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magnus
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Reged: 03/17/04

Loc: Visby, Sweden
eta Crb & gammaCrb
      #3766449 - 04/25/10 12:13 AM

Tonight I observed eta Crb & gamma Crb with my 8" OOUK Newt. Etas airy disks were kind of "kissin & touching" at 576X but no black space between them. So neg. clear split. To my eyes both components appeared yellowish.
Gamma was neg., I could not detect any hint of two airydisks. Maybe `cause they differ somewhat 1,5 mag.
Anyone who knows the sep. at the moment?

Magnus


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Catapoman
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Reged: 06/06/03

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Re: eta Crb & gammaCrb new [Re: magnus]
      #3766732 - 04/25/10 07:46 AM

Magnus,

Congrats on your observations. Based on data from the Sixth Orbital Catalog the current separation for eta CrB is approx. 0.606 and gamma CrB is approx. 0.685. Your difficulty with gamma is probably due to mag. differences and the components will be closing over the coming years.

Edited by Catapoman (04/25/10 08:08 AM)


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magnus
professor emeritus


Reged: 03/17/04

Loc: Visby, Sweden
Re: eta Crb & gammaCrb new [Re: Catapoman]
      #3768309 - 04/25/10 08:09 PM

Pernel,

Thank you for your information!
According to Haas`book "Double Stars" 0,9" is the limit for 200mm to detect seperation when magnitude difference is 1.5.
0.606" makes sense as I find eta a tiny bit more obvious than zeta Boötes (0.53" I believe)

Magnus

Edited by magnus (04/25/10 08:12 PM)


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Jon Isaacs
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Reged: 06/16/04

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Re: eta Crb & gammaCrb new [Re: magnus]
      #3768444 - 04/25/10 09:19 PM

Quote:

Tonight I observed eta Crb & gamma Crb with my 8" OOUK Newt. Etas airy disks were kind of "kissin & touching" at 576X but no black space between them. So neg. clear split. To my eyes both components appeared yellowish.
Gamma was neg., I could not detect any hint of two airydisks. Maybe `cause they differ somewhat 1,5 mag.
Anyone who knows the sep. at the moment?

Magnus




Magnus:

From Skytools 3 which computes the orbital data:

Quote:

Eta CrB

AB: 4.98+6.02 mag, STF1937, ADS 9617, B=STF1937B
Definitive Orbit: P=41.6 yr, a=0.87"
PA 174° Sep 0.61" (2010.3)




Quote:

Gamma Crb

AB: 3.85+5.62 mag, STF1967, ADS 9757
Good Orbit: P=92.9 yr, a=0.74"
PA 112° Sep 0.69" (2010.3)




I have split Gamma with my 10 inch F/5 and both my 12.5 inch scopes, usually at over 600x. It was not an easy split, Y0u are doing a great job to split eta with an 8 inch. (Thomas, I don't think you will get this one with your 63mm... )

I suggest purchasing Skytools 3, I bought it just for its double star capabilities. Its about $100.

Jon


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Catapoman
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Re: eta Crb & gammaCrb new [Re: magnus]
      #3768662 - 04/25/10 11:10 PM

You are welcome, Magnus. This region has been one of my favorite for binary observing. For my location it's directly overhead and challenges like these two are always on my list.

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Astrojensen
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: eta Crb & gammaCrb new [Re: magnus]
      #3768896 - 04/26/10 03:50 AM

Hi Magnus

Sissy Haas had an article about a mathematical formula invented by Chris Lord several years ago in S&T. Does the limit mentioned in her book come from that formula? If so, I can tell you it's flawed. I have tested it and can best it by quite a large margin.

For example, the double Theta Auriga is in the S&T article mentioned as being undetectable with a 60mm "no 60mm will ever split it", according to Haas. Well, my 60mm's haven't heard that, because I can split (or detect it, whatever) it with my Telemator quite easily on a fine evening. And not just that. On two occasions I have split it with a 42mm aperture stop on my 80mm f/15 Vixen, using 133x. The companion is extremely difficult, but visible as a tiny bump on the first diffraction ring. Conditions must all be truly first class.

