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Zeta Bootis - a Tough Test

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#1 Rustler46

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Posted 02 May 2018 - 01:41 AM

In my thread in the refractor forum (From 11-inch SCT to APO -Options), I have just reported on the first night's observation of double stars with an Astro Tech AT115EDT APO refractor. 

 

I was able to observe over a dozen nice double stars, the closest of  2 arc-seconds separation or less. One double Zeta Bootis, reportedly of 1 arc-second separation (a test for a 115 mm aperture) was at best elongated. While the components are of equal brightness (magnitude 4.5) seeing wasn't perfect. By way of comparison 50 years ago I was able to resolve this pair with my home made 8-inch Newtonian reflector. My observing log for August 30,1967 noted "1.1 arc-second; easy to resolve, image jumping in field; sometimes multiple images, each of which is double". Of course for an 8 incher that double was well within its capability, even on a night of questionable seeing.

 

But in defense of the 115 mm APO, SkySafari says that Zeta Bootis has a very eccentric orbit with a period of 123 years. At closest approach (3 years from now in 2021) the pair are "visually inseparable". Also they are easily separated at the widest separation (in 2082). The last time they were at widest was in 1959 (2082 minus 123 years). My 1967 observation was just 8 years later. My little APO refractor was at a great disadvantage in trying to resolve this double in 2018.

 

But for last night's observing with the little APO refractor I was able to employ powers up to 230X (50 per inch of aperture). The observing experience was not approached from a viewpoint of which of my telescopes gives a “better” image. Rather I was just enjoying what this little APO refractor can deliver. But Zeta Bootis was too much to ask of it.

 

Has anyone else had experience with Zeta Bootis, either recently or some time back?

 

According to Jim Kaler's website (Stars), Zeta Bootis is quite exceptional:

 

"The stars loop about each other on hugely elongated orbits, which carries them from (from the computed orbit) 64 Astronomical Units apart (half again Pluto's distance from the Sun) to a mere 1.4, close to Earth's distance, which is close to a record for such ellipticity. The double thus is separable by telescope during farthest separation, and quite inseparable at close passage, which occurred in 1897 and which will take place again in 2021. Your best view will come in 2082. With a stellar invasion like this one, no planets would be at all possible. The large eccentricity suggests some kind of violent encounter with another star. Perhaps a third member was lost in the process. We will never know."

 

Information like this is what makes astronomy in general and double star observing in particular so fascinating. While I find equipment forum discussions to be interesting, it's observing celestial objects that has held my interest for so long.


Edited by Rustler46, 02 May 2018 - 01:46 AM.

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#2 james7ca

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Posted 02 May 2018 - 02:18 AM

Interesting writeup, thanks.

 

However, SkySafari list Zeta as having a current separation of 0.3 arc seconds, so I doubt that you could see an elongation with your 115mm refractor.


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#3 Redbetter

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Posted 02 May 2018 - 02:35 AM

There have been a few threads on Zeta Boo.  It wasn't hard to see the elongation last year in the Z10 in fair seeing, but the separation is quite tight.  As a result it has taken my 20" to actually resolve it, despite marginal seeing for the attempt.  It is at 0.315" separation now per my calculations based on the orbital parameters.  I split it back in the late 90's when it was a nice test for my 8" SCT. 

 

The parameters I have from about a year ago show a period of 125.24 years so the orbit has probably been revised.  If the parameters are correct and haven't been revised again, it will briefly close to about 0.012" in Sept. 2023. 


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#4 Rustler46

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Posted 02 May 2018 - 03:27 AM

Interesting writeup, thanks.

 

However, SkySafari list Zeta as having a current separation of 0.3 arc seconds, so I doubt that you could see an elongation with your 115mm refractor.

Yeah, I saw that. What led me to examining Zeta Bootis in the first place is that SkySafari listed it under "Tonight's Best". As far as the 0.3 arc-seconds listed, we don't really know what year that figure was for. It could be for some year prior to 2018 making it even closer now. My observation of "possible elongation at best" was made at a lower power. When a higher power was used, there was no elongation visible.

