10″ Dob First Light

Unbelievably it’s actually six weeks since I managed this, but I got around to writing it up in the end…

Very unexpectedly, having finished my dob build the day before, around midnight there was a break in the cloud and as I’d left it in the workshop the dob was suitably cooled already. The seeing was pretty poor, the ground was sodden, rain was still dripping from the trees and the moon was still bright, but I was determined to test my new scope out.

My first problem was immediate: “How on earth do you tell where it’s pointing?” It’s nowhere near as easy to “aim” as a fast ‘frac and the optical finder didn’t show me enough of the sky to work out where I was. Eventually I settled for sighting down the top truss tube on a brightish star which would then be in the eyepiece. A telrad or RDF at the very least is on the cards I think. The next issue was that the low-profile focuser didn’t have enough back-focus for the eyepieces to reach focus. I had to add an extension to sort that.

I’d only lifted the scope to just outside the door, so had a view of the south-western sky from the zenith down to about thirty degrees, and round from Ursa Major to the point where the moon washed everything out. Obviously then my first target had to be M13, especially as I can find it even with only a few visible reference points.




Absolutely nothing had prepared me for the quite astonishing view, especially given the poor seeing. I’ve been using the ST120 a lot of late and with it M13 is mostly a fuzzy grey blob with the odd star around the outside. I could see stars, more stars, and even more stars. Cackling like a witch I ran up the eyepiece focal lengths to 9mm and it just looked even better. Probably just as well everyone else was in bed, really. The detail was astonishing. I couldn’t make out the propeller, but in the conditions there was no way I expected to.

Time was ticking on and I’d not intended to be out at all, so I swung around to Sagitta which was barely visible in the moonlight. Hop up the “arrow” and right a bit; oh, no, hang on, left and right are the correct way around; it’s up and down that are reversed now. Ok, right a bit and “Whoa! That’s the Dumbell?! It’s *huge*!” I’d hardly claim it was stunning, half washed out by moonlight as it was, but the definition was very impressive.

Last DSO to try was M57. I skipped over to Vega which appeared astonishingly bright in the eyepiece and then star-hopped my way down. With the ST120, M57 is, well, a bit underwhelming, to be fair. With the 10″ it looked clear and bright and again very well defined as far as I cared to push the magnification.

As the moon had cleared the house I couldn’t not have a look at that too. Foolish of me not to fit the moon filter. I nearly blinded myself. Pushing the magnification up to 200x helped a bit, but keeping something in the field of view at that kind of magnification is clearly going to require a little practice on my “nudging” technique. Nonetheless I could easily make out the central peak of Tycho and the terraces inside the crater walls. Scooting about the surface to features I’ve viewed before showed up so much more detail, though getting my head around the opposite image inversion to a ‘frac caught me out quite a few times.

At 1am the clouds started to reappear and I called it a night. Overall I’m really happy with the optics and the functioning of the alt bearings. I’m not totally happy with the az bearings and those will require some adjustment to get right. And I need a non-magnifying finder. Probably the biggest issue is that I really do need a set of wide-angle eyepieces now. I was using my BGOs for the higher magnifications and whilst they produced very crisp views they just don’t have the field of view required for this scope. Oh, and collimation. It was easy to do with the exception of finding the secondary adjusters with an allen key in the dark. Bob’s knobs or something similar are going to be required there.

I can’t wait to see my first galaxy with it.

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