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What do I need to split binaries?

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#1 Dante 34:139

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Posted 27 September 2020 - 10:41 AM

Aperture? A specific optical scheme? A fancy lens treatment? A deep wallet? All the previous plus a deeper wallet? And by splitting I mean proper split, as in "separation", with at least a black line in between. 

Posting in Refractors instead of Beginners because I'm talking refractors. Buzz off all you 16'' Dobsonians or Edge14 owners laugh.gif


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#2 Augustus

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Posted 27 September 2020 - 10:44 AM

Good high power eyepieces, good optics and good seeing.
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#3 gnowellsct

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Posted 27 September 2020 - 10:54 AM

Mainly you need to pick double stars within the capacity of your telescope.  There are hundreds of double stars in Skiff and Tirion's Bright Star Atlas that are pretty easy splits for small apertures.  Many of them are quite beautiful too.  

 

The Bright Star Atlas also lists several hundred deep sky objects that are within reach of three and four inch apertures.

 

For $10.00 including shipping it's an amazing bargain.  

 

Greg N


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#4 db2005

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Posted 27 September 2020 - 11:05 AM

3-4 inches quality refractor aperture will keep you happy for a long, long time. Slow (quality) achromats also work very well in this game. Some observers (like myself) enjoy challenges like "how little aperture do I need to separate [X] double star," deliberately masking down the aperture to maybe 50 or 60 mm. A very steady mount is important too, as steady views improve the visual acuity of the eyes. I have found that binoviewers also sometimes help.


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#5 gnowellsct

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Posted 27 September 2020 - 11:07 AM

You need to be aware that the smaller the telescope's aperture the lower the magnification at which it will turn a star into an Airy disk.   An Airy disk is an optical artifact brought to us courtesy of the laws of physics.  You start to see Airy disks at powers roughly > 100x though I imagine less than that in an 80 or 70mm.

 

Some refractor owners are absolutely in love with Airy disks they take it as a sign of perfection when they jack up the power really high on a double and see two glowing miniature poker chips.    I'd never seen one till I got my first 4" refractor.  When I polled Yahoo Bigdob group about Airy disks > 10 years ago no one answered to say they had seen them.   I've never understood the infatuation with Airy disks from users who are in love with "pinpoint stars."

 

So if you want high power in small aperture you better learn to love the Airy disk it is a fact of life.

 

On the other hand if the separations are big enough you won't need the high magnifications that turn stars into glowing poker chips.  The separations don't have to be huge to be doable with magnifications under 100x.  But the split may be less satisfying.   I was looking at E Lyrae in my 130mm at 80x and was getting a clean split but I'm not sure it would have satisfied a beginner.  When I am showing newcomers E Lyrae I might opt for 200x to make it plain and obvious....not in a refractor, so no Airy.  Then show them how it looks in a 92mm refractor (since I almost always have two scopes operating together).  

 

Greg N


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#6 Hesiod

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Posted 27 September 2020 - 11:10 AM

I'd say that a decent knowledge is probably the most important thing, since there are double stars fit for every aperture and wallet.

Of course it is just a personal preference, but having a telescope with good optics makes the observation the more pleasant; also such telescope will allow you "cleaner" splits of those systems really close to the telescope's limit.


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#7 Cotts

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Posted 27 September 2020 - 11:11 AM

Any telescope will do.   

 

The trick will be to select targets that suit the instrument you are using.  (You do not state the aperture of your telescope....)

 

Let's pretend you have a 4-inch / 100mm refractor.

 

The Rayleigh Limit for a true 'dark sky between' split of  two equal stars is 5.45/D with D being in inches. So, 5.45/4 = 1.36".

 

The 4-inch scope has the ability to split down to 1.36" - round that to 1.4".  So, theoretically, any pair that wide or wider could be split.  There are tens of thousands of pairs awaiting your scope....Huge caveat - the splits gets more difficult if the stars are of widely differing magnitudes...and if the seeing is really awful......

 

For a 4-inch scope you'll want an eyepiece that will give between 150x and 200x (up to 50x for each inch of aperture....) in order to examine the very tight pairs.  Less magnification for wider pairs, of course.  Part of the fun is to try splitting a pair with different eyepieces, finding the least magnification to do the job.. 

 

And be sure to hang out on cloudy Night's own double Star forum!!!!  Lots of lists and advice there!!!

 

Dave


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#8 sg6

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Posted 27 September 2020 - 11:15 AM

Not possible to answer, there will always be a double that is too narrow to split, and splitting a double is not having a black line between - it is having something like 2 circles just merging.

