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Another Level of Hard

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#1 Asbytec

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Posted 08 January 2013 - 10:25 AM

If you think 42 Ori is hard to split, stay well clear of BU 1238 in Tau. You have been warned, do not try this one without consulting a professional. BU 1238 is an 7.5 magnitude primary and an 10.5 magnitude secondary at 1.5" arc separation and PA 017. Easy, right? Well, BU 1238 gives new meaning to the term, "hard." It's almost impossible. Almost.

Tonight's seeing was fluctuating around 8/10 and transparency was mag 5 with intermittent bursts of cloudiness. BU 1238 central disc was mostly well defined and its very dim first diffraction ring was seen periodically: sometimes well formed, other times just there. The entire star image seemed to jump around the FOV while staying in tact. Weird, but I suspect some larger level of seeing.

It was one of those nights one sits alone in the dark with an eyeball pressed against the eye lens occasionally whispering, "Ah!" to yourself. It was one of those observing sessions you really learn to appreciate good eyepieces.

There was no sign of a split at 174x (10mm UP HD Ortho 1.6x Barlow.) More magnification (262x with 12mm UP HD Ortho and 1.6x Barlow) and more patients...the first, "Ah!" broke the silence. Over the course of about 30 minutes, a few more "Ah!" moments begged for closer inspection. Popped a 10mm Plossl (313x) into the Barlow and had three "Ah!" moments in rapid succession. Then a long pause before the next one. But, man, I really missed the view through the 12mm Ortho. The Plossl was just too tight.

Okay, back down to 262x and then up to 320x with a 6mm TMB II (no Barlow) resulted in a few more barely audible "Ah!" breaking the monotonous silence. For the brief period observing with the TMB II, I was pretty sure I captured it just once. Seeing seemed to be deteriorating and some clouds were in the western sky. Had to hurry...

Yea, BU 1238 is an a higher plane of "hard." Made more so by the contortions of my hands blocking every photon of stray ambient light, pushing my eyeball deeper into the eye lens, and keeping centered over small exit pupils. I think the eye guard is still stuck to my temple. Remember, do not try this at home, get to a dark site cuz you might need it.

Yea, not easy...it's hard. But, I really think there were enough "Ah!" moments to be reasonably sure I managed to see the 11th magnitude companion sitting pretty much straight north of the first ring. It was hard enough for one night, didn't try another one. You wanna know how hard it was? That's how hard it was.

#2 Patricko

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Posted 08 January 2013 - 02:52 PM

Awesome job and thanks so much for the detailed post. Your 150mm MCT does a really good job on those doubles. I'll give it a try with my C6 SCT when conditions allow. Are there any finder charts for this double? Are the powers of 174x (10mm UP HD Ortho 1.6x Barlow) and (262x with 12mm UP HD Ortho and 1.6x Barlow) reversed in the post?



#3 WRAK

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Posted 08 January 2013 - 03:59 PM

Congratulation, if my current RoT model is any good you managed to split a double at the about 15% split probability range for a 6" refractor - seems to be near the upper limit of what is possible with your scope.
Wilfried

#4 Darren Drake

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Posted 08 January 2013 - 06:03 PM

Will try in the 18 next chance I get. Should be quite doable if the seeing is really good...

#5 fred1871

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Posted 08 January 2013 - 06:12 PM

Norme, my older, simple RoT would give this one as being at the limit for a 6-inch scope, preferably unobstructed. So you've been able to (just) make it out despite a middling-large CO. Very well done. :D

I'll try it myself at first opportunity - only 140mm aperture, but no CO. If I succeed I've shown a need to change my simple RoT - or to take on Wilfried's probability-based model. Or to treat the simple RoT as a "what you can see without having to spend 30-60 minutes straining to see" rule. Or ??? :question:

But this observation is certainly a useful data point on the issue of Refractors versus SCT/Maks and the effect of CO for resolving uneven pairs.

#6 Asbytec

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Posted 08 January 2013 - 09:45 PM

Congratulation, if my current RoT model is any good you managed to split a double at the about 15% split probability range for a 6" refractor - seems to be near the upper limit of what is possible with your scope.
Wilfried


Wilfred, you know, having thought about the observation, I think you are spot on. Yes, it was so difficult to tease out some hint of the companion over a long period of time. It really does have to be at the limit with only a small probability of success. That's what happened. It was not like I enjoyed a high probability of success every time and the star was readily visible. Those probabilities were few and far between.

