Home > Uncategorized > Review: William Optics Star 71 Imaging Refractor

Review: William Optics Star 71 Imaging Refractor

I can’t speak for everyone, but my criteria for evaluating a wide-field imaging scope are:

  • Fast optics:  since the goal is typically to capture large, diffuse, and dim objects (usually nebulae), focal ratio is key.
  • Quality optics:  stars should be sharp to the corners, it should be truly apochromatic, and the field is evenly illuminated.
  • A good focuser:  the zone of critical focus is very narrow at fast focal ratios, so any focus shift is unacceptable.

Living under light-polluted New Jersey skies, my preference is to shoot narrowband objects, as that negates most of the effect of the light pollution.  I have an affection for small refractors, and I particularly enjoyed having a Borg 77EDII, which was a super-fast f/4.3, until I recently sold it.  There’s something nice about the simplicity of small refractors.  They are light, easy to balance, easy to align.  Give me a fall evening, a wide-field scope, and a camera set up for narrowband imaging, and I’m happy.

The William Optics Star 71 recently caught my eye as a replacement for my old Borg 77.  I signed up for the pre-order, and Agena AstroProducts delivered my scope this week.  The WO Star 71 has a stated focal ration of f/4.9 and a focal length of about 354 mm* (though this is not explicitly stated anywhere I can find), which nicely frames many large showpiece objects on common sensors. For an APS-C sized sensor, the field of view is 3.6 x 2.4 degrees. For a 4/3 format sensor like the KAF-8300, the field of view is about 2.9 x 2.2 degrees.  I plan to use it mostly with the ST-8300, and this field of view nicely frames a lot of narrowband targets I’m after this year.

(* Based on the field of view calculated by astrometry.net (3.61 degrees) and Canon’s stated 22.3 mm sensor measurement on the long side for their APS-C sensor, I calculated a focal length of 354 mm, which leads to a focal ratio of 4.98 if the aperture is truly 71 mm.  If they fudged a bit, and it’s 72 mm, the ratio would be 4.91, which would match the stated 4.9.)

The evening I got the scope, it was actually clear.  Now, it was a lousy night for imaging because it was over 80 degrees, the winds were gusting, and high humidity made for terrible transparency.  Worse, the waning gibbous moon would only allow for about two hours of dark skies.  So it was a terrible night for imaging… but perfect for taking a few test shots.

Look and Feel

Before it gets dark, let’s have a look at the scope.

The WO Star 71

The WO Star 71

This is classic William Optics:  white and gold.  The focuser has a thermometer–neat, but I’m not sure how much I’ll use that.  The scope comes with a nice pair of rings, a Vixen-style dovetail, and a M48 to Canon adapter.  So even visually, it’s clear that this is an imaging scope.  The focuser terminates in M48 threads, not a 2″ eyepiece holder.  (Note, that’s M48, not the standard T-thread of M42.)  If you want to use this scope for visual use, you have to buy a special adapter, which is fine with me, because imagers prefer to keep all the connections threaded.  For scale, you can see the Orion 50 mm guidescope mounted on top, and the Canon 450D/XSi DSLR.  For a 71 mm scope, it’s actually pretty big.  I don’t have my Borg 77EDII anymore, but I’m almost certain this is bigger.

What you can’t see in the photo is how heavy the scope is.  WO claims it’s 5.3 lbs with the rings, and that seems about right, but until you handle the scope, you don’t realize how dense that is.  There is clearly a lot of glass in there, and not just at the objective end.  It’s the same impression I got the first time I picked up my Tak 106ED.  This thing is solid.  WO notes that FPL-53 glass is used, and that there are a total of five elements in three groups.  People seem to attribute almost mystical powers to FPL-53, and while it is found in the best optics in the world, the glass type doesn’t matter if the elements aren’t well matched or poorly figured.  But between the weight and glass type, we’re off to a good start.

Imaging Performance

Let’s get down to brass tacks.  How did it perform for imaging?

Bear in mind that this was just a chance to take some quick test shots, so hopefully I’ll be able to do more extensive imaging work with it later, but first let’s check the overall field of view with a DSLR.

Field of view around M81/82

Field of view around M81/82

This is a single 30-second exposure of the area around M81 and M82, just to give a sense of scale.  Yeah, that’s a wide field.

“Great,” you say.  “But is the field of view flat?”  Let’s check.  Here are extreme close-ups of stars at the corners and center.  (These are from a different, 15-second exposure, since wind gusts kept streaking the stars a little.  The color was dropped to help reduce noise.)

