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Epsilon (ε) Draconis – bit of a challenge

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.

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4 Responses

  1. Well, hold on to your hat a bit longer, Greg, because I haven’t had much luck with this one so far, using a 60mm, the 76mm Tasco, and a pair of 80mm F12 scopes.

    I did — finally — after three or four different nights of trying, nail it with a 102mm refractor. The seeing was highly variable – at times reasonably stable, and at other times – on a scale of 1 to 5, with 5 the best – it would deteriorate to about a minus four. Colder air was approaching from the Pacific, and as the currents mixed into the atmosphere between me and Epsilon Draconis, it would suddenly balloon up to the size of Jupiter.

    My first attempts to find it were without the advantage of knowing what the position angle was. Several times I thought I could see something begin to split from the top of the primary, which would be a PA of about 200 degrees. But since additional magnification wasn’t bringing it out, I checked the PA and found it was on the opposite side of the star at twenty degrees!

    After a few more attempts to see it, I finally spied it protruding at the base of the primary with a 7mm Ortho that provided me with 126x – again, no small achievement in that kind of seeing. And, as often happens when you know exactly were to look, I found I could drop back to an 11m Plossl (80x) and still detect it. But that was an absolute minimum.

    Can it be done with a 60mm? Stay tuned!

    • Sept. 3rd, 2010, 1 AM

      OK, another try on this devious little pair of stars. Based on the numbers, this one should not be as hard as it is – but it is.

      The goal this morning was to see if I could split Epsilon with a 60mm scope. First, though, I used my 105mm F15, which is the lower half of a 105mm-60mm combination. The secondary was just poking it’s head out from the edge of the primary at 107x in a 14mm Radian. I switched to 125x with a 12mm Radian and gained some separation between the two stars, so I leaped all the way up to a 10mm (150x), but the seeing was too poor to hold it for long – I could glimpse the secondary, but it was bouncing around so much I went back to the 12mm.

      Now, the whole reason I came here tonight:

      I put a 15mm Plossl (67x) in the 60mm F16.7 just to get a first look. Epsilon is an attractive golden yellow color in both scopes, and against the black sky, it radiates a stream of pleasing photons into the observing eye. I worked my way up through a range of eyepieces and magnifications: 11mm (91x), 9mm (111x), 7mm (143x), and finally stopped with a 6mm (167x). All of this took a good thirty minutes of careful inspection and observation – this is not a task for a person in a hurry to finish and attack a late night (or early morning) snack in the kitchen. I could hear my stomach growling, but dedication, not to mention fanaticism, comes before food.

      I had a glimpse of the secondary, finally, in the 6mm, but I thought I could just detect it with the 7mm and 9mm eyepieces. I stopped with the 6mm because the seeing made it impossible to go any higher – in fact, it was a real battle with the 6mm. The night before I had managed to use a 4mm eyepiece in a 60/900 for 225x, and had essentially the same results – just a glimpse. The problem with these high magnifications in a 60mm scope is the photons have to be carefully divided up, lined up, and then fired directly into those little eyepiece lenses that are the size of a pin hole. In other words, the view was getting pretty dim. And the seeing was so rough that the image came close to bouncing beyond the edge of the field stop and out of sight.

      So, yes Virginia, there is a secondary visible in a 60mm scope – but it won’t reveal its presence without a struggle. What is really required is excellent seeing and probably a reasonably moisture-free atmosphere to eliminate the glare you get from the primary that gets in the way at the higher magnifications.

      The next time I get those ideal conditions – which actually do take place several times a year – mainly when the moon is full – I’ll give it another try. I would like to see a clean split with black sky between the two stars, and an image that isn’t bouncing around so much you can almost hear it caroming off of the sides of the eyepiece.

  2. September 25th, 2010 – 1 AM

    Arghhhhhh! I must be a masochist. I keep coming back to this devil of a double with small aperture scopes and get very little satisfaction for all my efforts – or, as was the case early this morning, no satisfaction at all.

    I had a pair of 90mm scopes I was comparing side by side – one an F6.7 apo, the other an F10 achro. I worked my way up to about 200x on both of them, but nothing I would call definite here – pretty similar to what I experienced above with the 60mm scope. Maybe a glimpse, but I wouldn’t want to describe it as definite.

    Seeing was really working against me, though. The image was bouncing around so badly that I almost had to hold on the the side of my chair to avoid getting seasick. Really not the way to try for a difficult double.

    So far, then, the only really satisfying split has been at 102mm and above. I think 90mm should be able to accomplish it, too, but I’ll need better seeing to get there.

    Being a masochist and a fanatic of the fantastically difficult, I’ll be back, though. If nothing else, the golden yellow color of Epsilon Draconis is very pleasing to linger over, so there is more than one reason to return.

  3. October 3rd, 10 PM

    Skies were very murky tonight, but the seeing was excellent, so I thought I would try one more time with a 60mm scope on this difficult pair of stars.

    I had my 105mm f15 in action again, with the 60mm f16.7 attached to it. After getting the 105mm lined up on Epsilon, I found I could see the secondary as just a small point of dim light right up against that very yellow primary with an 18mm Radian (84x) in the scope – significantly less magnification than I’ve needed in the past on this star with this scope. That got my hopes up, but I remained completely calm and carefully cautious.

    So ….. I had a 17mm version of an older Celestron Plossl (59x) sitting in the diagonal of the 60mm. A long, hard look in it didn’t yield any results. I replaced it with a recently acquired 12.5mm version of the same line of eyepiece (80x), and took another long look. I could see it! It was sitting there just beyond the edge of the single diffraction ring. I had to look very closely – and I do mean closely – but it was there! I could actually see it without it bouncing around and in and out of the diffraction ring. It was a steady, dim pinpoint of light!

    Next I tried a pair of 7.5mm eyepieces (133x) – first another of the older Celestron Plossls and then a Tak LE – and had to look just a bit harder because the increased magnification/photon depletion resulted in a dimmer image. Even at the higher magnification, the secondary was still lying very close to the diffraction ring – and it was fainter than a photon taking a long, unhurried scenic detour through the Milky Way. The transparency had improved a bit by this time in that area of the sky, which no doubt, in combination with the very steady seeing, made it possible to pick out this very dim point of light almost over-powered by the diffraction ring.

    So persistence pays off once again! I calmly stood up, threw my hat in the air, raised the three Plossls and the Tak to the sky gods in thanks, and watched the clouds come drifting back in.

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