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Wednesday, 1 January 2014

When I sawed through my telescope

(You read it right; not "saw" but "sawed".)

Four years ago, after decades of prevarication, I finally bought the telescope that I had promised myself since childhood. Despite its modest price tag, the Chinese-built "Skywatcher" reflecting telescope is an impressively precise piece of equipment. Its mirror's surface is formed into a parabolic curve to an accuracy better than a quarter of a wavelength of light across its entire 4½ inch diameter. This optical perfection means that all of the light collected from a star is brought to an impeccable focus with no loss of brightness due to cancellation of light waves arriving out of kilter.

This incredible achievement of human civilization has shown me the wonders of the universe, channelling ancient light from distant galaxies into my pupil. For four years I have mollycoddled its pristine optical surfaces which would fail if scratched by any abrasives such as metal dust.

So, it was with some trepidation that I took a hacksaw to my pride and joy.



I had been toying with this drastic measure for a couple of years, ever since realising that the 'scope could not be coaxed into focussing starlight directly onto the sensor of my camera. The idea was always swiftly dismissed, knowing that one slip of the saw would write the thing off, and that even a successful cut would still leave me with a pile of telescope pieces and no experience of assembling and perfectly aligning them.

When, at last, I had a week's holiday to take at home, while the children were still at school, I decided to take the plunge and indulge two solitary obsessions: metalwork and astronomy. This blog entry takes the form of a photo diary and how-to manual for the foolhardy amateur astronomer.


How it works

You see, a telescope's job is to concentrate each star's light into a bright pinpoint. It actually creates a miniature reconstruction of the astronomical original, blazing away just inside the eyepiece holder. The astronomer then views this bijou stellar facsimilie using the eyepiece which is nothing more than a high-quality magnifying glass, making it appear at a comfortable viewing distance. The overall effect is both to magnify (spreading out the angular separations of the stars) and to brighten the stars. It gives a brighter view than the naked eye alone by collecting all of the light that falls through the big hole at the front of the telescope, and squeezing it through the much smaller area of the human pupil that limits the naked eye's light-collecting power.

This is all summarized by the picture below, that shows some of the wavefronts of starlight being concentrated and funnelled into an observers eye. This picture demonstrates a refracting, rather than a reflecting telescope, because it allows the wavefronts to be shown more clearly. The objective mirror of a refracting telescope does exactly the same job as the big objective lens at the front of the refractor, except that it also sends the light back the way it came, to be viewed through an eyepiece near the top of the telescope tube. This would make the diagram more cluttered, with incoming and outgoing light overlayed in the tube's interior.



Astrophotography

So much for using a telescope with your eye. After doing so for a while, I could no longer contain my flabbergastedness (is that a word?) and wanted to share it. So, how do you photograph what the telescope sees? There are basically two options:

(1) Point a camera at the eyepiece, in place of your eye. This technique goes by the grandiose name of afocal astrophotography, (nicely explained here) and it supports an industry of various brackets and adapters to hold the camera steady and align it with the eyepiece. I used this method for a while, with some success. It allows you to choose the magnification of the final picture by selecting an appropriate eyepiece. But it has some drawbacks. A small fraction of the meagre starlight is lost due to imperfect transmission at all of the glass surfaces in the eypiece and in the camera's lens. Significantly more can be lost due to various apertures and lens-stops in the light-path.

A more crucial drawback is that anchoring a heavy camera to the end of a long eyepiece and then swiveling it around to follow the sky can put excessive bending forces on the poor old telescope, which was not bred for heavy lifting. This was a particular problem for my light-weight entry-level Skywatcher Skyhawk which, despite its top-quality optics, holds the eyepiece in a nasty plastic focusser.

The focusser had to go. I bought a well-engineered replacement for the plastic tat. Unfortunately, it was designed to fit a bigger scope, so had to be shortened to avoid it crashing into the secondary mirror when wound fully in. This meant re-making some of its parts.




Also, the new focusser barrel was wider, requiring a bigger hole in the side of the scope. So, after disassembling the optics and protecting most of the paintwork from scratches using masking tape and duct tape, I filed away at the thin aluminium.

The focusser's base-plate was designed to fit the cyllindrical surface of a larger-diameter 'scope. Re-shaping and fitting it involved making some bespoke aluminium brackets...

...and resin to fill the gaps, molded to the right shape by pressing it against the telescope itself. Greaseproof paper prevented it sticking to the paintwork.


A better way

(2) Rather than settling for the afocal technique, most astrophotographers prefer to remove the unwieldy eyepiece altogether, with a technique known as principal focus astrophotography.

It's very simple. You remove the camera's own lens and use the telescope as its lens. This means placing the camera's light-sensitive surface (the film in an old-fashioned camera, or the CMOS or CCD sensor nowadays) at the focal plane of the telescope, where all the starlight is concentrated. The focal plane is normally located inside the eyepiece-holder. It needs to be shifted if its going to fall on the camera's sensor, which can be done by moving the telescope's objective mirror (or lens).

Unfortunately, in the Skywatcher Skyhawk, the mirror's adjusting bolts won't move it far enough. So there was only one thing for it: to shorten the telescope by just the right amount. Too little, and the effort would be wasted as the focussed light would still not reach the sensor. Too much, and the focusser would never extend far enough to focus an eyepiece for normal operation.

Working out how much to cut off is easier said than done, since it's tricky to measure the current position of the focal plane, or the distance to the camera's sensor. These things can be calculated by focusing on a nearby object and measuring its distance from the mirror. The measurement needs to be accurate and the calculations error-free. How confident would you feel about sawing off the right amount, based on these scribblings?

A little saucepan came to the rescue.

By lucky chance, this saucepan was a perfect press-fit inside the telescope tube. So, by turning it into a temporary mount for the mirror...

...I could slide the mirror into a new position, to try it out before doing irrevocable violence to my treasured toy. The resulting affront to nature, henceforth known as the Saucescope, confirmed my calculation, that a little over an inch needed to be removed.

So, with everything carefully marked out...
...and heart in mouth, I took saw in hand and gingerly cut into the blue-anodized aluminium. There was no margin for error, because the cut end would not be hidden, but would simply butt up against the mirror housing, with any scratches or wobbles in full view.

With the nerve-wracking part of the job done, all that remained was to clean off every speck of glass-damaging metal dust, rebuild and collimate the scope.


The camera now fits snugly in the new focal plane.


On the subsequent clear nights, I was relieved to find that the work had all been worthwhile and I had not ruined my beautiful telescope. What could have been a time-consuming disaster, given a moment's lapse in concentration, turned out to be the most satisfying project I've undertaken recently. Here are some of the results, photographed with the new set-up:

The Dumbbell Nebula

The Pleiades

The Orion Nebula

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