Magic with mirrors ...

Although I'm fairly used to having unconventional days at work, I had absolutely no idea on this particular Tuesday that I would, later that day, be roaring down the runway at Woodford in search of a very generous man with some rather exciting optical equipment.

It all started that morning with a call from Stephen and Diana Willcox - Would I be interested in some large mirrors from a flight simulator that was being decommissioned at BAE? If not they would be in the skip by the end of the day.

Well I needed no encouragement, work was forgotten and within an hour my colleague and I were scratching our heads on how we were going to fit four of the largest (and probably the most expensive) mirrors I had ever seen into what was quite obviously too small a van. With three in the back and one strapped to the roof we eagerly made our exit.

To give some idea of what these things are like they are roughly rectangular in shape but with a curved top; each mirror weighs approximately 25 kg. The curvature appeared to be spherical and this seemed to be confirmed by measuring the sagitta in both directions which equated to a focal ratio of f1. The next and obvious question was what on earth to do with an f1 mirror! I must point out I am very much a novice with regards to making optical equipment, I'm halfway through grinding my first (and possibly my last!) mirror but even I realised that they were highly unlikely to be parabolised and that the aberrations would be more than significant.

For days I puzzled over this until the first clear night when I had chance to get one in the open. We only have a small yard at the back of the house and this is directly over looked by a street light – I wasn't hoping for much. I placed the mirror on the floor looking directly upwards and went inside for some bits and pieces and a cup of tea. As I returned I nearly dropped the lot ... The Plough was floating about 3' off the ground in the centre of my yard! It was utterly amazing – I looked up and the four stars of the bowl were now only just visible as a large patch of cirrus had all but obscured them but in front of me stretched the whole constellation of Ursa Major. It looked for all the world like I could pluck these tiny sparkling jewels from out the air in front of me; with a little prestidigitation I was able to have the plough floating within my hands – quite remarkable!

On investigation the mirror forms a real image at the focal plane which is so large that you can quite easily fit the largest of constellations within the image however the resolution is greatly increased – in February we had the whole of Orion in it and trapezium was visible as the four individual stars. In the summer months it really came into it own; the Milky Way was mesmerizing, as it resolved itself into countless stars – The colours in particular where quite striking. As a small exercise I had counted the number of stars visible between the bowl stars in the plough. Naked eye from my home there were none visible in the sky – in the real image created by the mirror about 20 on a good night – at Hardraw our dark site in the Yorkshire Dales I lost count at about 80 with magnitudes of 8.8 being reasonably obvious. The beauty of the system is that as you are not looking through an eyepiece, it's a very relaxing way to observe – it has its limitations – deep sky objects seem to be particularly elusive – but perhaps we haven't had the right conditions. The most remarkable instance was when observing with Chris Hill at Hardraw – we were just having a last look when a wonderful slow meteor passed through the air (seemingly) a few inches in front of us. It had the most remarkable green colouration whilst leaving a lovely and persistent fire trail after it.

To date we have only used one of the mirrors (this had been damaged slightly) which I've placed in a large mount – not unlike a Dobsonian – but what to do with the others? Watch this space!

Jason Woolley

Click on an image to see larger version.

Wow, look at that mirror!
by Diana Willcox

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