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Zeta Bootis, tough rascal!

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#1 Redbetter

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Posted 02 April 2016 - 06:09 AM

I've been trying to split Zeta Bootis for over a week, mostly when it was still too low in the sky, and with poor to mediocre seeing.  Despite this the 20" f/5 consistently hinted at its double nature by giving some odd "non-stellar" diffraction patterns even though the airy disk(s) were not seen.  It is hard to describe, but you've probably seen these sort of interfering diffraction patterns with other tight doubles in various apertures.

 

Finally tonight I succeeded as the double neared zenith.  The seeing settled somewhat although still not good enough to yield stable 1st diffraction rings or stable airy disks in the 20" (which was well equilibrated, fan off for the observation.)  The 8" SCT would sometimes get a relatively stable diffraction ring for a few seconds, so the seeing might have nudged 7/10 in the best moments, but more like 5/10 was the norm.  The SCT was showing an elongated airy disk above 500x when the seeing would allow, but it wasn't steady enough to provide satisfying proof of the double. 

 

The 20" finally paired with moments of steady seeing at 625x and 833x to cleanly split the airy disks for several seconds at a time.  This was repeated several more times over the next 20 minutes.  "Cleanly" is a relative term in this case because it means that a dark line was evident between the two clearly seen disks and always with the same orientation.  There was still all sorts of motion around the disks but they were staying fixed relative to one another.  As for PA's, I couldn't tell which one was brighter.  They are very closely matched.  However, they were offset about 10 degrees from the diffraction spike which was roughly aligned on the preceding/following axis as they drifted.   

 

Separation was tight.  It appeared over 0.3" but I couldn't estimate how much more because the diffraction rings were never stable enough to determine where they were centered relative to the center of airy disks.  WDS shows 0.5" measured in 2015.  Based on the appearance in the 8" and 20" scopes I guestimate it closer to 0.4" separation.  What is the current prediction?


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#2 fred1871

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Posted 02 April 2016 - 07:00 AM

The 6th Orbit Catalogue ephemeris gives 0.400" for 2016.0, and 0.365" for 2017.0.

So you could take it as being 0.39" at the present time.

 

The orbit is grade 2, which means it's likely to be  pretty accurate, and the listed period 125 years.

 

As the Rayleigh Criterion for a 20-inch aperture is 0.27", splitting this near-equal pair should be a matter of seeing conditions for that aperture.

 

Zeta Bootis will currently show slight elongation with an 8-inch; the usual limit for elongation is about 0.5Rayleigh, which is 0.34" for an 8-inch.

 

If we treat 0.39" as the Dawes limit, that gives an aperture of 11.7-inches for a Dawes type touching or tangent discs appearance; 0.39" as Rayleigh gives 13.9 inches, near enough to a C14 or similar, to get discs with an appearance of being just apart.

 

Because there are effects of perception as well as the optical factors in these matters, some observers will have visual experiences that differ a little from the above. That doesn't show that optical theory is useless. It shows there's an interaction between what optical systems do and how different perceivers see it. It's another of the YMMV situations (within reasonable limits).

 

Zeta Bootis will continue to close for some years. Those with larger scopes can follow this as the seeing allows. But it will be out of reach even for 20-inch scopes at minimum - less than 0.05". It's a very elongated orbit, the real size has been estimated as varying between 1.4AU and 64AU (Jim Kaler). Minimum separation is around 5 years from now. It should be back to 0.5" by about 2030, and keep increasing for many years thereafter.


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#3 junomike

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Posted 02 April 2016 - 07:45 AM

I know very little about the theory behind the "separation" but last year I found it interesting that I could clearly separate it with my C14 but not with my C11.

Wonder if the C14 will split it this year?

 

Mike



#4 Organic Astrochemist

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Posted 02 April 2016 - 08:54 AM

Repeated attempts while near zenith -- that is great advice for trying to split the tightest doubles.
Last year I was very happy to see very noticeable elongation in Zeta Bootis with my 8". This year will probably harder to perceive.

#5 drollere

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Posted 03 April 2016 - 02:57 PM

it's not an easy system to split. it is also a great system to use as a test the seeing at moderate magnification. you can tell immediately, just by looking at, it whether it's going to be a night for close pairs.



