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(IAAC) Obj: Nu Draconis, Mu Bootes, Iota Bootes, Epsilon Lyrae - Inst: Binoculars 7x35, 10x50; refractor 20x60
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Observer: Matt Leo
Your skills: Beginner (< one year)
Date/time of observation: 1998.6.4 22:45 EDT
Location of site: Melrose, MA USA (Lat 42o28", , Elev )
Site classification: Suburban
Sky darkness: 5 <Limiting magnitude>
Seeing: 5 <1-10 Seeing Scale (10 best)>
Moon presence: Minor - crescent or far from object
Instrument: Binoculars 7x35, 10x50; refractor 20x60
Magnification: 7,10,20
Filter(s):
Object(s): Nu Draconis, Mu Bootes, Iota Bootes, Epsilon Lyrae
Category: Multiple star.
Class:
Constellation: DRA,LYR,BOO
Data: mag size
Position: RA : DEC :
Description:
After stumbling on the double star Nu Draconis the other evening (see my prior
entry), I headed out with several instruments to observe it again, along with
similar nearby multiple stars. My hope was to get an idea of what kind of
double stars make good binocular targets.
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My targets were:
Nu Draconis -- separation 62 seconds, magnitude 5/5
Epsilon Lyrae -- separation 208 seconds, magnitude 5/5
Mu Bootes -- separation 108 seconds, magnitude 4.3, 6.5
Iota Bootes -- separation 39 seconds, magnitude 4.9, 7.5
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My first task was to compare the views of Nu Draconis at different
magnifications. First, I relocated the star using the 10x50, at the flattened
top (NW) of the kite marking the dragon's head. Once again, the 10x50s once
focused split this quite handily, despite shaking at 10x. Switching to 7x35,
I could also split this double, but the separation was not as easily seen
despite the lesser shaking. This may partly be due to the fact that despite
the lesser shaking, the separation was correspondingly smaller. On the other
hand, the 7x35's I used are not as good optically (BK-7 instead of BAK-4,
poorer coatings). Despite this, I'd think that even in a good pair of 7x
glasses this double would not be as obvious as at 10x. A good pair of 8x or 9x
binoculars may be ideal for this pair; at 7x the split is somewhat subtle.
Using my slapdash homebuilt 20x64 split this pair much more cleanly, although
I think the view is nicest at 10x.
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My next attempt was Iota Bootes -- a much trickier target having only 2/3 the
separation of Nu Draconis and a fainter companing -- mag 7.5. The first
challenge is finding this star. It is nowhere near the main body of the
constellation, far up on the NW border by Ursa Major. The easiest strategy
for finding this star is to star hop from Ursa Major. Start at the second star
from the end of the handle of the Big Dipper -- Zeta Ursae Majoris. From
there, scan east to Nu Ursae Majoris, the end of the handle; take a northward
ninety degree turn and proceed about 2/3 of the distance between Zeta and
Nu. This takes you into Bootes roughly in the neighborhood of iota Bootes.
--
I was unable to obtain even a hint of elongation with the 7x35 or
10x35s. Because of the trickiness of locating this object, I didn't attempt it
with my 20x refractor, which is mounted on a tripod with serious backlash
problems. After my failure to split this double I moved on to Mu Bootes.
--
Mu Bootes is off of the the herdsman's right shoulder (i.e. to your left as
you face him). Mu is easily seen at my location without any optical aid and
is the brightest star within a few degrees in any direction; if you imagine a
flattened isoceles triangle with Beta Bootes (the herdsman's head) on the
right and delta Bootes (the herdsman's right shoulder, to your left) on the
base, Mu would be slightly to the left of the apex.
--
I spent a considerable time with this double. Sometimes I could see it very
clearly in the 10x50s, then after resting my neck (the star was very close to
the zenith), I'd try again and couldn't see the dim companion; a few minutes
later I'd be able to see it again. The problem is not splitting this double,
but seeing the dim 6.5 magnitude companion at all. I suppose that 6.5 is right
on the edge of what I can detect in my 10x50s, and subtle differences in
atmospheric transparency from minute to minute caused it to appear and
disappear. I also tried with my 7x35's, but given the inferior BK-7 prisms
and coatings it was pretty much hopeless. 7x50s might be the best size for
this double, given the large separation and dimmness of the companion
--
Given that I could only see the 6.5 magnitude companion to Mu, it's pretty
clar that I had no hope of seeing the 7.9 magnitude companion to iota. I'm
curious as to how _relative_ and absolute brightness interact with angular
separation to make a pair easy or difficult to split. Perhaps once I have
more observations I'll be able to plot this.
--
Both Mu and Iota would be nice targets for people with small refractors.
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Finally, I took a stab at Epsilon Lyrae, the famous "double-double". The two
main stars can easily be split in 7x35s, in fact at 208 arc seconds of
separation I'd suspect you could almost split these without aid if you had
unusually good vision. By way of comparison Nu Draconis has a little less
than a third the separation and can be split, if somewhat tentatively, at 7x.
3 times the separation at one seventh the power means that without aid Epsilon
Bootes without optical aid has an apparent separation bit less than half as
much as Nu Draconis in 7x glasses. Maybe I'll find a youngster with keen
eyesight to test this out.
--
The primary pair is so easy to split, I much prefer the 7x view to the 10x.
--
Of course, the real question is how much power I need to split each member of
the pair. Since this double is so near to Vega, it was quite easy to locate
in my 20x64, despite my tripod problems. However, there wasn't the tiniest
hint of separation in either of the pair at 20x. This is not too surprising,
since I've heard it takes 100x to split each member of the pair, but one can
always hope.
--
I'd tentatively conclude that for my location, any angular separation of 40
arc seconds can readily be split at 10x and tentatively at 7x. A pair with an
angular separation somewhat larger than 30 arc seconds probably could be split
in 10x glasses, given that the companion star is not too dim. The test of
this will be Albireo in a few weeks when it gets over the trees on my eastern
horizon at a reasonable hour. Albireo has 34 arc second separation that I
think will be just within my 10x binocular's ability to split, althoug I may
need a tripod. The 5th magnitude certainly bright enough to see.
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