Splitting the Double-Double

A couple of nights ago I got the chance to set my Vixen A80MF telescope up for a session under the stars. It’s been a while since I last used it, I’ve been using a newtonian and binoculars recently, but the it’s reminded me why I like refractors so much.

This isn’t an instrument for wide field views normally, it’s my double star scope, but with a 16mm Skywatcher Nirvana eye piece (82 degree AFOV) the view of the double cluster was fantastic.

It’s so easy to get the focus just right on these ‘slow’ scopes, and it tends to stay right whatever the sky condition… almost. Lovely sharp stars with a hint of the circular diffraction pattern.

This is a great scope for general use – in the same mold as the Skywatcher Evostar range – but I’m not entirely satisfied with it’s performance at high magnification on double stars. So decided to try it out on the double-double (epsilon Lyrae) which has become a standard for testing optics and handy at the moment near the zenith.

Finder map of the double-double (epsilon Lyrae)

There’s a very widely spaced pair – eps01 and eps02 – that can be seen with binoculars and even the naked eye, but each of these is a double pair too, and that’s the test.

With my 7mm SW Nirvana in the diagonal, giving angular magnification of 130 times and about 38 arcmins TFOV, I could cleanly split both pairs.

What I found interesting is that the southerly eps02, comprising the C and D components of the double-double, was much easier to split even though it’s the closer of the two.

True, it’s components are separated by 2.3 arcseconds whilst the components of eps01 (A and B) are 2.4 arcseconds apart, but the visible difference was huge. I was using a 2x barlow on the 7mm eye piece to get a clean split between A and B with an angular magnification of 260 times.

The difference was clearly down to the apparent magnitudes of the component stars. The CD pair are very even in magnitude at about 5.5, whilst the AB pair have a difference of about a magnitude (A at 5.2 and B at 6.1).

It’s impressive how much difference that makes to splitting a double star, and what it says about the limiting performance of telescopes given the traditional methods of calculating angular resolution. These are based on even pairs.

Sissy Haas has a project to empirically derive a correlation for telescope performance on uneven double stars. It’s open to, reliant in fact on, contributions from the amateur public, so why not have a go.

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