cpsTN
professor emeritus
Reged: 04/26/07
Posts: 742
Loc: Smyrna, TN USA
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I have been going over tables of DS lately and I find that it is common to see separations of 1" up to 10" or even 14". Since my 8 inch dob can easily split DS as close as 1.5" and 2" DS, most of the time anyway, I consider tight as being LESS than 1". What do you guys think of as being tight?
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Charles P. Sands
Zhumell 8" f/5.9 dob
Looking Up since 1982
The heavens declare the glory of God;
and the firmament sheweth His handywork
(Psalm 19:1)
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asteroid7
Post Laureate
Reged: 10/19/04
Posts: 3716
Loc: CT
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Depends on your size telescope, sky seeing (urban or rural site. Living in the city and using a 4.5 refractor or 8
CPC, I consider tight as 1.2" or so. It also depends on the brightness of the doubles, if one is much brighter than the other.
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Clear Nights "Make My Day"
4.5" f/7 APM (TMB) APO Refractor on CG5 GoTo with Orion extension tube w/2" WO diagonal
5" Celestron Nexstar
8" Celestron Nexstar (8Ni)
8" Celestron CPC
Eyepieces:
40mm Pentax; 24mm Tele Vue Panoptic, 18mm Tele Vue Radian, 18mm HD Ortho, 15mm Celestron Omini,14mm Pentax,10mm Tele Vue Radian, 7mm Nagler,7.4mm Tele Vue Plossl.
2.5 Tele Vue Powermate
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VanJan
super member
Reged: 07/09/08
Posts: 116
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For me and my 8" reflector, I consider, in general, anything under 2" as tight; from there up to about 5" as close. No standards as to the terms, though, which is why it is important to state the stats, as it were, in descriptive observational reports. Good question! I'll be interested in reading how others define the term.
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cpsTN
professor emeritus
Reged: 04/26/07
Posts: 742
Loc: Smyrna, TN USA
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Thank you. I reserve all of my "bad" questions for my inlaws. 
I am interested to see what others will say, too, of coarse.
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Charles P. Sands
Zhumell 8" f/5.9 dob
Looking Up since 1982
The heavens declare the glory of God;
and the firmament sheweth His handywork
(Psalm 19:1)
Edited by cpsTN (08/10/08 07:37 AM)
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walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2417
Loc: Doylestown, PA
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With the poor seeing that is common here under 2" separation is getting tight, even with my 18. When the seeing is good than 1".
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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RLTYS
Carpal Tunnel
Reged: 12/18/04
Posts: 1741
Loc: New York (Long Island)
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Assumeing that the two components are about the same brightness I would consider any seperation of about 1 arc sec or less to be tight with my 10" refl.
Clear Skies.
Rich (RLTYS)
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10" F4.8 Refl.
4" F5 Refr.
50mm F12 Refr. (Tasco #6TE-5)
12x63 and 10x50 Binoculars.
"I want to do more then just look."
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Hal_Coward
super member
Reged: 11/26/06
Posts: 111
Loc: Cloudy Southeast Texas
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As other said, depends particulary on relative brightness of the components, for a given instrument. In hunting up targets I always look at relative brightness in deciding whether to observe the object.
With a Mewlon 180 I once had, sub-arcsecond was often achieveable on stars of the same magnitude.
OTOH, I could never split 78 Uma (1.4 sec sep, 5 primary, 7.4 secondary) until I tried with a 15" Obsession.
With my TV102, anything within a few tenths of one arcsecond gives me a thrill.
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Hal Coward
TeleVue 102 refractor on Gibraltar mount
27mm Panoptic
3,4,5,6,12,18mm Radians
"The pumps don't work 'cause the vandals took the handles" - Bob Dylan
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Achernar
Post Laureate
Reged: 02/25/06
Posts: 3720
Loc: Alabama, USA
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Where I live, anything less than 2 arc-seconds is tight, even for my 10-inch due to the usually bad seeing, especially if the stars are very unequal in magnitude. The tightest double stars I've separated so far have apparent separations of .8 or .9 arc seconds.
Taras
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10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
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John Fitzgerald
In Focus
Reged: 01/04/04
Posts: 1253
Loc: AR
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Anything less than 1.5" or so, mainly due to seeing conditions.
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?
Observing since 1966
Messier Cert #898
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12578
Loc: Cumberland, R I , USA42N71.4W
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I observe doubles in at least 10 different sizes of instruments. just a few listed here.
in an 80mm scope tight is 2.5-3". Extremely tight is under 2".
In my 5" tight is 1.5-2". Extremely tight is 1.3-1.5"
In my binoculars tight is entirely dependant on the power of the binocular, not the aperture.
In 25x100, limit is 6", very tight is 8"
in a 20x80, limit is 7.8", tight is 9-10"
in a 16x70, limit is 9.8", tight is less than 12"
in a 10x50, limit is 14.2", tight is under 20"
in a 8x40, limit is 18"-20", tight is under 30"
Mag 8 pairs and mag 9 pairs need to be much wider and still appear tight.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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ABC
sage
Reged: 10/22/06
Posts: 421
Loc: Heidelberg, Germany
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At good seeing my 6" f/9 refractor splits doubles down to 0.8" (i.e 14 Ori, 16 Vul). Therefore I consider for this particualr scope "tight" as beeing less than 1".
