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Three Toughies in Tau

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#1 Asbytec

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Posted 16 January 2013 - 10:15 AM

...and one in Orion.

Seeing conditions about 8/10 and NELM 4.5 to 5. Observations were with 12mm UO HD Ortho and 1.6x Barlow (263x) and 6mm TMB II (320x.) At times I did use 8mm TMB II with 1.6x Barlow (384x.)

STF770: Pretty difficult 8.8 mag primary with 9.8 mag companion separated by 1.1 arc sec. At first, I didn't think I would succeed, found myself using a bit of averted vision. But, the companion could be made out as a very faint, intermittent speck sitting to the NW either on or near the first ring (not clearly seen.) Success at 163x and 320x.

BU536: Right in the heart of the Pleiades, how cool is that? There are two stars aligned EW with BU 536 trailing and a third field star north. BU 536 is 8.1 mag with a 9.4 mag companion about 1 arc sec separation. Again, a faint and intermittent speck was seen sitting just south of the primary. At first, I thought I glimpsed something at 263x and confirmed it at 320x and 384x. The companion was about the distance of the first ring.

BU 1040: moving up the scale of difficulty, this one was very difficult. Its a 7.8 mag primary, I cannot call a good split on this one. It reminded me much of BU 1238 in that a faint flash of light was noticed just north. I was seeing 13th mag field stars but could not really see it's 11.5 mag companion 3.5" arc away. I was at 263x and 320x on this one.

(EDIT: After checking the maps, maybe one 12th mag star field star a bit away from the primary was the companion. Not sure, though, seems a bit far for 3.5" arc. There was a field star out NW on a line the companion would be. I am confused at the moment as to whether I split this one without knowing it.)

HO 20 (in Orion): This one sounds hard. After star hopping to it, I was unsure which of the two stars it was within the triangle I used to find it. So, I checked both stars thoroughly at 263x and 320x. For the life of me, I could not split either one. Apparently, according to the charts, both of the stars are doubles.

Upon double checking my position, well, no wonder...I was on the wrong triangle of stars. Okay, got that squared away and was treated to an easy split. HO 20 is a 6.8 mag primary and 11.8 mag companion separated by nearly 8 arc seconds and sitting about about PA 280. In fact, it was so easy I thought something was wrong...especially after coming off of those close and difficult pairs. Turns out, this one was a treat in comparison. Easy split at 163x. In fact, I got a hint of orange color for the primary, but wasn't sure if it was simply the dimmer companion looking rather pale that gave the impression. Turns out, it is orange-ish.

(EDIT: I am confused about this one, too. In both cases, there is a field star at the proper PA and approximately the right magnitude. Just seems the separation is quite far. Truthfully, I am not sure whether or not either HO 20 or BU 1040 were split. Checking Triatlas Mag 13 charts and Stellarium is simply confusing.) :mad:

#2 Asbytec

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Posted 16 January 2013 - 12:49 PM

BU 1040 cleared up. The field stars easily seen are in green, hinted at or fleetingly noticed in red. That would put north up and west to the left. The scale is only a bit smaller than my TFOV. Mystery solved, no resolution. I believe the companion is in yellow

Image from:
http://aladin.u-stra...58 59.9183&a...

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#3 Darren Drake

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Posted 16 January 2013 - 12:55 PM

I will give these a shot next time I have the 18 out. I did try the one in M45 a while back and think I did get a split but it was hard to confirm. It is fun attempting these challenging doubles when you have good optics.

#4 Asbytec

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Posted 16 January 2013 - 01:04 PM

HO 20 mystery solved. No resolution, the approx location of the companion is in yellow.

Image from:
http://simbad.u-stra...ubmit=SIMBAD...

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#5 Asbytec

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Posted 16 January 2013 - 01:16 PM

I want to try HO 20, again. How hard can it be? :lol:

I picked HO 20 from the list because it looked pretty easy with a bright primary. I just didn't feel like chasing a dimmer star and it's companion seemed far enough removed to be seen at 12th magnitude. Its rated about the same difficulty as STF 770 and BU 536. I guess not. Wow.

I am pretty sure BU 1040 is a lot like BU 1238, extremely difficult with no distinct disc. Just a brief hint or flash of light in the general vicinity of the companion. I am not going to call that a split. That one was just too difficult.

Darren, good luck. I'd love to know you got them. They are very difficult.

Here's another image of HO 20.

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#6 fred1871

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Posted 16 January 2013 - 07:04 PM

Norme, I quoted my observation of BU 1040 in another thread you started (Some doubles in Tau...) - the 285x there was meant to be 230X, as I find after referring to my original notes - clearly seen at that power, glimpsed with 160x after locating it with 230x. That's with the 140mm refractor, no moon, dark sky, good seeing conditions though not perfect. When the separation gets out to 3.5" I find pairs are a lot less difficult than closer not bright ones even when the Dm is less.

It's no way comparable with BU 1238, given the numbers - that one is MUCH closer and will be much tougher, despite a less faint companion. My weather this week has turned to flarey, slow-boiling stars - 160x useable, above that no point trying, so the tougher doubles like BU 1238 will have to wait.

If both stars of BU 1238 were brighter, but the same Delta-m and separation, I'd expect it to be less difficult. Not easy, but I've seen a few brighter pairs with similar Dm and sep with the 140mm refractor. Tough rather than near impossible.

#7 Asbytec

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Posted 16 January 2013 - 09:10 PM

Fred, you're correct, by the numbers they don't compare. I should have been more clear meaning only in terms of /apparent/ difficulty. I would think when a star gets out from the brighter rings it would be easier, too. I agree with you, it should be easier. At 3.5" arc, though, it does sit on my fourth diffraction ring (but not seen, only in brighter stars.)

I am not sure why I could easily see field stars down to 13th mag (even pushing 14th mag - guessing - circled in red) and not split BU 1040. Rather not see the companion easily. I did catch a fleeting glimpse of something in that vicinity, but not enough to be sure or to call a split (which drew my comparison to BU 1238.) Even the stars in red were only rarely glimpsed. Only the primary and the two stars in green were seen easily, the others were just not visible consistently.

Maybe this one deserves another look, too. But, it was a pretty good night and time is short with the moon coming around.

I chose HO 20 from the list because it seems rating the for 144mm seems to be pretty close to a limit, based on some stars observed, for me. At that sep and magnitude, it should have been easier. Curious.

#8 Asbytec

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Posted 19 January 2013 - 10:37 AM

Revisit of HO 20 resulted in a successful split. The faint companion could be held steady in 9/10 seeing under mag 4 skies.

BU 1040 was a fail, again.






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