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gregory93
super member


Reged: 07/03/12

Double star observing newbie
      #5910972 - 06/09/13 08:24 AM

Hello fellow CNers!
I would like to ask a few questions regarding double stars.
My current equipment is a XT8" refl and from eyepieces a 12T4,Hyp zoom ,MaxVision 24mm( Meade SWA 5k).
With this equipment,what are the limits regarding double stars?For example, what is the lowest separation i can resolve,regarding that i live in a mild light polluted area?
I mainly observe galaxies,but i would like to see what the fuss is all about

Thank you and clear skies!


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Nucleophile
super member


Reged: 05/24/13

Loc: United States
Re: Double star observing newbie new [Re: gregory93]
      #5911028 - 06/09/13 09:29 AM

I have the same scope and you may get some idea of what is possible via my posts (although I often also report results of viewing through a larger instrument).

In short, you will see more than you think you can with continued use of this telescope and training on "how to see" double stars, particularly those that are tightly coupled and/or contain large magnitude contrasts. The Dawes limit for this scope is about 0.57" which applies to mag 6 stars of equal magnitude. You should read through EdZ's notes listed in the stickies section and also David Knisely's article which he recently reposted here to familiarize yourself with some of the terms commonly used describe observation of varying degrees of "doubleness".


As a start, try for doubles that are no fainter than mag 8 and whose separation is between 1 and 2 arcseconds--work your way down to the tighter ones to see what kind of limit you can establish for yourself. Burnham's Celestial Handbook (Volumes I-III) is a great resource for sourcing targets--the Washington Double Star (WDS) database will give the current information on the pair. I also use the excellent WDS search engine Stelle Doppie.

Here are some (easier) gems in the sky now to try: Polaris, Porrima, Izar, Algieba, Cor Caroli, Mizar.


Welcome to the adventure!

Edited by Nucleophile (06/09/13 09:39 AM)


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: Double star observing newbie new [Re: Nucleophile]
      #5911555 - 06/09/13 03:42 PM

Light pollution is of minor influence when observing double stars - you loose maybe 1 or 2mag from your telescope magnitude limit but you would in most cases anyway not want to observe doubles this faint. This is one good reason why double star observing is an interesting field - you can do it with severe light pollution and even with full moon.
Else what is the fuss about - good question. Besides a lot of doubles of unquestionable aesthetical visual appeal there remains a wide field of optical challenge of your equipment and your observation skills especially when it comes to resolving unequal doubles - this where it gets really interesting. And then double star observing is one of the few remaining astronimical areas where amateurs can still contribute results of scientific value.
And then ...
Wilfried


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azure1961p
Postmaster
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Reged: 01/17/09

Loc: USA
Re: Double star observing newbie new [Re: WRAK]
      #5911842 - 06/09/13 07:23 PM

I agree magnitude 8 is a comfortable faintness limit for a newbie. Too near equal magnitude doubles make things easier as well as the 1-2" seperation. A lot can be said for some stunning low power doubles at 70x with 5" seperation. Super cut tight. Then there's the easier obvious 35" doubles.

One thing worth mentioning is the ability to see finer and finer doubles of challenging magnitude spread becomes easier and easier as experience builds and some of those super difficult ones we saw when new now kinda seem a little obvious.

Challenging doubles can have magnitudes if 11 and seps of 1.5" and still split!!! The mag 8 limit is a nice guideline for now though. Brighter than Mag 6 however I think is just too too bright . Each aperture has its magnitude sweet spot. I think 6.5-7 is ideal for an 8".

Good luck.

Pete


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StarDusty
sage


Reged: 10/02/07

Loc: Parsippany, NJ
Re: Double star observing newbie new [Re: azure1961p]
      #5912070 - 06/09/13 09:19 PM

I would recommend trying the Astronomical League's list. It is a list of 100 doubles that you can observe over a 9 to 12 month period. I found it was a good mix of doubles. Some beautiful, some challenging and some just plain interesting.

Go here for their site:

http://www.astroleague.org/al/obsclubs/dblstar/dblstar1.html

I was able to complete their list a few years ago. Go here for my observing list and logs

http://www.clearskyobserver.com/index.php/component/content/article/22-double...


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C_Moon
Pooh-Bah
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Reged: 10/23/09

Loc: Beneath the arms of Cassiopeia
Re: Double star observing newbie new [Re: gregory93]
      #5912077 - 06/09/13 09:23 PM

Quote:

Hello fellow CNers!
I would like to ask a few questions regarding double stars.
My current equipment is a XT8" refl and from eyepieces a 12T4,Hyp zoom ,MaxVision 24mm( Meade SWA 5k).
With this equipment,what are the limits regarding double stars?For example, what is the lowest separation i can resolve,regarding that i live in a mild light polluted area?
I mainly observe galaxies,but i would like to see what the fuss is all about

Thank you and clear skies!