#6 Redbetter

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Posted 03 April 2016 - 04:05 PM

The 6th Orbit Catalogue ephemeris gives 0.400" for 2016.0, and 0.365" for 2017.0.

So you could take it as being 0.39" at the present time.

 

The orbit is grade 2, which means it's likely to be  pretty accurate, and the listed period 125 years.

 

As the Rayleigh Criterion for a 20-inch aperture is 0.27", splitting this near-equal pair should be a matter of seeing conditions for that aperture.

 

Zeta Bootis will currently show slight elongation with an 8-inch; the usual limit for elongation is about 0.5Rayleigh, which is 0.34" for an 8-inch.

 

If we treat 0.39" as the Dawes limit, that gives an aperture of 11.7-inches for a Dawes type touching or tangent discs appearance; 0.39" as Rayleigh gives 13.9 inches, near enough to a C14 or similar, to get discs with an appearance of being just apart.

 

Because there are effects of perception as well as the optical factors in these matters, some observers will have visual experiences that differ a little from the above. That doesn't show that optical theory is useless. It shows there's an interaction between what optical systems do and how different perceivers see it. It's another of the YMMV situations (within reasonable limits).

 

Zeta Bootis will continue to close for some years. Those with larger scopes can follow this as the seeing allows. But it will be out of reach even for 20-inch scopes at minimum - less than 0.05". It's a very elongated orbit, the real size has been estimated as varying between 1.4AU and 64AU (Jim Kaler). Minimum separation is around 5 years from now. It should be back to 0.5" by about 2030, and keep increasing for many years thereafter.

 

It is definitely a seeing test for the 20".   That was one of the reasons to keep checking it as a way of qualifying what sort of seeing could accomplish the task and how much magnification I would need to examine the higher resolution presented by smaller airy disks and diffraction rings.  I knew that the 20" could do the job within any range it was likely to be in at this time, the biggest question I had was how good would the seeing have to be to make it happen.  Negative results can be as valuable in training the eye as the positive results.  It also gave me some opportunity to experiment with the TV 3-6mm zoom.

 

I found it valuable that even in poorer seeing and lower on the horizon at high power I could regularly tell that the pair was not a single star when compared to the images presented by similar magnitude solitary stars.  This was still true at times when it was impossible to tell which way the pair might be oriented because the airy disks were not visible for this or single stars.

 

Being ~5th magnitude probably made Zeta Boo more difficult than a 7th magnitude pair would have been for the aperture in the same seeing.  They were so bright that the intense dancing/shifting diffraction rings usually wiped out the airy disks. 

 

I'm finding I like having the 8" set up with the Dob for comparison.  I would like to get a night of real 8-10 seeing here for both of these.  I already know the 8" is showing the elongation in about 7/10 seeing, but the resultant stretched disk was not steady enough to examine closely or to get a PA estimate from the elongation.



#7 Organic Astrochemist

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Posted 14 April 2016 - 06:28 PM

I had a good look at Zeta Bootis at 500X in my 8" last night. Transparency was quite good and seeing was 4/5. To estimate the position angle I just watch the pair drift and estimate the position from the preceding star to that following (an error of at least +/- 10 degrees).
First I practiced with Izar B which was leading and well separated from Izar A that followed from the SE at approximately 160 degrees.
I could clearly see the elongation in Zeta Bootis and at times I glimpsed a peanut shape. I couldn't tell which component was leading (actually B) but the other followed at approximately 110 degrees in the SE.
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#8 Redbetter

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Posted 15 April 2016 - 12:20 AM

I had another night about a week ago when the seeing reached about 6-7/10 and was able to split it again more easily/cleanly than before.  I only had the 20" set up, so no 8" SCT comparison this time. 

 

Can't get good seeing here...the above is the best I've had.   Not good enough to fine tune the SCT's collimation.  Not good enough to star test the new 10"...especially with 30 mph gusts at present. 



#9 Joe Bergeron

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Posted 15 April 2016 - 10:48 AM

I miss Zeta. Several years ago I used to split it regularly with my 6" refractor, but then it closed up too much. I don't suppose I'll last long enough to see it again, unless it's with the 32" scope on Mount Lemmon.



