Tegmine - Zeta Cancri: Resolved but not Split with a 4 inch

[John]

You will often see Tegmine / Zeta Cancri named as a target in observing reports around this time of the year.

It is a fine triple star and also presents an interesting challenge for the 4-5 inch aperture scope. 

The close pair currently has a separation (source: Stelle Doppie database) of 1.098 arc seconds. The 3rd member of the group is separated from this pair by around 6 arc seconds, so relatively easy to split from the close pair. The close pair are of magnitude 5.3 and magnitude 6.25 respectively.

The challenge for the 4 inch scope is that the Dawes resolution limit for that aperture is 1.14 arc seconds. The, perhaps more realistic, Rayleigh limit for a 4 inch scope is 1.36 arc seconds. So theoretically a 4 inch scope cannot show the close Tegmine pair as clearly split, whichever limit you choose to apply. These figures apply to stars of equal brightness I believe.

What a good 4 inch scope can do however is to show this close pair as resolved. I am using the definition illustrated below by David Knisely:

Tonight my 102mm F/6.5 refractor clearly showed the tight Tegmine pair as a resolved or notched pair of apparently touching airy disks. I take this to mean that the scope is performing to the Dawes limit under the fairly steady seeing conditions.

When I observed Tegmine a few nights back with my 120mm F/7.5 refractor I could see that elusive sliver of black between the airy disks - the pair was split and the additional aperture was doing it's job.

I have seen reports of the close Tegmine pair being split with a 4 inch aperture scope. I guess a sharper eyed observer than me, observing under excellent conditions might be able to see the pair as split. Or maybe the terms resolved pair and split pair were being used differently 🤔

Anyway, I've never been able to quite see a clear split with this pair with either my 3.9 or 4 inch refractors but I was quite happy with my resolved / notched pairing with the 4 inch refractor this evening. The scope is doing it's very best, especially when the slightly uneven brightness of the close Tegmine pair is taken into account🙂

Incidentally, I found 221x magnification the optimum for this target tonight. I tried more but the definition of the close pair didn't get any cleaner. Lower magnifications made seeing the "notch" effect harder.

Another interesting factoid is that the name Tegmine is derived from the latin Tegmens, which means Crab's Shell, which is apt of course 🙂

 

Edited March 6 by John

[The60mmKid]

That's very cool! Can I ask what you used to take the picture? I like it very much.

[John]

Just now, The60mmKid said:

That's very cool! Can I ask what you used to take the picture? I like it very much.

Thanks but the image is not mine (I don't image). I'll see if I can find who did though and let you know 🙂

I would imagine that imaging close double stars is a fairly specialised business and pretty tricky !

[The60mmKid]

Just now, John said:

Thanks but the image is not mine (I don't image). I'll see if I can find who did though and let you know 🙂

I would imagine that imaging close double stars is a fairly specialised business and pretty tricky !

I see. No worries: I didn't mean to give you work to do 🙂 I like it because it's what one would see at the eyepiece. I've often wished I had a way to capture the view of doubles. I'm also not an imager, and I haven't been satisfied with my attempts at sketching.

[Stu]

Great info John, thanks.

Coincidentally I was looking at Tegmine through my FC-100DC this evening. The seeing wasn’t brilliant but I could clearly see the tight pair resolved. At times I thought I could see a very fine black line giving a split but your info is now making me doubt this. I will give it another go with hopefully better seeing.

The aperture benefits of the FS-128 are clear though, because this showed a clear split with three lovely airy disks, a really nice view.

[DirkSteele]

Nicely done!  I have often been a little loose and easy with my language over the years with splits / resolved etc.  I have tried in more recent years to use the convention you shared and be consistent in my reports.

Will have to try this with my 100mm and 105mm scopes.

[John]

31 minutes ago, Stu said:

Great info John, thanks.

Coincidentally I was looking at Tegmine through my FC-100DC this evening. The seeing wasn’t brilliant but I could clearly see the tight pair resolved. At times I thought I could see a very fine black line giving a split but your info is now making me doubt this. I will give it another go with hopefully better seeing.

The aperture benefits of the FS-128 are clear though, because this showed a clear split with three lovely airy disks, a really nice view.

Stu - I saw those fine lines with my 102mm from time to time as well. At least I thought I did, or might have. I wasn't sure if it was my eye playing tricks though - maybe joining up the two notches ? More observations needed ! 🙂

 

 

Edited March 6 by John

[Stu]

5 minutes ago, John said:

Stu - I saw those fine lines with my 102mm from time to time as well. At least I thought I did, or might have. I wasn't sure if it was my eye playing tricks though - maybe joining up the two notches ? More observations needed ! 🙂

🤣🤣 that sounds exactly like my experience 🤪. As you say, more observations needed, better seeing would certainly help.

[John]

1 hour ago, The60mmKid said:

I see. No worries: I didn't mean to give you work to do 🙂 I like it because it's what one would see at the eyepiece. I've often wished I had a way to capture the view of doubles. I'm also not an imager, and I haven't been satisfied with my attempts at sketching.

No problem.

The imager is an Italian astronomer called Vincenzo della Vecchia. The equipment used was a CFF 30 cm Cassegrain and the QHY5III-200M (I assume the last is the camera ?). 

The image appeared in this CN thread and there is some discussion in it on the settings etc used which might make more sense to you than they do to me !

