Monday, February 14, 2011

aligning with SkyTools

During (visual) observing session preparation with the Celestron-Vixen Super Polaris mount, I usually just put Polaris somewhere inside the polar 'scope field and leave it at that. I'm not doing photography (or I haven't so far) thus a highly accurate alignment isn't necessary.

However I have noticed on some occasions that my tracking is spot on; other times not. Mildly annoying. If I wasn't thorough or careful, I supposed the alignment could be off by a degree or so.

Suddenly, I wondered if I could improve on it with the use of SkyTools 3. I added Polaris to the observing list!

If I always include Polaris, then I can show it on a built-in sky atlas display. With the North Star on the electronic chart shown specifically at the time of set-up, I'd be able see the relative position of the north celestial pole (NCP) as well as the distance between (ha ha, position angle and separation). I still have a bad taste in my mouth with the low bandwidth way of using the little paper star wheel guide thing.

Then, it occurred me that it would be helpful to simulate the field of view as presented by the built-in polar axis 'scope. I did a bit of web stumbling to try to get that number but couldn't turn up anything. I thought it roughly same as my cheapo binoculars or the old 6x30 finder scope. So about 7°.

I looked closely at the reticule again. I know the radius of the inner circle is 48 arc-minutes. The inner circle appeared to consume about 1/5th of the FOV which roughly makes the field 500' in diameter. Quickly crunched some numbers in a spreadsheet. Then made a "fake" refractor telescope in ST3: 51mm aperture with a 250mm focal length with a 40mm eyepiece sporting a 60° apparent field of view. It approximated the power and field of the little pole 'scope.

I turned on the "flipped" option but not the "mirrored." A little foggy about that again but I knew I could apply that later.

Opened up the Interactive Atlas on Polaris. Set the time to Now. The Little Dipper was off to the right, almost horizontal, matching how I saw it in the sky, naked eye.

Activated the Context Viewer. Told it to use the new "Vixen Polar Scope" with the matching eyepiece. Switched the Mirror Diagonal option on. Dragged the orientation handle so that west was straight up. Voila. Rotated view with δ (delta) UMi off to the top-left!

OK. Where's NCP now? I wondered. I popped into the View Controls and looked for a little NCP cross hair option... Didn't see anything so I set the Coordinate Grid to Equatorial Apparent with the Fine option. Ugh. Changed the colour from white to red in the Chart Preferences. It would do, even though the grid leaves a donut hole in the middle. So, now I knew that the NCP was a bit to the left of Polaris and, happily, at that moment, almost perfectly horizontal.

How far? I zoomed in. Then right-clicked in the donut hole and chose the Angular Measure option. Hovered on Polaris. Curiously, I found that NCP and Polaris are about 40' apart! Hey. Not 48, like what the reticule is marked at. That's weird, I thought we were moving further away...

Anyway. It was working. I wanted to know where NCP was in relation to α (alpha) Ursae Minoris presented in a way that would match the tiny polar axis telescope. I set the mount so that it was aimed a bit left of the North Star.

We'll see how it holds up.

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