Synta/GSO Finder Bracket for Astro-Physics AP130GTX

If you've ever had a hankering to attach a cheap and cheerful Synta or GSO finder scope (or red dot finder) to your spendy Astro-Physics AP130GTX triplet refractor,  then this 3D printed design is for you.

I had forgotten (or did not want..) to buy the fancy Baader Vario finder and rings from Astro-Physics when I bought my AP130GTX. Since I had a 3D printer and some cheap no-name China red dot finders, I figured I'd design a finder bracket.

There's a bit of a complication because the AP refractor has its finder bracket screws coming out at a 20-degree angle, and the two screw holes are quite far apart. I also did not want to secure the finder bracket with a single screw. This bracket is appropriately curved to fit the AP refractor, and the screw holes are also tilted for proper alignment.

Right-Angle Finder Adapter for Takahashi Polar Scope

This is a Yashica/Contax right angle viewfinder, which can be obtained from ebay for under $20:

I decided to 3D-print an adapter allowing this finder to be attached to a Takahashi polar scope, so when using the polar scope an observer can avoid a stiff neck.  The adapter can be found here.

Here is the Contax right-angle viewfinder attached to the polar scope on a Takahashi Space Boy mount. The adapter has two holes that need to be tapped for M4 for the set screws. 3D printing technology isn't repeatable enough at a sub-0.5mm scale to allow a tight press-fit on both the Contax finder and the Takahashi polar scope, at least when using PLA (succeeding prints may be too loose or too tight, hence the need for the set screws).

Potentially if using ABS or HIPS it might be possible to exploit the material's flexibility for a press-fit on both the Contax finder and the polar scope eyepiece end.

Takahashi EM-1S Ersatz Polar Scope

I bought an old Takahashi EM-1S mount from Yahoo Japan, but it did not come with a polar scope.  Since this mount is 25+ years old, I was not even sure if a polar scope could be bought for it. Also, the Takahashi polar scope is expensive, and since I got the EM-1S fairly cheap (for a Takahashi..) I figured it would not be worth it to buy the current polar scope.

As luck would have it, I had a Celestron 6x30 "Long Eye Relief" finder scope, and I thought it might be possible to adapt this finder scope as a primitive polar scope for the EM-1S. The outer diameter of the eyepiece portion of this finder is about 24mm, and the barrel itself is 32mm in diameter. After three attempts, I came up with this design for an adapter, and 3D-printed it. The finder scope is a bit of a loose fit, so I added two masking tape shims:

There are three 3mm diameter holes in the circumference of the adapter, which must be tapped with an M4 tap and grub screws inserted to hold the finder scope in place.

Here is the ersatz polar scope in position. The adapter is a very tight fit in the Takahashi mount's RA bore. A strap wrench would be needed to remove the adapter. So it is important to place the finder scope into the adapter, and secure it with the grub screws, before inserting the adapter into the mount's RA bore. Some whacking was necessary to get the adapter to seat (although a previous version I made did not require whacking, so it's probably down to non-repeatability at sub-mm level in the 3D printer).

The polar scope manhole cover (cap) still threads on nicely:

Almost Full Frame: Fuji X Camera Lens Turbo Adapter (Ver II) from Mitakon

I very much like the form factor and UI of the Fuji X-mount cameras: they are very much the poor man's Leica.

A challenge is that wide lenses are hard to find, due to the 1.5X crop factor, since the Fuji cameras are not full-frame. The only full-frame mirrorless cameras to date are those from Sony ($$$) and Leica ($$$$$) which aren't an option for me. One could use LTM lenses such as the 15mm Voigtländer Super-Wide Heliar in LTM, but it too is rather expensive.

I've used some LTM lenses (Leica Elmar 90mm f/4 and a bunch of Russian lenses) on the Fuji XE-2, and these are fine - but still no wide-angle lens. To get say a 28mm equivalent would require an 18mm focal length. Fuji makes a very nice native X-mount 18mm lens, but it's quite expensive. All of the Fuji lenses are expensive, even the kit lens.

