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What minimum degree of binary star separation can I expect from a quality mid size APO

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#1 gustave

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Posted 10 October 2020 - 03:38 PM

What minimum degree of binary star separation can I expect from a quality mid size (115-127MM) APO.  

 

Thanks



#2 Astrojensen

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Posted 10 October 2020 - 04:00 PM

That depends on many factors. Resolution is tied to the aperture, but the brightness difference between the components in a pair is also a major factor, as is the seeing, obviously. Not less important are your observing skills, especially when dealing with unequal pairs.

 

 

Clear skies!

Thomas, Denmark


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#3 Stozhary

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Posted 10 October 2020 - 04:47 PM

Do you observing visual or use camera. Both have "+" and "-". Digitally you can reduce exposure time and this way you have more choices of objects. I think seeing condition is main limit. Try to see zeta Aquarius 4.5-4.6 mag with 3 arcsec separation. Star atlas or planetarium software could give you idea what to look.

Clear skies!

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#4 BillP

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Posted 10 October 2020 - 05:34 PM

What minimum degree of binary star separation can I expect from a quality mid size (115-127MM) APO. 

The theoretical resolution for scopes in that aperture range would be: 1.00-0.91 arcsec.  Personally I find doubles observing more fun with the smaller apertures as they cool down fast so show steady views quicker, and they can achieve the classic airy disk pattern on a star at lower and more comfortable magnifications that look aesthetically nice in less than great seeing.  There are so many multiple stars up there to see that no shortage of challenges, if you want challenges, for any aperture scope, so IMO one is not missing anything to worry about with smaller apertures.


Edited by BillP, 10 October 2020 - 05:41 PM.

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#5 PETER DREW

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Posted 11 October 2020 - 06:57 AM

I think you could "expect" to see 2 arc second separation.  Closer, which is achievable with the apertures mentioned, will depend on other factors, particularly seeing conditions and experience.



#6 BillP

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Posted 11 October 2020 - 10:18 AM

I think you could "expect" to see 2 arc second separation.  Closer, which is achievable with the apertures mentioned, will depend on other factors, particularly seeing conditions and experience.

Agree.  That should be easy under most conditions.  The closer components of Epsilon Lyrae (STF 2382 AB & STF 2383 CD) which in 2018 was around 2.3" separation is always an easy spilt most any evening for me in my 4" scope, which has a resolution of 1.13".


Edited by BillP, 11 October 2020 - 10:20 AM.

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#7 daquad

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Posted 13 October 2020 - 07:02 PM

A better question is:  For a given aperture, what magnification is required to split a double star at the Dawes limit?  To be clear the Dawes limit is the minimum separation in seconds of arc that a telescope can distinguish the separate Airy discs of equal 6th magnitude stars.

 

For an apparent separation of 4 minutes of arc (about right for normal eyesight)  a magnification of 53X/inch is required.  Keep in mind that the Airy discs will appear as overlapping.  That is, there will not be black sky between the components.

 

For a 4" refractor (resolving power = 1.1")  try 4X53 = 212X to split 52 Orions, separation 1.1", both components mag 6.2.

 

Dom Q.


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#8 daveCollins

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Posted 14 October 2020 - 01:38 PM

I agree with comments above that 2" will be doable. With my GTX 130 I can get down to 2.0" from Washington DC. Going below this gets challenging and depends on the magnitudes and magnitude differences. Obviously the seeing conditions are of primary importance when splitting binaries. My seeing is generally so-so. With my 175 f/8 I've gotten below 1" from Washington DC. I am observing from the balcony of a concrete building which has AC/Heating exhausts above and below me. So I wait for great seeing holes which occur on a cyclic type basis of around 60 seconds ... that is, every 60 seconds I can get better seeing for a few seconds.


Edited by daveCollins, 14 October 2020 - 01:44 PM.


#9 Jon Isaacs

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Posted 14 October 2020 - 06:19 PM

A better question is:  For a given aperture, what magnification is required to split a double star at the Dawes limit?  To be clear the Dawes limit is the minimum separation in seconds of arc that a telescope can distinguish the separate Airy discs of equal 6th magnitude stars.

 

For an apparent separation of 4 minutes of arc (about right for normal eyesight)  a magnification of 53X/inch is required.  Keep in mind that the Airy discs will appear as overlapping.  That is, there will not be black sky between the components.

 

For a 4" refractor (resolving power = 1.1")  try 4X53 = 212X to split 52 Orions, separation 1.1", both components mag 6.2.

 

Dom Q.

 

Doubles are about seeing. I usually use around 80/inch for doubles in the Rayleigh Criterion/ Dawes limit range.  Sidgwick in the Amateur Astronomers handbook also suggests 80/inch.  In the doubles forum I've seen recommendations of pushing it over 100x/inch.

 

Give I'd be try.. 

