Visual Astronomy

Cleaning the Lukehurst 20" f4.0 Dobson

Well, my Dobson is now nearly 5 years old but still going strong. Unfortunately the other day, while I was at work, some builders pulled down an old chimney on the barn. Low and behold, this chimney led down to my garage, so the entire garage was engulfed in dust which settled to about 3mm. Now the Dobson does have a shroud and the mirror is always kept covered but none the less did get some dust on it. Also it had never been cleaned since new, so I thought, now is the time. I was a little apprehensive ( a 20" optical mirror is not cheap ) and I emailed Dave Lukehurst, the manufacturer, with some questions, which he quickly and expertly answered. The bottom line was, he recommended removing the mirror from the mirror cell.
So I began by stripping down the Dobson, the shroud and secondary covers got a wash. There were simply 4 bolts in the base which held in the mirror cell, once removed, the whole cell could be slid out of a gap in the side of the main box. The mirror was held in place simply with three clamps, these were removed and the mirror lifted out and put on a towel. The mirror was then tilted slightly and rinsed with DI Water, followed by DI Water with a little detergent ( one drop ). The mirror was layed flat and filled with the soapy water and left a few minutes. Then lightly swabbed with cotton balls underneath the liquid. Finally again rinsed with DI Water and left to drain. The few drops left were gently absorbed onto a microfiber cloth.
The mirror was replaced and everything then in reverse order rebuilt. The reinstallation was carried out in reverse order. On completion, I recollimated the optics, to my amazement, I only had to move the secondary mirror a little to get the lightpath centered, the collomation had hardly changed.

The mirror cell, the gap bottom front is where the mirror is removed

The support for the 20" mirror

The mirror removed and being cleaned