New Mirror First Light and Calibration Ahead of New Tube

[Captain Scarlet]

I’m not sure whether this is an Observing Report or more about the kit. Oh well. I plan to write up my whole “mirror experience” more fully in due course, so below is a taster.

A few days ago I took delivery of a brand new 1/10 wave Orion Optics 300mm mirror, after a 5 month wait. I plan to specify a new carbon tube for it, as this mirror has a slightly longer focal-length than the one it’s ostensibly replacing, my blue-tube SW 300p. The new mirror requires the secondary to be further up the tube than the current one will allow.

But in order to properly specify the new tube dimensions, I needed to be certain of its focal length. Orion said “they generally come out 1590mm +/- 10mm” but I wanted to be sure. So I planned to just plonk the new mirror into the cell and use it “as is” for now, and photograph the environs of Vega to establish the FL, albeit operating at only about 270mm effective aperture instead of 300mm.

I did the mirror-swap over the weekend and set up last night, Tuesday (pre-US Election), with lovely clear skies. The Milky Way was fully on view ahead of Moonrise. Having spent half an hour re-collimating everything, I was ready to go. Because of the extra 90mm focal length, and still the same tube, I needed all the adapters I possessed to achieve focus, resulting in a ridiculously long stack. Knowing how flexible these tubes can be, I suspect the arrangement wasn’t the best for collimation!

First Light

Of course with a new toy I had to do SOME observing, so I immediately headed off to Mars. I’ve looked at Mars quite a lot this season, as we all have, and had some very decent views. But none like this. I don’t know the names of any features except the Polar Cap, but at times it seemed as if I was looking at an atlas of Mars in a book: from time to time everything would freeze into an incredible clarity of detail. The dark features across its middle looked like a “walking man” with arms outstretched, torso and legs. Quite extraordinary, and looking back over my house as well!

How much was due to the new mirror I don’t know, but from looking at Polaris it was plain the seeing wasn’t the best. I moved over to the Moon, and that too was as good as I can remember, I even said “wow” again.

I slewed to Lyra in prep for my calibration-photograph and of course couldn’t resist the Double-Double. They were easily split but Lyra was starting to sink into lower-down murk at the time and it wasn’t the best view I’ve had of it. I took my photographs and estimated a focal length of 1587 mm, although I couldn't quite (though very nearly), achieve focus with my DSLR, given the combination of adapters I had: too far out or too far in! I'll do it again. Funnily enough, the calibration picture I did take shows the coma inherent in a Newtonian quite well. The doughnuts start to become skewed really not very far from the centre. Vega's doughnut itself is round and central, showing there was no coma in the middle.

Cheers, Magnus

 

Edited November 4, 2020 by Captain Magenta