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!

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.

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.

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?

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.

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

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.

How to install a ZWO EAF on the William Optics Star71

I bought an EAF and found that that installing it on my WO Star71 was less than straightforward, so I’m posting this tutorial to help others in this situation. It’s quite doable, and a quick trip to the hardware store can make it even better.

1. Removing the course focuser knob (the one with the thermometer) is tricky because the screws to loosen it are actually under another screw.  Remove the grub screw (it’s just to cover the hole) closest to the knob.  Here, I’ve set it next to the hole so you can see it, but you’ll want to put it away in a little bag somewhere in case you ever take the EAF off.  Look through the hole and turn the knob until you see one of the two screws that holds it on.  For both these tasks, the provided 2.5 mm Allen wrench is fine.
2. Here you can see the knob removed, showing the two screws that hold it on.  They are about 90 degrees apart. 
3. Get one of the provided grub screws.
4. Screw it into the flexible coupling.  For the Star71, you need the second largest of the four couplings provided.
5. It needs to be almost flush in order to fit it into the hole where the knob was.
6. This is where the coupler is going.  The grub screw will need to tighten on the flat part.
7. Turn the focuser so the grub screw is aligned with the hole and put the Allen wrench through the hole to tighten it.
8. Get another grub screw and use it to attach the EAF to the coupling.  It’s not shown here, but you can’t put the coupling all the way into the EAF because the shaft is not flattened across its whole length.  The point shown here is about right.
9. Attach the bracket using one of the provided M4 screws to the second from left screw hole on the focuser.  You’ll need to take off the focus lock screw, but since it’s a little longer, you can actually use it on the far right side.  Use two more of the provided M4 screws to attach the bracket to the EAF.  Here is my initial setup, using only parts from the kit. 
10. I put two washers under the left side so the bracket wasn’t at so much of an angle.  I would have used three washers, but the screw provided isn’t long enough.  It’s not perfect, but it works.  You can see why the bracket won’t sit flush with this WO focuser.
11. MINOR IMPROVEMENT:  I didn’t love the slight angle of the bracket, so I went to the hardware store and bought an M4 screw that is 10 mm long (the ones provided are 8 mm) and a 1/4″ nylon spacer.  For a total cost of $1.18, this really improved things.  Much better.

So there you have it.  It is possible to install the EAF on a William Optics Star71 using the provided parts, though a little less than ideal.  With a very cheap addition of a longer screw and spacer, it can be improved.

TUTORIAL: How to eliminate star halos in PixInsight

Bright stars next to dim deep-sky objects are one of the more challenging issues to deal with when processing an image, especially when there are reflection halos off a filter.  This is especially true with narrowband imaging, where we are stretching the image aggressively.  This can make what would otherwise be an inconspicuous reflection look like a giant circle drawn over your image.  The most common example is Alnitak when imaging the Horsehead Nebula, but this issue crops up in other objects.  In this case, with Gamma Cassiopeia when trying to image IC59 and IC63.

Here is one quick way to address this with PixInsight’s PixelMath process.  There are several other tutorials online that describe a similar approach, but they seemed needlessly complex, so I’ve written up what I think is a faster way to do it.

Here is our offending image, an H-alpha image of IC59/IC63 with Gamma Cass lighting up the whole center area.

Here are the steps we’ll follow:

1. Get the left, right, top, and bottom values of the halo.
2. Clone the image and tell PixelMath to generate a mask from the clone.
3. Blue the mask.
4. Apply the mask to our original image, and use HistogramTransformation to remove the halo.

STEP ONE:

We just need to get four numbers:  the X values for the left and right edges of the halo and the Y values for the top and bottom.  In PixInsight, you can move the cursor to the appropriate spot and read these values from the bottom panel.  In the image below, you can see that the left edge has an X value of about 1928.

STEP TWO:

First, clone your image by dragging the image identifier tab on the left side to anywhere on the workspace.  Now we’re going to use PixelMath to create a mask for the halo with these four numbers in one step.  Here’s the formula to enter on the RGB/K line:

iif(sqrt((x()-(R+L)/2)^2 + (y()-(B+T)/2)^2) < (R-L)/2, 1, 0)

This is just an application of the formula for a circle to an IIF statement that turns the pixel white if it’s within this circle and black if it’s not. From the quick measurements I took above, I can enter the left, right, top, and bottom values on the Symbols line.  Your values will differ depending on your image, but you just need to assign them each to their appropriate letter symbol:

L=1928, R=3342, T=912, B=2334

It should look like this:

Apply this process to your cloned image, and you should get a mask that looks something like this.

STEP THREE:

As with any mask, it needs to have softer edges, to open up the Convolution process and apply a little blur.  In this case, I’ll try a Std Dev of 20 pixels.

Much better.

STEP FOUR:

Now apply the mask to the original image by dragging the image identifier tab on the left up to the original image’s gray bar on the left (or you can use the menu options if that’s what you’re used to).  Leave the mask enabled, but unclick  the “Show Mask” button so you can see what your doing.  Open the HistogramTransformation process and click the check mark so it’s tracking the currently active view.  Now, open the Real-Time Preview by clicking the little circle at the bottom of the process.  Drag the black point slider to the right until the halo disappears.  Apply to the image once you are happy with the setting.  That’s it!