I discovered the Mitakon Camera Lens Turbo Adapter (Ver II) which is a 0.726X focal reducer. I've used reducers on telescopes; what these devices do is reduce the effective focal length of an attached lens, increasing the f-ratio in the process (since the focal length shortens, but the lens' physical front aperture does not change). Focal reducers are a great idea in theory, but they also decrease back focus. This means the focal reducers can only be used with SLR lenses (which have longer back focus than the X-mount). LTM lenses cannot be reduced (so no full-frame Leica lenses on a non-full frame camera).

A concern with focal reducers is that they may reduce image quality; after all, we are introducing another 2-3 lens elements between the original lens and the camera sensor. To test this theory, I took a known-good lens (the Canon 180mm f/3.5L Macro) and mounted it on both a Canon 6D full-frame camera, and the Fuji XE-2 with the Mitakon Camera Lens Turbo Adapter. And here are the results:

Here's the full-frame image with the Canon 6D and 180mm lens:

And here's the same scene with the Fuji XE-2 and Mitakon focal reducer (I literally just removed the Canon DSLR body and substituted the Fuji body, since the lens was mounted using its tripod foot to a tripod). Note the obvious vignetting in the corners, and the slightly smaller field of view. The Canon 180mm Macro with the 0.726X reduction and 1.5X APS-C crop factor is effectively a 196mm lens:

Here's the center of the Canon image (with the pants hanging out of the window). I took several shots with both auto-focus and manual focus using Live View, and this was the sharpest. The lens was wide-open and the shutter speed around 1/1000 second, with everything on a tripod.

and here's the same crop with the Fuji and Mitakon adapter. It actually looks sharper than the Canon image (and is slightly narrow in FOV). This is the same lens, although I manually focused using the Fuji's focus-peaking feature. I think the Canon image can be sharper if I had taken a RAW image. Fuji JPEG processing really does seem better than Canon's, and the X-Trans sensor probably also helps.

Here's the corner of the Canon image (top-left, but not the extreme corner):

and the same from the Fuji. Note it's a bit darker and the vignetting is obvious. Sharpness seems about the same as the Canon (note that the macro lens is wide-open, since you can't stop down the aperture on a Canon EF lens if it's not attached to an EOS body):

Long story short: at least on this sample of the Canon 180mm f/3.5L macro lens, the Mitakon focal reducer does not obviously reduce image quality, when used with a Fuji XE-2 body. I'm sure the Fuji's X-Trans sensor and JPEG processing has something to do with this. It's entirely possible that the Mitakon reduces image quality from the lens, but the Fuji's in-body processing compensates for this.

I'm not inclined to test the Canon 180mm lens "native" (with no reducer) on the Fuji body, since I don't intend to use any SLR lenses un-reduced on the Fuji body. Also, the Mitakon reducer adds a layer of protection over the Fuji sensor.

Here's the Canon lens on the Fuji body, it does look a bit ridiculous:

There's just one wrinkle with the Mitakon reducer: certain lenses don't work (they don't reach infinity with the adapter) due to projecting rear elements. Here are three Pentax screw-mount lenses that I have: the Super-Takumar 50mm f/1.4, Super-Macro-Takumar 50mm f/4, and Super-Takumar 35mm f/3.5 - and only the middle lens reaches infinity with the Mitakon reducer in place. The 50mm f/1.4 and 35mm f/3.5 have projecting rear elements that strike the Mitakon's front element (bad..) when racked to infinity:

There's a more complete list (80+ lenses) by A. Hillyard (with a Google sheet) that enumerates both compatible and incompatible lenses.

In summary, the Mitakon Camera Lens Turbo Adapter (Ver II) looks like a good choice for "almost full frame" on Fuji X-mount cameras, so long as you are willing to live without autofocus. It is also generally quite inexpensive (US$ 150).