Splitting Dubhe?

[Nik271]

I recently learned that Dubhe (Alpha U Ma) is actually a close double. Consulting Stelle Doppie shows that the separation is close to the maximum of just over 0.8'' and will increase a little bit until 2023. The orbital period is 44 years and the components are currently about 30 AU apart.

I thought this will make an interesting challenge for my 180 Skymax since the Big dipper is now close to the zenith in early night. 

I'm just worried that I may be setting the bar too high for a 7 inch scope. At this separation the secondary will sit right in the first diffraction ring. I'll be interested to hear what scopes people have used to split Dubhe. My guess is that a 6 inch refractor is probably the best tool for the job...

 

[John]

I have not split this one but I'll certainly give it a go !

My 12 inch dobsonian and my 130mm refractor will be the best tools I think.

Very close separation and quite an uneven brightness pair - very challenging I would think !

Might have to settle for just getting some elongation with the refractor but we will see. At least it's easy to find and nice a high in the sky.

Thanks for the suggestion

 

Edited May 1, 2021 by John

[rl]

I noticed this post in early May and have been trying to split Dubhe on a few good nights since. I've been using my 14" Dob but seeing has usually been awful with the star just a splodge at *230. But last night was tantalisingly close...after an hour of cooling, occasional diffraction rings were visible over an hour around midnight. There were hints of doubleness (duplicity?) with the secondary looking suspiciously like the same colour as the primary. However I can't claim it yet. With the forecast looking good this week I might take the scope out away from the buildings on the housing estate where I live and give it its best shot..

I've got a 6" refractor but I think the close separation, while being on the Dawes Limit will be too close given the difference in magnitude. 

[Nik271]

Thats awsome! Looks like you are very close. Good luck! I did give it a try with my 180mm Skymax in May but did not even get a hint of elongation. Since then the weather has been poor.  I will give it a try this weekend. It will need excellent seeing of course. 🤞

[John]

Thanks for the reminder - I'd forgotten about trying this one when I next have my 12 inch dob or 130mm out

The 130mm split the .91 arc second Lambda Cygni a few days back it might show some signs that Dubhe is a double at least.

 

[kirkster501]

I'll give this a go too if the permacloud ever clears.

[F15Rules]

I tried it a few times with my Tak FS128 (5"), but never on a night of really good seeing.

I've never got near to splitting it, and that could well be as much to do with my Mk I eyeballs not being what they were as with the scope's objective (which although only 5", is of known excellence 🥴😊)..

I think your Skymax at 7" aperture might manage it on a great night, if totally thermally acclimatised. I've read that these Maks are slightly stopped down to c170mm, but the thread below (which is very technical but very well written!) suggests otherwise I think, and might interest you..it will certainly keep you occupied on one of our all too regular cloudy nights!😁

Dave

Edited July 14, 2021 by F15Rules

[Jiggy 67]

Dubhe is actually a quadruple system……from SkySafari………

The star is orbited every 44 years at a distance of about 23 AU by a warmer, much dimmer, and less massive class F0 V star. Over 400 times farther away, at a distance of about 8000 AU, is another class F8 star that also has a companion with a 6-day period - making Dubhe a quadruple system.

[lunator]

This will be a real challenge. I will be interested to see what can be achieved.

just to give some insight, the latest info from WDS on the pair is primary is magnitude 2.02, secondary is magnitude 4.95. Separation 0.8.

Based on the MacEvoy  rule of thumb you would need about x425 magnification on a 150mm scope.

To give you some hope on a night of excellent seeing it might be do-able at x250-300.

Cheers

Ian

[Ags]

1 hour ago, lunator said:

Based on the MacEvoy  rule of thumb

What is this rule? Googling has drawn a blank...

[John]

1 hour ago, lunator said:

This will be a real challenge. I will be interested to see what can be achieved.

just to give some insight, the latest info from WDS on the pair is primary is magnitude 2.02, secondary is magnitude 4.95. Separation 0.8.

Based on the MacEvoy  rule of thumb you would need about x425 magnification on a 150mm scope.

To give you some hope on a night of excellent seeing it might be do-able at x250-300.

Cheers

Ian

I've got the 130mm refractor out cooling and I'll give it a go with that tonight. More in hope than expectation though

 

[lunator]

50 minutes ago, Ags said:

What is this rule? Googling has drawn a blank...

Hi Ags

It is in the Cambridge double star atlas 2nd ed. 

This link should take you to his website.

https://www.handprint.com/ASTRO/bineye1.html

Cheers

Ian

[John]

Well the seeing is very steady tonight. Zeta Herculis is as clearly split as I've ever seen it, right up to 600x with the 130mm triplet refractor.

Dubhe ? - well, so far, the most I can say is that I think that I can detect a slight brightening / thickening within the 1st diffraction ring at more or less the correct P.A. and right up against the airy disk of the primary star.

Nothing more than that though. I'll wait and try again when it's risen a little higher in the sky.

Zeta Herc is really excellent tonight though. Not as tough as Dubhe of course !

Seems a little odd to be starting at 300x and going up with the 4mm-2mm Nagler zoom but the conditions and the optics seem to be playing ball

Edited July 14, 2021 by John

[Les Ewan]

I didn't know Dubhe was a binary, I will give it a go with with my 16" Newt some time.

[Nik271]

8 hours ago, John said:

Well the seeing is very steady tonight. Zeta Herculis is as clearly split as I've ever seen it, right up to 600x with the 130mm triplet refractor.

Dubhe ? - well, so far, the most I can say is that I think that I can detect a slight brightening / thickening within the 1st diffraction ring at more or less the correct P.A. and right up against the airy disk of the primary star.