You mentioned 15%, well, if I can hazard a guess at it, I doubt I got glimpses of the companion 5% of the time. Boring for a lot of folks, but for me the challenge. The only reason I am reasonably sure I even succeeded is sheer persistence. I was there when the probabilities offered the best chance of success. Even then, it was not like there was a clear, well defined Airy disc. Just a faint point-like brightening. Again, far from beautiful, but a brief speck of light none the less.

So, yes, a very difficult observation of a brief speck of dim light about 5% of the time to be only reasonably sure it was there. :lol: (Made myself chuckle thinking why we do such things.)

So you've been able to (just) make it out...

...what you can see without having to spend 30-60 minutes straining to see...

Emphasis on "just," put a lot of "u"'s between the "j" and the "s." Yes, Fred, that was very difficult, in fact being difficult seemed its most important trait. I'd love to hear you succeeded in your 140mm. Such an observation can tell us something.

Interesting how a 11th mag star is so difficult against an apparently dark background near a moderately dim star. It did not sit on the first ring, close...just outside it. Pondering the observation and delta M. That's fascinating.

You ask a big question, does a RoT describe what is theoretically possible during an hour or what might be enjoyed by the general observing population within a few minutes. Dunno, but I am beginning to think the boundaries we set are soft boundaries, anyway.

Probabilities near a limit keep the observation usually beyond reach, except for every once in a while. If one variable were off, success becomes less often. At some point it just becomes so highly improbable: technically, and in all likelihood, completely impossible. Is that were we set the RoT?

And what should the view look like. What constitutions resolution: a brief ring brightening/flaring or a distinct Airy disc.

#7 Asbytec

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Posted 08 January 2013 - 11:43 PM

One of the interesting aspects of double star observing is the level of difficulty, beyond color and beauty. But, why is it so difficult to split a star at 1.6" arc and why is it so difficult to see an otherwise pretty easy 11th magnitude star so close to it's primary component? Both the magnitude and separation should be rather easy, per se.

Is there a gradient of light falling off between the first ring and some radius further out? Well, yes, its diffraction. But, if it is not apparently visible and an 11th magnitude star should be easily visible, what affect is causing it to be apparently so dim?

Well, contrast of course. Does contrast even apply if it's below the visible threshold? E and F Trapezium are set against a righter background and can be challenging (but easier, by far, than BU 1238.) The companion sits outside the first ring and near the second minimum in a 6" scope. Is the first ring the culprit? Is imperceptible glare from the primary the culprit?

I am wondering if something more sinister is occurring. The primary's diffraction image is caused by a series of constructive and destructive interference. If a companion falls on a minimum, does that destructive interference inhibit the primary's image formation, as well, for the same reasons the minimum is black? Maybe.

Anyway, to reiterate, I think the probability of detection played a role in making the observation possible. The probabilities of success were very low most of the time. This was reflected in the longer periods of waiting and not seeing the companion. Only briefly and rarely were probabilities favorable for detection resulting in those "Ah!" moments.

Success requires patients and persistence. That being said, one might argue this particular double IS beyond the capabilities of a 150mm obstructed scope - very nearly all of the time.

#8 azure1961p

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Posted 09 January 2013 - 12:11 AM

Okay a new marque de Sade double more difficult than before. Trouble is I kno what ill probably end up with in seeing so I don't know if I can make this one out. I think it's great though that you are pushing your boundaries. What is the smallest sep I can still get an elongation on? Have you tried that?

St anyrate I'm putting this one on my bucket list.

Ciao you Mak kinda guy.

Prte

#9 azure1961p

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Posted 09 January 2013 - 12:12 AM

Okay a new marque de Sade double more difficult than before. Trouble is I kno what ill probably end up with in seeing so I don't know if I can make this one out. I think it's great though that you are pushing your boundaries. What is the smallest sep I can still get an elongation on? Have you tried that?

St anyrate I'm putting this one on my bucket list.

Ciao you Mak kinda guy.