 

I prefer reflectors with some aperture because in good seeing, they'll make a 1" split wide and clean. My backyard is quite often under 1", these past weeks, there's been several nights when goodtostargaze.com spec'd the seeing at 0.6". I've split doubles under 0.5" with my Dobs.

 

It's about seeing, it's about being prepared. Refractors are less trouble but in great seeing they're limited by their aperture.

 

Jon


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#10 daquad

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Posted 14 October 2020 - 06:43 PM

In my New England skies, even under relatively poor seeing (5-6/10 Pickering) I can reach the Dawes limit at 53X/inch on 52 Orionis (as mentioned above) with my 4" Jaegers f/15.5.  The double nature is obvious, even though the image is moving around (more like jiggling) in the field.

 

For unequal doubles, higher power and a relaxing of the resolving limit is in order.

 

Dom Q.



#11 John Fitzgerald

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Posted 14 October 2020 - 06:53 PM

The more aperture you get, the more pairs that can be successfully resolved.


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#12 Ihtegla Sar

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Posted 14 October 2020 - 08:49 PM

I need to find some more closer double stars of a similar magnitude that are within the range of a 4" apo. In reviewing my notes, I haven't really tried anything closer than epsilon Lyra. I've never had trouble getting a clean split of both pairs of epsilon Lyra regardless of seeing, which typically isn't great where I live. According to my notes I got a split of both pairs last year with several eyepieces, including my 11mm ES 82, which is about 82x in my f/9. That was the lowest power that gave me a clean split of both pairs on that night. But I would say that 2" separation with similar magnitudes is easily within reach of a modest apo, and on nights of good seeing Im confident a closet split would not be a problem. Now I just need to find time to test that theory. But any break in the clouds is going to be used on Mars for the next month or two. So it may be a while before I can test the limits if my 4" apos on tight doubles.

#13 Jon Isaacs

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Posted 15 October 2020 - 12:34 PM

I need to find some more closer double stars of a similar magnitude that are within the range of a 4" apo. In reviewing my notes, I haven't really tried anything closer than epsilon Lyra. I've never had trouble getting a clean split of both pairs of epsilon Lyra regardless of seeing, which typically isn't great where I live. According to my notes I got a split of both pairs last year with several eyepieces, including my 11mm ES 82, which is about 82x in my f/9. That was the lowest power that gave me a clean split of both pairs on that night. But I would say that 2" separation with similar magnitudes is easily within reach of a modest apo, and on nights of good seeing Im confident a closet split would not be a problem. Now I just need to find time to test that theory. But any break in the clouds is going to be used on Mars for the next month or two. So it may be a while before I can test the limits if my 4" apos on tight doubles.

 

I use Sky Safari. The Planner function lets me search for doubles based on separation, magnitude and region of the sky. 

 

For example, I can search for all the doubles in Hercules brighter than magnitude 7 with separations between 0.4" and 3". (There are 7 ) it takes about 30 seconds and the search results can be turned into and observing list and the stars highlighted in the chart view of desired.

 

I have a series of doubles in Orion I use to estimate the seeing:

 

Rigel

Zeta Orionis 2.2" but mag 1.7-3.7

Eta Orionis 1.8"

32 Orionis 1.4"

52 Orionis 1.0"-1.1"

 

Stelladoppie says 1.0 " for zeta Orionis but I've seen reports it's actually 1.1"

 

https://www.stelledo...?iddoppia=21652

 

Jon

 


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#14 gnowellsct

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Posted 15 October 2020 - 01:27 PM

$10 bucks shipped hundreds of double stars accessible in a 4 inch telescope.  And many many deep sky objects also accessible in small apertures.  And all of them, or almost all, very attractive in larger scopes.

 

https://www.willbell...TLAS/atlas1.htm



#15 John Fitzgerald

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Posted 15 October 2020 - 01:35 PM

I have a 6" apo.  I can access thousands of pairs, but some I want to see are so on the edge of resolution, it leaves me wanting a 9" or 10" apo.  Then, there would be others, so I would want a 12" apo.........



#16 Jon Isaacs

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Posted 15 October 2020 - 01:37 PM

I have a 6" apo.  I can access thousands of pairs, but some I want to see are so on the edge of resolution, it leaves me wanting a 9" or 10" apo.  Then, there would be others, so I would want a 12" apo.........

That's why God invented the Newtonian.. :)

 

Jon


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#17 Jon Isaacs

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Posted 15 October 2020 - 01:46 PM

$10 bucks shipped hundreds of double stars accessible in a 4 inch telescope.  And many many deep sky objects also accessible in small apertures.  And all of them, or almost all, very attractive in larger scopes.

 

https://www.willbell...TLAS/atlas1.htm

 

 

Books can be frustrating because their information is not current.  Short period binaries change over the years so one can end up trying to make an impossible split.

 

For example, Zeta Boo was 0.8" is 2000, today it's 0.2".

 

Sky Safari computes the separation for the current time for these short period binaries using the orbital elements.

