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Cosmic Challenge: Izar [Epsilon (ε) Boötis]
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Cosmic
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This
month's suggested |
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3- to
5-inch |
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Target |
Type |
RA |
DEC |
Const. |
Magnitudes |
Sep. |
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Izar |
Binary |
14h |
+27° |
Boötes |
2.5, 5.0 |
2.9" |
Finding this month's challenge object is no challenge at all unless you are trying to starhop to it from the inner city. That can be tough, but for everyone else, Izar (Epsilon [ε] Boötis) is visible easily by eye to the northeast of brilliant Arcturus (Alpha [α] Boötis) as one of six stars that make up the constellation's distinctive kite shape. Swing your telescope its way and it still looks like a single star, as it does to the naked eye. So, what's the attraction?
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Above: Early evening star map. Credit: Map adapted from Star Watch by Phil Harrington. |
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Above: Finder chart for this month's Cosmic Challenge. Credit:
Chart adapted from Cosmic Challenge:
The Ultimate Observing List for Amateurs by
Phil Harrington.
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Izar is one of the more challenging binary stars in the northern spring sky for small telescopes. Low-power binoculars readily show it to be accompanied by 34 Boötis, a red 5th-magnitude sun 39' to its southwest. They form an attractive pairing, even though they have no relation to each other.
The challenge presented here is to see Izar's actual companion, 5th-magnitude Izar B. Izar B is 2.9" away from the system's primary star, 3rd-magnitude Izar A. Observers usually describe Izar A as looking either yellowish or pale orange, while Izar B ranges from bluish to sea green or emerald.
Two stars separated by 2.9" sounds as though they are wide enough to drive a truck through until you consider the circumstances. The problem is due to the optical properties of light. Because light travels in waves, stars never appear as perfect points through telescopes. Instead, a star will focus to a small central disk -- the Airy disk -- encircled by one or more dim rings called diffraction rings. The distance to the first, most prominent diffraction ring is about six times the area of the Airy disk itself. In the aperture range set for this month's challenge, Izar A's first diffraction ring is approximately 3 arc-seconds away from its Airy disk. As a result, the companion lies nearly superimposed on the primary star's first diffraction ring, lowering image contrast and creating the challenge.
With this in mind, I headed out with my 4-inch (102-mm) f/10 refractor to my suburban backyard a few years ago to see what I could see. Popping in a 7-mm eyepiece (143x), I could readily split the pair, with Izar B appearing as a bright spot on A's diffraction ring. It helped that this particular instrument has very good image contrast and that the seeing was steady. Had I been viewing through a similar size telescope with an obstructed optical design (that is, a reflector or a catadioptric), the resulting lower contrast could have prevented me from seeing the companion.
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Above: Izar as portrayed through the author's 4-inch (10.2 cm) telescope. |
Like many
refractors, mine came with an objective lens cap that has a smaller central
opening. In this case, the smaller opening, which is normally covered by a
plastic dust cap, measures 1.75 inches (
To be successful at spotting Izar B, you'll need high quality optics, both in your telescope and in the eyepiece, and steady seeing. Light pollution is of little or no concern once Izar is in view since both stars are so bright.
Now, I know there are going to be readers out there who say that splitting Izar is easy. Sure, if you are viewing it with a telescope larger than the specified aperture range, Izar poses little challenge. So, for those readers, I am offering a bonus challenge, the binary star A570 (ADS 9301). You will see that it is also plotted on the chart above, not quite 3° west of Izar . Here, we find two stars, magnitudes 6.0 and 6.5, separated by only 0.2". That's a challenge for any scope!
Good luck with this month's challenges! And be sure to post your results in this column's discussion forum.
Remember that half of the fun is the thrill of the chase. Game on!
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About
the Author: |
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Phil Harrington's Cosmic Challenge is copyright 2022 by Philip S. Harrington. All rights reserved. No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder. |
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- CollinofAlabama, Jon Isaacs, John O'Hara and 7 others like this
44 Comments
Very nice. It's not particularly challenging in a small refractor, though, if the scope is of good quality. Never had trouble with it in my 50mm and 63mm Zeiss refractors, with magnifications 100x and up. A friend and I had a small competition some years ago, where we would try and detect it as a double, with smaller and smaller aperture stops. We could *just* manage to see a bit of the companion with 30mm aperture.
If someone likes an additional challenge, then try observing it in daytime! Yes, it's possible to see it split in the daytime. I've done so with my 63mm Zeiss, using a 6mm ortho, giving 140x. Here's a photo of it, taken through the Zeiss, with my old Sony Xperia phone. Technically, the photo wasn't taken during the day, because the Sun had set five minutes earlier. The biggest challenge is simply finding it, especially if you're doing it old school, and not use GOTO mounts!
Clear skies!
Thomas, Denmark
I've resolved this one in my 60 mm f/11.8 Swift Aerolite refractor, at full aperture and stopped down to 2 inches. In fact, I made the attempt at Phil's suggestion while he was still writing Cosmic Challenge. It is a very nice binary for small scopes.
