Astronomy
McNaught by day, IC443 by night . . . Plus a little lathe work.
Jan 10. Frustrated trying to see this bright new comet in the twilight, I tried it at midday. Success!
Tonight I set the deepsky outfit up after dark, after supper. Went together relatively smoothly, but something is not right. I'm getting guide errors in both axes; very consistent trailing. Maybe poor polar alignment, maybe something slipping.
I tried the guide scope with and without the Antares 0.5x compressor (it produces a very wide field and so many candidate guide stars in this part of the sky that the calibration routines become confused).
No world-beaters tonight, but images were plenty good enough to measure fields and focal lengths.
Ponpoint Astrometry in Maxim derived effective focal lengths of the ST2000XM / CFW-8a combination behind the A-P at its native focal length and with what is nominally a 0.75x A-P telecompressor (but whose exact compression depends on spacing):
Native focal length (SBIG at prime focus):
760.2mm / F6 / 1.00x
SBIG mounted on Orion 2-inch adapter:
602.1 mm / F4.74 / 0.79x
Using the A-P 2-inch adapter:
593.2 mm / F4.67 / 0.78x
A-P 2-inch adapter + Nikon M-ring:
565.6 mm / F4.45 / 0.74x
IC443.
A-P 5" @ F4.45
10nm H-a 3x600s
There is too much flex in the first two telecompressed arrangements owing to excess extension of the drawtube and too much in the third from the combined slack in the two Nikon couplers. It's time to machine a one-piece adapter to set the optimum spacing with no intermediate connectors.
IC 443 has become my standard candle. Late in tonight's run, tracking went to hell despite small guide errors. That has to mean that something is slipping out of line between the guidescope and the CCD. Maybe one fix for everything...
Jan 11. I machined a one-piece aluminum adapter to maintain close to maximum seperation between the telecompressor and the sensor (the more seperation, the more compression, and the more in-travel required, and there is only so much of the latter available). The inner surfaces of the adapter are turned to accept and hold in solid alignment the A-P 2-inch camera adapter at the sensor end and the 2-inch adapter tube from the Orion adapter at the insertion end. The total extension behind the telecompressor is slightly greater than provided by the A-P adapter combined with the Nikon M extension tube. There is room for doubt that the camera can reach focus. If not, I have room to turn up to 1/4 inch off the tube, but I need some stars to test.
Jan 13. I caught the Pleiades through balmy twilight haze and checked to see whether the new adapter for the ST2000 / CFW-8a would allow the combination to reach focus in both white- and hydrogen-alpha light. It does!
In white light, by Pinpoint astrometry:
547.1 mm / F4.31 / 0.70x / 2.79" per pixel.
In H-a:
548.3 mm / F4.31 / 0.70x / 2.78" per pixel
The telescope's focusser is racked out hardly at all -- so little that none of the blue drawtube is visible behind the focusser body. That short arm will contribute to rigidity. And I torqued down all the screws that hold the Mandel widefield adapter together; a couple were not entirely snug. Play is substantially reduced by the cumulative effect of all these mods.
Comet McNaught, 3:00PM EST, a little less
than
11 degrees from the Sun.
A-P F6, Canon 20D, 12x0.0005s
One piece aluminum spacer/adapter for
telecompressed
CCD imaging shown
at focussed position. (Over-long socket head
screws have been replaced
by set screws.)
Brass G11 counterweight extension / toe
saver.
A 3/8x16 rod connector turned to fit inside
provides threads; the #10-24 set screw is for paranoia;
thread a bolt or threaded rod through the connector
and screw onto counterweight shaft.
10-12% more moment arm.
We'll see how
much it matters.