Dob Repaint pt3

By Antony McEwan

Customisation

As I’ve already said, my reflector has never been completely stripped down before, so I thought about other things I could do at the same time to improve its performance even more. Something that is often recommended is blackening the inside of the tube. This can be done by applying black self-adhesive material, or simply by painting it. This serves to increase the contrast in the view by reducing any reflection inside the tube. Originally the tube interior was painted a semi-shiny grey colour, but after a couple of coats of Blackboard Paint it is now definitely much darker and non-reflective. (Pic to left shows the difference, but note - flash was used to take this picture)

Since my last report on this ‘scope, I have also added an Orion Accufocus electric focuser. This is simply a small motor that allows the user to focus electrically instead of actually turning the focus wheels by hand. It may sound like overkill, but focusing by hand inevitably introduces vibration to the image, and it can be frustrating at high magnifications making a small adjustment, waiting for the image to settle, making another adjustment, waiting for the image to settle, etc. The motor attaches to the focuser shaft once one of the wheels is removed, and stays put by having two grub screws tightened against it. The motor connects to a battery box which has two buttons; one for focusing inwards and one for focusing outwards. There’s also a knob that can be turned to vary the speed of the travel. I’ve found this to be extremely useful, especially when viewing the planets, and it has completely removed any focusing vibration or oscillation. Not bad for £42 and 15 minutes to fit.

The mirrors came back from Orion Optics within a week of being sent to them. They usually suggest that it takes up to two weeks, but I was just lucky! I installed them when I was putting all the components back in the telescope tube, but I first re-marked the centre of the primary mirror for ease of collimation. Details of how I did this can be found here. The mirror cell on this telescope has always been a very tight fit, and it still was, even after I had sanded down the part of the cell that slips inside the tube interior. If I’m ever able to remove it again, I’ll also sand down the inside edge of the tube!

While I was in a sanding mood, I also had a go at the little nubs around the edge of the protective end-cap. These are supposed to make the cap fit snugly inside the end ring, but they were slightly oversized, meaning the fit was often too tight. A few strokes with medium sandpaper, and it is now a perfect fit.

I also noticed some spots on the end rings; top and bottom, where the grey mottle-effect paint had been chipped or scratched away. I had a tin of black Hammerite paint, and guess what – it’s EXACTLY the right shade to touch up areas like that and get them looking brand new again. A few spots with a small paintbrush were all that were needed.

The final modification was the addition of a Telrad reflex finder. I had so much trouble over the last season with my optical finderscopes dewing up that I decided to invest in one of these useful zero-power finders. It projects three concentric circles onto a small transparent screen that you look at the sky through. The rings have effective diameters of four degrees, two degrees and half a degree. When looking at the night sky through the screen, these rings are superimposed over the area of sky that you are looking at. You can then use the rings to work out distances from bright stars to dim deep sky objects (or even planets), and line up the telescope on them.

The finished 'scope, shown before Telrad was fitted.

Job Done!

So with everything stuck back together again, and the telescope sitting on it’s rocker box, I could finally say the job was finished and that I was very happy with the result! Collimation was required from scratch, and took a while because the secondary mirror support had been completely removed, and needed to be accurately centred within the tube when it was being reinstalled. Now it’s just a matter of waiting for viewing season to begin again so I can put the Dob through its paces again and see what differences the optical upgrade has made. The change in colour will make no difference to its viewing performance of course, but it will look nice while setting up in the cool evening dusk before dark autumn nights. And at least I won’t have to worry about the dreaded rust eating through the tube. At least, not for a while anyway…

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