Tuesday, February 15, 2011

fake 'scope

Here are the details of the "telescope" and "eyepiece" I put into SkyTools3 to help me simulate the view through the polar axis finder scope.

aperture: 20mm
telescope focal length: 108mm
eyepiece focal length: 18mm
apparent field of view: 48°

The aperture I found noted on a couple of web sites. And I measured it with my ruler.

The magnification number (6) and true field of view (8) also came from the web. I'm hoping they are correct.

I took the polar axis scope apart, that is I removed the eyepiece from the little scope shaft. Spotted the reticule cross hair plate thing. Sitting in a field stop. Ah ha! That field stop was 15mm in diameter.

I calculated the telescope focal length by multiplying the field stop by the magic number 57.3 (found at Tele Vue) and divided by the TFOV: 108mm (rounding up).

AFOV comes from the product of the mag and TFOV.

While I was goofing around with the tiny telescope, quite by accident I beamed the clear image of the reticule onto my desk. So I grabbed a piece of paper and did a rough tracing. Then I measured the inner circle to the outer field: 5 times. Like what I guessed yesterday.

So, I'm pretty happy with all these numbers. It's close enough for my improved polar alignments.

The key thing now is that I can know where the NCP is, in advance.



You can still buy it... In a kit. Looks like there's bits for different models of telescopes. Not cheap.

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