Pete

#10 Asbytec

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Posted 09 January 2013 - 12:32 AM

Pete, yea pushing critical observations is something I got from you. :)

This one is far enough separated where it's not an elongation problem, like 72 Pegasi and Io. Titan deserves another shot, being that it's larger that 1/4 the Airy diameter and therefore an extended object (broader PSF) in a 6" scope.

Still thinking about why close unequal pairs are so difficult. The answer is contrast, of course. But why. If destructive interference played a role, one might imagine that would reap havoc at the Raleigh limit where the center of one sits on the other's point of maximum destructive interference. In this case, maybe the companion would destruct and not be visible at all. Likewise for the primary because it sits on the companion's point of maximum destruction, too. So, maybe all Raleigh limit doubles should just disappear completely. :lol:

If that's true, then we're left with a lumpy gradient of diffracted light intensity falling away from the peak intensity of the primary. The peak intensity of the companion must contend with this added gradient against an otherwise pure black background. The companion needs to be that bright, at least, and maybe even brighter by 5% to offer sufficient contrast for the human eye to discern it.

But, if this gradient is below the visible threshold, well an 11th magnitude star is not. It should pop at some separation well beyond the Raleigh limit and by virtue of being above the limiting magnitude of the scope. Yet, it doesn't. So, why doesn't it? Arg!

#11 Asbytec

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Posted 09 January 2013 - 12:39 AM

...(10mm UP HD Ortho 1.6x Barlow) and (262x with 12mm UP HD Ortho and 1.6x Barlow) reversed in the post?


Oh, I am sorry, it was an 18mm Ortho, not 10mm. Yea, thank you for catching that. Focal length is about 1950mm with the Barlow.

#12 Asbytec

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Posted 09 January 2013 - 02:41 AM

Alright, another thought on blur size, especially in the presence of some residual SA. At best focus, there is a small halo of blur surrounding the Airy disc. In well corrected or balance conditions, this blur is very weak. However, for pure spherical, unbalanced optics the blur at best focus can be smaller and more intense. APOs and Maks need the balanced form to correct for higher order SA, as I understand it. They use highly curved spherical surfaces which give off lots of high order SA resulting in a small, but compact blur around best focus.

Surely APOs are well balanced, some Maks may or may not be. I /suspect/ mine is balanced to come degree after star testing for that very thing. The result is a broader, less bright blur around best focus. So, maybe the amount of correction, especially in fine APO samples, enables detection of faint, unequal binary stars because the blur intensity is greatly reduced.

I am not sure how blur and diffraction are related, they may be the same thing caused by the longer caustic focus. I think they are not related, however, because even a perfect parabola will have diffraction but no blur at best focus. (Advantage reflector?)

#13 WRAK

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Posted 09 January 2013 - 03:45 AM

... my older, simple RoT would give this one as being at the limit for a 6-inch scope, preferably unobstructed. ...

Fred, as far as I remember this would go 116/1.6*2.96 = 214.6mm for reflectors with CO and 2/3 of this value = 143mm for refractors - this would give you a good chance with your 140mm. But this calculation does not consider the effects of increasing faintness of primary and secondary as the result of this rule is the same if the primary is +5mag or +9mag and this makes certainly a difference.
Norme, may be we should discuss when you count an observation as a split. When you have now and then a faint flicker in the diffraction pattern at the "right" position (and I assume sometimes also at wrong positions as usually in a dancing diffraction pattern) then this would be a successful personal experience but not an "formal" split - this would in my opinion require at least short periods of a stable picture for some seconds.
Wilfried

#14 Asbytec

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Posted 09 January 2013 - 05:18 AM

Wilfried, I might agree. A stable Airy disc is a much better criteria for a split using a RoT. That did not happen with BU 1238. I never got a stable central disc on the companion, unlike the other tight doubles from the previous observation. Each of those was rated at 150mm and less.

#15 Patricko

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Posted 09 January 2013 - 05:36 PM

Does anyone have a finder chart for this double?