#10 cildastun

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Posted 17 April 2016 - 02:10 AM

I've never looked at this one before as I assumed it was beyond my aperture. However, last night with excellent seeing (4.5/5) and Zeta Boo well placed for me, I spent an hour on it with my 180 Mak. Clear elongation at x350 with hints of a peanut waist. With x540 (the highest I've been yet with this scope), the shape and diffraction rings were stable centre field, and showing the double nature very clearly. I was surprised at how little sharpness dropped off at the higher mag.

 

Chris



#11 Organic Astrochemist

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Posted 17 April 2016 - 02:28 PM

Because Zeta Boo is both close and bright (mag 3.8) it is special and rewarding. Resolution in the human eye depends both on angular size and surface brightness. Increasing magnification renders details large enough to perceive. Unfortunately magnification also decreases the surface brightness of the image, degrading the eye's ability to perceive details.
The optimum magnification affords maximum size and maximum surface brightness. At 540X, you must have gotten very close to optimum magnification of Zeta Bootis with your 180 Mak. Great job.
Too bad there aren't more doubles like this pair. Much harder to do with dimmer stars.

#12 Konstantin 1980

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Posted 17 April 2016 - 02:35 PM

2 years ago I saw the ellipticity in the 4-inch diameter telescope. А year ago I telescope 254 mm in diameter was seen almost separate 2 disks


Edited by Konstantin 1980, 17 April 2016 - 02:36 PM.


#13 fred1871

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Posted 17 April 2016 - 10:15 PM

Konstantin, your description from last year with 254mm telescope sounds like what would be expected.

 

But the two years ago with a 4inch is not plausible. To get elongation on Zeta Bootis with a 4-inch it would have to be further in the past, when the pair was a little wider. Two years ago the 6th Orbit Catalogue ephemeris gives 0.45" as the separation. The Rayleigh Criterion for a 4-inch is 1.36"; and 0.5-R, the usually accepted limit for detecting elongation, is 0.68".

 

To detect elongation at 0.33-R is quite simply not something that suggests a credible observation - unless the telescope has astigmatism, of flare effects due to seeing mimic slight elongation.

 

Looking up the older ephemeris for Zeta Bootis, the listed separation for 2008 is about 0.57" at this time of the year. Even that would be extraordinary with a 4-inch. It might be possible back then with a filter cutting out wavelengths above about 450nm. At blue/violet wavelengths resolution is potentially better. But even that technique would not be adequate for 2 years ago, in 2014. 



#14 Konstantin 1980

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Posted 17 April 2016 - 10:42 PM

 

 

Konstantin, your description from last year with 254mm telescope sounds like what would be expected.

 

But the two years ago with a 4inch is not plausible. To get elongation on Zeta Bootis with a 4-inch it would have to be further in the past, when the pair was a little wider. Two years ago the 6th Orbit Catalogue ephemeris gives 0.45" as the separation. The Rayleigh Criterion for a 4-inch is 1.36"; and 0.5-R, the usually accepted limit for detecting elongation, is 0.68".

 

To detect elongation at 0.33-R is quite simply not something that suggests a credible observation - unless the telescope has astigmatism, of flare effects due to seeing mimic slight elongation.

 

Looking up the older ephemeris for Zeta Bootis, the listed separation for 2008 is about 0.57" at this time of the year. Even that would be extraordinary with a 4-inch. It might be possible back then with a filter cutting out wavelengths above about 450nm. At blue/violet wavelengths resolution is potentially better. But even that technique would not be adequate for 2 years ago, in 2014. 

I watched a lot of  stars in the 4-inch telescope at the time. Zeta  was the most difficult, I spent  time on it, but often I could see  perceptible ellipticity disc 4-inch reflector. Stars from a distance of 0.52 "- 0, 57" could be seen easily. In all cases it was wholly a coincidence with the orbit to within a few degrees. I am very keen observer, Zeta Bootes not give me 2 days, until the third day, when the atmosphere was very calm,  4-inch (110 mm) Soviet reflector (Mizar) ( I observed July 20, 2014 Zeta Bootes)


Edited by Konstantin 1980, 17 April 2016 - 11:03 PM.