In this case, here’s the final image:

If the mask was off center or needs to be adjusted, you can undo and go back to tweak your values, especially the amount of blur that needs to be applied.  If you have inner halos due to other reflections, you can just repeat the process using their coordinates.

The ‘puckered’ halo around Gamma Cass above is an artifact of the microlenses on the sensor… alas there is not a simple solution for these other than layering in data from a shorted exposure or directly editing the area in another image processing tool.

 

The story (and stats) behind the APOD

Yesterday’s Astronomy Picture Of the Day was a collaboration between me, Mladen Dugec, and Max Whitby.  We have been working together on an astrophotography app, and this image was part of that work.  Wolf’s Cave is not a frequently imaged area due to its faintness, but there are a variety of objects in the area that we’ve captured in this widefield.

First, the image stats:

• Exposures:
  • Luminance 25×300s
  • Red 26×300s
  • Green 24×300s
  • Blue 24×300s
  • Total exposure time:  8 hours 25 minutes
  • Taken 26th and 28th August 2019 by Max Whitby in Northumberland, UK

• Telescope: Takahashi FSQ106EDX4 (f/5 530 mm)
• Camera: SBIG STX-16803 with Astrodon Series E Gen 2 filters
• Mount: Software Bisque Paramount MyT
• Guiding: unguided (!)
• Processing: PixInsight and Photoshop by Charlie Bracken and Mladen Dugec

For such a faint set of objects, 8 hours of exposure time is on the short side, but Max’s dark skies in Northumberland, UK were a big asset.  As usual, the individual subs didn’t reveal much, but the first time I saw the initial integrated image, I knew we had something special.

So what are we looking at here?  The part known as Wolf’s Cave is in the center.  It’s cataloged as reflection nebula vdB 152 (read “van den Bergh 152”), and the long dark nebula that trails behind it is B175 (read “Barnard 175”).  Astronomer Max Wolf announced this object in 1908, describing it as a “cave-nebula” at “the end of a long starless lacuna.” Later, he goes on,

“All over the cave lies a network of still darker spots and channels. This raises the hope that we may understand the interesting process more thoroughly at some future time, when we can photograph the region in greater detail with more optical power.”

Well, Dr. Wolf, that time has clearly arrived.  If he were still alive, I’m sure he would be amazed that this could now be accomplished by non-professional astronomers with much smaller equipment (his original discovery image was taken with a 28-inch reflector).  The area is indeed dense with molecular clouds, and the densest patch in the upper right is cataloged as LDN 1221—it’s surprising that Barnard missed this one, but Beverly Lynd picked it up in his catalog later.

There is a really colorful little planetary nebula near the center called Dengel-Hartl 5. DeHt5 was not cataloged until 1980, based on the Palomar survey plates.  The big mystery for us was the thin filament of H-alpha nebulosity that runs diagonally across the image.  It could be LBN 528, but Bob Franke notes that it is part of a much larger supernova remnant called SNR 110.3+11.3, on which there is very little information.

The Iris Nebula (NGC7023, vdB139)

This is one of those objects that I seem to have imaged several times over the years, but each time something went wrong.  Tracking issues, moon issues, dark frame issues, you name it.  So until now, I never really had a decent image of this premier object.

Two nights ago, I had a really still, moonless night.  I looked up the culmination time of the Iris Nebula: 11:30pm.  All I had to do was stay up to manage the meridian flip, and I could get 6-7 hours of good data.  And indeed, that’s what I did.  This was one of those nights where everything worked, the sky stayed completely clear, and every sub was good.

The image below represents 89x180s luminance and 10x180s each R, G, and B taken with an ASI1600MM-Pro through an FSQ106 on an EM-200 mount.  Processing was done entirely in PixInsight.  This image is half the resolution of the original to keep the size manageable.

The Moon and Old Faithful

What to do when a first quarter moon interferes with your plans to shoot the Milky Way in Yellowstone National Park?  Incorporate it into the shot!

On the one clear night I got during my trip to Yellowstone, the moon was too bright to capture the Milky Way like I’d hoped to, but I was able to walk around to the back side of Old Faithful to bring it into the shot.  This is a series of 15-second exposures at ISO1600 gain using a Rokinon 14 mm lens at f/2.8 on a Canon 6D.

Seeing a geyser at night was an beautiful experience.  It was quiet and calm, with very few people out.  If you are ever in Yellowstone, I recommend staying up late, when you’ll have Old Faithful all to yourself (and maybe a bear or two).

Custom digital editions of The Astrophotography Planner now available!

I am now able to create a custom digital edition of The Astrophotography Planner for you.  It’s the same price as the print book ($19.99), but instead, you get a custom pdf of over 160 pages with data based on your specific location. It also includes additional details like moon rise/set data incorporated into the charts and an overview chart showing how many Quality Imaging Hours are available for all 76 areas in one view.  Plus, you get charts that go an extra year beyond the print book:  2019-2021.

If this is something you are interested in, please send $19.99 via Paypal to deepskyprimer@gmail.com with:

• Your name
• Your latitude and longitude (rounded to the nearest degree is fine)
• Your time zone

Turnaround is 1-4 days, since each one requires some manual steps to create.