Similarly, a friend, David jenkins, and I have tried our hand at splitting/detecting Epsilon Bootes and Dave could split/detect it with a 35mm and I with a 30mm. Some years ago, Luis Arguelles and another observer used a 4" f/10 Vixen achromat to split/detect a 0.35" unequal double. I can't remember which double it was, but I believe it was Epsilon Hydra, but I am not sure and can't find the article anymore. Both observers detected the double.

Also, the formula indicates that around 2005 (or was it earlier? Suddenly I can't remember) it would have taken at least a 30" - 40" telescope to see Sirius B, but that year it was seen by Sue French at the WSP with a 4.1" AP refractor and the following years, lots of observations began to pour in.

Detecting very unequal doubles depend strongly on many factors, some of which are under the observers control, some of which aren't. First, obviously, the seeing needs to be *perfect*, but secondly, the transparency also needs to be very good or excellent, as you want no stray light near the main star to mask the faint companion. This also means your telescope must be unusually perfect. You also need good dark adaption, in order to see the faint diffraction ring and any irregularity in it. This is especially critical in a small instrument. Your eyepieces also need to be super clean and free of scatter, etc, etc.

A good observing trick here is to use *very* high power. 100x/inch is not too much, as it will serve to magnify the image to such an extent that small irregularities in the diffraction rings and any deviation from a perfect circle in the airy disk will be much easier to see.


Clear skies!
Thomas, Denmark


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Clive Gibbons
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Re: eta Crb & gammaCrb new [Re: Astrojensen]
      #3769399 - 04/26/10 11:57 AM

Quote:

Some years ago, Luis Arguelles and another observer used a 4" f/10 Vixen achromat to split/detect a 0.35" unequal double.




Lordy, that's some feat.

Definitely wouldn't be a split.
But just seeing the slight bump on the primary's Airy disk amounts to something of a visual wonder.


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magnus
professor emeritus


Reged: 03/17/04

Loc: Visby, Sweden
Re: eta Crb & gammaCrb new [Re: Clive Gibbons]
      #3774378 - 04/28/10 08:38 PM

Hej Thomas!

The formular mentioned in Haas book (pg5) comes from a French work called "Revue des Constellations" and the formulas that consistently match them were developed by W. Dawes, PJ Treanor and Christopher Lord. As Haas mention in her book it should just be used as aguide. If I understand her right it is not written in stone. Just as you suggest.
Thanks for your advice regarding split of tight unequal mag. binaries; never considered things as transparancy (thought it was only a DS issue)and dark adaption. The latter has only been something I thought of when starhopping to faint binaries.
I thoght it was only pure seeng that ruled!
I`ll also consider your advice using as high magnification as possible.
The obs. you are really tremendius. Did not think it was possible to achieve.
Have you split Antares with your 63mm Zeiss Telemator and keen eyes?
I have never even bothered to try due to it`s low altitude up here at 57*N.

/Magnus


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Astrojensen
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Re: eta Crb & gammaCrb new [Re: magnus]
      #3774835 - 04/29/10 04:01 AM

Tjenare, Magnus!

Well, from Bornholm, which is at 55 degrees north latitude, Antares only climbs 8.5 degrees above the horizon. Enough to see it easily, but not enough to split it as a double and I have never seen its airy disk, only a smeared blob, surrounded by strong blue and red from the atmospheric refraction. Below around 15 degrees above the horizon the seeing is almost always very poor here, even if it is near perfect near the zenith. In summer we can sometimes get superb seeing.


Clear skies!
Thomas, Denmark


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magnus
professor emeritus


Reged: 03/17/04

Loc: Visby, Sweden
Re: eta Crb & gammaCrb new [Re: Astrojensen]
      #3774863 - 04/29/10 05:26 AM

Hejsan Thomas!
I recognize myself in your description. Well we are almost in the same island...sorry, boat here in the Baltic Sea!

Magnus


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Astrojensen
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Re: eta Crb & gammaCrb new [Re: magnus]
      #3775416 - 04/29/10 12:26 PM

Hej Magnus

Well, since Bornholm and Gotland are both placed in the Baltic Sea some distance from other landmasses and have comparable sizes, I'd think our observing conditions should be very much alike. I see from your avatar info that you're from Visby. How's the skies in your neighborhood when it comes to darkness?

