Splitting Dubhe?

[chiltonstar]

I made a simulation this morning with Aberrator* and it was gratifyingly similar to the visual observation last night, except for the fact that the magnification of the image looks a bit too high:-

Chris

* 180 mm 30% obstructed aperture, f15

 

[Nik271]

9 hours 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

 

That's a great result, Chris! I had a look with my 180 Mak last night but it was close after sunset and the seeing was not stable in that part of the sky where Dubhe was. The diffraction ring was wobbly and I couldn't spot the secondary at my maximal available power which is also x450. Later in the night I moved to Hercules and Cygnus and managed to split Zeta Herc with ease and with some effort even Lambda Cygni, both at x450. The fact that they are higher in the sky than Dubhe must have helped.  Tonight seems to be another good night, I will try for Dubhe a bit later, maybe around 11pm when the ground air has cooled.   

With these good conditions I haven't even bothered removing the  6mm EP from the diagonal

[John]

3 hours ago, Voyager 3 said:

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

I went up to 600x but I reckon I was starting to see indications of the secondary component at around 350x. Crazy power for a 130mm aperture really !

 

[Nik271]

30 minutes ago, John said:

I went up to 600x but I reckon I was starting to see indications of the secondary component at around 350x. Crazy power for a 130mm aperture really !

 

Indeed, that is over twice the 50x per inch rule but I believe there are real benefits of going that high. I think the old rule of 50x per inch of maximal usable resolution is based on the fact that around this magnification the radius of the Airy disc (up to the first minimum) looks about 2'  in the EP. And various sources claim that in practice the resolving limit of the average human eye is about 3'.

However not everybody can spot fine details at separation of 3'. I know that I can't with dim objects. The Double Double's two pairs are 3.5' apart and I haven't  split them with naked eye.

So on nights of excellent seeing there are benefits of going above 50 per inch, unless you have the eye of an eagle. I think for me even 100 per inch could still deliver some visual benefit over 50 per inch.

 

Nik

[lunator]

7 hours ago, John said:

I went up to 600x but I reckon I was starting to see indications of the secondary component at around 350x. Crazy power for a 130mm aperture really !

 

Hi John,

Paul Couteau used to say that you needed 2x your aperture before you really started doing the tight visual double stars 😀

He was speaking about measuring them but there is some truth in it for observing as well. For your 130 a x260 mag is just the beginning 😁

Cheers

Ian

 

[John]

2 hours ago, lunator said:

Hi John,

Paul Couteau used to say that you needed 2x your aperture before you really started doing the right visual double stars 😀

He was speaking about measuring them but there is some truth in it for observing as well. For your 130 a x260 mag is just the beginning 😁

Cheers

Ian

 

Thanks Ian

When I bought my 2mm-4mm Nagler zoom I thought it would only get occasional use but with the Tak 100 and the TMB/LZOS 130 it's often the first eyepiece in the focuser when I'm observing double stars and sometimes the only one I use. I'll be wearing it out at this rate !

 

 

[Nik271]

I'm pleased to say that I managed to resolve Dubhe last night with the 180 Skymax and 6mm EP, at x450 magnification. The seeing was superb around 11pm, and Dubhe presented very steady view. The diffraction rings were elongated and more dense towards NW and in short periods I could spot a blob forming on the first diffraction ring  I wouldn't call this a clean split because it was never there for long periods of time and did not separate from the Airy disc of the primary but I'm pretty happy with this result! Dubhe was getting low at 40 degrees elevation and dispersion was starting to show up, so it's position is not ideal right now. I think in winter this will be even better when it is high overhead.

This high pressure system has brought us very good seeing, at least in the south east!

Edited July 19, 2021 by Nik271
forgot the mention magnification used.