#15 Cotts

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Posted 04 July 2016 - 03:23 PM

Had a good look last night with the Lockwood 12.5" f/6.5.   With my highest power, 5mm Nagler giving 423x, I could clearly see a notched pair or two disks overlapping by about 25%....  I did have to wait for 'those moments' of better seeing, though......   

 

Tough at 0.4" or a bit less....

 

Dave


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#16 junomike

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Posted 04 July 2016 - 03:56 PM

Had a good look last night with the Lockwood 12.5" f/6.5.   With my highest power, 5mm Nagler giving 423x, I could clearly see a notched pair or two disks overlapping by about 25%....  I did have to wait for 'those moments' of better seeing, though......   

 

Tough at 0.4" or a bit less....

 

Dave

I had a similar experience last week with my C14 but it was far more difficult ton ascertain the split than it was last year (in the C14 also).  I've never made the split in anything smaller than my C14.

 

Mike



#17 glmorri

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Posted 07 July 2016 - 04:04 PM

My attempt at rendering an impression of the elongation of zeta bootis on 5/23 & 24 using a C8 with SAFIX and a Pentax XO 2.5 (approx 800X).  Seeing was 3.5-4/5.  It was the 'catch' of the night for both evenings.

 

 image.jpeg



#18 Cotts

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Posted 07 July 2016 - 11:14 PM

My attempt at rendering an impression of the elongation of zeta bootis on 5/23 & 24 using a C8 with SAFIX and a Pentax XO 2.5 (approx 800X).  Seeing was 3.5-4/5.  It was the 'catch' of the night for both evenings.

 

 attachicon.gifimage.jpeg

?  posted wrong photo?  

 

What are we seeing here?

 

Dave



#19 glmorri

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Posted 08 July 2016 - 12:30 AM

Dave,

 

     An enlargement of a quick sketch of zeta bootis, attempting to show the orientation of the elongation, and the broken first diffraction ring.  North is up.



#20 drollere

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Posted 09 July 2016 - 11:51 AM

nice drawing, glmorri.

 

CDSA2 gives the separation as 0.5" measured in 2012 (the most recent listed in WDS 2015.01), which is now inaccurate as the system is in a highly eccentric orbit, approaching periastron and closing rapidly. note that the ephemeride at stelle doppie is corrupt, but the pair is very likely below 0.4" by now according to 6th orbits.



#21 fred1871

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Posted 09 July 2016 - 09:09 PM

I've had a closer look at data regarding Zeta Bootis, following the remarks I made in another thread on Lambda Ophiuchi. To the point, I checked the "Last precise Measure" list on the USNO/WDS site, where it gives 0.460" for 2015. One might assume 2015 being somewhere near 2016.5 as this is a measure, not an ephemeris number.

 

The 6th Orbit catalogue ephemeris has

2014.0   291.7  at 0.464"

2015.0  290.5   at 0.433"

2016.0  289.2   at 0.400"

2017.0  287.3   at 0.365"

 

So there's a small difference between the ephemeris (2015.5 would give ~0.417") and the 0.46" of the measure.

 

Likewise, the PA differs slightly - the measure in 2015 is 287.3, the ephemeris suggests ~289.9.

 

How precise exactly is the 2015 measure? That's something to evaluate over time as new measures are made in the coming years. More measures will need to be made over the next decade in particular, as the pair move through periastron, in order to determine if the current orbit calculation needs tweaking and by how much (as with Lambda Ophiuchi, it's grade 2, so not regarded as "definitive" at present). "How much" is the likely result; it won't be huge.

 

What comes out of this is that we can't be certain - at the level of precision rather than of close approximation - of the exact separation at a given date. Pretty close, yes. But is the pair currently at 0.38"? (ephemeris); or perhaps a tiny bit wider, at, say, 0.42" (based on the 2015 measure). It will matter to an orbit calculator. And it might matter to someone looking for boasting rights, "my scope split (whatever they understand by that term) a 0.38" pair" :grin: .



#22 Redbetter

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Posted 10 July 2016 - 02:14 AM

Looking back, Zeta Boo was a clean split in my 8" SCT...back in '96.  Seeing was 8/10.  At the time I even noted one star as being brighter than the other in my sketch at 825x.  I found my log of this some time after I made the original post. 




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