 

I find that SkySafari has a lot of errors, or at least inconsistencies (see* below). The information given on the left panel of the information page often doesn't match what is given in the paragraphs on the right. Much of the latter information is taken from Jim Kaler's website, which I more trust over what is listed on the left. The source of what is shown on the left is uncertain. I guess the real test is what can be seen with one's telescope. And Zeta Bootis definitely was not resolved by my 4-1/2 inch refractor.

 

*An example of the errors or inconsistencies in SkySafari is in the information page left panel listing the components of Zeta Bootis as magnitudes 3.77 and 4.46. But according to Jim Kaler's website 3.78 is the combined magnitude of both, with each component being around magnitude 4.5. So one has to be cautious in using the information presented there.

 

But don't get me wrong - overall SkySafari is a wonderful resource, one that I make frequent use of. I particularly enjoy using it to control my G-11 mount wirelessly. It's much easier than using the Losmandy control paddle.



#5 Rustler46

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Posted 02 May 2018 - 03:33 AM

There have been a few threads on Zeta Boo.  It wasn't hard to see the elongation last year in the Z10 in fair seeing, but the separation is quite tight.  As a result it has taken my 20" to actually resolve it, despite marginal seeing for the attempt.  It is at 0.315" separation now per my calculations based on the orbital parameters.  I split it back in the late 90's when it was a nice test for my 8" SCT. 

 

The parameters I have from about a year ago show a period of 125.24 years so the orbit has probably been revised.  If the parameters are correct and haven't been revised again, it will briefly close to about 0.012" in Sept. 2023. 

Thanks for the interesting reply! While Burnham's Celestial Handbook doesn't have much of a writeup on this star, it does list the period as being around 125 years. Based on what you have shared, it would not presently be resolvable even with my C-11. Even so it is quite an interesting star. I'll do a search on CN to see what has been reported.



#6 flt158

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Posted 02 May 2018 - 05:23 PM

Well I am sorry to stick a big pin in our very large balloon. But if you all would check Zeta Bootis on the stelledoppie.it website you will see that the current separation is 0.074 arc seconds.

That's less than 0.1". 

Which means it is far too tight for any of us. 

The PA is 243.5 degrees. 

In 2027 the separation will be 1.106". 

 

Clear skies from Aubrey.  



#7 Redbetter

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Posted 02 May 2018 - 06:28 PM

Well I am sorry to stick a big pin in our very large balloon. But if you all would check Zeta Bootis on the stelledoppie.it website you will see that the current separation is 0.074 arc seconds.

That's less than 0.1". 

Which means it is far too tight for any of us. 

The PA is 243.5 degrees. 

 

 

Those numbers are wrong.  Stelle Doppie has problems with Zeta Boo's orbit and some others, particularly some with high eccentricity.   Take a look at the disjointed orbit display.

 

The developer of the site has noted here when problems with Zeta Boo were noted before that there are some limits to the mathematical precision of the language being used for the calculations and it is causing problems with a few of these.   I thought it might be a convergence problem, as my own spreadsheet required more iterations to converge for certain parts of the orbit, but he says the problems are with the precision of the functions, which makes sense. 



#8 fred1871

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Posted 02 May 2018 - 08:45 PM

We've had some threads in the last few years about Zeta Bootis, which continues to close year by year, heading for minimum separation - below 0.05" - in a few years from now.

 

The Ephemeris from the 6th Orbit Catalogue of the US Naval Observatory - the source material for stelle doppie and others - gives a current separation of (interpolated) 0.31". Ephemeris numbers in the 6th OC are for the zero point of the year: so 2018.0, 2019.0, etc. Keep that in mind when using the Ephemeris.

 

A few years ago, at separation ~0.50", I could still get a Dawes Limit type resolution of the components with my C9.25 at high power (touching discs at tangent). Now, very much closer, I'll be trying it again to see if I can get a hint of elongation with that telescope, the current separation being just wider than 0.5-Rayleigh for that aperture. And 0.5-Rayleigh as I've discussed in other threads is a reasonable fuzzy limit for detecting elongation with equal pairs.