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Kind regards, Christian
Meade 178 ED
Meade 152 ED
Intes Micro Alter M500
Bresser 70/700 Achromat
CGE
EQ-6
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ABC
sage
Reged: 10/22/06
Posts: 421
Loc: Heidelberg, Germany
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At good seeing my 6" f/9 refractor splits doubles down to 0.8" (i.e 14 Ori, 16 Vul). Therefore I consider for this particular scope "tight" as beeing less than 1".
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Kind regards, Christian
Meade 178 ED
Meade 152 ED
Intes Micro Alter M500
Bresser 70/700 Achromat
CGE
EQ-6
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dmdouglass
member
Reged: 12/23/07
Posts: 89
Loc: Tempe, AZ
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"Tight" is a relative term. The discussions above here seem to focus on the size of the telescope. But there is another element, and that is the magnification power being used. And that is where I am looking for guidance. I am starting our clubs (Arizona EVAC) double star observing program, and doing some research into what is needed for each observation, both for viewing, and possible imaging.
So, lets expand this a little (hopefully). The scopes involved with me are an 8" Meade LX200, and a Meade 80mm ED APO. For anything wider than 25", no strain. The 8" does well for visual with a 26mm lens (X77 magnification). The 80mm does well there for imaging (need a 2x barlow for 25-50", then barefoot after that.).
But now for the smaller stuff. Down to about 1.5". What magnification is generally recommeneded for splitting the doubles. Say 1.5-5", 6-10", 10-15" 15-20", 20-25".
Hope I did not "steal the tread". I think this is (or should be) part of the general question. Thanks.
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David M. Douglass
Tempe, Arizona
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12578
Loc: Cumberland, R I , USA42N71.4W
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Quote:
But now for the smaller stuff. Down to about 1.5". What magnification is generally recommeneded for splitting the doubles. Say 1.5-5", 6-10", 10-15" 15-20", 20-25".
I think I covered that farily well in my reply, regarding binoculars. With the exception of 1.5" to 5", all the other separations you listed are suitable for binocular magnifications.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Patricko
professor emeritus
Reged: 01/30/07
Posts: 742
Loc: around the corner
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Hi Charles,
I consider doubles closer than 2" of unequal magnitudes to be tight in a small (~70mm) refractor. With 10X50 handheld binoculars DS of 30" seconds or less or equal brightness are tight for me. 
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Clear skies,
Patrick
 "Life is too short, go collect some photons!" - Me, myself, and I
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dmdouglass
member
Reged: 12/23/07
Posts: 89
Loc: Tempe, AZ
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Thank you edz for your reply. I went back, and re-read your post. I think you are saying...
08x for over 30
10x for 20-30
16x for 12-20
20x for 10-12
25x for 8-10
Interesting. Are you saying tht with those magnifications, you will actually "fully split" the double? Hmmmmm. More testing is in order for me. I have been viewing many of the sketches here on the forum, referencing 60-100x and some at 300x and higher being needed to fully split (and seperate).
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David M. Douglass
Tempe, Arizona
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12578
Loc: Cumberland, R I , USA42N71.4W
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I've written several articles on observing doubles, one of those on doubles for binoculars. I have cleanly split doubles as I've stated above. I use doubles to test binoculars.
I was out last night with a 10x42 roof prism binocular. 100 Herc at 14.2" was a challenge, it was quite obviously double, but not sure if it was cleanly split or not. Stf 2690 in the tail of Delphinus at 16.7" was cleanly split.
Theta Serpens, near Aquila, at 22" was an easy wide split in the 10x42 that could still be seen cleanly split even 40-50% out from center in the binocular. In a Nikon Prostar 7x50, this same 22" double can be seen split within 10-20% of the edge of field of view.
With a 25x100 binocular I have split 95 Herc, 6.3"
As you appraoch the diffraction limit of a scope, magnification needed goes up at a very steep curve. No binocular is operating anywhere near it's diffraction limit. The power needed to split a double is controlled by my acuity, 140-150 arcseconds apparent. That is the case with ALL low powered doubles.
Binocular Doubles - 75 Targets - some very close
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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dmdouglass
member
Reged: 12/23/07
Posts: 89
Loc: Tempe, AZ
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Ed,
Thanks again for the reply. I did some more tests last night. The image below reflects those tests, and this is what I was referencing. "Tight" can be "controlled" with magnification change (applies only to telescopes, i.e. change the eyepiece...).
I used the 80mm with a 26mm eyepiece (480mm/26mm = 18x), and the view was very close to the 80mm image (Direct), or just barely fully split. Seems that your guide is very good.
Thank you again for your help here, and willingness to share information with others. It helps..... A lot !
Edit: I should add that these images are at the same camera scale. But they are NOT the "full image". I cropped each to concentrate on the double, and conserve on space and size of file for this post.
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David M. Douglass
Tempe, Arizona
Edited by dmdouglass (08/20/08 03:51 PM)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12578
Loc: Cumberland, R I , USA42N71.4W
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David, I didn't mention Albireo, but that was another i looked at last night with my 10x42. Actually, it looked a lot more like your second picture, quite widely separated thru the 10x42.
Your photo "direct" is how I would describe Gamma Delphinus (9.6") in my 15x70s.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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