 

If you had a scope where a 2 arcsecond separation was counted as "a split" in standard terms you are limiting yourself to doubles that are likely a minimum of 4 arcsec separation because of the black line requirement.

 

For what you seem to want I would not start with less then a good 100mm ED, then some good eyepieces and you will need a fair selection. No use having a step of 8mm to 12mm you will likely need the "planetary" approach of 1mm increments.

 

But it depends on the separation of the doubles. Mizar and Alcor can be split by a pair of good eyes and was a test of eyesight in (I think) the Persian army.

 

So too many variables, and requirements, for one answer.

If "wider" doubles then maybe an 80mm ED, but still a bag full of good eyepieces.


Edited by sg6, 27 September 2020 - 11:16 AM.

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#9 Dante 34:139

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Posted 27 September 2020 - 11:39 AM

Any telescope will do.   
 
The trick will be to select targets that suit the instrument you are using.  (You do not state the aperture of your telescope....)
 
Let's pretend you have a 4-inch / 100mm refractor.


That's because I don't have one. Yet. I'm in the market for my first and I thought I needed something really fancy, like those oh-so-tempting 152mm cannons doublets (APM/Lunt). But those would require a CEM70 while I'm pretty fond of the CEM40, thus limiting my choice to instruments well below the 10kg threshold. That's a convoluted way to say 5 inches doublets or 4 inches triplets. I'm glad to hear that in either case I'll be able to split. So...
 

The Rayleigh Limit for a true 'dark sky between' split of  two equal stars is 5.45/D with D being in inches. So, 5.45/4 = 1.36".
 
The 4-inch scope has the ability to split down to 1.36" - round that to 1.4".  So, theoretically, any pair that wide or wider could be split.  There are tens of thousands of pairs awaiting your scope....Huge caveat - the splits gets more difficult if the stars are of widely differing magnitudes...and if the seeing is really awful......
 
For a 4-inch scope you'll want an eyepiece that will give between 150x and 200x (up to 50x for each inch of aperture....) in order to examine the very tight pairs.  Less magnification for wider pairs, of course.  Part of the fun is to try splitting a pair with different eyepieces, finding the least magnification to do the job..


... I guess now the question is more about the eyepieces and the magnification.
 

And be sure to hang out on cloudy Night's own double Star forum!!!!  Lots of lists and advice there!!!
 
Dave

Holey moley I never noticed that one! waytogo.gif



#10 gnowellsct

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Posted 27 September 2020 - 12:22 PM

The other thought is that double stars are infinite.  That is to say that something like 60% of all stars are double stars and so there is no fixed list of do-able stars.  The list of doable stars scales up with aperture and seeing.  The Dawes on a 60mm is about 2 arc seconds which is not going to be favorable to splitting stars unless they have generous spacing.  See Bright Star Atlas.  



#11 Codbear

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Posted 27 September 2020 - 01:20 PM

Any telescope will do.   

 

The trick will be to select targets that suit the instrument you are using.  (You do not state the aperture of your telescope....)

 

Let's pretend you have a 4-inch / 100mm refractor.

 

The Rayleigh Limit for a true 'dark sky between' split of  two equal stars is 5.45/D with D being in inches. So, 5.45/4 = 1.36".

 

The 4-inch scope has the ability to split down to 1.36" - round that to 1.4".  So, theoretically, any pair that wide or wider could be split.  There are tens of thousands of pairs awaiting your scope....Huge caveat - the splits gets more difficult if the stars are of widely differing magnitudes...and if the seeing is really awful......

 

For a 4-inch scope you'll want an eyepiece that will give between 150x and 200x (up to 50x for each inch of aperture....) in order to examine the very tight pairs.  Less magnification for wider pairs, of course.  Part of the fun is to try splitting a pair with different eyepieces, finding the least magnification to do the job.. 

 

And be sure to hang out on cloudy Night's own double Star forum!!!!  Lots of lists and advice there!!!

 

Dave

 

 

 

 

Holey moley I never noticed that one! waytogo.gif

 

Most people don't...they just read a few posts and then "split"!lol.gif

 

In a local forum I belong to one of its members posts the most prolific amount of double star splitting than any member I've seen in that forum or on CN, with some targets being very tight doubles. His instrument? a 10" F5.7 dob.

 

His secret is that he has good optics that are collimated down to the gnats...well let's just say he has that thing collimated incredibly well. If the seeing isn't up to snuff on a given night he can just mask the dob down to 4-5". 

