Epsilon (ε) Draconis
RA: 19h 48m Dec: +70° 16′
Mag: 4.0, 6.9 Sep: 3.2″ PA: 20°
Dist: 147 ly
Spectral Classification: G7
I’m with Sissy Haas on this one all the way! In Double Stars for Small Telescopes she writes:
Fantastic contrast for the separation! A brilliant star with a little dot on the edge in the beautiful colors of Sun yellow and powder blue.
Yep! And that description comes from using a 125mm at 200X. OK – I had just finished giving a lesson and was using the ES 127mm APO so the student would have no trouble seeing the companion of Polaris, so it seemed like the perfect scope to use on Epsilon (ε) Draconis. And it was. In fact, this was an interesting lesson for me on how the magnitude difference interacts with the separation. In this case we have a magnitude difference of 2.9 and a separation of 3.2 seconds of arc.
Now according to a handy table on page 5 pf the Haas book a 100mm should be able to separate two stars with a magnitude difference of 3 – almost the case here – and a separation as small as 2.3 seconds. So this should be a piece of cake for the 127mm scope and it was. It took a 7mm eyepiece (136X) to get a clean split and the best split came with the 4mm click stop on the Nagler 6-3 zoom – that’s 238X. Admittedly, at that power it was in and out as seeing went from better to worse and back to average in a few seconds. But in those rare moments of good seeing – and they were brief – it was very nice. Ah – but when I put the 127mm away and switched to the Tasco 76mm it was a different story. Now the dynamics change. The Haas table doesn’t give parameters for a 76mm, but presumably they would be a bit better than a 60mm. Now a 60mm should be able to handle a 3.7″ separation when there’s a difference of three magnitudes between the primary and secondary. But we have a difference of 3.2 seconds with Epsilon Draconis – so extrapolating from this table I would say maybe – just maybe – on a night of better seeing I’ll be able to split it. Now watch John go split it with a 60mm! I’ll tip my hat to you , John, if you do 😉 Better yet, one of us should do the math and adapt this table for a 76mm scope! Or maybe that’s a job for Klaus?
In any event, I had a related experience a bit later when I split two stars that were even closer together with the 76mm – but these two stars were the same magnitude! (More on that here. )
All of these figures are estimates, of course, that work pretty well when the primary is brighter than 6.5.