 

The reverse happens as well. Porrima was about 1.5" In 2010 ( I think), today it's about 3.0".

 

Jon


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#18 John Fitzgerald

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Posted 15 October 2020 - 01:47 PM

That's why God invented the Newtonian.. smile.gif

 

Jon

I have had big Newts.  I have a 12 inch now.  Their performance is lacking on the close, unequal doubles I like most to view and challenge myself with.  It's probably a combination of central obstruction, fast focal ratio, and spider vanes.  A slow, unobstructed reflector design might work, but it would be as long, and might be nearly as costly as the equivalent apo, if the optics are custom made by an expert mirror maker.

A big Newt has worked ok for me on the unequal ones that have wider spacing, like Xi Peg, or the very tight equal doubles.  They fail on double stars like 90 Herculis, at least in my experience.



#19 noisejammer

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Posted 15 October 2020 - 04:08 PM

That's why God invented the Newtonian.. smile.gif

I thought Newton invented the Dobsonian. :p



#20 noisejammer

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Posted 15 October 2020 - 04:16 PM

Assuming equal brightness, under good seeing, a 115 mm should split about 1 arcsec visually.

 

If you use lucky imaging and non-linear processing to boost the high spatial frequencies, you might get down to about 0.7 arcsec. If you use Bayesian sharpening and over-sample the field, you may even get down to ~ 0.3 arcsec. That's pretty hard work though.



#21 daquad

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Posted 15 October 2020 - 06:23 PM

I use Sky Safari. The Planner function lets me search for doubles based on separation, magnitude and region of the sky. 

 

For example, I can search for all the doubles in Hercules brighter than magnitude 7 with separations between 0.4" and 3". (There are 7 ) it takes about 30 seconds and the search results can be turned into and observing list and the stars highlighted in the chart view of desired.

 

I have a series of doubles in Orion I use to estimate the seeing:

 

Rigel

Zeta Orionis 2.2" but mag 1.7-3.7

Eta Orionis 1.8"

32 Orionis 1.4"

52 Orionis 1.0"-1.1"

 

Stelladoppie says 1.0 " for zeta Orionis but I've seen reports it's actually 1.1"

 

https://www.stelledo...?iddoppia=21652

 

Jon

I've split every one of those with my Jaegers 4" f/15.5 at 225X in 6/10 seeing, except Zeta.  Haven't tried that one yet.

 

Dom Q.



#22 daquad

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Posted 15 October 2020 - 06:35 PM

$10 bucks shipped hundreds of double stars accessible in a 4 inch telescope.  And many many deep sky objects also accessible in small apertures.  And all of them, or almost all, very attractive in larger scopes.

 

https://www.willbell...TLAS/atlas1.htm

True, but many of the more challenging doubles are not listed in Tirion's "Bright Star Atlas.  Eg. 52 Orionis, while indicated as double on the chart is not listed, because the authors' criteria for listing doubles is that the brighter star must be  brighter that mag 6.5 and the companion brighter than 8.0 and the separation between 2 and 30 arc seconds.

 

Dom Q.



#23 John Huntley

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Posted 15 October 2020 - 07:15 PM

I find the unequal brightness binary stars a really good challenge for my refractors. I got Zeta Herculis with my ED120 refractor back in 2016. It's a little easier now but still quite challenging for 4-5 inch scopes.

 

I think the separation is around 1.4 arc seconds now. The brighter star is mag 2.95 and the dimmer companion mag 5.4.


Edited by John Huntley, 15 October 2020 - 07:16 PM.


#24 Uwe Pilz

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Posted 16 October 2020 - 12:32 AM

I observe double at a regular base.

The answer to your question depends on what you count as "separated".

 

I count as separated if I see it is a double and can measure / gauge the position of the secondary, its angle.

 

The most important thing for such a task is having a well-collimated instrument, without coma or pinching. It is hard to collimate the short instruments, which are mostly in use today. My 4-inch refractor is collimated to around lambda/6 which is not bad. Bad I have trouble with it for close doubles.

 

My new to me 120 mm Skywatcher ED is very well collimated. The retailer (TS) looked for an exemplar which very ok. 

 

I looked for doubles of similar magnitude. I have an excellent article in S&T, "find your personal Dawes limit", which has a list. I have it not here, but may be the list at this site may be similar.

 

With 120 mm, it was hard to separate a double of 0.70". The Airy disc was slightly longish, which I could see at a magnification of around 300x.

It was a joy to separate a double of 0.75". At around 150x it was clearly separated, at around 300x it looked like a snowman. No black between the stars, but at waistline.

 

With a well-made instrument you may separate doubles close than you think first.



#25 Voyager 3

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Posted 16 October 2020 - 09:08 AM

I thought Newton invented the Dobsonian. tongue2.gif

I thought John Dobson invented dobsonian....lol.gif

I bet you know this lol.gif flowerred.gif .




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