 

As Cotts said, "Any telescope will do."


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#12 gezak22

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Posted 27 September 2020 - 02:40 PM

As counter intuitive as it is, a 60 mm refractor can be a joy for double stars, while larger apertures can easily struggle.

 

Here's some mandatory reading.


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#13 Jeff B

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Posted 27 September 2020 - 02:58 PM

Aperture? A specific optical scheme? A fancy lens treatment? A deep wallet? All the previous plus a deeper wallet? And by splitting I mean proper split, as in "separation", with at least a black line in between. 

Posting in Refractors instead of Beginners because I'm talking refractors. Buzz off all you 16'' Dobsonians or Edge14 owners laugh.gif

Actually a nice, used SW 120ED (F7.5 FPL-53 based doublet) does just fine:

 

1. "Cheap" used at ~$900 to $1200 depending on vintage, condition and included extras.

 

2. Typically very good optics.  Mine is excellent and I have the star and bench test data to prove it.

 

3. Lightweight, can be used quite well with Vixen Super Polaris class mount or similar Alt/azimuth mount.

 

4. "Cheap"

 

5. Sub-arch second resolution, putting all kinds of doubles within reach.

 

6. Larger aperture for good brightness

 

7. Well color corrected for good star color contrasts

 

8.  Plenty of samples on used markets

 

9. Plenty of aftermarket focusers in case you want to upgrade that.

 

10. Works very well with bino-viewers

 

11. "Cheap"

 

Just to name a few advantages...

 

Jeff

 

7. "Cheap

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#14 aeajr

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Posted 27 September 2020 - 02:59 PM

Some double stars can be split with binoculars.  Mizar and Alcor for example.

 

Some can be split at low power like Albiero which splits nicely at about 25X

 

Others have to be taken up to very very high power and can only be split under the best of conditions.

 

Aperture helps in that the larger the aperture the greater the resolving power. 

 

I have split doubles with all of my scopes.   As Cotts said, it is just a matter of selecting targets that are suitable to your equipment and the "seeing" conditions that night.   This is something you will come to learn over time.

 

A 4" refractor or a 5" Mak make good double splitters, but I have usually used an 8" or 12" Newtonian/Dob. 


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#15 Dante 34:139

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Posted 27 September 2020 - 03:07 PM

As counter intuitive as it is, a 60 mm refractor can be a joy for double stars, while larger apertures can easily struggle.
 
Here's some mandatory reading.

That was a really interesting and surprising read. So, given seeing conditions are boss, the quality of the optics are way more important than the aperture in this splitting business. I'd better add a Tak 60 to my shortlist.


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#16 ris242

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Posted 27 September 2020 - 04:13 PM

I need 'seeing' around the 1-1.2 arc second.........which I haven't had for a while........without wind or clouds.



#17 John Fitzgerald

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Posted 27 September 2020 - 04:51 PM

For double stars, the telescope needs to be solidly mounted.  It won't hurt to go up a size larger than the recommended mount for the scope you choose.  An Orion Atlas (EQ6) for a 4 inch refractor, for example.



#18 gnowellsct

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Posted 27 September 2020 - 04:57 PM

Dante 34:139

 

This?

 

We mounted up, he first and I the second,
  Till I beheld through a round aperture
  Some of the beauteous things that Heaven doth bear;
Thence we came forth to rebehold the stars.

 

salimmo sù, el primo e io secondo,
tanto ch'i' vidi de le cose belle
che porta 'l ciel, per un pertugio tondo.
E quindi uscimmo a riveder le stelle.


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#19 gezak22

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Posted 27 September 2020 - 04:58 PM

Well OK.  Daniel M. has a big fan base here.  Personally I have found that I look closely at what he recommends and then do the opposite, I'm usually better for it.

 

Greg N

I'm not trying to start a flame war, but how do you know you are "better for it" if you never tried to follow his advice?

 

I used to have an 11" Edge and even after many hours of cool-down, it never matched the results of the 140 mm refractor on binaries. It's just a casual data point (I don't observe that many binaries), but when combined with the thread I lined above, I think Daniel M. (and all the other binary observers who use small refractors, before Daniel M shared his opinions) is onto something.



#20 BlueMoon

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Posted 27 September 2020 - 05:01 PM

Another text that I like for understanding the "why's and how's" to split multiple stars is James Mullaney's "Double and Multiple Stars and How to Observe Them". I really enjoyed reading this not only for understanding the telescope physics involved, but the physics of the stars themselves and how multiple star systems can evolve.

