A Simple Helical Crayford Focuser

How simple can a focuser be?

This has been a bit of a design challence for me… Just how small, how simple, can I make a focuser and yet still provide excellent usability. My latest Helix 1-14 design is my best yet.

A simple helical crayford telescope focuser
A simple helical crayford focuser

As my 3D printer design skills improve I applied lessons learned and the unique capabilities of an FDM printer to the problem of a focuser. After a bit of a journey I have arrived at a design I can consider finished. Is it the final design? I may tinker some more, but I am satified for the moment.

The design is based on a few ideas from here and there, products I have seen or used, internet postings from others faced with the same challenge. I am not one for just printing someone else’s design, I rarely do that, I want to do my own, and maybe make it better.

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The ScopeStuff 2″ Crayford Focuser

When shopping for a focuser you face a lot of choices. From the cheap eBay Chinese options to the top-of-the-line Moonlights and Feather Touch. I once again perused these options when buying a new focuser for the Cave Astrola.

ScopeStuff 2" Crayford Focuser
The ScopeStuff 2″ Crayford Focuser mounted to the 8″ Cave Astrola
It has been a while since I looked at what was out there. So I spent some time shopping and wandering website re-educating myself on what the options were.

I considered the cheapest end of the market, a few options found on eBay, and again at the top-of-the-line, machined works of art that cost as much as some telescopes. This time I was limiting the budget for restoring the Astrola, not wanting to spend much more than a hundred on the focuser to keep the cost of the whole effort down.

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Some Chinese Eyepieces

A Celestron FirstScope in need of eyepieces. Time to find a couple inexpensive eyepieces. With the telescope destined for an eight year old girl I really did not expect the eyepieces to live up to any demanding standards. But neither do I want the views to be truly horrible either, it would be nice if the telescope is properly enjoyable.

Chinese Eyepieces
A set of cheap Chinese eyepieces
Thus I went to the source of cheap optics and found a set of Chinese eyepieces on eBay that might just do the job. For $22 they were the right price. The set includes three eyepieces, a 23mm, a 10mm and a 4mm for high power, a nice selection of focal lengths. The right price, but how about image quality?

As the eBay sale is from a US seller the eyepieces arrive in just a few days. Unpacking the eyepieces is the first pleasant surprise… The quality is not bad, metal barrels, decently molded rubber bits, decent looking glass. An ebay sale often a bit of a gamble, but this does not look bad at all.

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Astronomical Intensification

Visual astronomy is the practice of pushing our built in optical detectors to the limits of performance. Our eyes are surprisingly good optical instruments, and until the advent of film about a century ago were the only means we had of observing the universe. Even now, in the age of sensitive electronic detectors there are those who appreciate the view of the heavens through our own eyes.

Intensified M104
The edge on spiral M104 photographed with a Collins I3 Image Intensifier
This does not keep some from trying other ways to improve the view. Night vision technology, devices pioneered by the military that amplify the available light, offer intriguing possibilities. Available in compact packages these amplifier image tubes have been incorporated into an eyepiece sized package that can replace a standard eyepiece and offer an amplified image.

The image tube operates by charging a grid to a high voltage inside a small vacuum tube. Incoming light (photons) strike this grid and create a shower of electrons that continue onwards to strike a phosphor screen at the rear of the tube. A single photon can create a shower of hundreds of electrons, a very large signal gain. The phosphor screen glows where struck by photons, creating a image of the amplified signal.

A bright image is seen on the screen of the object, hundreds of times brighter than the original. The image is green, as result of the phosphor, this amplification is a monochromatic process. There is some noise in the image, random “sparkles” called scintillation that result when electrons leave the charged grid in a random fashion.

The images shown here are taken with the Collins I3 eyepiece or I3piece. The device is a very nicely built unit that is about the same size as a modern high quality eyepiece. Actually it is much smaller than some of the large designer eyepieces seen on some telescopes. An internal battery means there are no cables resulting in a neat package. The intensifier has a standard 1.25″ or 2″ nose piece threaded to accept standard astronomy filters.

The image intensifier is not a panacea, there are some objects where the intensifier works well, and others where it does not perform. Globular clusters and planetary nebulae are quite dramatically represented in the intensified view. It is faint, low surface brightness objects like galaxies and extended nebulae that are often better appreciated with a normal eyepiece. Switching back and forth is generally a poor idea as using the intensifier decreases dark adaptation.

Intensified M51
The edge on spiral M51 photographed with a Collins I3 Image Intensifier
During personal observations of galaxy views in two large telescopes side by side, I noted more detail in the un-amplified images compared to those using the intensifier, particularly where subtle detail was concerned. I have had opportunity to observe the same galaxy in my 18″ f/4.5 followed by the view in a 24″ with the intensifier. The images in the intensifier were much brighter, but the contrast range seem compressed, such that HII regions and similar low contrast details disappeared.

One place where the value of the intensifier is undeniable is in public outreach. The live views of bright galaxies show far more detail to the inexperienced observer that they would otherwise have missed in the eyepiece view. A spiral galaxy is clearly a spiral galaxy, even to a first time observer. In addition the intensifier can be used to provide views of faint objects under less that ideal conditions. Addition of a narrowband filter can increase the signal to noise and allow viewing of emission nebulae even with substantial light pollution from natural (e.g. the Moon) or artificial sources.

Unfortunately the Collins intensified eyepieces are no longer available from the manufacturer, though the website still appears functional. There is an equivalent product from BIPH which uses the same technology. At nearly four thousand dollars these devices are not for everyone. they can be used to good effect under the right conditions.