 

So, yes, it's still detectable as a double with mid-size scopes, and should still split with Redbetter's 20-inch for which the separation at present is just above 1.0-Rayleigh. 


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#9 Redbetter

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Posted 03 May 2018 - 03:22 AM

I gave it a look in the 110ED tonight just out of curiosity.  Seeing was poor/mediocre so I didn't expect much.  The 110 has some coma from misalignment/miscollimation that is somewhat variable in nature.  Tonight it was worse than normal and nearly aligned with the calculated position angle of the pair, which was unfortunate.  I should have rotated the tube. I guess I need to take a shot at collimating the objective, I've been holding back not wanting to make it worse since it gives pretty good images anyway.  The previous owner had never noticed the issue despite owning many scopes and teaching astronomy for many decades. 

 

I did see subtle elongation of the airy disk in the same direction as the drift and the coma, primarily at 308 and 385x.   However, Aberrator shows that this is likely to be far more heavily influenced by the coma angle than the very slight elongation of the merged disks for this aperture.  I suspect the level of elongation predicted by Aberrator might be barely distinguishable with a 6".  However, to confirm it one would need to make certain the collimation was spot on and to check another similar magnitude star to see if it revealed elongation in the same orientation.



#10 fred1871

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Posted 03 May 2018 - 03:56 AM

Unlikely detectable, except falsely as a seeing artefact or slight astigmatism, with only a 6-inch: if my calculator worked properly, that's 0.34-Rayleigh, and I'm totally sceptical about that as a possibility. At what I'd take to be a barely credible limit, 0.4 Rayleigh, the numbers work out for 7-inch aperture. 8-inch should do it (~0.45-Rayleigh) if you are able to  break the 0.5-Rayleigh barrier.

 

If someone has a blue filter (cutting off longer wavelengths) they might do better, as resolution is wave-length dependent, so at shorter wavelengths the resolution criterion becomes closer for a given aperture. At 450nm it's about 18% closer than at 550nm. How blue-sensitive are your eyes? And how much light do you have available with such narrow-band viewing? : better with blue-white stars one would think. And the Zeta Bootis stars are type A0, so they're good in the right spectral region.


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#11 flt158

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Posted 03 May 2018 - 08:11 AM

What a pity Stelle Doppie does not do separations and position angles month by month. 

Still it is good the website gives year by year. 

The measurements presumably are for January 1st each year. 

But I am greatly surprised that Redbetter blatantly judges that Stelle Doppie is wrong about Zeta Bootis. 

If the website is unreliable on this binary, then can it be trusted on other stars which are changing constantly. 

Still everyone is entitled to his or her opinion.

Let us wait and see if Redbetter's 20" Obsession will have success some time over the next few months   

 

Aubrey. 



#12 Redbetter

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Posted 03 May 2018 - 03:47 PM

 

But I am greatly surprised that Redbetter blatantly judges that Stelle Doppie is wrong about Zeta Bootis. 

 

 

It isn't hard to see that it is mistaken if one cross checks with the WDS listings Fred noted (or uses the posted parameters at the site to calculate the orbit as I have done.)  The current depiction of the orbital diagram (as well as table) at the Stelle Doppie site reveals that Zeta Boo's separations and PA's are off in a few instances and this makes a mess of the diagram.

 

If you want further confirmation read post #12 from this Feb. 2017 discussion of the problem with the pair.  Per Gianluca's statement there:

I've spent some hours on this problem and I'm now convinced it's due to a lack of precision in language's mathematical functions. This is the only explanation. The algorithm is ok. Even if I've increased its precision digits, that's not enought.

sin/cos functions are buggy for tiny numbers, I suspect. I've imported all the official WDS ephemerides in a database and compared to my tables some decimals vary in some rows, but not in all rows.

Excel was built for high precision calculations, I'm not surprised it's performing better.

Perhaps when I'll switch to PHP 7.x in a few months, things may change.

Or, I could rewrite the procedure in C, but unluckly my time in not a infinite resource.

 

Gianluca

 

Of the pairs I have noticed with problems, all are apparent from the discontinuities in the orbital diagram.  As I said, it seems to be more of a problem for certain orbits with high eccentricities, but it is still somewhat scattered as to which are impacted and which are not.  It is a fairly small number of orbits that have problems from what I have seen actually looking for the problem. 

