The FSQ-106 is the premier small refractor for astro-imaging. It is only about 4.2" in diameter and 16.7" long (airline-portable). It is a 4-element modified Petzval design that uses two fluorite glass ("extra low dispersion", or "ED") elements. In the Petzval design , the front lens is well-corrected for spherical abberation, but introduces coma (elongation of stars at the corners of the field). The second doublet and field stop corrects for astigmatism, but introduces field curvature and vignetting. Takahashi's "modified" design is claimed to virtually elminate all of these optical distortions, as well as correcting the chromatic abberations which produce a "secondary spectrum". Theoretically, at least three optical elements are required to fully correct for chromatic abberations; the FSQ uses a 4-element optical system to produce a very large, flat-field, color-corrected image at the focal plane.

The layout of the FSQ is shown above. It includes (from left to right) an extendable dew shield, a 4" focuser (shown in the photo below), a guide scope (if so equipped), a camera angle adjuster (manual camera rotator), and adapters for oculars and photographic gear. As you can see in the photos, for a 4" scope, the FSQ is incredibly robust. It weighs more than 15 lbs (7 Kg), and by the time you add a large-format camera, filter wheel, rotator, focuser, remote guide head, etc. it probably weighs more than 25 lbs (11.5 Kg). Thus, for serious imaging, this scope requires a mount capable of at least 40-50 lbs capacity.

You can see on this side of the scope the 10:1 focus knobs. There is also a focus locking lever underneath the scope. A RoboFocus motor is attached via a special bracket to the other side of the focuser, and connects directly to the FSQ focusing shaft. [I will add a photograph later showing the mounting of the RoboFocus motor on the FSQ focuser] The critical focus zone ("CFZ") of a telescope is determined by the formula:

CFZ (microns) = 2.2 * f#^2      (where f# = FL/Dia)

In other words, the criticality of focus position depends only on the f# of the scope (i.e., the focal length divided by the diameter). Therefore, the CFZ of the FSQ-106ED - at an f ratio of 5 - is about 55 microns, so focusing can be very challenging - as the focal position also changes with temperature (and therefore changes throughout the night, as the scope cools down).

The FSQ-106 has a huge image circle of 88mm allowing it to work with very large format CCDs, or film up to 6x7 cm size. Nevertheless, there is light falloff moving off-axis (i.e., away from the center of the image), as shown in the graph below:

The STL-11000 CCD camera that I am currently using has a 42 mm (diagonal) chip, and thus would be at the 21 mm position on the above graph - with less than 10% light falloff, even in the corners of the image. In addition, the field is extremely flat; I have not yet made good measurements of the field flatness using CCDInspector, but I believe that it is less than about 6-8% across the STL-11000 chip. [I will add this, as I do more testing]

The optical and mechanical specifications of the FSQ-106 are shown below:

The "spot size" diagram below shows the optical distortions of a point source (e.g., star) at various distances from the center of the field, up to 44 mm. The diagram compares the spot sizes of the original FSQ-106N (top series) and the "new Q" (FSQ-106ED). The improvement in optical correction is easily seen.

The "secondary spectrum" graph below shows the extent to which chromatic (color) abberations have been [un]corrected in the FSQ-106ED. These corrections are an improvement over the original FSQ-106N, which did not use fluorite ("ED") glass for two of the elements. The vertical scale is "focus difference" in millimeters; you can see that this never exceeds about 0.11mm, or about 110 microns. This corresponds to about 2x the critical focus zone.

I have both the Extender-QE and Reducer-QE optical accessories for the FSQ-106ED scope. The Extender-QE multiplies the focal length by a factor of 1.6x, yielding an 850 mm focal length at f/8. In contrast, the Reducer-QE multiplies the focal length by a factor of 0.73x, yielding a focal length of 365 mm at f/3.6. Both the extender and reducer have a smaller image circle of 44 mm (slightly more than the 42 mm diagonal of the STL-11000 chip). With the STL-11000, the field of the native focal length is about 2.6x3.9 degrees - which is quite widefield. However, with the reducer, this becomes 3.6x5.3 degrees - which is "ultra" widefield. The availability of 365 mm, 530 mm, and 850 mm focal lengths makes the system very versatile for various astro targets.

[to add pictures of the Extender-QE and Reducer-QE]

The extender and reducer modify the optical properties of the scope - both in terms of spot diagram, and in terms of chromatic abberations. The data from Takahashi (Japan) is shown below for reference:

As seen in the top photo of the scope, the FSQ-106ED is very robust mechanically. The cross-sectional views of the scope (again from Takahashi) are shown in the drawings below:

Even more striking is the comparison between the FSQ-106ED and my previous scope, the Megrez 90. Although they are both fairly similar in diameter (90 mm vs. 106 mm), they are quite different in construction, as can be seen in the images below:

One "challenge" (and additional expense) with the Takahashi scopes is the number of adapters required to hook-up visual or photographic accessories ... and none of these adapters is actually labeled (so after a while it is very difficult to determine which adapter is which, and what they're for). There are "system charts" for the FSQ-106ED (different from the original FSQ-106N) at its native focal length, and using the extender or the reducer, as shown below:

Overall, I am quite excited about having the FSQ-106ED (which I purchased in May 2010). This has allowed me to re-enter the widefield (and ultra-widefield) imaging arena with a near-professional scope ... vs. the Megrez 90, which showed distortions even on a 15x15 mm chip. Please check out the images on my Recent Images 5 and Recent Images 3 pages for samples of what the FSQ can do (with and without the reducer).