When is a split not a split?

[Zermelo]

When I've been double splitting, and pushing the bounds of my equipment and the observing conditions, I sometimes see the following kind of thing:

(not an actual image - I recreated this with Aberrator)

The target would be a tight (<2") double with a significant difference in magnitudes. I'm usually using my mak 127. If the seeing is good, but not excellent, then for the brighter star I get an Airy disc and one or more diff rings, but the inner ring isn't continuous - it usually appears as three arcs as shown.

I look for the fainter companion at the given PA, and it's not there. But what I do sometimes see is a brightening of the ring arc in the correct direction (6 o'clock in the fabricated image above). I conclude that the companion is "hiding" in the diff ring. Whether or not I count the star as split depends on the obviousness of the companion - it varies with the circumstances. I'm interested to hear what others think in these cases.

Also - do the relative distances of companion and diff ring change with magnification, so that you can move the companion away from the ring? I don't seem to get any improvement by changing eyepiece f/l.

 

[cajen2]

I don't consider it as a split unless I can see a bit of darkness between the stars. Thus I can't split some doubles which other people say they've 'split'.

I've always found higher mags make for easier splits IF the seeing is good enough and, as you say, the secondary isn't hidden by diffraction spikes or ring.

Edited September 8, 2022 by cajen2

[Zermelo]

10 minutes ago, cajen2 said:

I don't consider it as a split unless I can see a bit of darkness between the stars.

Yes, I'm with you on that.

In the cases I'm talking about here, there is a clear gap (the gap between the Airy disc and the first diff ring) but the question is whether the fainter companion is sufficiently distinct within the diff ring.

[cajen2]

Too subjective to be definitive...🤔

[Stu]

@chiltonstar may be able to add some useful info to this.

To me a lot depends on the stability of the view and if I can convince myself that the brightening on the diffraction ring is consistently in the same place or not. Sometimes I can get Zeta Herc in a 4” apo often looks like this although more distinct than your image. Izar in a tiny scope like the TAL Alkor or Telementor also shows the secondary on the first diff ring but in that case it is blindingly obvious, and very beautiful, like a gem on a ring.

So to me it just depends on the consistency and stability of the brightening on the ring as to whether I take it as a an observation of the secondary or not.

[Zermelo]

44 minutes ago, cajen2 said:

Too subjective to be definitive...🤔

Yes, definitely subjective. As you say, the "resolved vs. split" distinction does at least depend on the appearance of some black between the two. But I'm just interested in any opinions.

 

43 minutes ago, Stu said:

more distinct than your image

Yes, I probably tweaked my simulation a bit far - I was aiming for a borderline case, but as you say, it's also about consistency and stability over time. The observer's individual confidence in a split will vary according to the particular double and their previous experience.

 

 

[Stu]

37 minutes ago, Zermelo said:

The observer's individual confidence in a split will vary according to the particular double and their previous experience.

Past experience and building up a picture of what to expect really is important. I’m sure Zeta Herc had probably been visible to me a good few times before I was actually able to pin it down in a 4”, so I had to see it fully resolved in the Mewlon 210 before seeing it on the Vixen FL102S.

[cloudsweeper]

I think the three "arcs" indicate that seeing is having a big effect.  Key to this is more aperture - you can get greater mag before seeing ruins everything, and mag can take the sec away from the diff ring.

Doug.

[chiltonstar]

16 hours ago, Stu said:

@chiltonstar may be able to add some useful info to this.

 

If I can see the secondary clearly I feel I've split it - sometimes increasing the mag does make the secondary clearer, either by moving it away from the diff ring (if it is close to it, not actually embedded in it), or by making the diff ring fainter and the Airy disk of the secondary more apparent. I often use a zoom EP to play with the mag until I have the best view.

The attached stacked image is Zeta Her with a 127 Mak - I would say it is "split".

Chris

 

[Stu]

25 minutes ago, chiltonstar said:

If I can see the secondary clearly I feel I've split it - sometimes increasing the mag does make the secondary clearer, either by moving it away from the diff ring (if it is close to it, not actually embedded in it), or by making the diff ring fainter and the Airy disk of the secondary more apparent. I often use a zoom EP to play with the mag until I have the best view.

The attached stacked image is Zeta Her with a 127 Mak - I would say it is "split".

Chris

 

Thanks Chris. I guess the question is, how do you classify the ones where the secondary is plonked on the first diffraction ring and is not resolved as an airy disk, but as a brightening on the ring. Is that a split for you?

[chiltonstar]

A definite "maybe".