 

There is a problem, but no reason to "throw the baby out with the bathwater" as the saying goes.  One just needs to look at the numbers and diagram a little to see if they make sense or if there are discontinuities. 


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#13 Redbetter

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Posted 03 May 2018 - 04:30 PM

Unlikely detectable, except falsely as a seeing artefact or slight astigmatism, with only a 6-inch: if my calculator worked properly, that's 0.34-Rayleigh, and I'm totally sceptical about that as a possibility. At what I'd take to be a barely credible limit, 0.4 Rayleigh, the numbers work out for 7-inch aperture. 8-inch should do it (~0.45-Rayleigh) if you are able to  break the 0.5-Rayleigh barrier.

 

If someone has a blue filter (cutting off longer wavelengths) they might do better, as resolution is wave-length dependent, so at shorter wavelengths the resolution criterion becomes closer for a given aperture. At 450nm it's about 18% closer than at 550nm. How blue-sensitive are your eyes? And how much light do you have available with such narrow-band viewing? : better with blue-white stars one would think. And the Zeta Bootis stars are type A0, so they're good in the right spectral region.

 

Accuracy of the observation comes down to whether one does the sort of careful cross checks as suggested:  verifying spot on collimation on stars in the center of the field, and using at least one or more similar magnitude stars as a check for elongation on the same axis orientation.  Turning the head 90 degrees during the observation and checks is also a valuable check for the eye itself. 

 

Now an observer can always fool him or herself or fail to do any confirmation checks wanting the result to be right (a frequent problem as we know), but striving for an honest evaluation can lead to a credible report.  My result above at 110mm is an example of how one can quickly discard an observation based on such checks--when I saw the level of coma I recognized it would make further confirmation futile.  As a result I skipped doing some comparison star checks as I had intended since I wouldn't have trusted the result regardless.  Instead I went inside and matched the coma pattern of the rings in Aberrator to verify the magnitude of the impact on the airy disk.  Matching the pattern of the rings confirmed the elongation of the disk on the opposite side. 

 

The Aberrator generated image shows rather apparent elongation at 150mm and ~400 to 500x, which is the sort of magnification I would employ for the aperture if I had it.  The problem is I don't have a 6" scope, and more importantly, a 6" driven scope.  For this level of scrutiny/slight elongation the star needs to be in the center of the field and carefully examined, as do the confirmation check star(s).  While I could see elongation easily with the undriven Z10 last year, there will be a point where the inability to keep the star well centered at increasingly high power overwhelms any accuracy/visual acuity.  

 

The minimum ratio is an interesting problem that would probably best be evaluated on a wider equal mag pair with even smaller aperture (with better collimation obviously.)   This would reduce seeing dependence considerably, and one might experiment with aperture stops as well.



#14 Redbetter

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Posted 04 May 2018 - 05:35 AM

Gave it a try in the Z10 tonight after some Jupiter viewing but the seeing really wasn't satisfactory.  I could not get clean enough in-focus diffraction patterns for anything conclusive.  Despite that I did consistently get some noticeable elongation of the airy disk in the correct PA as the binary drifted across the field at 313 and 417x.  The elongation was seen in the same orientation all the way across whenever the merged disk was distinct.  In better seeing and with a tracking scope of similar aperture this one should not be too difficult. 

 

Edit:  Also, collimated my 110ED successfully tonight as well.


Edited by Redbetter, 04 May 2018 - 05:36 AM.

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#15 Redbetter

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Posted 05 May 2018 - 02:47 AM

Thin high level cloud nixed a DSO trip to dark skies, so I set the 20" f/5 up in the backyard, primarily for planetary later tonight.  I took a look at Zeta and Pi Bootis.  I had a breeze blowing again so the seeing was running somewhat less than 3/10 in the 20"--not good enough for a distinct airy disk even though the disk can be discerned at times at high power. 