 

You can find a copy at Springer: https://link.springe...7/1-84628-180-6

 

Clear skies.


Edited by BlueMoon, 27 September 2020 - 05:38 PM.


#21 gnowellsct

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Posted 27 September 2020 - 05:02 PM

I'm not trying to start a flame war, but how do you know you are "better for it" if you never tried to follow his advice?

 

I used to have an 11" Edge and even after many hours of cool-down, it never matched the results of the 140 mm refractor on binaries. It's just a casual data point (I don't observe that many binaries), but when combined with the thread I lined above, I think Daniel M. (and all the other binary observers who use small refractors, before Daniel M shared his opinions) is onto something.

Never mind.   I will delete the post.   I do have apos, so I guess that is "following his advice."  I also have some fantastic SCTs, which is very much not his advice. 


Edited by gnowellsct, 27 September 2020 - 05:04 PM.


#22 Nippon

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Posted 27 September 2020 - 05:26 PM

You need to be aware that the smaller the telescope's aperture the lower the magnification at which it will turn a star into an Airy disk.   An Airy disk is an optical artifact brought to us courtesy of the laws of physics.  You start to see Airy disks at powers roughly > 100x though I imagine less than that in an 80 or 70mm.

 

Some refractor owners are absolutely in love with Airy disks they take it as a sign of perfection when they jack up the power really high on a double and see two glowing miniature poker chips.    I'd never seen one till I got my first 4" refractor.  When I polled Yahoo Bigdob group about Airy disks > 10 years ago no one answered to say they had seen them.   I've never understood the infatuation with Airy disks from users who are in love with "pinpoint stars."

 

So if you want high power in small aperture you better learn to love the Airy disk it is a fact of life.

 

On the other hand if the separations are big enough you won't need the high magnifications that turn stars into glowing poker chips.  The separations don't have to be huge to be doable with magnifications under 100x.  But the split may be less satisfying.   I was looking at E Lyrae in my 130mm at 80x and was getting a clean split but I'm not sure it would have satisfied a beginner.  When I am showing newcomers E Lyrae I might opt for 200x to make it plain and obvious....not in a refractor, so no Airy.  Then show them how it looks in a 92mm refractor (since I almost always have two scopes operating together).  

 

Greg N

Very true. My 4" refractor shows obvious Airy disks at about x160 or so but my Mewlon 180 they appear obvious at x270. Both scopes do well on binaries but the refractor on any typical night gives the more pleasing view. 


Edited by Nippon, 27 September 2020 - 05:33 PM.


#23 gnowellsct

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Posted 27 September 2020 - 06:45 PM

this is how I test aperture differences in the field--one of the ways anyhow.  

 

GT 130 and 81s Landis May 8 2018jpg.jpg

 

 



#24 gnowellsct

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Posted 27 September 2020 - 06:52 PM

Very true. My 4" refractor shows obvious Airy disks at about x160 or so but my Mewlon 180 they appear obvious at x270. Both scopes do well on binaries but the refractor on any typical night gives the more pleasing view. 

When I had a 4" apertures (had two) the airy disks became evident to my eye at about 120-130 at 160x I was sort of into the shrugging to myself: why do people do this.  But the Tiron and Skiff doubles in 4" or 3.6" are really fun and quite beautiful.  No argument.  I prefer my doubles to split BEFORE they become Airy disks however.   

 

I don't have anything to say against refractors.  I own four good ones and have owned others.  They represent perhaps 50% of every nickel of outlay I have made in astronomy.   I encourage people to buy refractors.  I don't think they are a cure for bad seeing or a miracle drug whereby the smaller aperture beats the large one.  I wouldn't buy one as a "double star scope" or a "planetary scope."

 

Greg N



#25 Spikey131

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Posted 27 September 2020 - 07:08 PM

This is fun.  I love doubles, and often seek them out with a C8 mounted in tandem with an NP101.  I use the double star files for Sky Safari gleaned from Burnham by CN’s own Rustler46 to find them, constellation by constellation.  See here:  https://www.cloudyni...constellations/

 

Anyway, the fun part is how the different scopes show doubles.  Some pairs of widely different magnitudes can best be seen in the refractor.  And a lot of doubles appear more attractive in the refractor.  But for really close and really dim pairs, the C8 rules.  Sometimes I am struggling to split a close pair in the refractor, then move my eye to the C8 and say to myself, out loud, “there it is” because it is just obvious.  Then, sometimes I can go back to the 4” and make it out, once I have seen it in the SCT.

 

 

 

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