Splitting Izar (Epsilon Boötis)

[AndyWB]

I split the Double-Double the other week - at x260 in my 130 heritage there was a clear separation in both pairs (though the stars weren't pin-sharp by a long way, and by 'eck they were shifting!). However, I've struggled with Izar, even though it's got a greater angular separation. Is that normal? I wondered if it was due to the magnitude of the 2 stars being quite different - that one is bright enough to hide the other?

I think I just about managed Izar last night at x200, but I can't say it looked that remarkable - certainly I didn't see much colour difference, which I've heard mentioned. It's enough that I did re-check that I was pointing at the right star several times. Perhaps I'm being optimistic for the small scope?

In short, should I be able to achieve a split of Izar with a 130mm scope in light polluted skies?

[NGC 1502]

I would have thought that a 130mm reflector should split Izar, if the atmosphere is fairly steady, and your scope is well collimated.

I don't think that light pollution will stop your observations of double or multiple stars, they are one type of object least affected.

My 90mm Maksutov, and my 70mm refractor will both split Izar nicely, in fact I like the pinpoint stars they produce better than through my 10" Dob.

Not saying your collimation is off, but worth a check, you could view nearby Arcturus, at medium to high power the defocused view, kept in the centre of the field of view, will confirm.

I wouldn't rush to get a larger scope either, my smaller scopes get used more. A lot can be seen with patience, far less cooldown needed, you can shift the scope easily to avoid trees or buildings, I do that several times during each observing session, and is one reason I don't have a 'go-to', I would spend a lot of time realigning each time I shift the scope.

Regards, Ed.

[chiltonstar]

I would agree with Ed, your scope should split Izar fairly easily, given good seeing (my 5" Mak shows the pair in all their coloured glory on most nights, and my 80mm frac ditto).

Keep trying!

Chris

[Stu]

Agreed, should be possible with good seeing and a well cooled and collimation scope. Depending on which ep's you are using, possibly better quality ones may help.

Just got it tonight in my 76mm frac at x160. The blue secondary was sitting right on the first diffraction ring of the primary but quite clear still. Double double split cleanly too at x80

Stu

[PorkyB]

I often find that the colours of doubles are pretty subtle. My notes describe Izar as "pale orange and green" but when I saw it the other night (at 200x using my 200p) I would describe it as white and blue-grey. Everyone's perception of colour is different. The magnitude difference between the two components certainly makes it more challenging to split.

[AndyWB]

Hmm. Well, I'm pretty sure that my collimation was good, and doubt it shifted since splitting the double-double, which I guess in a way is the puzzler for me, and the scope had been out about 25 minutes (Saturn had to be the first stop of the night!)

When I think I split it what I saw seemed white and blue-grey too, so maybe. I'll give it another try the next chance I get - it could be user error, either in pointing at the wrong thing (which I doubt) or in interpreting what I'm seeing (more likely).

Oh, Stu, when you say "diffraction ring" - does that mean the ring that you get around stars at high magnification? That's normal then? (That's one thing that was making be question my optics). I've just looked it up on Wikipedia, but must confess, my physics isn't that good anymore.

And yeah, not getting a larger scope in a hurry - I want one, but I think not living in a flat would be a priority!

[Stu]

Hi Andy. Yes, diffraction rings around stars are an unavoidable side effect of physics unfortunately.

With Newtonian scopes, at focus you get diffraction spikes off the secondary mirror supports, but I guess you get rings too (I'm not a newt expert, don't use them often).

Without wanting to start a newt vs refractor debate, good refractors give lovely pure star shapes with clear diffraction rings which tends to make splitting doubles easier. They are also smaller aperture than newts generally so can cut through poorer seeing better. There are times when a large newt won't split the double double for instance, when a small one or a frac will.

With Izar in the 76, it was less obvious because the secondary was right on the first diffraction ring but the colour difference made it clear that it was there so sounds like you probably got it.

Stu

[Donkeiller]

You should certainly be able to split the "double-double" at alot less than x260, unless the seeing, or your collimation (or both) are poor.

I can get very clear splits at x77 on a decent night and down to x61 on a good night with my 102mm refractor.

Izar is also an easy split at anything over X87- you should be seing (well i am) a orange/yellow primary with a smaller light blue secondary. They really are a pretty pair.

When I look at Izar in my 6 or 8SE they are brighter, but less colourful.

[umadog]

You're right that the magnitude difference on Izar matters. The brighter primary tends to wash out the secondary, so Izar is a little harder than it would have been if both companions were equal brightness.

As Stu says, diffraction always occurs when light passes through an aperture. There is more diffraction in a smaller scope, and this makes the diffraction rings and the Airy disk more prominent at higher power in a smaller scope. Since refractors are typically smaller, you're more likely to see a diffraction ring in a typical refractor than a typical reflector (which are larger). You should get equally "nice" diffraction rings in either scope design, however. The out of focus diffraction pattern, which is the so-called star test, will reveal optical issues in the telescope.

The Airy disk places a limit on the smallest thing you can resolve, so a larger aperture with its smaller Airy disk allows you to resolve smaller things. The problem, however, is that as you start to pass about 8" or so, you're mostly resolving the fine structure of atmospheric turbulence. So a larger scope will see a horrible mess on a night of below average seeing: swollen, "furry", stars; but a smaller scope will still see a nice Airy disk because it's smoothing out all the bad seeing that's being resolved in the larger scope.

