Friday, December 2, 2016

DIY 2x54 ultra wide field monocular

One lucky day I have discovered on the forum that a really great optical tool for the truly wide field observing of the starry sky could be made for less than $30 out of the very common Nikon TC-E2 2x zoom camera adaptor!

With the help of 3D printer, I've managed to turn it into a really convenient to use monocular.
The Nikon optics is very clear and producing the straight (not inverted) sharp image over the entire ~26 degrees field of view. The Galilean optical scheme works very well here, also reducing the number of "light eating" reflective surfaces. 

It's especially pleasing to use in my silver white light pollution zone near the San Francisco, as it reveals plenty of stars down to 6m (after just a short darkness adaptation period), while without it I could barely see anything fainter than 2.5 magnitude stars. It's frankly teleporting me hundreds of miles away to some Blue LPZ...


You can use the lens as is. But to make the observing experience with it even more pleasing I have designed and printed the 3 parts plastic enclosure for it. The 7/8 finished result is pictured above.

Dew shield

The first problem with the bare lens is that it's very short as necessary for convenient use with the point and shoot Nikon camera. So I have designed the dew/stray light shield for the front part, which is also comfortable to hold in the hand and provides a strong hand/neck lanyard's eyelet on the side. The shield's design implements 3 important features:
  1. In the narrow end of it, I have a short conical lip wich stops the lens in the well-aligned coaxial position while it's pulled through from the wider front end. No aligning guesswork.
  2. In order to avoid vignetting, the outer end had been initially test-printed much wider than estimated in order to measure the actual linear field of view at that exact distance with the lens installed.
  3. The inside diameter of the narrow end of the resulting shield cone made a bit wider than the lens' barrel. Which allows the original Nikon front cap to fit and be operated easily 
The dew shield extends 50mm from the lens' front and has 91mm front opening. 

The size is selected so the combined OTA length provides perfect grip for my palm, so I have no fingers in the view while holding it, and it's still not too large. The white PLA plastic is then spray-painted black, however, I'm surely planning on flocking the insides later.

The front and back of the cone have reinforcement rings.


The second problem is the lack of the eyepiece's eye guard. It's easy to "poke" your eye with the threaded end. I have plenty of various rubber guards, so the initial idea was to just make a matching adapter ring for one, however, there is another problem - the lens is extremely sensitive to the optical axis alignment with the eye. The spot providing perfect views through the entire field is quite narrow. So, I went trough 4 different eye guard designs in PLA plastic in order to mate my scull's eye socket with the eyepiece lens precisely. A 3D scanner could come really handy here. The final geometry is pictured, but it's not painted yet, as the next important step is heat treating it in order to smooth all the imperfections. I'm using an ordinary soldering iron for that. I will post the final image later.

The 3 most important EP guard design features are:
  1. The lens' threaded end has a narrow lip just below the thread and then a 5mm of narrower lens body (neck) before it expands to the objective lens holder. I'm using that feature to hold the guard with the split ring under the lip (white edge of it is visible on the second image above). The top part of the guard is press-fitted over that ring very snuggly with the rubber mallet but still, allows the eye guard to rotate in order to accommodate various lanyard positions.    
  2. The eye guard has a wide long "temple lip" which is not only to cover the eye from the side light, but also for fast positioning of the "sweet spot" between the temple and the nose bridge side.
  3. The top edge of the temple's part is parallel with the optical axis to aid with the quick naked eye pointing of the monocular as you bringinging it to your eye.
  4. The eye side hole is slightly wider than the Nikon back end screw-cap (comes with the lens), so the later can be used as intended easily. It also serves as the guard's alignment check (the cap must be flush with the guard's back when the later is press fitted in place).
Also, I've found that having a bit of the metal body exposed (between the front dewshield and back eyeguard pieces) helps to avoid fogging, as the lens is naturally and quickly warming up under my hand while in use.

In order to make the eyecup easier to use and to avoid losing it, I have mounted a rustic two screws aluminum eyelet on its back, which I had scavenged from some other equipment. It's also helping to hang the monocular vertically while wearing it over the neck.

The lanyard has an adjustable stop to regulate its length so you can wear it in over the neck, over the wrist, or tighten to the palm of your hand.

Even though the construction looks beefy, it's actually very lightweight, as it's not really a solid piece of plastic. I'm even thinking of making a hands-free harness so I can wear it as a zoom eyeglass...

I can definitely publish the printing data files on request, however, due to the nature of the design (individual face fit and often unique PLA type / printing conditions combination effect on the plastic's expansion adjustments necessary for tight fitting the lens) I would recommend just doing it yourself from scratch for better results. It's really a no-brainer, just test print until everything fits well.

To be continued soon.

To do

  1. Heat-shaping the eye guard (done with solderin iron and custom smooth bit).
  2. Eyeguard sanding, polishing, painting (done, painted with matte spray paint).
  3. Flocking inside?
  4. External cap to reinforce the rim more while transporting (done).
  5. Hands-free harness?
  6. Front cap lanyard (done).
  7. Publish STL's and drafts.

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