Wednesday, March 16, 2016

astrometric calibration

We're in for a spell of clear weather for the next several days, so last night I finally started my double star astrometry adventure.  While having the 12.5-inch on a permanent backyard installation on a driven mount (in my case an equatorial platform) is a major benefit of building the roll off shed last year, doing double star astrometry was the primary force to drive me into the effort.  I have modest hopes: it's for my personal challenge, not necessarily for publication.  Last night proved it would be a challenge indeed.

I put my astrometric eyepiece in a 3x barlow and pointed the scope to Sirius to get the finders aligned, and of course to give "first light" of the assemblage to the Pup.  And there is was, pretty widely separated in the field (531x, 0.6 exit pupil, and 0.08° TFOV – can that be right?).  Seeing was variable and since I was viewing after full darkness the diffraction was much larger.

I then turned to Iota Cancri to take some separation measures.  It's a lovely orange and blue pair; I resolved airy disks but the surrounding diffraction rings were quite bright -- perhaps a sign of the mirror's overcorrection?  The need for a solid mount with very fine controls became immediately apparent.  I could not center the stars in the center scale of the eyepiece; the Zlomotion apparatus I bought last year moves the stars too far in the field and the mount has a jerky motion which then wobbles for several seconds.  And this when there was no wind whatsoever.  I had to let the stars drift into the center scale -- meaning the equatorial platform is not tracking so very precisely.  With patience I was able to collect the needed 10 observations to make an initial calibration for this and also Beta Monocerotis, a triple with nearly equally bright white stars. 

I will consider what to do to reduce the wobble.  I know my central pivot is somehow not tight enough.  And I think my altitude bearings are too small.  For certain the tube is not balanced with relation to the mount.  Fixing these will probably help, but not fully solve the problem -- I'd need better tracking and better slow motion controls, for which I may need to resort to electronics.  But I'll give this some time (I can't afford to spend more on this right now).  Patience and perseverance will prevail.  Aside from doing this as a home project, I'm interested to try this set-up on the 30-inch Challenger telescope -- we'll see how steady that mount is!

I tried Sigma Orionis to see whether I could get the AB split, but not -- Orion is well past meridian.  A sheet of thin marine layer was moving in, so transparency was hurt, but seeing was still good.  I had a long stare at the moon at 553x, so much detail.  At some point I'll spend more time to learn the names of all these features.  Jupiter was overall soft at 340x but there were waves of good seeing every 7-8 seconds where cresting waves of festoons in one of the equatorial bands became clear, along with nearby large oval spots -- one of them could have been the GRS but it lacked color.  Strange how the seeing was so predictable like that.  At one point an airliner on the landing path to Oakland airport passed by near to Jupiter, and the ripples from its jet exhaust disturbed the view. 

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