Home > Uncategorized > Review: Takahashi FS-60C Reducer C-0.72x (TKA20580B/TRD0060)

Review: Takahashi FS-60C Reducer C-0.72x (TKA20580B/TRD0060)

The Takahashi FS-60C is a cute little fluorite doublet that has been around for years, with several improvements along the way.  But I’d always heard that its color correction and sharpness were not up to par with its larger cousins.  This makes sense, given that a doublet–even one with a fluorite element–cannot be corrected for color as well as a triplet or quadruplet design.

I have a weakness for small widefield scopes, but I hesitated on the FS-60C not only because of the above, but because its native focal length of 355 mm is redundant with another, faster scope I have (the WO Star71).  This was until I read several posts online noting that the new reducer “C” substantially improved the sharpness and color correction, while reducing the focal ratio to a very fast f/4.2.  Finally, I had the excuse I needed to get one!

Unlike previous reducers that were used with the FS-60 (e.g. the Sky90 reducer), this reducer is designed specifically to match.  It’s also expensive, at $570, which is 66% of the cost of the FS-60 itself.  But think of them together as a $1400 mini astrograph.  This review aims to show a few examples of their performance together.  I didn’t try the FS-60 without the reducer, so I can’t compare how much it improves things, but I’ll take the internet’s consensus that it’s a bit underwhelming for astrophotography.

First, a pretty picture.  This is the Rosette Nebula, taken with the FS-60C and reducer-C with a ZWO ASI 1600.  This is exactly one hour per channel (SII, H-alpha, and OIII) in three-minute subexposures.  F/4.2 indeed!

Rosette SHO Final copy

Obviously, it’s a very wide field, even on a 4/3″ size sensor.  Click the picture for a full resolution (though JPEG) image, and yes, you’ll see that some of the small stars are blocky.  Even with the relatively small photosites of the ASI1600, we’re still imaging at 3 arcseconds per pixel.  But guiding is easy, and the data are a joy to process.  The stars are equally sharp in the corners as the middle, even though I just eyeballed the focus manually.  (The “blue bloat” you see around some stars is because the OIII filter was way off focus, but I dealt with it in processing by using the H-alpha as luminance.)

What about vignetting?  Here is a flat.example flat no stretch

Okay, that’s not helpful.  Let’s stretch it so you can see any variance.  I’ve added percentage of the central ADU count to quantify the falloff.

example flat stretched copy

That’s really good.  No more than 8% falloff to the edges.  I didn’t put values in the corners because the vignetting there is due to the filterwheel.  (It’s about 12% falloff there.)

So for a 4/3″ sensor, this is an excellent astrograph.  But what about a larger sensor?

Before I go on, know that Takahashi’s specs for the FS-60C + reducer-C are for a 40 mm image circle.  That image circle is defined as having 60% of full illumination.  I’m not sure I’d want to go down to 60% illumination if I could avoid it, but last fall I attached my full-frame Canon 6D.

I should mention that the reducer-C is one of the easiest bits of Takahashi kit I’ve used.  Normally, it seems you have to buy a custom spacer for every new Tak configuration, but in this case, you just screw the EOS wide adapter onto the reducer, and you’re done.

First, let’s look at the stars.  Can it deliver sharp stars from edge to edge of a full frame (36 x 24 mm) sensor?

light_mosaic

Yes.

And what a wide field it creates with a full frame camera.  You can capture the heart of Cygnus in one shot.  Here is a single, uncalibrated 60-second exposure of the area around Sadr.

stretched light

What’s that you say?  Vignetting?  Yes, that is substantial vignetting.  Let’s have a look at a flat (stretched).

FF flat stretched copy

It stays within the 60% illumination threshold on the left and right edges, but barely.  I didn’t mark the corners, but the illumination there is about 38%.  But again, the reducer was not designed to work with a full frame sensor (where the diagonal is 43 mm, exceeding the 40 mm image circle in the Takahashi specs).  I just thought I could get away with trying the 6D for fun and then cropping the images.

What this does tell us is that the reducer-C should be perfectly fine for an APS-sized sensor, probably with less than 15% falloff.

In summary, the reducer-C seems to complement the 60-C well, making it a neat little astrograph.  I’ll definitely be hanging on to mine, especially for the summer nebulae.

PROS:

  • Stars seem sharp across a very large field.
  • Vignetting is minimal, even for APS sensors.  (Not designed for full frame, though.)
  • Imaging at F/4.2 is a joy.
  • Very easy to use–simply screw on the EOS wide adapter.
  • Solid connection, as it screws onto the scope.

CONS:

  • Pricey (though Tak=high quality)
  • Not a lot of objects are framed well at 255 mm focal length with most sensor sizes.
  • Even for small-pixel sensors, image scale is likely to be around 3″ per pixel.
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