Phil:
Good choice.. Izar is one of my favorites. The colors and the difference in brightness (2.6-4.8) make it especially attractive. It's unequal magnitudes make it more of a challenge than equal magnitude doubles of similar separation.
A current "project" is splitting Izar with my 50 mm F/5 Astro-Tech finder scope. It has unusually good optics for a finder, I've split Porrima at 3.0" and I've thought I've seen a split on Izar a couple of times but I'm not confident. I'm using 120x-140x, 60x-70x/inch.
Jon
Jon,
If my experience is an indicator, you should be able to do it. We're rooting for you!
John
John:
There's a big difference between an 60mm F/12 stopped down to 50 mm and a 50 mm F/5 finder scope push to magnifications well over 50x/inch but I keep trying.
Jon
Splitting it with a 70mm F/10 ain't all that easy for me, either. It is a challenge with this particular telescope. It takes a very steady sky and 155X, at an exit pupil 0.45mm. Far more often than not with mediocre seeing, this scope will not split it.
With an 80mm ED, I've split this double a couple of times more comfortably at 133X, but the sky steadiness still must be very good, but usually I have to goose up the power to 160X and that is with an exit pupil of 0.5mm. I'm maxed out visually, too much like work. I have more fun pursuing other doubles and the color contrast of Almach and Rasalgethi has always appealed to me much more.
For me and small telescopes, this double is a challenge and isn't my favorite. Yes, with an 6 inch, F/8 reflector or larger telescope Izar is Duck Soup and I do enjoy viewing it.
I believe I was using a 5.5 Plossl for about 130x with my Swift 60 mm f/11.8 Aerolite refractor, both at full aperture and stopped down to 2 inches. This little refractor has a killer objective.
Of course, you're right. I read through your post quickly and thought you had a small premium apo 50 mm. I'll be interested in how you do with it.
that's gonna be easy!
It was tight but still pretty easy in my 90mm f/13.9 Mak-Cassegrain (32% central obstruction) as that scope typically does pretty well on doubles. Indeed, even with the 24 Panoptic in the scope (52x) I could still tell that it was a double. It was slightly harder in my 100mm f/6 achromat due to the secondary color of that shorter f/ratio doublet, but still quite doable. Seeing the color contrast was problematic in the refractor, but the primary was yellowish white with the companion showing only a hint of a bluish hue. In anything significantly larger than 4 inches, the color contrast is quite prominent.
Funny, just minutes before discovering this post I was looking at Izar which was an easy split. I'm not sure what the lowest magnification I would have needed to split it since I went straight for my 5mm XW for 163x then the 3.5mm WX for 233x which made short work of this double, both beautiful unwavering discs. A wonderful double Izar is with its slight green/bluish pup, I find that even when seeing isn't that great I could still split this pair with my TSA-102 (my little darling star splitter) which lands square within this aperture challenge range.
Here is something that has always bothered me about viewing the famous and pretty "double double," Epsilon Lyria. Several times at star parties I have pointed out the object to people at star parties. I have then asked them; "do you see the two separate stars where when you first looked up you might see just one?" Usually they simply tell me they do, since they usually still have younger and less damaged eyes than I do, and can split objects a full 102 arcsec apart.. On the other hand, more than a few people have said things like; "Of course I see that, you could drive a truck through that separation, but do YOU see that both of the two stars are also double stars themselves?" To which I tell them that I know they, but only because I have seen them all through 4" and larger aperture telescopes, I could never even come close to splitting them naked eye, even back when my eyes were still pretty good. To that, or something similar, comes a reply saying something like; "Oh it's not so hard to split them," after which they give me the angles of inclination, and usually quite CORRECTLY! How is that POSSIBLE when 2 arcsec resolution like, which is at least 25 times better than what my favorite expert, Sam Brown, says is humanly possible, indeed a resolving ability that should require a scope of at least around two inches aperture?
All I can guess is that the images of these close doubles must "smear" slightly through diffraction in the direction of the axis between them, so they are not actually resolving either pair but merely seeing slightly oblated images. I think that might be a solution, since if they actually had the ability to separate stars only 2 arcsec apart, then other people through history obviously should also have had the same type of nearly preternatural resolving ability as well. And that means, in turn, that many such people ought also to have had the ability to discover the moons around Jupiter, especially if they had simply used a small obstruction at the center of their field to block out the intense glare from the vastly brighter Jupiter, But there are no reports of anyone making such a cosmos-transforming observation before Galileo turned his crude handmade telescope upon them in 1609. ..... So ..... does anyone know what is going on here??
Another possibility, which only came t me now, is that the people who knew the inclinations of the separations, knew them from having previously seen these objects through a telescope. Whether intentionally or unintentionally, then, they retained a memory of the inclinations. .... And yet the people I remember giving me these correct answers seemed to be newcomers to astronomy, and not the type of people who would be "putting me on," in which case they would probably have told me that at some point. So I am still baffled.
