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DSC-60: Delta Herculis – will-o’-the-wisp

This is a DSC-60 Project observation –  for project details go here.

Stats from the Double Star Club List:

Delta Herculis 17h 15m.0 +24° 50′ 3.1, 8.2 8.9″ 236°

For a finder charts and a look at how Delta Hercules fares with other scopes read John’s post on it here, as well as the comments on that post.

The Double Star Club listing  treats this as a double only and the listing appears out of date. Again, John’s post deal with all four stars and gives more recent stats for the B component which show a separation of 12.4 seconds and a PA of 286°.  Given those numbers – in fact, given the earlier numbers even, you might expect this to be easy. It isn’t.  The C and D components are easy, even though they’re a couple magnitudes fainter, because they are each roughly three minutes from the primary.

Locating that B component – the object you need for the Double Star Club, is much more difficult.  With the 60mm and my  5.5 skies it was a will-o’-the-wisp. I saw it – but I suspect I saw it only because I had seen it a few nights before  using an 85mm and 102mm, so  I knew exactly where to find it. And with the little TV60 it definitely required averted vision and good dark adaption. I used a 4-2mm zoom and I could see it at the 4mm setting (90X), but only with averted vision. It pops in and out – more often out of view than in view.

So I would count this one as seen – barely – and also as a lesson in how difficult it is to see even a magnitude 8 star when it has a companion just 12 seconds away that is five magnitudes brighter. Much the same challenge you have with Polaris where the difference is greater, but so is the separation.


3 Responses

  1. Had my first look at Delta Herculis (Sarin) August 19 with my 90mm refractor and B was very well seen at 89x despite mediocre atmospheric transparency. On this hazy evening Polaris’ companion required averted vision to spot but the companion of Sarin was held steady. While viewing this double at 9:48 pm CST I was treated to a 4th-5th magnitude satellite crossing the eyepiece field rapidly traveling S to N. Incredibly, about 5 minutes later, an appoximate 9th magnitude object transited the 1/2 degree field in about 7-8 secs moving west to east..Talk about a coincidence! Near Earth space must be getting really crowded.

    Clear & steady skies


  2. By the way, the latest (2010) WDS data on Delta Herculis gives a separation of 12.3″ PA 288 degrees.


  3. Hi Karl,

    Just changed my post to reflect the 2010 WDS figures — thanks for that info. I thought I was current with the 2009 figures! The “B” component has shifted one tenth of an arcsecond away from the primary, and two degrees in PA, in one short year.

    Absolutely amazing that we can see that much motion in so short a time.


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