Corner sharpness

Corner sharpness

These are 200×200 pixel areas from the edges and center.  The stars are sharp, and that’s a pretty flat field, especially for f/4.9.  This substantially outperforms my old Borg 77EDII (though to be fair, that scope was f/4.3 and not known for exceptional flatness of field or perfect apochromatism).  It’s probably not as good as the Tak 106ED at f/5.0, but it’s slightly better than the Tak at f/3.7.

What about vignetting?  WO claims the scope will deliver a 45 mm usable imaging circle. That would be large enough for a full-frame 36 x 24 mm sensor.  That’s always a bit of a judgement call, because the manufacturers never seem to state what level of light falloff is acceptable in determining the size of the usable imaging circle, but okay.  An APS-C sensor needs about a 27 mm circle, so that’s all I can test for now.  I took a flat frame, and then I measured the 8-bit brightness level at the corners and in the center.  I’m sure someone with CCDInspector could provide a pretty 3D map, but this method will tell you what you need to know.



The flat field image was exposed to stay in the linear range of the sensor, and we can see that there is only about a 3-4% falloff from the center to the edge.  That’s really good.  (NOTE:  see next post above.  Photoshop applies a gamma curve when importing CR2 raw data, so the vignetting is actually higher than this, but the performance still holds up very well.)

All right, final test… what about apochromatism?  Do all the colors come into focus at the same plane?  Sorry, I’ll have to update this review later with that information once I get a full night out with this scope and my ST-8300.

And what about the focuser?  I monitored focus through a Bahtinov mask, and as expected, focus is really touchy at f/4.9.  The critical moment came once I had lined up focus and turned the focus lock knob.  This is where cheap focusers will suddenly throw the image out of focus, leading to an annoying game of “How far out of focus do I need to start so that it will be in focus after tightening?”  (Oh, you’ve played that game, too I see.)  In fact, I used to have an early Zenithstar II from William Optics whose focuser was… well, it kinda sucked.  And judging from comments on forums, WO has had some challenges with their focusers.

The Star 71’s rack and pinion focuser performed well.  The image stayed in focus after tightening the locking knob.  I have to say that the focuser does not feel as tight as a Moonlight or Feathertouch, but it did the job admirably.

Summary and Final Thoughts

My initial impressions of this scope are very good.  I’m really pleased so far, and I hope to provide a fuller review with images at a later date.

In my view, here are the WO Star 71’s strengths:

  • A sharp, flat field
  • Exceptional fit and finish
  • Nice focuser
  • Threaded fittings
  • Solid tube rings and a Canon adapter are included
  • Fast enough focal ratio

I would have been thrilled with f/4 or even f/4.5.  An 80 mm objective, but still at around 350 mm focal length would have been amazing, but I’ll take it at f/4.9.  It’s substantially better than some similar scopes.  (I never bought the AT65EDQ because no matter how sharp it is, at f/6.5 I’d never get a narrowband image finished.)

If I had to name a few “opportunities for improvement,” I’d say:

  • I would rather have had an M48 to M42 T-thread adapter included
  • No case is included
  • The dew shield only extends 1.5 inches past the objective lens cell.  This is too short, especially for those of us in humid climates.

And I’m not sure how I feel about the price of $998.  ($898 for the first few sold.)  I know this is a new, patented optical design.  And the optical performance looks really good.  And it’s f/4.9.  But it’s also only 71 mm of aperture.  The price compares well to the Borg 77ED or 71FL, and the Televue 76.  But WO sells their own Zenithstar 71 + 0.8x focal reducer for $568, and that would be an f/4.9 system as well.  Granted, I don’t know if the field is as flat, the optics are as sharp, or the focuser as good, but it makes for an interesting comparison at over 40% less.  To be fair, WO explicitly notes that the Star 71 is a  “[n]ew, unique, patented design – not a conventional objective/flattener/reducer design.”  But having another 71 mm f/4.9 scope out there plants a little seed of doubt.

I have a unique consideration when reviewing this scope.  I originally ordered two with the plan to create a dual-scope narrowband imaging set-up.  I reasoned that I only get a few nights a month of decent weather, so why not double up on the exposure time with two scopes?  But I’ve had a hard time finding another used ST-8300, and once I started pricing everything I’d need, I started to chicken out and reduced my order size to one scope.  (Oh, so that’s why no one seems to be imaging with a dual-scope setup…)  So the question for me now is not whether I’m keeping the Star 71 (I am), it’s whether I’m going to buy another one.

  1. mfrissen
    June 25, 2014 at 4:27 am

    Mine is on the UPS van today. I guess there’ll be clouds for the coming nights.

  2. August 3, 2014 at 5:22 pm

    Hoping to see mine in a few weeks. Which M48 to m42 adapter did you use? With my STF-8300 fw/OAG, I need slightly under 10mm.

