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#1 cildastun

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Posted 26 May 2013 - 04:08 AM

Anyone got a decent split of Dubhe? With my 5" Mak, I can see the third star clearly (violet) and see that the primaries are a pair of yellowish stars of unequal brightness, but with my small scope the Airy disks touch each other and the diffraction rings overlap - what size scopes have folk used to completely split the primary pair?

Chris

#2 Rich (RLTYS)

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Posted 26 May 2013 - 06:59 AM

Wasn't aware that Alpha UMa had a third member. With my 10" refl at 38x A = Straw Yellow, B = Grayish.

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#3 Cotts

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Posted 26 May 2013 - 09:43 AM

Dubhe AB is far too difficult to split with any telescope at 38x.

From the WDS:

11037+6145BU 1077AB 1889 2010 73 326 18 0.9 0.6 2.02 4.95

That's 0.6" and a delta mag of 3! I would think a scope of 12 inches and a night of perfect seeing might have a chance.

The AC pair has a separation of 308". It is undoubtedly this pair you are reporting.

Dave

#4 fred1871

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Posted 26 May 2013 - 06:41 PM

Ummm.. are you sure you were looking at Dubhe (Alpha UMa)?? - as Cotts has pointed out, the AB pair is very (very) close and has a large brightness difference. His suggestion of at least a 12-inch scope seems to me not at all pessimistic - I'd be inclined to think 16-inches a better chance. So your description of stars with discs touching, with a 5-inch scope, suggests a double at twice Dubhe's separation, and with less unequal stars.

#5 Rich (RLTYS)

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Posted 27 May 2013 - 07:05 AM

As stated earlier I wasn't aware of a third component so I assumed the two brighter components were the A and B pair. I will correct my notes. :foreheadslap:

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#6 cildastun

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Posted 27 May 2013 - 01:30 PM

Ummm.. are you sure you were looking at Dubhe (Alpha UMa)?? - as Cotts has pointed out, the AB pair is very (very) close and has a large brightness difference. His suggestion of at least a 12-inch scope seems to me not at all pessimistic - I'd be inclined to think 16-inches a better chance. So your description of stars with discs touching, with a 5-inch scope, suggests a double at twice Dubhe's separation, and with less unequal stars.


Definitely Dubhe. My scope has a Dawes limit of 0.9 arcsec, so with doubles around 0.7 arcsec sep, I usually get a "figure of eight", although I agree here with a mag difference of 2.8 that's more difficult.

chris

#7 WRAK

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Posted 27 May 2013 - 04:55 PM

Sorry that I have to join in here - current WDS data is 0.6" +2.02/4.95mag. For Dawes you need here already more than 190mm aperture - and at Dawes you can see in this magnitude range no more than a notched rod. The very best you can see here with an 5" scope is an egg hopefully pointing into the right direction but I doubt even this.
Wilfried

#8 7331Peg

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Posted 27 May 2013 - 11:31 PM

If you look at the WDS ephemerides (here -- it will come up as Bu 1077), you'll get the separation for 2013, which is listed at 0.683", widening to 0.710" next year.

I've looked at Dubhe several times with my six inch f/10 at magnifications up to 608x and only had the slightest inkling of an elongation. The 2.93 magnitude difference adds a huge degree of difficulty because of the tight separation.

On the other hand, Dubhe has a beautiful yellow-orange color that more than makes up for all the time I've spent on it.


John :refractor:






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