by Jeff Barbour
With a 2/3rds Moon floodlighting the sky, went on a double star splitting spree. Started around 8:30PDST and wrapped things up at 11pm (but left 150mm MCT Argo parked outside to catch Selene later as she clears the foliage.) By quitting time, could just hold the three stars comprising the Herdsman's club (east of the Big Dipper's handle). So even with Luna keeping as much of the sky to herself as possible, could just manage stars unaided (and direct) down to magnitude 4.5.
Telescopically, things were better. At 210x (the only magnification used all evening), was able to just hold a 12.7 magnitude test star in Praesepe. Referencing http://astro.geekjoy.com/calcs/maglim and inputing all the right numbers (150mm, 50mm obstructed catadioptic at 210x under 6/10 seeing stability skies) gives an actual sky transparency of 5.4. Thus, if my eyes were as sharp as Argo's, and had the baffling needed to eliminate the bulk of the lunar sheen, I'd probably be seeing stars unaided and direct almost a full magnitude deeper!
Having backgrounded on the seeing, and equipment, let's move on to get an idea of what I did and did not see this evening...
Polaris: Well before skydark, just able to hold the Pole Star's faint 18 arcsec separated companion at 220x. This just after beginning the evening's activities. At that time, it was pretty tough seeing the dimmest star in the Big Dipper - magnitude 3.4 Delta UMA (Megrez). I know, I know Polaris is EZ - but check it out sometime at high magnification through a bright sky and before dark adapted. It's much easier at low power when its really dark out.
While in the Little Bear attempted KU 1. Here's the data for this pair out of http://www.virtualcolony.com/sac/star-search-form the Saguarro Astronomy Clubs Double Star Database:
Name Star RA Dec Comp Mag1 Mag2 Sep PA U2000 Spec Notes
KU 1-AB 16 43.0 77 30 AB 6.1 9.4 2.7 184 11 F2 ALSO HU 917
With a magnitudinal delta of 3.3 and separation 2.7 arcsecs, this baby definitely qualifies as a tightish disparate. Given an early hour and relatively disadvantaged sky position, it's not surprising I was unable to locate the secondary - though I thought I saw something trailing and to the north... (But Wrong PA!)
So from Ursa Minor moved to the Major. Started with:
HJ 2477-AB IOTA UMA 08 59.3 48 02 AB 3.9 9.5 4.5 24 70 A5 AXBC: Orbit calc\'d. A is a Delta Scuti var and sp-bin.
This super-deltified (5.6 magnitude), closish (4.5 arcsec) disparate has plagued me for over a year now. Of course, I only seem to go after it when there's a moon - but that really is no excuse... In Iota's case, I consistently seem to pick up something on the visual threshold, leading and to the south. Again wrong PA!
This same issue applies to the even dimmer companion of Upsilon UMA. At best I seem to pick out a threshold secondary leading and to the south. And tonight was no exception. (BTW: No scope I've ever looked through has shown 3.9 magnitude Upsilon's, 11 arcsec separated 11.5 magnitude companion. Give it a shot! And email me if you succeed.)
So just to get some kind of feeling of success, dropped by 23 UMA for a look at a nice wide yellow-blue pair with secondary leading due west. This double has a nice color contrast and warrants a view whenever heading further north to take in the M81/82 Great Galactic Duet.
Next on my list:
STF 1306-AB SIGMA UMA 2 09 10.4 67 07 AB 5.0 8.2 3.0 3 23 F8 \"P = 1067
yr., a = 6.20\'\', motion retrograde.\"
That taste of success payed off! Had no trouble distinguishing this yellow and blue-grey pair. Secondary pretty much due north, close but with a clear gap. The fifth magnitude primary showed a well-defined airy disk with walking first diffraction ring and only a little flare. This in contrast with the virtual images of the brighter fourth magnitude primaries of the evening whose airy disks, though perceptible, were engulfed in randomly scattered luminosity ejecting visibly outside what didn't really pass muster as a first diffraction ring...
Thence on to:
STT 199-AB 09 20.7 51 15 AB 6.1 10.2 5.7 133 44 F2
Thought I caught a faint secondary maybe 5 arcsecs south of and slightly leading the primary across the sky (PA~200 degrees). Although separation was pretty accurate, (I don't take double star data with me to the eyepiece) position angle off significantly. So a no go - unless the database is awry or out of date...
Continuing to:
HO 377-AB 51 UMA 11 04.6 38 14 AB 6.1 12.6 8.2 250 73/ 106 A2
Here I made out a pearlescent (bluish white color - rare in UMA) primary led to the south by a detection threshold level faint secondary at about 8 arcsecs. Both PA and separation are quite accurate BUT, 12.6 magnitude is very dim! This one definitely needs a darker sky. (Despite the fact that Argo held stars down to 12.7, there is always some sky glow close to 6th magnitude stars.)
STF 1523 XI UMA 11 18.2 31 32 4.3 4.8 3.0 129 106 G0 Also 53 UMA
Although the current separation is now closer to 2.0 arc secs, Xi remains an easy split for Argo at this magnification - even through 6/10 skies. Both stars are yellow suns, and of approximately the same brightness (a matched pair). Visibly fainter secondary leads to the south (PA ~ 235 degrees - an obvious example of out of date data.)
Next study:
STF 1524 NU UMA 11 18.5 33 05 3.7 10.1 7.3 147 106 K0
Nu proved to be a remarkably definite resolve! The faint grey secondary could be seen some 8 arcsecs trailing and south of a brightly flashing yellow primary.
Then to:
STF 1543-AB 57 UMA 11 29.1 39 20 AB 5.4 8.4 5.5 358 73/ 106 A2 A is a
sp-bin.
A second pearlescent primary. This time led slightly to the north by a purplish secondary some 5 arcsecs distant. This is a fun one for a three inch and shows a satisfying color contrast through six inch instruments. High power not necessary, but I stayed lazy throughout the night. (The old 3x configured barlow and longish eye relief 15mm Ultrascopic eyepiece inertia trick.)
Of course, to balance out that recent string of successes moved on to:
BU 919 11 59.3 33 09 6.0 12.0 4.5 16 107 K0
Thought I caught a faint companion some 5 arcsecs trailing and to the north (~PA 70). Either another mis-perception or out of date PA. Needs to be revisited on a moonless night...
And now the real treat of the evening:
STF 1695-AB 12 56.3 54 05 AB 6.0 7.9 3.7 280 48 A2 A sp-bin.
Easy locate just south of Epsilon UMA. Lovely colors! Warm yellow primary with blue-green secondary. Both stars show a diffraction disk (though the secondary's is almost pointilistic and there is no diffraction ring - sign of threshold airy). Secondary leads the primary to the west. If you are going to view only one double from this list, this is the one!
Well folks, there's more where these came from. I put together a list of similar type pairs found on the SAC database that lie within Leo. Give the Moon a couple of days then track down some of the following:
BU 105-AB KAPPA LEO 09 24.6 26 11 AB 4.6 10.4 2.4 211 143 K0 Y/B
STT 523 39 LEO 10 17.3 23 06 5.8 11.4 7.6 299 144 F5 Mt. Wilson spectral types are F1s and dM1.
STF 1424-AB GAMMA LEO 10 20.0 19 50 AB 2.6 3.8 4.5 123 144/ 145 K0 AB: Premature orbits calc'd. Y/B
STF 1450 49 LEO 10 35.1 08 39 5.8 8.5 2.2 157 190 A0 A is the Algol-type system TX Leo.
KUI 54-AB CHI LEO 11 05.1 07 20 AB 4.7 11.0 3.6 268 191 F0
STF 1536 IOTA LEO 11 23.9 10 32 4.1 7.3 1.3 141 191 F5 "P = 192 yr., a = 1.92'', motion retrograde."
STF 1552-AB 90 LEO 11 34.7 16 47 AB 6.1 7.4 3.4 208 147/ 192 B3 B is a sp-bin. W/B
STF 1555-AB 11 36.3 27 46 AB 6.4 6.8 0.6 143 106/ 107/ 147 A3 A is a sp-bin
If you don't have an atlas that shows where these babies are located, plug in the RA and DEC in your DSC's or do as I did, plot the coordinates on your favorite star chart and find the closest star of the appropriate magnitude.