Clear skies,
Patrick

#16 fred1871

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Posted 09 January 2013 - 07:00 PM

Alright, another thought on blur size, especially in the presence of some residual SA. At best focus, there is a small halo of blur surrounding the Airy disc. In well corrected or balance conditions, this blur is very weak. However, for pure spherical, unbalanced optics the blur at best focus can be smaller and more intense. APOs and Maks need the balanced form to correct for higher order SA, as I understand it. They use highly curved spherical surfaces which give off lots of high order SA resulting in a small, but compact blur around best focus.

Surely APOs are well balanced, some Maks may or may not be. I /suspect/ mine is balanced to come degree after star testing for that very thing. The result is a broader, less bright blur around best focus. So, maybe the amount of correction, especially in fine APO samples, enables detection of faint, unequal binary stars because the blur intensity is greatly reduced.

I am not sure how blur and diffraction are related, they may be the same thing caused by the longer caustic focus. I think they are not related, however, because even a perfect parabola will have diffraction but no blur at best focus. (Advantage reflector?)


Maks plainly raise issues that, say, Newtonians don't, because of the requirement in Maks to balance different orders of SA. Whereas in a Newt, you've a much simpler optical system, that depends on other factors only (eg quality of making) for the form of the image at the centre - off-axis in the Newt, you have increasing coma, and astigmatism eventually begins to show a fair way out.

In the case of an apo refractor it will depend on how the colour curves (focus with wavelength) have been designed as to what kind of final image you get - some apos, and some modified achros, may favour corrections that are not optimal for visual observing, because the response of the eye interacts with that creating other issues.

The human eye is not achromatic and obviously has less sensitivity at some wavelengths than at others.

One curious issue is that reflectors (simple, Newtonian) are still not of stable focal length, as Paul Couteau puts it. Because all colours come to the same focus, and red is obviously of longer wavelength than blue, a star image will have a red halo around the blue focused light, though very small. Suiter discusses this also. It leads Couteau to suggest that the achromatic refractor is better for double stars than reflectors because "the light that actually contributes to the diffraction disk is limited to a narrow range of wavelength, about 200 Angstroms [20nm]. The other wavelengths are dispersed ...and form a large violet halo..."
This very spread out light has less interfering effect than light which is only a little out of focus, and therefore of much greater intensity.

Yes, as the optics gurus have started pointing out (on Cloudy NIghts), we're into a complex and multi-factor-interactional area.

Even so, I'm hopeful of finding some rules of thumb for uneven pairs. Though I'm now wondering if we consign BU 1238 to the area of "normally beyond 6-inches with OC" with the RoT suggesting a slightly less hard normal limit.

#17 fred1871

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Posted 09 January 2013 - 07:47 PM

Stable discs on secondary stars - seems to depend on a few factors, one being where the star is in relation to the diffraction rings of the primary; another is air steadiness/seeing effects.

And I'm wondering if factor 3 is "how dim is the secondary star"; and if factor 4 is "CO or not CO"?

The most difficult double I've so far split with my 140mm refractor is Upsilon Gruis. Back in late September, I was checking out Theta Gruis again for the Haas project, and Theta split nicely with the 140mm.

The night gave stable images at 400x, so I thought I'd try the more difficult Upsilon Gruis, mags 5.70 and 8.24 (delta-m of 2.54 mags) at only 0.9" separation!! I had split this one years ago with a 7-inch refractor. It's much harder than Theta Gru, because Theta has slightly smaller delta-m (mags 4.45, 6.60, so Dm 2.15 mags) and is much wider (1.5").

I was pleasantly surprised that Upsilon showed its companion, a clear tiny speck right beside the primary at 400x, clear in the steadiest moments. And I didn't have to spend 30 minutes or more watching, or I would not have succeeded - I don't have that level of patience.

Upsilon's companion was not seen at less than 400x. Theta's (much wider one) was.

Theta is within range for my (old, rough) RoT. Upsilon is not. With Upsilon, apart from no CO (refractor), I suspect the better brightness of the secondary star helped greatly. At mag 8.24 it's going to be much further inside the brightness ability of a 140mm telescope, and less subject to various "noise" effects that make fainter companions harder to see.