Nothing more than that though. I'll wait and try again when it's risen a little higher in the sky.

Zeta Herc is really excellent tonight though. Not as tough as Dubhe of course !

Seems a little odd to be starting at 300x and going up with the 4mm-2mm Nagler zoom but the conditions and the optics seem to be playing ball

Well done John! With 130mm the secondary should be touching the primary within the first diff ring, just as you report. I made a simulation with Aberrator

   Ignore the PA in the picture, primary is mag 2, secondary is mag5 at 0.8'

This corresponds well with your observation.

I was not ready last night and forgot my small 'solar' Mak on the mount until late evening by which stage it was to late to cool its big brother.

With only the the small Mak I focused on some 'easy' stuff. Izar, Mizar, Polaris and finally 65 U Ma, which was a first for me. 65 UMa is a nice triple visual system, which actually has 7 components. 

Tonight I'm not making changes, the storage bag with the big Mak is already out of the shed and in the shade 

 

[rl]

Bit of a change of plan last night. Life called off the dark sky / good seeing excursion with the 14" so I set up the 6" refractor in the garden to have a peek at Jupiter and Saturn. Dubhe is unfortunately behind the house. Planets a were not much good due to seeing, so I tool a leaf out of John's book and tried Lambda Cygni instead. It would be good practise since the separation is about the same (0.9 arcsec). The brightness difference between the two components is not so great as Dubhe but it would be a good training target to hone skills and manage expectations. 

Oh the joy of using a decent driven equatorial mount on close doubles! Whereas in the Dob the other night it was a right pain nudging the scope with one hand while trying to damp off the vibrations with the other...the star just sits where you want it with the eyepiece at a convenient height! Much easier to concentrate! Lambda Cygni was resolved as double quite easily if a bit messily using a 4.7mm Ethos (*320). The B component was a very striking blue compared to the white primary. Seeing good for round here but not spectacular. 

Inspired, I put the planetary camera on just to see if I could get an image of the split. To make the scope's resolution the limiting factor rather than the camera I put a *2 Barlow in the system but this resulted in a more than expected increase in focal length since the spacing is not as intended. The focuser had to be racked right out and the whole camera setup was clinging on by its fingernails...I'm not sure the optical axes are properly coaxial which explains some slightly oval star shapes not seen visually. The blue colour of the secondary was even more apparent on the monitor at night; it's not so obvious viewed in daylight. 

Video clip attached; this is 105 images taken with 1 second exposures on a 6" f/10 triplet,  Barlow stretched to officially f/20 but probably f/30 or f/40 in reality. No filters. 

I had rather dismissed the refractor for Dubhe as just a bit light on resolving power. But based on this it's worth a try. It's certainly going to be harder just down to the magnitude difference. A blue filter might help.

 

[JeremyS]

Great result @rl, but video won’t play

[rl]

Thanks @ JeremyS. Don't know why. 

One of the better frames:

 

[chiltonstar]

I've looked at it maybe half a dozen times with my 180 Mak. I see it as an unequal infinity sign, but with no dark space between the components.

Re the above comment about the true aperture of a 180 Mak, I think the clear aperture is nowadays close to 180 mm (they now have an oversized primary). There are very many threads on CN on the subject.

 I've put mine on an optical bench and get 179 mm.

Chris

[John]

I've got the 12 inch dob out tonight but the seeing seems poor compared to last night. I doubt that I'll get anywhere with Dubhe tonight unless it really settles down a lot.

 

[JeremyS]

Good luck John - hope it settles down

[chiltonstar]

56 minutes ago, John said:

I've got the 12 inch dob out tonight but the seeing seems poor compared to last night. I doubt that I'll get anywhere with Dubhe tonight unless it really settles down a lot.

 

The seeing here tonight is excellent+, with a stable sharp view at x450 (6 mm Baader Ortho) with the 180 Mak. The secondary seems to be oval shaped tonight and attached to the diffraction ring, but there is at least dark space between the primary and secondary! Better view than I've had before, down to the seeing I suppose. I think I might just claim that as a split........ 

Chris

 

[John]

1 hour ago, chiltonstar said:

The seeing here tonight is excellent+, with a stable sharp view at x450 (6 mm Baader Ortho) with the 180 Mak. The secondary seems to be oval shaped tonight and attached to the diffraction ring, but there is at least dark space between the primary and secondary! Better view than I've had before, down to the seeing I suppose. I think I might just claim that as a split........ 

Chris

 

Excellent stuff Chris

I forgot to go back to Dubhe tonight having got rather carried away planetary nebulae hunting in Aquila

 

 

[John]

19 hours ago, Nik271 said:

Well done John! With 130mm the secondary should be touching the primary within the first diff ring, just as you report. I made a simulation with Aberrator

   Ignore the PA in the picture, primary is mag 2, secondary is mag5 at 0.8'

This corresponds well with your observation.

I was not ready last night and forgot my small 'solar' Mak on the mount until late evening by which stage it was to late to cool its big brother.

With only the the small Mak I focused on some 'easy' stuff. Izar, Mizar, Polaris and finally 65 U Ma, which was a first for me. 65 UMa is a nice triple visual system, which actually has 7 components. 

Tonight I'm not making changes, the storage bag with the big Mak is already out of the shed and in the shade 

 

Thanks for this.

My 130mm refractor was, last night, showing a view quite similar to that simulation but with the E / W orientation reversed and a little shimmer in the diffraction rings. So like this:

Possibly about as good as the aperture can do ?

[Voyager 3]

At what magnification were you detecting the component John ? Fantastic observations @John and @chiltonstar  .