I have similar eyepieces, but you might want a 2x barlow for high magnifications. Unless I'm misinterpreting, the highest you can go w/ your current setup is 150x. I use the hyperion zoom with a barlow (gets me up to 300x) and it works quite well.

If you go for very tight doubles (< 1"), you may want higher than 300x (I know I do). I will go up to 500x sometimes to observe those types of doubles. You need good seeing, but you can get away with higher mag on doubles than you can on planets.

Of course, I'd check out the showpieces first, and 300x is plenty for those. See for example, the AL double star reqmts. Also, the Cambridge Double Star atlas has a nice list of "showpiece" doubles.

Good luck!


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Ed D
Carpal Tunnel
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Reged: 04/30/10

Loc: Sunny South Florida
Re: Double star observing newbie new [Re: azure1961p]
      #5912089 - 06/09/13 09:30 PM

Your XT8 is fine for doubles and can perform to the fullest of its capability. I like observing doubles from my home in Miami (very heavy LP) with my XT6. My favorites are colorful doubles, and a Google search for COLORFUL DOUBLE STARS will turn up websites with observing lists to get you started.

Ed D


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gregory93
super member


Reged: 07/03/12

Re: Double star observing newbie new [Re: Ed D]
      #5914790 - 06/11/13 10:34 AM

Thank you all for your helpful replies!Searching the internet i stumbled upon an article of sky & telescope titled "More pretty double stars". While reading it i read this "Experience has shown me that doubles look best when viewed at a magnification determined by dividing 750 by the separation of the pair, given in arcseconds. For example, the components of the star Kappa (k) Herculis, also known as Marfik, are separated by 27.1 arcseconds; thus the optimum magnification is 750/27.1, or approximately 28x. The optimum magnification is typically about two and a half times the minimum required to resolve any given pair. "

So for a double separated by 1-2 arcs i need about +x350 mag.Is this true? If that's the case i cannot go higher than x150,which means i can only resolve pairs with separation greater than 5 arcseconds.


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Ed D
Carpal Tunnel
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Reged: 04/30/10

Loc: Sunny South Florida
Re: Double star observing newbie new [Re: gregory93]
      #5915127 - 06/11/13 02:00 PM

You may want to read this short page about Dawe's Limit. The theoretical resolving power of your 8" scope is 0.57" (arc seconds). However, several factors affect the resolving power of an instrument, as noted in the writeup.

I read the article you referenced. In doing the math for two extremes I came up with 6.94x for Mu Bootis (108") and 535.71x for Pi Lupi (1.4"), both of which I have observed, but not at such extremely low or high magnifications. What I do is start low to find the double/multiple star, increase the magnification until I get a split, then tweak the magnification to get the most pleasing image.

I don't see any reason why you couldn't go any higher than 150x with your scope, not unless you don't have the eyepieces for it yet. I also don't see any reason why your scope couldn't split a 1.4" double on a good night.

My suggestion is not to over think this. Just get out under the stars and find some doubles. Play with magnifications and gain experience in observing them. BTW, conditions are everything, and some nights you may not split a double that was an easy split the previous night. Here's a link to a list of colorful doubles. Pick out a few that are visible this time of year where you are and have at it.

Ed D

ADDED: I re-read your original post. Although you are currently limited to 150x there are a lot of doubles that can be observed at 150x and lower. A 2x barlow, as suggested, would give you a great range of mags with your zoom.

Edited by Ed D (06/11/13 02:22 PM)


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fred1871
professor emeritus


Reged: 03/22/09

Loc: Australia
Re: Double star observing newbie new [Re: gregory93]
      #5916211 - 06/12/13 01:33 AM

I'm thinking the Sky & Telescope article's suggestion about magnifying doubles to an apparent separation of 750" (such as 5" at 150x) is a purely personal preference by the writer of that article. Some folk like their doubles barely separated; others prefer close but not-too-close; and some like 'em wide.

For a double in the 1-2" range I'd expect (fairly even magnitudes) a magnification in the range of 150x (2") to 300x (1") to be fine, unless your optics are not very good, or your eyes need extra resolution. Eyesight varies.

Pete above has suggested "Super cut tight" for a 5" pair at 70x; I agree, and two doubles I can see from south of the equator, Alpha Centauri and Alpha Crucis, both presently around 4", I find to look nicely split pairs and neatly close at 80x - so that would be ~320 divided by the separation.

Although 150x is a limitation for the closest doubles, there are thousands visible with that magnification or less, and without going to the really faint stuff (such as 10th magnitude and below).

But getting a means for higher power - such as a good barlow - to take you to 300x will cover nearly everything you're likely to want when starting off with doubles. Evenly bright pairs a bit closer than 1" should appear double at 300x; significantly uneven doubles that are close are another area, and vary a lot in splittability. We're discussing/debating those in other threads.

An 8-inch Newtonian is fine for doubles - main issues are likely to be collimation (makes a big difference) and having it settled to ambient temperature.