    • August 5, 2014 at 9:39 am

      I had two M48-M42 adapters I was able to borrow from focal reducers that were lying around, but I’ll have to order new ones soon. Agena carries them, but they were out of stock last time I checked. No need for spacers, since the focuser changes the distance from the rear lens element.

      Sadly, both of my Star 71s are temporarily out of commission because they were rained on for less than a minute, and the rainwater wicked up between the front two lens elements. I’m working on a solution with WO.

  3. Dodi
    August 22, 2014 at 7:07 pm

    It sounds nice, but is there a way to incorporate 48mm filters (like Hutech IDAS filter) somewhere in that tube? I mean, it ends in 48mm thread, and I can’t see using clip-in filter on my full frame camera’s. IDAS just doesn’t have them, and I swear to that brand 😉

    • August 22, 2014 at 10:32 pm

      I’d be worried if the filter could handle the strain of bearing the weight between the camera and scope, but I’m guessing if the the threads were deep enough it might work. Borg has a filter drawer they use on their focusers that might also work, but I doubt the threads match. Worse comes to worse, you could get a cheap, manual filter wheel with a T-thread to 48mm adapter, and put that between the scope and your camera.

    • Bill Eareckson
      October 2, 2014 at 10:12 am

      I have a question regarding the Star71. The drawing at WO and as stated earlier indicates that this scope uses an M48 thread connection point but the drawing shows this as M48x1mm thread pitch, whereas most adapters, such as at Agena, utilize M48x .75mm thread pitch which is finer. Since most SBIG and QSi cameras use M42x .75mm threads what adapter has anyone found that couples properly?


      • Bill Eareckson
        October 2, 2014 at 9:02 pm

        Never mind I found out that the pre-production models were initially M48x1 and current production was changed to M48x .75mm.

  4. Dodi
    August 23, 2014 at 4:51 am

    Maybe. But a filter is a rigid ring. How much in travel is left with a Canon in focus, to see whether an adapter could be made? If not, it has to fit inside the focuser. I think there is part of the focuser that can be unscrewed to mount the special diagonal, perhaps inside that unscrewable piece there is room for a 48mm filter?

  5. Dodi
    August 26, 2014 at 1:01 am

    I just got confirmation from a user and from WO: there is 48mm thread inside the focuser drawtube to install a filter.

  6. Henri-Julien Chartrand
    September 7, 2014 at 7:58 am

    For a newbie like myself, this is what I call a well written and useful review. I wish all reviews were less marketing and more practicle like yours.
    Thank you so much.
    Henri-Julien Chartrand, Montreal QC, Canada

    • September 7, 2014 at 8:48 am

      Thank you very much, Henri-Julien. I appreciate it.

  7. zaxh
    September 29, 2014 at 11:00 pm

    i have same scope, just wondering what kind of dovetail you put on top, it is the small black one, i could really use one myself. Thanks

    • September 30, 2014 at 7:08 am

      It’s a six-inch Vixen-style dovetail with four holes. Most online astronomy stores should have them.

  8. Marcel Dumeny
    January 9, 2015 at 8:03 pm

    Bummer about the rain water wicking up between the lens elements. Is there a fix for this or is this a continuing concern?

    • January 9, 2015 at 10:20 pm

      I don’t know if there has been any change to the design of the scope, but William did repair them for the cost of shipping to China. I am obviously very careful about the forecast before I put them out for imaging now!

  9. Kenneth Doninelli
    February 6, 2015 at 5:54 pm

    Do you plan to do the follow up?

    • February 6, 2015 at 7:34 pm

      My semi-scientific test for apochromatism has been to measure focus through different filters using a Bahtinov mask, which at f/5 reveals even tiny shifts in focus. The result is that focus is exactly the same between an H-alpha filter and an OIII or between red, green, and blue filters. WO will also send you a calculated spot diagram and lateral aberration diagram if you ask. The spot size is not as tight as a Tak, but aberrations appear very well controlled for a scope of this price.

  10. June 9, 2015 at 4:12 pm

    Hello under your experience.
    What do you recommend?
    W.O 71 5 elements 4.9 or borg similar specification.

    Jose Mundo

    • June 9, 2015 at 5:00 pm

      At least compared to the Borg 77EDII, the WO71 seems to be of slightly better optical quality, but at a much lower price. I haven’t tried the Borg 71FL, though. The benefits of the Borg 71 or 77 would be the ability to go down to f/3.9 and the ability to add the excellent Feathertouch focuser, but those options would put you over $2000.


      • June 11, 2015 at 1:03 pm

        Thanks a lot Charlie

  11. Kenneth Doninelli
    March 15, 2016 at 7:13 am

    Thank you very much Charlie. I love my WO71

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