 

I recollimated with the scope pointed high in the sky near Zeta, so it would be spot on for the attempt.  I used 625x and 833x on Zeta first.  It was a hot mess to put it mildly, but in watching it drift through the field of view many times it was apparent that this was a double star, not because I could resolve the disks from one another, but because of the elongated mass and the funky partial diffraction pattern just preceding it.  I've seen this before with very close doubles in various orientations, including Zeta several times in recent years, in poor seeing that would not allow resolution of the airy disks or first diffraction rings.  In this case the orientation was creating two ripples in front of the central mass, and in line with the PA.  (Possibly a defocus artifact of the seeing?  These can be oddly consistent in presentation.)  I could also see elongation of the bright center along the same line, but I could not really tell if there was separation to the center--too bright and messy in there.  

 

Too confirm that the pattern I was seeing was unique rather than the same as other similar stars, I turned to a nearby star that was only a little dimmer, this turned out to be the nearby double Pi.  I hadn't consulted a chart ahead of time so I was pleasantly surprised to see it was a double, 4.9 and 5.8 mag components 5.5" apart.  These were easily resolved of course, and using the same power the airy disk of each could be somewhat seen through the bloated, shattered diffraction patterns.  This is how I arrived at 3/10 Pickering rating. 

 

This provided the confirmation needed that the messy elongation and weird diffraction effect were indicative of the double.  It isn't the same as resolving the double or seeing clearly elongated disks though.  It is the sort of thing where if I didn't know it was double beforehand, I would assume that it was but that I lacked the seeing to resolve it. 


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#16 Redbetter

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Posted 08 May 2018 - 08:35 AM

 I had just slightly better seeing last night at a high altitude dark site, maybe as good as 4/10, although the airy disk(s) when visible was/were constantly moving about.  In the case of Zeta Boo the test went much as before at 833x in the 20".  There was the ripple in front and mess on the sides, but this time the airy disks could be discerned at times with a thin line between.  The disks weren't so much individually resolved as they were seen as a long rod with rounded ends and a thin but definite dark band between.   The rapidly moving seeing often meant multiple moving/fading/brightening images.  The better moments showed it more clearly and the impression was always the same. 

 

I was hoping for 5/10 seeing with less bodily motion.  That would have been sufficient for a clean split, but this was still good enough to say the scope was showing the two disks with some measure of resolution.


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#17 Mike Spooner

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Posted 08 May 2018 - 09:32 AM

Last night I was testing out a new 8" reflector OTA and checked Zeta Boo out with magnifications up to 890x. Seeing wasn't perfect but in the instants that the image solidified it looked to me like the airy disk extended SE (position angle 130 deg maybe?) a bit dimmer in this direction. Haven't looked up the PA so maybe I was seeing some spurious seeing stuff. I have been checking on and off this past week with a 6" and suspected the same orientation but I came away with a bit more confidence last night. With the seeing conditions I still wouldn't bet a lot on the result.

 

 I'll continue checking to see if I get better seeing. Would be a new personal best for me with smaller scope if it works out.

 

Mike Spooner 



#18 fred1871

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Posted 08 May 2018 - 10:35 AM

Magnitudes listed for Zeta Bootis suggest only 0.1 magnitude difference between the stars, so "a bit dimmer in this direction" seems unlikely.

 

PA from the Ephemeris is 285/105 depending on which way you read the primary/secondary. The measures list confirms that there's very very little difference between the ephemeris and the best recent measures (the last one of best quality listed that I found was in 2014; 2015 was out of the sequence slightly, and made with a rather small telescope).

 

False positives aren't unusual with this kind of limit observing, even with well experienced observers. Try for a repeat, Mike, using the 8-inch. 890x sounds like plenty : 0.23mm exit pupil, and angle subtended for 0.31" (if resolved as an extension) would be 276" on a straight geometric assumption, so the eye is in the right zone.

 

But I don't know what form the elongated image is supposed to take at this sub-0.5 Rayleigh point: Couteau in his double star book goes only down to 0.5-Rayleigh, and there are interesting variations on the way, with the image appearance varying from the geometry suggested by the true separation - in other words, it's not a linear sequence in terms of appearance.

 

Hmmm... could motivate me to try it with my (optically very good) Mewlon 210: pretty close to an 8-inch.