[AndyWB]

Hmm. I'll try them both again next time I'm out, having checked collimation. It may be optics, or it could be noobish not knowing what to look for. Near splits might not be obvious to me, or maybe i expect my splits to be well split!

(The first close double I saw, I thought it was a problem with the optics...)

Heck, I'm still worried I was pointing at the wrong thing.

[Stu]

Hi Andy,

Whilst I agree with Donkieller's comments, I would just add that a premium quality refractor will split these stars far easier than you should expect from the 130. I think under good conditions you will achieve a split at lower than the x260, but I would not be disappointed if you still need over x150. The lack of central obstruction and diffraction spikes gives apo, and long focal length achros, a big advantage here.

Before I get shot down by the newt crew :-), I am referring to the 130, not to some of the larger, higher spec optics out there, or longer focal length newts, those using curved vane secondary spiders or even those aperture masked to give a stopped down but unobstructed light path. Hopefully that is enough of a disclaimer to escape unscathed :-)

Let use know how you get on.

Cheers,

Stu

[Moonshane]

Hi Andy,

Whilst I agree with Donkieller's comments, I would just add that a premium quality refractor will split these stars far easier than you should expect from the 130. I think under good conditions you will achieve a split at lower than the x260, but I would not be disappointed if you still need over x150. The lack of central obstruction and diffraction spikes gives apo, and long focal length achros, a big advantage here.

Before I get shot down by the newt crew :-), I am referring to the 130, not to some of the larger, higher spec optics out there, or longer focal length newts, those using curved vane secondary spiders or even those aperture masked to give a stopped down but unobstructed light path. Hopefully that is enough of a disclaimer to escape unscathed :-)

Let use know how you get on.

Cheers,

Stu

I agree completely Stu (even about newts!). I can usually readily split Izar with any of my scopes but sometimes the seeing defeats me getting a clean split. when I use my aperture mask though it does make for a smaller exit pupil, unobstructed view and therefore a cleaner split so when seeing is worse the aperture mask helps a lot.

Izar splits generally at 150x or more for me and the colours range according to conditions. a good way to enhance colours in doubles is just to put the image slightly out of focus. this swells the stars which somehow makes the colours more obvious.

It is worth the effort as it's a stunning double when everything comes together.

[Stu]

I was about to ask about the relationship between diffraction rings and spikes on a newt as its been a long time since I looked through one. However, I found the following article which has some useful examples.

http://www.astronomyhints.com/spider.html

Stu

[AndyWB]

Thanks guys! Yes, according to my log, I think I split it at x160, but didn't see colour difference, and I wasn't entirely sure. Colour could be eye adaption too - this was is Reading, so it's never dark.

Yeah, cheapy wee scope, so maybe I'm just asking a lot - resolution, under LP, with my inexperienced eye.

That's a good article Stu, and I understand why curved vanes now (I had wondered). The only thing it misses is an example of a single arm support (!) - that's what the Hertiage 130 has. Weird.

I'd have gone out and given it a go last night, but I'm away with work. Clear skies, but no scope :/ But I'll feed back when I get the chance.

[Stu]

Judging by the example of the three vane spider, I would expect the single vane to show two diffraction spikes, one in line with the vane and the other exactly opposite. You could verify if this is correct next time.

Stu

[AndyWB]

Had a look last night and yes, at x160 in moments of good seeing I could see a sharp blue-grey dot just to the side of the primary. It was pretty much on the diffraction ring, and it wasn't always visible - the primary actually looked quite blurry a lot of the time, and it was a bit hazy with cloud last night. I checked out the Double-Double too, which split more easily, but was still more blurred than the last time I looked also.

I wasn't really able to spot any diffraction spikes, but to be honest, I was maybe not paying enough attention to that!

[NGC 1502]

Nice one

I just love double stars. I too was out last night, with my 70mm refractor went round all the summer doubles at between 23.00 & midnight when it was reasonably dark.

Only fainter object I looked at was M27 the Dumbbell, hard to see it with a 70mm scope plus Southend's light pollution and the time of year, but it was there, best view was 34x. Your 130mm will show it better.

Regards, Ed.

[Moonshane]

just for fun try this. cut a circle of material (black card, platic or cardboard) which will fit inside you tube end. cut a further small circle near the edge of about 55mm diameter and then stick the mask onto your secondary with a blob of blu tak or similar. this will reduce your aperture but for double stars will importantly increase your focal ratio (assuming 55mm it would be f12) so your exit pupil is smaller. this will make stars like Izar much easier to split.

[AndyWB]

Thanks Moonshane, but I'm not sure I get the bit about masking the secondary. Are you describing sticking something to the back of the secondary (where the screws are) to increase the size of the secondary occlusion? So the idea is to narrow the aperture leading into the OTA and to increase the occlusion size of the secondary?

Or are you saying cover the whole top of the tube and then have an off-centre 55mm hole (which would give roughly f12), and could take advantage to the lack of secondary supports...

Sounds crazy, might be interesting to try!

[Stu]

Hi Andy

It's definitely the last one! Basically masking the front of the tube but leaving a 55mm aperture in the space where there are no secondary supports. This gives an f12ish scope with no secondary obstruction so better contrast and no diffraction spikes. Definitely worth a go.

Stu

[Moonshane]

as Stu said, the second one what this does is dim the image but it makes it a lot cleaner so the double stars will split more easily. it effectively makes your scope a small slow refractor which have always been thought of as the best double star scopes about. here's the one I made for my big dob. http://stargazerslounge.com/topic/143780-aperture-mask-for-16-f4-dobsonian/