Chris

[RobertI]

I’m not an expert on doubles by any means, but have observed a few tricky doubles of this type, with the secondary on the diffraction ring. In general I have found that really high mags will eventually reveal the secondary clearly in the diffraction ring if the conditions are right, but these higher mags are trickier to reach in my small scopes, or become too dim. I guess my results have fallen into one of the following main categories, which are  still pretty subjective and probably not very useful to anyone! 😆

1. Not seen
2. Suspected brightening - glimpses of slight brightening of the diffraction ring in the correct position position angle 
3. Definite brightening  - obvious brightening at the correct position angle
4. Resolved - the secondary clearly stands out as a star in the diffraction ring 

To address the original question, I guess in this situation where the secondary is on the diffraction ring, and there is a clear gap, the term “split” isn’t really useful anymore and the above descriptions might be more useful? 

Edited September 9, 2022 by RobertI

[Sunshine]

On 08/09/2022 at 10:57, Stu said:

Past experience and building up a picture of what to expect really is important. I’m sure Zeta Herc had probably been visible to me a good few times before I was actually able to pin it down in a 4”, so I had to see it fully resolved in the Mewlon 210 before seeing it on the Vixen FL102S.

Very good point regarding experience on one particular double, Zeta Her looked much like the simulated image Zermelo shows in the OP on many many occasions where I have tried to split it. If I was satisfied with that as a split then you would be hearing from me weekly about splitting Zeta Her, but I myself am not completely satisfied with this and prefer to be patient and wait for that moment when it smacks me in the face with an unmistakable split, and when it does it really is special. This is only possible once maybe twice a season if I’m lucky.

[NGC 1502]

As Robertl has mentioned, PA (position angle) is one way to confirm seeing the secondary star.

Lots of sources to find PA, I most often consult my Cambridge Double Star Atlas, but not until AFTER a visual attempt or “confirmation bias” may creep in…….

Zeta Herc- my OOUK 10” Dob will often give a definite visual split, as will my Hinds 6” Dob if the seeing is not too bad. By a definite split I mean two independent dots of light.

Doubles will usually take very high power much better than other types of objects. So I’m not afraid to use “silly high power”.

Ed.

[Nik271]

In the case of close unequal doubles like Zeta Herc under suboptimal seeing I usually would say that the double has been 'detected' rather than split.  I'm not very consistent though. It's good to be as precise as possible but of course the view in the eyepiece varies so much with seeing, magnification, optics that a fixed word will not fit everything. I think it does not matter so much as long as we add a more detailed description what we have observed.

Stelle Doppie is a good source of up do date astrometry data, I believe it gets it directly from the Washington Double Star catalogue and presents it in a user friendly way. 

[Stu]

John used to regularly post this sketch of Zeta Herc, drawn when observing with a 120mm frac. It shows the secondary as an elongated brightening of the diffraction ring rather than a separate airy disk. Personally I would certainly count this as a definite observation of the secondary. I guess it is when it is a lot less clear that thing that doubt creeps in.

I’m assuming an increase in aperture would shift the position of the secondary relative to the secondary, or would it always remain in the same place?

[lunator]

Over the years I've observed doubles I have found that you can generally apply 3 descriptions .

Round/not resolved

Resolved - clearly 2 stars but no black space between them

Split - A clear black space between them. 

Obviously seeing can make the black space appear and disappear 😊.

If the pair you are looking at is below the Raleigh Criteria which is 138/D at peak response for the human eye then you will not see any back space, but it is still possible to resolve the pair.

Cheers

Ian

[Stu]

21 minutes ago, lunator said:

Over the years I've observed doubles I have found that you can generally apply 3 descriptions .

Round/not resolved

Resolved - clearly 2 stars but no black space between them

Split - A clear black space between them. 

Obviously seeing can make the black space appear and disappear 😊.

If the pair you are looking at is below the Raleigh Criteria which is 138/D at peak response for the human eye then you will not see any back space, but it is still possible to resolve the pair.

Cheers

Ian

In these cases Ian, there is clear separation because the secondary is lying on the first diffraction ring, but is a brightening of the ring, rather than being fully resolved into a separate airy disk. That doesn’t seem to be described in your three examples, how do you classify these?

[lunator]

Hi Stu 

I would classify them as split. There will always be some black space between the 2 discs of stars as this scenario is at the Raleigh Criteria so the eye can detect a black space as long as seeing permits. 

Cheers

Ian

[Zermelo]

4 hours ago, Stu said:

John used to regularly post this sketch of Zeta Herc, drawn when observing with a 120mm frac. It shows the secondary as an elongated brightening of the diffraction ring rather than a separate airy disk. Personally I would certainly count this as a definite observation of the secondary. I guess it is when it is a lot less clear that thing that doubt creeps in.

I’m assuming an increase in aperture would shift the position of the secondary relative to the secondary, or would it always remain in the same place?

 

I like his decription: "like a thin snake that has swallowed a tennis ball"