece end in the MK-IV. This new mechanical arrangement allows the large format 645 0.72x reducer to be fitted to the rear of the OTA without the need for any additional accessories.

The FSQ is a stocky unit.. With the dew shade retracted it is just 580mm long and weighs 7Kg . Not quite airline overhead bin compatible but close. While its mass will unlikely tax many mountings, the tiny star foot prints will herald even small tracking errors by an image full of elongated dots. A quality mount is strongly recommended.

Takahashi EDX telescopes leave the factory triple boxed, filled with copious ammounts of styrofoam beads (note to Takahashi: please protect of our planet and use biodegradable cellulose beads! ). Apart from providing excellent protection of the optics during shipping this makes the "unboxing" session seem like discovering a Russian Babuska doll set.

Despite this high level of packaging and the telescope tube arriving in pristine condition, the objective lens cap inexplicably arrived with some small surface scratches.

Takahashi were contacted about the dud cap, but declined to correct the problem, saying that it probably happened in transit and would need to be resolved with the (now defunct) dealer. Not exactly the response one would have expected from a premium manufacturer, given the shipping boxes had clearly never been opened and the OEM cost of a cap is probably about $5.00.

This telescope was one in the very last shipment to the former Australian dealer, AEC, in February 2023. From order to delivery took just over two years. The Takahashi dealership has since changed to Testar Australia.

Takahashi's 645 0.72x reducer deserves further mention. It makes the system a remarkable 380mm F3.6 instument, but it also adds very slight red fringing at the edge of field.

Below: Pixinsight's aberration inspector image of FSQ+ 645 reducer image
(no flat-field applied) .

APS sensors show no colour error with this reducer, but this is is not a cheap accessory! Hence any colour error remains niggling issue with full frame sensors (albeit easily fixed with Photoshop's lens correction filter).

The 98mm diameter rack-and-pinion focuser with a 1/7 microfocuser is smooth and precise, but in practice while fine for visual users, it is simply inadequate for ultra-precision focusing of large/heavy camera+CFW+OAG instrument trains. Slight focal plane tilt will likely be evident (depending on the camera used) and fine focus changes are frustratingly tedious to make with a heavy camera.

Takahashi market this telescope as an astrograph and would do well to design a new and functional focuser for imaging users: not just those wanting to observe visually. While third-party focusers such as the FLI Atlas and PrimaLucaLab fix these problems, they add to the price of an already expensive imaging system.

Takahashi have a suite of adapters for various accessories and cameras, so many infact that it has been said they have "adapters for adapters". Their threads are metric. This makes mating the optical tube assembly to USA manufactured cameras, filterwheels etc. impossible without a custom metric to imperial-unit adapter. While US imperial-unit hegemony is hardly Takahashi's fault, again this extra link often does add to the expense of a FSQ imaging system.

Thankfully as the FSQ has a very generous 178mm of backfocus, you can hang all manner of spacers, off-axis guiders, filter wheels and cameras at the back of this instrument and still comfortably reach focus.

A FLI Atlas focuser (step resolution of 0.083 microns) and Gerd Neumann tilt adjuster was added to this system to acheive the ultra-precise focus and focal plane orthogonality that the FSQ optics demand. This addition also converted accessiores to an Astro-Physics 2.7" thread, for which there are many rings, spacers and camera adapters available at a modest price point.

Despite using parfocal (i.e. same thickness) filters, the FSQ clearly shows differing critical focus points for Red Green & Blue exposures. Testing put this difference an the focal plane of 83 microns (i.e. 0.083 mm) from blue to red exposures..hardly a fatal result, but if not accounted for will likely result in slightly bloated star images.

Star testing showed the optical correction to be as advertised.

Stars snapped into focus with no hint of astigmatism, coma, spherical error...or any other error listed above for that matter. The anti-freflection coatings of the four element FSQ delivered excellent contrast, with no halos or odd reflections to be seen.

The FSQEDXIV's level of image quality is ethereal but when experienced it's difficult to accept anything less. Faint star-fields look like sparking star dust. Real filigree details, rather than processing artifacts, are revealed. Bright stars remain tight and compact with a very subtle edge glow in deeper images.

The theoretical RMS spot size of the FSQEDX-IV varies from about 2 microns (center) to 4.5 microns (edge) across a 35mm frame. Testing here unfortunately could not verifiy those remarkably tight numbers as atmospheric turbulence proved to be the limiting factor. In any event, star profiles were point-like across 60 megapixel test images.

There was vignetting evident in the full frame 35mm test images that all displayed a modest brightness roll-off (about 30% ) at the extreme corners. This was easily corrected however with suiltable flat field calibration frames.

Below: vignetting graph

This very wide-field, high resolution imaging system is *very sensitive* to any focal plane tilt. It took considerable effort to perfectly square-on the camera sensor to deliver tight stellar profiles fully across the 60Mp/35mm test frames.

Indeed after two nights of trying to adjust the camera tilt I found one corner or another would show a tiny residual ammount of astigmatism that was nigh impossible to totally erradicate. That said, stellar profiles had minisucle footprints in the FSQ. There was little doubt that atmospheric seeing was the overiding factor to being able to perfectly null-out the system.

As mentioned earlier, adding the Takahashi 645 reducer does introduce a small amount of coma at the corners of a 35mm frame sensor. This is almost invisible at 30Mp, but very detectable with a 60Mp sensor. The 5 degree field of view is however simply remarkable. Stars look like perfect points across a 4K screen.

It also reduces the backfocus to around 56mm. This is fine for DSLR and Mirrorless cameras, but is only barely enough for a short backfocus Astro CCD or CMOS camera with filterwheel. You would do well to check the focal plane distance of your camera before purchasing this not inexpensive accessory.

As for the results, the images and test data below are typical examples of what the FSQ106EDX-IV can deliver.

When coupled with a full frame sensor, the 3.9 degree field of the FSQ106 allows wide vistas to be captured with remarkably high resolution, particularly if paired with 60 megapixel camera. Images show needle-like stars to the very edge edge of frame. A feat few optics can approach let alone replicate.

At $AUD five figures, the Takahashi FSQEDX-IV is not cheap.

There was an old Scotch advert that solved this dilemma: the merchant asks this simple question: "Do you want to take home the Scotch you want to pay for, or the one you want to drink?"

With every FSQ imaging session, you'll be reminded you chose wisely.

Suffice to say, the FSQ106EDX-IV is one of the finest wide-field astrographs *available.

(or in this case, currently *unavailble, as from September 2022 Takahashi Japan have stopped taking any new FSQ106EDXIV orders, until further notice ).

Below: Eta Carina, 7nm H-alpha, FSQ106 @F5.0, full frame QHY600M 6x 300 second exposures.

Below: Pixinsight Aberration inspector image of the FSQ106 at F5.0.
60mp QHY600M frame taken with 7nm H-alpha filter.

Below: Extreme lower right corner crop, 35mm frame, of the Eta Carina region Ha image above.
(rescaled to approx 60% )