Home > Uncategorized > Testing the William Optics AFR-IV (aka PFLAT4) focal reducer/field flattener with EON120 and TMB80SS

Testing the William Optics AFR-IV (aka PFLAT4) focal reducer/field flattener with EON120 and TMB80SS

Since it’s a full moon, and I’m not well placed for the total lunar eclipse tonight, I took advantage of the clear skies to test out a new piece of equipment: the WO AFR-IV.  I already have an excellent field flattener, the AstroTech AT2FF, but I was looking for a focal reducer, especially for the EON 120, which at f7.5 is a little slower than I’d like.

The AFR-IV bills itself as an adjustable flattener reducer (0.8x) designed for refractors between 500-1000mm. It also claims to cover the full DSLR sensor area with a 50mm image circle.This claim is interesting because the adjustable back focus settings range from 66-86mm.  A Canon DSLR has a flange to sensor distance of 44mm.  Add on 5mm or so for the t-ring, and you’ve got around 50mm for a standard DSLR, well below the 66mm minimum setting on the AFR-IV.  As we’ll see, this causes some issues.

Here, I’ve tested it with two refractors:  the Orion EON120 (900mm FL, 120mm aperture = f7.5) and the TMB 80 Signature Series triplet (500mm FL, 80mm aperture = f6.3).  The camera is a Canon EOS XSi DSLR.  I shot the region around Deneb in order to get on the other side of the sky from the moon.  Let’s go right to the results. All images are 15 seconds, uncalibrated, with only minimal processing (blackpoint was set).

First up, the EON120.  As a baseline, I took a frame with the AT2FF.  Here are 400 pixel squares from each corner.  You may have to click it to get a full resolution view.  As you can see, the AT2FF works really well with this scope.  (Actually, it works well with all three of my scopes!)

EON120 with AT2FF

I was really excited to see how the AFR-IV worked with the EON.  With the AFR-IV, it would be around 720mm and f6, which is a very exciting prospect.  Unfortunately, the AFR-IV fails here.  Or maybe the EON is the failure.  Either way, they don’t work together.  The EON needs about 2mm more in-focus to work with the AFR-IV at its lowest setting, at least on a DSLR.  2 freakin’ millimeters!  In fact, you can see how close it is in this view through the Bahtinov mask with the focuser racked all the way in.

EON120 with the AFR-IV (so close!)

It should work with a focuser that has more in-focus, perhaps a Moonlite?  Alternately, it probably just requires more back focus, in which case a $17 spacer ring from ScopeStuff will solve the problem.  With a CCD (+filter wheel, +OAG) that naturally has more back focus than a DSLR, the AFR-IV’s 86mm maximum setting is probably a blessing.  I’ll hopefully get the chance to find out soon.  But for now, let it be known that the EON120 does not work with the AFR-IV attached to a DSLR, at least with the stock focuser.  I’ll order a spacer and see if this remedies the problem.

Next up is the smaller refractor, the Thomas M. Back 80mm Signature Series triplet.  It’s a great little scope at 500mm and f6.3, but at 400 mm and f5, I’m even more excited!  Again, let’s start with the AT2FF, which works beautifully with this scope.  Here are the 400 pixel squares from each corner:

TMB with the AT2FFNice, right?  I was focusing by eye here, since I’ve lost my smaller Bahtinov mask, so it’s not perfect, but you can see that the stars are not stretched on the corners.  The full image is 4200 pixels across, so these crops are just the extreme edges.

Now, cross your fingers for the AFR-IV… it works!  There’s not a lot of in-focus left, but it works.  At the minimal setting of 66mm, you can see that it doesn’t fully correct the field:

TMB AFR-IV Corners at 66

TMB AFR-IV Corners at 66

But somewhere around 76mm, it works very nicely:


Because of the in-focus issue, the TMB was racked in all the way when the AFR was around 78mm, but it corrects well at that point, so it’s not a problem.

It lives up to its reducer billing too.  The field of view is obviously wider, and the moonlit background sky is brighter as well.  Using the ruler in Photoshop, I calculate that at this setting on this scope with the DSLR, it operates as an 0.82x focal reducer, right in line with spec.

The adjustability of the AFR-IV is a nice feature no one else offers, but the required in-focus or back focus may be a problem for some people.  I really wanted it for the EON, so it’s disappointing that it won’t work there.  I’ll try the spacer ring to lengthen the back focus and see if that helps.

The AT2FF remains amazingly sharp and versatile as a flattener (it even works on my AstroTech AT8RC), but it doesn’t have any reduction power to it.  I’ll be keeping both for now.

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