[John]

18 minutes ago, Nik271 said:

I'm pleased to say that I managed to resolve Dubhe last night with the 180 Skymax and 6mm EP, at x450 magnification. The seeing was superb around 11pm, and Dubhe presented very steady view. The diffraction rings were elongated and more dense towards NW and in short periods I could spot a blob forming on the first diffraction ring  I wouldn't call this a clean split because it was never there for long periods of time and did not separate from the Airy disc of the primary but I'm pretty happy with this result! Dubhe was getting low at 40 degrees elevation and dispersion was starting to show up, so it's position is not ideal right now. I think in winter this will be even better when it is high overhead.

This high pressure system has brought us very good seeing, at least in the south east!

Great stuff Nik

I could not get anywhere with it with my 12 inch dob so I'll have another go with my 130mm refractor when I get a chance. I'd like to try when Dubhe is somewhat higher in the sky too.

 

[markse68]

I’ve been trying past few nights but it’s not been stable enough to call- hints of a split but inconclusive. Seeing is everything on such a tight split

Mark

[John]

Actually tried with the ED120 tonight. May have been hints of something in the right position in the steadier moments - like something stuck on the side of the primary star airy disk. Probably "averted imagination" though. Trouble is, once you have seen simulations of what it might look like, it's difficult to un-see them !

Nice night though - quite a steady image even at 450x with the 4.7 inch frac.

The easy ones like Iota Cass are looking gorgeous

[Nik271]

I saw something like this in my Mak at x450

 

 

Except that the diffraction rings were thicker because of the central obstruction and much fainter after the first one.

Just checked that Dubhe is listed as BU 1077 so it was discovered by S. Burnham. I wonder if he did it with his own Clark 6 inch refractor or had to use the big refractors at Lick or Yerkes.

[John]

On 16/07/2021 at 02:34, John said:

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 ?

Good seeing at the moment with the 130mm refractor again (1st time for a while actually ). Getting something similar again on Dubhe. 400x is holding up quite well.

The "dome" of the secondary star on the side of the primary seems a distinctive mid-grey tone compared with the primaries pale golden (to my eye) tint.

In terms of resolution I guess it's somewhere between "elongated" and "resolved" on the illustration below. "Partially Notched" perhaps ?

Not a "split" though.

 

Edited August 12, 2021 by John

[Nik271]

Very nice, John! I think a clean split will require more aperture, perhaps 6inch refractor or 7-8 inches for an obstructed scope because of the brighter diffraction rings.

Dubhe gest behind a tree for me at this time of the year. I mean to look at it again in some autumn early morning, when it's well positioned. 

I was out last night but looking in the opposite direction to Ophiuci. Managed to split Tau Oph at 180x with the 180 Skymax. Separation is 1.4'' and the two components have almost equal brightness, so not really a tough double in good conditions but the seeing was so-so and made it challenging. I gave up on doubles for the rest of the evening and looked at some globulars instead. M13 was gorgeous, I could resolve stars down to the central core at x180. 

[John]

1 hour ago, Nik271 said:

Very nice, John! I think a clean split will require more aperture, perhaps 6inch refractor or 7-8 inches for an obstructed scope because of the brighter diffraction rings.

Dubhe gest behind a tree for me at this time of the year. I mean to look at it again in some autumn early morning, when it's well positioned. 

I was out last night but looking in the opposite direction to Ophiuci. Managed to split Tau Oph at 180x with the 180 Skymax. Separation is 1.4'' and the two components have almost equal brightness, so not really a tough double in good conditions but the seeing was so-so and made it challenging. I gave up on doubles for the rest of the evening and looked at some globulars instead. M13 was gorgeous, I could resolve stars down to the central core at x180. 

My best split last night was Zeta Hercules. 1.47 arc seconds but quite uneven brightness. I've been following it over the years and have seen the PA change noticably. It used to be an "on the edge" challenge for my ED120 but practice has made it a little easier now and the 130mm triplet gives an wonderful view of the pair.

The seeing lower down was quite mediocre though.

I'll have a look at Tau Oph next time out.