Zeta Cancri split - Double Star Observing - Cloudy Nights

[Nik271]

I made several quick simulations with Aberrator:

Tegmine AB with 100mm unobstructed aperture:

 

And with 128mm aperture:

 

I also simulated what will Tegmine look if it was exactly on the Raleigh limit (1.36'' for 100mm aperture):

 

 

The theory shows that pairs of almost equal stars will be resolved at the Dawes limit but in order to be split they need to be at the Raleigh limit (or further).

 

 

 

 

Edited March 7 by Nik271

[Stu]

Thanks for those @Nik271. I often find that the reality is a little better than these simulations for some reason. It maybe that as the brightness falls away from the centre of the airy disk, it shows in the simulation but is not picked up by the eye? Anyway, in the 128 it was definitely clearer to my eye than this, with two separate disks and a black line between them.

[Saganite]

14 minutes ago, Stu said:

 Anyway, in the 128 it was definitely clearer to my eye than this, with two separate disks and a black line between them.

Ever the pessimist , on Monday evening  with my 152, I saw a thick black line with a disc either side.....

Edited March 7 by Saganite

[RobertI]

Interesting post @John, and a very useful diagram for clarifying the terminology. I looked back through my observing log and my notes say with my 102ED at 320x the pair was resolved (peanut shaped) and occasionally a clear split in moments of clarity. Hmmm. On an previous occasion with my 150PL at 240x it was clearly split although it came and went with the seeing.

In my limited experience, the gap is usually very distinct, it’s either there or it isn’t, but I do wonder whether with these dimmer pairs at the limit of resolution, whether there might be an optical illusion at play - looking at Nick’s diagrams you might be led to that conclusion. 
 

 

[josefk]

Thats a really useful post for me @John. I'm not sure i've paid enough attention to the intermediate states between "unresolved" and "split". I certainly don't record a description of the precise "not split" state - only "not split". I must try harder! 🙂 

[Stu]

43 minutes ago, josefk said:

Thats a really useful post for me @John. I'm not sure i've paid enough attention to the intermediate states between "unresolved" and "split". I certainly don't record a description of the precise "not split" state - only "not split". I must try harder! 🙂 

Likewise! If I can’t see black between them then I just sulk and class it as not split 😉. Resolved is actually a really useful classification and makes a lot of sense.

[Mark at Beaufort]

I decided to check when I saw Tegmine and split it into 3 stars. Well it was April 2020. I was using my 12" Dob which I no longer have.

First chance that I have I will use my StellaMira 125mm ED Doublet and see what I can see and whether I can split the close double.

[Mr Spock]

I remember having an easy split last year, but haven't written it down. It was probably with the 12". I'll give the 4" a go this year and see what happens.

[Mr Spock]

That's really odd. I've just looked through my observing book and there's nothing. I remember having a long session in CMi and the area around, and remember commenting about the large number of easy 10-11mag doubles in the bottom of CMi. I've got one session in Hydra, then the next is Leo/Gemini, with nothing in-between.

Strange...

[Zermelo]

(i) There is inevitably some subjectivity in deciding whether or not there is a "black" dividing line. As already mentioned, as you move away from the centre of the Airy disc, the brightness tails off over a finite distance, it isn't immediate. So, for pairs that are on the cusp of being split, it will be a judgement as to whether or not the point half way between the stars is at the same level of blackness as the rest of the field. Obviously some observers will be more optimistic than others in making this call. I remember a previous thread where someone posted a graphic showing the light curve dimming and brightening between the stars; possibly @vlaiv  (but then I always think it was him).

(ii) on the seeing: yes, I'm sure this can affect decisions too. When operating at high magnifications in less than perfect conditions, the stars can be jumping about quite a bit. In a very brief moment of stillness, I think the eye can be tricked into seeing a gap where there was none. I've also experienced observations where a star has appeared briefly to be - at the very least - resolved, but I suspect that it was another artefact of the seeing - the image of the star has been "flicked" very quickly to one side, and back again, and the persistence of vision suggests two adjacent discs.
I expect the actual effects of imperfect seeing depend on the size and motion of the cells in the air column, and probably also the size of the instrument objective.

[Stu]

Some excellent images here from Mr Peach, as you would expect:

https://www.damianpeach.com/binaries.htm

[runway77]

Thanks John for your tip to try to resolve the Tegmine. I did try it yesterday with my 6 inch f/5 Newtonian and it was definitely a challenge to resolve the close pair. It could be "resolved" similarly like on the examples the Nik271 posted but I was unable to see a clear separation between these two. I used a T6 5mm Nagler eyepiece. A very interesting triple though. 

Edited March 8 by runway77

[josefk]

17 hours ago, Stu said:

Some excellent images here from Mr Peach, as you would expect:

https://www.damianpeach.com/binaries.htm

Off topic but that’s a good representation of Sirius A/B on there (the larger pic without diffraction spikes). I’m encouraged that I was closer than I thought with my little scope the other night. A little semi steady knot of brightness against other scintillating brightness. I still can’t “tick it” but still very encouraging. Thanks. 

[Mr Spock]

I notice even Mr peach can't spell Albireo 

[Saganite]

7 minutes ago, Mr Spock said:

I notice even Mr peach can't spell Albireo 

And you can't spell his name correctly...Mr Peach....

[Stu]

18 minutes ago, Mr Spock said:

I notice even Mr peach can't spell Albireo 

Seems ok to me?