#19 Redbetter

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Posted 10 May 2018 - 08:56 AM

Finally!  Had some 5/10+ seeing last night at the dark site, and this time the airy disks were steady enough to be seen consistently and well formed despite some motion.  833x showed them well.  There was another observer at the site and he had no trouble seeing them through the 20" either.   

 

Per my orbit calcs the separation is now down to 0.314" at position angle of 285.0 degrees.  This time next year it will be down to 0.274". 


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#20 Konstantin 1980

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Posted 17 May 2018 - 05:52 AM

I watched the star yesterday through a telescope with a diameter of 254 mm. the Diffraction disk is noticeably lengthened( at the moment approximately 0.3"). This data is absolutely accurate . The minimum diameter of the telescope in order to see that this is not a round disk and a little elongated, about 120 mm , But it will be very difficult and will require great experience of supervision. About two years ago I saw with confidence that the disk was extended into a telescope with a diameter of 100 mm, then the separation was about 0.39 ".

 

In my land, the atmosphere every night allows you to see through a telescope with a diameter of 254 mm that this star is significantly stretched , it is very easy, the vision is constantly and very confidently. The minimum increase which is very clearly noticeable - approximately 400 x ( when it is visible so that we can say for sure ) . If you raise it, the requirements for the atmosphere are greatly increased, that is too large increase - bad. For a telescope with a diameter of up to 250 mm, it makes no sense to put an increase of more than 500x, that is, you can put it, but the visibility is likely to be worse than at a smaller increase , because the atmosphere is restless


Edited by Konstantin 1980, 17 May 2018 - 06:24 AM.

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#21 cildastun

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Posted 22 May 2018 - 01:41 AM

It certainly looks slightly elongated to me in the right direction (PA) in my 180 Mak at x450. Running a simulation with Aberrator using 0.315 arcsec as the separation does produce a slightly elongated image, although to my eye, it looked slightly more elongated than the simulation shows:-

 

Chris

 

zeta boo sim2.jpg


Edited by cildastun, 22 May 2018 - 01:42 AM.

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#22 Nucleophile

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Posted 02 June 2018 - 09:57 AM

I was imaging near Zeta Boo last night and decided to give it a quick try with an optical train I have been using to measure faint, tight doubles.  I do not have firm numbers as yet, but this setup gives about f/13.  I used an integration time of 1.5 milliseconds and a gain of 300.  I also used an orange filter.

 

While most of the frames presented as clearly oblong, one very special frame gave a hint of notch.  This is shown below.

 

I used REDUC to sort through FITS frames to get the best 5% of over 5000 total--these were converted to a PSD cube and analyzed using the Symmetry function of STB (also shown).  A crude measure gave a sep of 0.38"  I should note that in my hands the symmetry function of STB overestimates separation values (vs using Reference Star); hence, the actual separation value can be expected to be less than this.

 

Anyway, I thought it was fun to be able to show duplicity in an image for this very challenging double star.  If conditions are equally dreadful tonight (haze and humidity), I may give Zeta Boo a try visually using the Pentax 2.5 XO eyepiece.

 

      ZetaBOO_CN.jpg


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#23 Nucleophile

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Posted 02 June 2018 - 10:02 AM

and here is a zoomed in version of the lucky image:

 

 

    ZetaBOO_CN_Zoom.jpg


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#24 Nucleophile

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Posted 02 June 2018 - 11:39 PM

I had a good look tonight at 798x with the 15" reflector:  the object was at best extended/barely rod-shaped.  The photos I took show much more detail than what I can see with my eyes. 

 

Zeta Boo is decidedly more difficult to resolve now than it was when I made this report two years ago:

 

https://tinyurl.com/y8bp4wtm


Edited by Nucleophile, 02 June 2018 - 11:40 PM.

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#25 Cotts

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Posted 09 June 2018 - 09:41 AM

This pair will definitely be among the first lucky imaging targets for my nearly-finished 12.5" f/6.5 Lockwood Serrurier Truss Imaging Newt in my nearly finished roll-off-roof obsy.  Probably up and running in about 2 weeks.....

 

Dave




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