by Jeff Barbour
Once again last evening (Wednesday, May 1) clouds dominating the daytime sky rolled away and I had a solid night of observing. And even more importantly the "spring fever" of the last few weeks subsided and sky stability returned to the usual 7+/10 seen most nights here in the Santa Cruz Mountains.
Evening started early. By 8:15, with Argo well-chilled and no sign of tube currents, easily picked Venus out of the western dusk. Planet showed the typical luminosity gradient. Again, some 30 degrees supra-horizontal, no real atmo-chromaticism. Interestingly, despite my best efforts to bring the planet to focus, found that once I shifted to a well-positioned Jupiter had to refocus. So stability in that part of the sky less than 5/10.
Jupiter however, lay well within the sky's middle third. At 210x quite sharp. Aside from the usual compliment of belts (xEBs, xTBs, xPR-shading) caught a huge white oval just past the CM on the NEB. Also several reasonably contrasty NEB incursions into the EZ. For some reason, didn't catch the SEB rift until I bumped the magnification to 360x. Made a check at 540x too! White oval difficult but could still made out along with the EB and belt edge irregularities. Aesthetically the planet looked best at 210x. Slightly better views of discrete details at 360 however.
Saturn now lies no more than 10 degrees east of Venus along the ecliptic. Cassini appeared an "unsharpened pencil line". SEB's northern frontier relatively distinct while the south blended into the temperate region. Shadowing effects though obvious, lacked presence. So maybe 6/10 seeing in that part of the sky.
Had hoped to follow Polaris-B's emergence at various magnifications. But as a result of steady skies found that every magnification revealed the companion as soon as I tracked the pair down (around quarter to 9:00PDT). The companion however was clearly sharper at lower magnifications than higher. Meanwhile the background sky was significantly darker. Overall the edge - to my eye - leaned towards the lower magnifications.
Speaking of higher magnifications - able to resolve Polaris-A airy disk this evening. However, the diffraction ring appeared less than text book. Airy disks were quite nice throughout the evening. For instance while viewing 3.9 magnitude Upsilon UMA, actually caught the first diffraction ring in perfect concentricity and equi-luminosity. Beautiful! Must have been a rare moment of 9/10 stability. (The fourth harmonic was also visible as a "ghostly presence" - something that only appears on very dark and steady nights.)
After monitoring Polaris, went on to a well-placed Iota Leonis. Inspected the pair under a variety of magnifications. Found I could get better than hairline separation at 180x while 120x showed pair-elongation. Although this is a little out of range for sepCalc, the values suggest the pair is at minimum 1.2 arc seconds separated. Meanwhile at 360x, I could plainly see the blue-white secondary trailing to north just outside the first diffraction ring. Since the first diffraction ring lies at roughly 170/Dmm (D=aperture) from the center of a star's airy disk, this method also suggests a separation of about 1.2 arcsecs. So yah, no wonder this particular disparete double (magnitudes 4.1/7.3, published separation 1.3) requires 6+/10 seeing to resolve!
While in the neighborhood, swung north to 90 Leonis. Immediately identified the pair based on a distant 11th magnitude come due north. Like Iota, this double resolved cleanly at 180 and elongated at 120x. Again like Iota, the pair gave a fine resolve at higher magnifications (210 - 540x). And as previously, position angle (trailing to north ~70 degrees) is quite a bit awry from the published 208 degrees. Meanwhile, the separation has narrowed significantly as well.
By the time I observed my next pair, the sky had darkened considerably. 5.3 magnitude SAO 81054 (located between Epsilon and Mu Leonis) could just be held direct. And this was a very good thing too! For Chi Leonis (mags 4.7/11.0, sep 3.6, PA 270) proved to be a very difficult but satisfying resolve, One that was characteristic of an "on the edge" disparate.
First off, 70x magnification kept turning up a variety of "phantom secondaries". Meanwhile, 120x showed nothing suggestive of a companion. 180x was a bit more promising while at 210x the 11th mag companion was suspected. This was also true of 540x. Only 360x gave a confident view of this extremely faint and relatively close companion. Yowser! There's real food for thought here! Under 7/10 stability, and 5.3 ULM skies at 360x, Argo's calculated telescopic limiting magnitude is roughly 13.6. Meanwhile, some 3.6 arcsecs from a 4.7 magnitude star the glow is so intense that an 11th magnitude star can just be detected with any confidence. Finally 540x magnification was not supported by sky conditions (causing the secondary to "dissolve" under 7/10 seeing). So its obvious you can't just throw magnification at these babies and expect resolution...
Also took in three doubles in Ursa Major tonight. Neither Upsilon (mags: 3.9/11.5, sep: 11.3, PA: 295) nor Iota (mags: 3.9/9.5, sep: 4.5, PA: 24) gave any hint of a secondary (despite whatever magnification I through in their general direction). However, I did revisit Nu (mags: 3.7/10.1, sep: 7.3, PA: 147) to get a sense of how various magnifications effected this widely disparate pair.
Once again, was able to pick out Nu's companion at the expected PA (trailing to south), but only at magnifications in the range 70 - 210x. In fact, the best view was at 120x, while at 70x the secondary to "jump about" a bit and higher magnifications appeared to increase the intensity of the 3.9 magnitude primary's "glow". The fact that there remained at least some range of available magnifications suggests to me that an even dimmer secondary may have been seen at this separation. However - not much dimmer!
