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Two Tight Twins in Tau

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#1 Asbytec

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Posted 04 January 2013 - 10:38 AM

I like tight doubles, and a tropical disturbance clearing seemed the perfect opportunity. But tonight wasn't the best. I tried for 7 Tau, a nearly equal magnitude pair, and managed a dark space in 6 to 7/10 seeing. During the best moments, anyway. The Spurious discs were fleetingly visible and a dark space at times.

Due to the fainter companion being a bit south, it reminded me very much of 31 Tau. It gave the /impression/ it was a bit tighter than 31 Tau. So, I checked. Truth is, I just could not tell in those fleeting moments if it was tighter or not.

Apparently, according to the database below, what I took to be 170 PA from the brighter to dimmer star, visually, is actually reversed. Apparently 7 Tau is 352 PA and ~.074" arc sep, mag 6.6 and 6.8. 31 Tau is PA 204, sep ~0.8" arc and mag 6.2 and 6.5.

Actually the data base I reference has two values for the sep, one at 0.74" arc and another at 0.54" arc and PA 11. I tend to believe the former. It's a great database, but only as good as the data in it, I guess. Also, the magnitudes for 31 Tau primary and companion seem reversed. The northern component appears brighter, as it did in 7 Tau.

http://stelledoppie....2.php?section=1

Anyway, Jupiter was a blurry mess, so was BU 87A about an arc minute SW of Jupiter.

#2 fred1871

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Posted 04 January 2013 - 07:44 PM

Norme, it's surprising that stelledoppie gives the 0.54" separation for 7 Tauri - the larger figure, 0.74" is obviously right.

Looking up the WDS the 2010 position is PA 354 and separation 0.7" (rounded) - the pair is widening at present, and will continue to do so for many years. So I'd think ~0.75" is about right for current separation. Pretty much the Dawes Limit for your 6-inch. :cool:

The WDS 6th orbit catalog gives a gd 3 orbit for this one, which means they think it reasonably accurate, and the orbit diagram they have which includes all the measures shows the pattern - closing in the years after discovery, widening for quite a while since. So it'll get easier over time. If the orbital period is near correct (given as 522 years!) we have a while to wait to see maximum separation, not a lot wider at about 1.0".

I looked at 7 Tauri about 6 weeks ago, and with 140mm aperture could not get a clean split, more of a discs overlapping effect (285x, 400x). That fits for 0.74" or thereabouts; 0.54" would be a short rod image, based on other pairs that close.

31 Tauri? - similar separation, similar effect, though a cleaner "figure-8" effect, at 320x and 460x (2x barlow instead of 2.5x Powermate used for 7 Tau).

For these even and close pairs maybe I need to add a central obstruction to my refractor to make the diffraction disc smaller? Or, since they're bright, I could use a narrowband blue filter to get extra resolution - the spectral types, A2 and B5, suggest the blue light from both is brighter than for Solar type stars (G). Hmmm. I think it's time to try Neil English's suggestion, because these pairs fall into the limited number of doubles where that approach could work, despite, as Thomas pointed out, the low sensitivity of the eye to blue light. (see the discussion late in the topic, Putting the "Rule of Thumb" to test).

#3 Asbytec

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Posted 04 January 2013 - 08:40 PM

Tonight looks better to observe them again, maybe the storm has cleared the area and seeing will be better. But, yea, 7 Tau reminded me so much of 31 Tau. In the best moments, I clearly saw both discs. I could not say how dark the split was, nor really get a good look at it to compare with 31 Tau. In that seeing, those fleeting glimpses simply gave me the /impression/ of being twins. 7 Tau is about a half magnitude dimmer, maybe that made the split appear wider. I cannot wait 522 years for it to get easier. :lol:

You got a cleaner split on 31 Tau. Yea, that sound right, really, I should have too. I wanna better, more critical look tonight. Hopefully the seeing will normalize, the tropics can be pretty good almost nightly. But the generally N-S PA, the magnitude, one star slightly dimmer, and the dark space all resembled 31 Tau. I just could not gauge accurately the cleanness of each split in that seeing.

I like challenging observations, so picked 7 Tau and was just pleased when seeing settled and the dark space showed itself. Two tiny white specks floating in space with a very faint figure "8" diffraction ring. (Well, not fully closed across the middle, just kind of bent around both stars. Almost oval, I guess. Will check that, again, too.)

BU 87 was just not doable. I mean, maybe there was something, but not enough to get excited over. It was kind of cool seeing it that close to Jupiter, though.

You know, I dunno anything about filters for resolving close doubles. I would think the intent is to push back the peak intensity at the edges making the Airy disc smaller. Might work. The intensity curve falls off pretty rapidly toward the first minimum, not sure how much gain one might get.

As for an obstruction, well, maybe. This is one reason I chose this pair, too see how high frequency resolution is affected by the CO. If it exceeded Dawes, well that's interesting. But, to make that determination, one has to really be accurate in terms of separation and contrast between the two. I just can't be that accurate just yet, but I suspect it's happening. And MTF Theory supports it, actually. (Hence my tongue in cheek aperture + CO resolution limit. Really, it should be about 9% better than clear aperture and not follow the aperture - CO rule in this frequency range.)

Looking for more tough doubles, especially some close unequal pairs. Those are kind of enjoyable, pushing yourself and your scope.

#4 Asbytec

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Posted 05 January 2013 - 08:13 AM

Well, in more consistent, slightly better seeing 7 Tau was not as wide as 31 Tau. Still not uniformly bright across both discs with a slight darkening between them. Actually, it looks like what I think the Dawes split might look like.

Strangely, both discs appeared to display some limb shading with an occasional bright pin point of light dancing near their center. The darker space between 31 Tau components was a bit more pronounced.

Neither diffraction ring appeared pinched. Both observations at 320x (6mm TMB II) and 380x (8mm TMB II 1.6x Barlow.) Each was cleaner at 380x.

Still, a good challenge.

#5 Asbytec

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Posted 05 January 2013 - 08:35 PM

Sketch added: 7 Tau (top) 31 Tau (bottom) at 320x and 380x.

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#6 inZet

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Posted 26 February 2013 - 07:25 PM

There's an obvious error in Sep. Now field. Orbit computation and extimation for 2013 seems correct.
I'll fix that as soon as possibile.

Gianluca






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