Others have already offered you good ideas - the Cambridge Double Star Atlas, the various resource articles about doubles here on CN, the Astronomical League's list etc

AS Wilfried has said above, mild light pollution is pretty much a non-issue for doubles. Doubles, like planets, need steady air rather than pristine darkness.


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C_Moon
Pooh-Bah
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Reged: 10/23/09

Loc: Beneath the arms of Cassiopeia
Re: Double star observing newbie new [Re: fred1871]
      #5917863 - 06/12/13 10:25 PM

Quote:

I'm thinking the Sky & Telescope article's suggestion about magnifying doubles to an apparent separation of 750" (such as 5" at 150x) is a purely personal preference by the writer of that article. Some folk like their doubles barely separated; others prefer close but not-too-close; and some like 'em wide.




I agree. In fact, that's one of the nice things about the zoom, you can easily change magnifications and see what view you like best. I have found some times a color contrast is more pronounced when the stars are barely split, and other times I can see the colors easier at a higher magnification. This is one aspect of the fun for me...


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City Kid
Carpal Tunnel
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Reged: 05/06/09

Loc: Northern Indiana
Re: Double star observing newbie new [Re: StarDusty]
      #5948516 - 06/30/13 07:09 PM

Quote:

I would recommend trying the Astronomical League's list. It is a list of 100 doubles that you can observe over a 9 to 12 month period. I found it was a good mix of doubles. Some beautiful, some challenging and some just plain interesting.

Go here for their site:

http://www.astroleague.org/al/obsclubs/dblstar/dblstar1.html



I agree. If you want to see what all the fuss is about this is a good list to check out. I completed this list and even though I'm still not a big double star observer I really enjoyed working through the list. The best part is you don't have to have dark skies to make these observations.


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Cotts
Just Wondering
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Reged: 10/10/05

Loc: Toronto, Ontario
Re: Double star observing newbie new [Re: StarDusty]
      #5948674 - 06/30/13 08:56 PM

Quote:

I would recommend trying the Astronomical League's list. It is a list of 100 doubles that you can observe over a 9 to 12 month period. I found it was a good mix of doubles. Some beautiful, some challenging and some just plain interesting.

Go here for their site:

http://www.astroleague.org/al/obsclubs/dblstar/dblstar1.html

I was able to complete their list a few years ago. Go here for my observing list and logs

http://www.clearskyobserver.com/index.php/component/content/article/22-double...




+2 on this. Look at as many types as possible to gain experience. Very close equal pairs (Pi Aquilae, 1.4" is good). close unequal pairs (delta cygni and Epsilon Bootes come to mind). Wide pairs with colour(Albireo). Multiple stars (iota Cass and epsilon lyrae). Pairs with historical significance (Mizar, 61 Cygni, Xi Ursae Majoris) Use every magnification you have on every pair to see what works best and what your thresholds are. TAKE NOTES!


......and post here to let us know how you're doing

Dave, clouded out again.....


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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: Double star observing newbie new [Re: Ed D]
      #5951152 - 07/02/13 11:19 AM

Ed.D posted:

Quote:

You may want to read this short page about Dawe's Limit. The theoretical resolving power of your 8" scope is 0.57" (arc seconds). However, several factors affect the resolving power of an instrument, as noted in the writeup.




Well, that article is a little misleading. If the center of one star's diffraction disk falls exactly in the middle of the second star's first dark ring (the first "minimum" of the diffraction pattern), the disks do tend to overlap, but the separation is *not* at Dawes Limit. The proper limit for that situation is the Rayleigh resolution limit:

r = 1.22*Lambda/D, where r is the separation in radians, Lambda is the wavelength of the light, and D is the aperture of the telescope. For visible light having a wavelength of 5500 angstroms, separations in arc seconds, and apertures in inches, it comes out to be:

Rayleigh Limit: r = 5.45/D

where D is the aperture of the telescope in inches and r is the separation in arc seconds (the Metric form is r = 138.4/D, where D is the aperture in millimeters). Dawes Limit (r = 4.56/D) is just an empirical limit based purely on the visual observation of various stars, and is about 84% of the Rayleigh separation.

If you want to read Dawes original paper on the subject, it can be found here:

Dawes Catalog of Double Stars and Dawes Limit

Clear skies to you.


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haywool
member
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Reged: 05/07/12

Loc: Illinois, U.S.A.
Re: Double star observing newbie new [Re: David Knisely]
      #5978563 - 07/18/13 07:42 PM

Ya know ...
Sometimes, when I have a particularly rough time with the doubles, be it collimation, magnification, or especially just plain seeing, I usually make a trip to that little cabnet down in the cellar. No problem ... then everything's double. Whooooaah !

Rich


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labmand
sage
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Reged: 02/08/11

Loc: Michigan USA
Re: Double star observing newbie new [Re: haywool]
      #5979396 - 07/19/13 09:07 AM

haywool
Salud! my friend


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