Re: Pole of MIR 'ORBIT'

From: Astronomy Ireland <ai_at_iol.ie>
Date: Thu, 22 Jun 1995 22:54:10 -0400

Russell Eberst <R.Eberst_at_roe.ac.uk> wrote:
> In order to track MIR by rotating camera/telescope about one axis
> (which points to a 'pole'), the pole required is NOT the pole of the
> orbit (as given in TLEs)[That would only work if your camera were at the
> centre of the Earth]. What is required is the pole of the transit-path
> as seen from your particular site (which does depend on latitude, etc.)

He's right!

Set my super polaris mount at the pole of Mir's orbit a couple of nights
ago and had to make large declination adjustments.

Set it at the pole of the Great Circle that is Mir's track across the sky
and barely had to touch the declination! And I had not aligned with the
'pole' accurately!

I derived a formula to calculate the ra/dec of the pole of Mir's track
across the sky and give it below to help others. If you've got a super
polaris mount you really must try this!

Take two ra/dec positions for a pass of Mir. I use 30 seconds either side
of Mir being at its highest. Call these ra1,dec1 and ra1,dec2.

Let x = arctan( ( cos(ra2-ra1)-tan(dec1)/tan(dec2) ) / sin(ra2-ra1) )
Decl. of pole is arctan( -cos(x)/tan(dec2) )
r.a. of pole is x+ra2
example:
ra1,dec1=14h29m23s,+03d18'30"
ra2,dec2=16h40m33s,-04d17'21"
=> x=71.4225degrees
whence decl. of pole=76.7529
       r.a. of pole=21.4373 hours.

P.S. I'm no expert at spherical trigonometry. If there's a more elegant
solution please let me know! I get the Mir ra/dec positions from TRAKSAT
v 2.80. It does the job. Hope you find my formulae useful.

--
David Moore BSc FRAS, Editor of "Astronomy & Space" magazine.
(ai_at_iol.ie) Chairman, Astronomy Ireland, P.O.Box 2888, Dublin 1.
Tel: +353-1-459 8883. Fax: +353-1-459 9933.
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Received on Thu Jun 22 1995 - 23:02:00 UTC

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