Re: Epsilon Lyrae. I can usually split the wide 3.5 arc minute pair with my unaided eye, although with the faintness of the two stars, it isn't exactly a piece of cake for a lot of people. In the telescope, I can start to see hints of the narrow components of the system at only about 52x or so, but to get a good "split", I have to go past 83x. Clear skies to you.
You mean to tell me some people can split Epsilon Lyrae naked eye? (Not the four components of course) but yeah I didn’t think that was possible. As mentioned above, the moons of Jupiter would have been documented long before Galileo if that was the case (great point on that) not buying that one.
I posted a comment on this list yesterday, -- and it definitely did post -- but it isn't here today. Does anyone know what might have happened to it? I said nothing subversive or even have naughty. And obviously I thought my comment about this subject would be of interest to others. Is CN now following facebook and Twitter down the primrose path of ever greater censorship? What gives?
So this time when I returned to this page my earlier post suddenly showed up. I guess I had an older screen still up and hadn't reloaded it. Sorry about that! ......... Nevermind.
I just posted again on this topic, but due to another foolish mistake on my part, it doesn't seem to have gone up. It was a bit too complicated for me to wish to retype it all again now, but the gist was:
1.If not for Jupiter's glare, Callisto, the Galilean moon farthest from Jupiter, would be easily separable from its planet. The distance is ~607 arcsec, which is roughly 10 times the separation of the two nearly equally bright components of Nu Draconis, which many people can split fairly easily with their "naked" eyes alone.
2. Contrary to what I had thought, Chinese astronomers, using nothing but their eyes, and probably something to occult Jupiter's glare, had already discovered Callisto well before the birth of Christ. There is, however, no evidence of their also having discovered any of the other three Galilean moons.
The wide components of Epsilon Lyrae are magnitudes 4.6 and 4.7 and about 3.5 arc minutes apart, so with halfway decent vision, they are well above the often-cited one arc minute lower "limit" for human eye resolution. My direct vision tops out at somewhere between magnitude 4.7 and 5.0 (my dark sky averted vision limit is between 6.5 and 7.2 typically) so I can look directly at them. As for the Jovian moons, they are so faint and usually close enough to Jupiter that is is not surprising that they were not really noticed until after the telescope was invented. As for myself, I can sometimes see Ganymede (and occasionally Callisto) at greatest elongations with my unaided eye, but use of a distant telephone wire to block out the glare helps a lot with this. However, a friend of mine has repeatedly been able to see three or four of the Galilean moons without optical aid or wire blocking methods (I tested him a number of times, as his naked-eye vision is far better than mine). Near opposition, Io can get as far away from Jupiter as 2.2 arc minutes, so this would support possibly a few people with exceptional vision being able to see all four, although more commonly, a low power binocular of finder scope is required. Clear skies to you.
When I was in my teens and 20s, I could split the two main components of the "Double Double" with my naked eye from within the city limits of Des Moines, IA. At the time, I had 20-10 vision without correction. However, I've not been able to do so for many years, even with corrective glasses.
1. Epsilon Bootes is easily split with a cheap 70mm spotting scope at 60X (I did it last night).
2. The wide, main components of Epsilon Lyra are pretty easy to see in a good dark sky (if I wear my glasses).
3. I had a friend in the 1970's (we were in our 20's) with exceptionally fine eyesight. He could spot distant aircraft naked eye faster than I could with binocs. He could see Gallilean moons naked eye - I tested him on it on a number of occasions by asking what orientation they were in relation to the planet. When he could see one (at elongation) he nailed it every time.
Izar is the first star that comes to my mind when I wonder which double is my favorite. Struve called her "pulcherrima", so I'm not alone in this ;D
I was not positive I'd split it before with my Vixen 3", so yesterday night I went out and did just that. I think I remember that it was well split around 63x but the best view was at ≈105x. No need for a Barlow anyway. This was the beginning of great night of lunar and doubles observing (during which I reviewed a few other favorites: Porrima, Zuben el Genubi – not exactly a challenge! – and Xi Scorpii).
Thanks for the idea, Phil!
This is a bit off the mark but on a mostly-cloudy Thursday night I was able to observe the Resurs 01 and the Helios 1A rockets passing close to Izar using a Canon IS 15x50 binocular.
Last night I gave the 50 mm Astro-Tech finder the old college try on Izar. The seeing was quite good, Izar was near the zenith, I was making beautiful splits of Izar in my 80 mm ED/at 115x but I just could make anything out at any magnification on 50 mm.
Splitting Porrima at 3.1" had given me hope but Porrima is an equal magnitude double, Izar's companion is 1/8 as bright as the primary.
Jon
Hey, you gave it a try! It would be interesting to hear of someone having success in one of those little plastic Galileo Scopes, but they'd have to mount it well and use a higher power eyepiece than the one provided with the units.
Here's one of my reports on this pair from May 2018 using an AT115EDT at 146X: