Push your telescope to the limit and put your eyes to the test with this series of tight-knit springtime double stars.
I know, I know. It's not spring yet, but since we'll be talking about pushing the limits of your telescope, I thought it appropriate to nudge the calendar along, too. With the Moon coming back to the evening sky, nebulae and galaxies will soon take a back seat to objects less affected by moonlight and light pollution. Double and multiple stars fit the bill, so I thought it would be fun to provide a graduated series of challenges to test both your telescope's resolution and the keenness of your eye.
First, let's consider the resolution of your telescope. The most stringent test of telescopic resolution uses the criteria of Dawes' limit given by the equation R = 4.56/D, where R is the resolution in arcseconds and D is the telescope's aperture in inches. If using millimeters instead, the formula becomes R = 116/D. Here are a few examples for common telescope apertures given in millimeters/inches (rounded):
60 mm / 2.4 inches — 2.2″
80 mm / 3 inches — 1.5″
100 mm/ 4 inches — 1.2″
150 mm/ 6 inches — 0.8″
200 mm/ 8 inches — 0.6″
250 mm/ 10 inches — 0.5″
320 mm/ 12.5 inches — 0.4″
400 mm/ 16 inches — 0.3″
Dawes' limit holds true for stellar pairs consisting of equally bright stars of about magnitude +6, where the diffraction disk, that little white dot surrounded by its quavering diffraction rings, touches its companion's disk. If the stars are much brighter or fainter, and especially if the double is unequal with a considerable difference in brightness between the primary and secondary, the limit will prove next to impossible. Even approaching the limit for your scope requires nights of steady seeing with calm star images along with the application of high magnification, sometimes in excess of 300×.
More realistic when it comes to standing at the telescope's side is the Rayleigh Criterion, given by R = 5.5/D (inches) and defined as when the outer edges of the two stars are separated by the width of the first diffraction ring — in other words, the duo is just resolved. Here are the Rayleigh values:
60 mm / 2.4 inches — 2.3″
80 mm / 3 inches — 1.8″
100 mm / 4 inches — 1.4″
150 mm / 6 inches — 0.9″
200 mm / 8 inches — 0.7″
250 mm / 10 inches — 0.55″
320 mm / 12.5 inches — 0.4″
400 mm / 16 inches — 0.35″
Whichever you choose, we've got you covered. Before beginning your star-splitting adventure, allow your optics to thoroughly cool down; otherwise, heat rising from the objective or primary mirror will make stars look like oozing daubs of gooey light. No one wants to bust an eye hammering away on Dawes' limit under those conditions! Nor do you want to push the limit on nights of lesser seeing. Since it's hard to know exactly what nights are best, when you do encounter ideal conditions, set aside some time to tackle a few of these close pairs.
I did some double digging and came up with a list of stars in good view in early spring. A few are easy and even particularly beautiful for any aperture, but then we ratchet up (or down?) to under 5 arcseconds, a single arcsecond, and finally, sub-arcsecond pairs.
For the closer pairs, I've tried to select examples with small differences in magnitude between primary and secondary that are relatively bright and easy to find. I also kept them all to one region of the sky, so you wouldn't be hopping all over the place.
Double stars are the jewelry of the sky, and exceptionally close pairs exceedingly so. Tiny beads trembling inside a nest of ever-shifting diffraction rings almost seem alive. Such delicacies await your gaze.
|14 Double Stars in Order of Decreasing Separation|
|Star||R.A.||Dec.||Mag 1||Mag 2||Sep.||P.A.||Comments||Scope|
|ι Cnc||08h 47m||+28° 46'||4.2||6.6||31"||308°||Spring Albireo!||Any|
|γ Leo||10h 20m||+19° 51'||2.2||3.5||4.5"||127°||Stunning, colorful!||60 mm|
|90 Leo||11h 35m||+16° 48'||6.0||7.3||3.3"||209°||3rd comp. 63″ SW||60 mm|
|38 Lyn||09h 19m||+36° 48'||3.9||6.1||2.5"||224°||Wide mag. spread||80 mm|
|γ Vir||12h 42m||–01° 27'||3.5||3.5||2.5"||2°||Easier than 38 Lyn||60 mm1|
|Σ1333||09h 18m||+35° 22'||6.5||6.7||1.8"||50°||---------------||80 mm|
|OΣ215||10h 16m||+17° 44'||7.2||7.5||1.6"||177°||Slowly widening||150 mm|
|57 Cnc||08h 54m||+30° 35'||6.1||6.4||1.4"||~316°||Orange, very nice||100 mm|
|Σ1338||09h 21m||+38° 11'||7.2||7.5||1.1"||~315°||Slowly widening||150 mm|
|OΣ186||08h 03m||+26° 16'||7.5||8.2||0.8"||73°||---------------||150-200 mm|
|Σ1555||11h 36m||+27° 47'||6.4||6.8||0.8"||148°||---------------||150-200 mm|
|ω Leo||09h 28m||+09° 03'||5.7||7.3||0.7"||103°||Tough one!||200 mm|
|Σ1517||11h 14m||+20° 08'||7.5||8.0||0.6"||115°||---------------||200-250 mm|
|φUMa||09h 52m||+54° 04'||5.3||5.4||0.45"||~300°||Equal mags. Ideal!||250-320 mm|
|Data are from the Washington Double Star Catalog and Sky Catalog 2000.0, Volume 2. Right ascension and declination are for equinox 2000.0|
1. γ Vir reached a minimum separation of 0.4″ in 2006. It's been widening since. The values shown are for 2017. Other double star values are also as current as possible.