Conclusion - roughly, that delta-m is part of the equation, but absolute levels of brightness, relative to aperture, also play a part. And I'm left wondering about the "Couteau Limit", mentioned in these threads a few times, that might impinge as we go to mag 10-11 or beyond. Bruce (drollere) has noted this with his 12-inch scope. Couteau's description of it -
"Experience shows that, whatever the aperture, magnitude 10 is a barrier. In a large instrument the images lose their sharpness and break up. Light is lost in the diffraction rings, and the eye does not receive very much more illumination".

This sounds like a seeing (air steadiness) related issue, from the description. But it's interesting, because Couteau had long experience observing and measuring doubles, with refractors of 38cm, 50cm and 76cm aperture (15, 20 and 30-inch).

Smaller scopes don't gather enough light; bigger ones have reduced benefits because of seeing effects. Hmmm - think I've heard about that before :grin:

#18 Asbytec

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Posted 09 January 2013 - 10:37 PM

Yes, the topic is quite complex. I though doubles were just beautiful. Truth is, they are complex beasts that need taming with a chair and whip.

Interesting to consider the Coutaeu limit. Hmmm...dim and scattered by air. BU 1238 was at that barrier. You know, if seeing gets better than previously, gotta hit that one again. Just curious. But, if I do not see it in a reasonable period of trying, move on...

#19 fred1871

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Posted 11 January 2013 - 11:48 PM

Norme, I thought I'd look up BU 1238 in Burnham's own catalog of his discoveries - turns out he discovered this one was double not with his 6-inch refractor, not with the Chicago 18.5-inch, but with the Lick 36-inch. :shocked:

Does that make "another level of hard" seem not so bad after all, now you've teased out the companion, with only a 6-inch? :grin:

I've not tried it yet. I'll report on what I do or don't see when I observe it.

Someone was asking for a finder chart - next best thing is coordinates, so you can plot it on a chart - it's at RA 05h 01.3m, Dec +26 32 (J2000, as per current common usage).

#20 Asbytec

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Posted 12 January 2013 - 04:30 AM

Fred, not sure what to make of that. It does put things inter perspective, one might imagine a double observed in a 36" refractor would most certainly be very hard in a 6" Mak. :lol:

I'd love to know how you fare observing it. That's how I found it, plotting the coordinates on a star chart and star hoping to it. Using the star patterns outlined in blue, it was pretty simple.

(Please tell me I was on it...it's the only 8th mag double on the chart in that area. If not, well, then that double was extremely hard. :))

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#21 Patricko

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Posted 12 January 2013 - 10:29 AM

Thank you! I want to give this a try. :)

#22 Darren Drake

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Posted 14 January 2013 - 11:33 PM

I gave this a good shot tonight in the 18 inch. At first when the seeing was about a 7.5 outa 10 I couldn't see the companion. After a while the seeing improved to about an 8.5 outa 10 so I went back. Success! The very faint companion was most definitely there. I was using a paracorr and a 5.5mm Apogee super easyview eyepiece for a mag of 410X. Experienced observer Dan Joyce was there to confirm. (I also showed him the resolved disk of Ceres.)This was fairly difficult in an 18 inch and I can only imagine how hard it would be in a 6 inch Mak. That observation takes some major dedication. Good job Norme!

#23 Asbytec

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Posted 15 January 2013 - 12:38 AM

Thank you Darren. I am happy to hear you report success and that there is indeed a companion. :)

You know, that kind of spurred a little debate on what it means to resolve a double (again.) Surely seeing a companion as a distinct disc is resolution, maybe a faint flicker of light is not. The former should be relatively less time consuming if the companion forms nicely. The latter, well, I did not realize I was observing it for nearly an hour. My problem is not liking to fail, so I'll persist if there is the slightest chance of success. But, in the end, really there might not have been a reasonable 'split' in the six inch: faint flicker of light as opposed to a steady disc.

A disc on Ceres? Now, that seems a feat. Congratulations, Darren.

#24 Darren Drake

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Posted 23 January 2013 - 09:12 AM

Am I the only one here to get this one? I am wondering who out there tried with no luck in good seeing. A negative result is also valuable info...

#25 Asbytec

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Posted 23 January 2013 - 10:03 AM

You got it? Cool! What scope? I am wondering if either of your 8" scopes can do it.

I've tried once since. You know, I guess it depends on what you call a split. For me, it's certainly not a nice looking pair, as it were.






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