Now tonight was not all about doubles! In fact I had selected three galaxies from the 6 inch optimal aperture list (M81, 82 & 51) for review at a variety of magnifications (70, 120 & 180x). The idea was to get a sense of how the peculiarities of image scale and magnitudinal reach impacted unresolvable studies of variegated surface brightness.
M81: At 70x, this large 2:1 aspect ratio galaxy (magnitude: 6.9, size: 20x9) gave a fully averted appearance of about 1/3rd the 45 arcmin field in major axis extension. So maybe 15 x 6 arcmins of the spiral was detectable. In terms of structure, a starlike nucleus, bright core region, and diffuse spiral extensions were seen. Bumping the magnification to 120x, revealed a secondary condensation half-way out from the nucleus to the west. At 180x, the core region became visibly spheroidal with perhaps a better defined eastern hemisphere. Skies were at best 5.0 ULM deep in the northern sky over Boulder Creek proper.
M82: At 70x, the irregular nature of this edge-on presented galaxy was instantly apparent. The galaxies primary condensation lay offset decisively to the east. The galaxy's southern frontier appeared visibly sharper. It's western extension appeared far more developed than that to the east. It's major axis appeared to displace maybe 1/7th the 45 arcmin field (or about 6 arcminutes in length and 2 arcmins in width).
120x didn't seem to do more than make the irregular profile of the galaxy more obvious. Interestingly, the galaxy now seemed to take up about 1/5th the 26 arcmin field along the major axis. So not a whole low more of the galaxy was "dissolved" in the process of magnification.
At 180x the ideosyncratic nature of the galaxy's profile revealed itself as a triumvirate of condensations along its profile. The vaguish core region resolved to an almost starlike node. This nuclear condensation was in turn echoed by two others. The one to the east appeared closer to the nucleus than that to the west. Meanwhile, the western condensation appeared visibly brighter than its eastern confrere.
The last study for the evening was M51 in Canes Venatici. But by this time, the sky had taken on that "gauzy" look that discourages extended time at the eyepiece. At magnitude 8.40 and apparent size: 9x8'. M51 took up about 1/7th the field of view at 70x. So maybe 6x5 armins of the galaxy were seen. And this 6 arcmins appeared visibly more overdeveloped to the west than the east. At 120x, a 13th magnitude "star" appeared maybe 3 arcmins southwest of the galaxy's starry nucleus. A "sense" of spiral was possible W-N and E-S. By 180x, the core rgion took on an "oblique" appearance as the roots of the spiral extensions revealed themselves.
Now the fact is that on at least one occasion, M51's spiral arms have been detected through 150mm Argo at 210x - along with various knots and condensations. But conditions must be very good to achieve the kind of contrast needed to achieve such a view.
Of course, there is another way to get such a view, but that would involve contracting a illness I'd rather avoid - "aperture fever"...
Just after my return to observing the night sky, I came to the conclusion that without a definite series of astronomical projects to pursue, my interest in the hobby would soon flag. Fortunately, there are no lack of such projects available to the avid amateur. Among those I've taken "to heart and mind" is the pursuit of "on-the-edge" observation of double stars.
There are in fact two areas of extreme duplicity that I personally tend to favor. One are the "sub-Dawesian matched pairs" class of doubles. While pursuing that line of investigation, ran into a wall somewhere between .55 arc second Beta Delphinus (magnitudes 3.6&4.6 .55") and .5 arcsec Gamma Andromeda C. It certainly would appear that no matter how much magnification you throw at ever tighter matched pairs, you eventually fail to detect elongation suggestive of duplicity. Thus a value of about 80/Dmm (D=aperture) may very well describe the frontier beyond which even the finest of scopes fails to reveal duplicity in matched double star pairs.
In the matter of disparate doubles (defined as pairs whose delta is greater than 1.5 magnitudes), the ability to state a hard and fast limit is exceedingly problematic. In general, the main limiting factor for matched pairs (outside of scope and observer) appears to be seeing stability. While that of disparates adds sky transparency to the mix as an essential factor and places far greater load on the refinement of the optics (freedom from glare and the ability to present high contrast views). Add to this careful selection of magnification on the part of the observer (as discussed earlier in this series) and you can see why resolving doubles of this type is such an intense challenge!
On the basis of this last series of observations, I've now "rolled back" my personal threshold of disparate double resolution along several fronts. Without making an exhaustive search of all observing records, it seems that the Nu UMA pair (magnitudes 3.7&10.1, separation 7.3") lies just this side of "no pairs land" in the 5 to 10 arcsec separation range. Meanwhile between 2.5 to 5 arcsecs, Chi Leonis (mags 4.7/11.0, sep 3.6") takes a similar post. Among disparates which include sub-3.0 magnitude primaries, the summer pair Eta Draconis (magnitudes 2.9&8.2, separation 4.8") proved difficult but probably surpassable. While the toughest pair at or near the first diffraction ring (Rayleigh's limit), Iota Leonis (4.1/7.3, 1.3") is also likely to be superceded as observations continue. Finally, right at Dawes limit (here defined as 120/Dmm), several mildly disparate pairs (delta=1.5 magnitudes) have proven resolvable in the past. These have alwaysrequired magnifications in the range 360 - 540x.
Of course, given the fact that "there will always be a better sky around the corner", it is unlikely that "the quest for the disparate doubles edge" will ever be consummated.
Just the kind of thing to keep me at the feet of Nuit for a lifetime!