Clear skies!
Thomas, Denmark


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magnus
professor emeritus


Reged: 03/17/04

Loc: Visby, Sweden
Re: eta Crb & gammaCrb new [Re: Astrojensen]
      #3776295 - 04/29/10 06:48 PM

Visby, were I live, is a small town with 25000 inhabitans. Still it`s by far the largest town on the island of Gotland. Best known for it`s medival citywall.
A 10-15 minutes drive with my car and I have pitch black skies. When I feel lazy I observe from my balcony and still get resonably dark sky in east and south directions. Guess Iam very fortunate. In Sweden we also have the "everymans right" (allemansrätten), meaning I can use whatever observing site which suites me.As long as don`t put my equipment on anybodys garden of course.
If I just could get a large dobsonian in my SAAB 900 car I would not hesitate to get one to really take advantage of the dark skies for DS.
Now & then I consider a collapsable SW 12" dob.....
Guess your skies in Bornholm are also very nice and dark?

Hälsningar,
Magnus

C80ED
120ST
MK66
8"f/6 Newt
Porta mount
CG-5
CG-5GT
and possibly a Lunt35Ha this summer


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Astrojensen
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: eta Crb & gammaCrb new [Re: magnus]
      #3777101 - 04/30/10 03:56 AM

Hej Magnus

The biggest town on Bornholm is Rønne, with around 14,000 inhabitants. The whole island is home to just above 42,000 people. I live in a small village of less than 1,000 inhabitants and my garden has excellent skies, though with a little stray light. But the biggest advantage of Bornholm is that nearly all street lights are turned off after midnight during most of the week and most of the streetlights are full-cutoff to begin with and private outdoor lights are also turned off, since electricity is expensive and crime virtually nonexistant. From midnight until 5.00 AM, we have near-pristine skies from most of the island and remember, we are on an island, which means we sit in the middle of the Baltic Sea, with at least 40km's to the nearest source of significant light pollution, which is Sweden. During these "blackouts" I can, on a very clear night, get mag 7 skies from my yard... Our biggest problem is the low altitude and being close to the sea, which means haze is visible on most evenings and limit us to mag 6 skies or worse, even if it is otherwise clear.

I have lived here all my life, so I never considered these conditions to be anything special, but after visiting starparties in other parts of Denmark and see them go crazy over what I would consider to be very mediocre conditions, I began to understand just how good my skies are. It's odd and a bit of a shame I am the only person here who observes deep-sky objects. I too, am beginning to consider a dobsonian or maybe just a bigger refractor. I am hopelessly in love with refractors, you see.


Clear skies!
Thomas, Denmark

PS. Should this post have inspired anyone to move to Bornholm, then please do so. I could use some company under the stars.


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magnus
professor emeritus


Reged: 03/17/04

Loc: Visby, Sweden
Re: eta Crb & gammaCrb new [Re: Astrojensen]
      #3777161 - 04/30/10 05:47 AM

Hej Thomas,

Nearly all streetlights turned off after midnight...
Sounds like the amateurastronomers "wet dream"!
Yeah maybe I take my wife with me and move to Bornholm when I am retiered from work, within 6-7 years I hope! Different place yet close to Sweden.

See you under the Bornholm Skies 2017,
Magnus


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NewAstronomer
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Re: eta Crb & gammaCrb new [Re: Astrojensen]
      #3802001 - 05/12/10 06:23 PM

Quote:

Hi Magnus

Sissy Haas had an article about a mathematical formula invented by Chris Lord several years ago in S&T. Does the limit mentioned in her book come from that formula? If so, I can tell you it's flawed.




That would not surprise me. He is very knowledgeable, but has made some serious mistakes in his various reviews posted on his website.


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Clive Gibbons
Mostly Harmless
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Reged: 05/26/05

Loc: Oort Cloud
Re: eta Crb & gammaCrb new [Re: NewAstronomer]
      #3842245 - 06/02/10 08:56 AM

Had a look at Eta last night with the ED120 at 270x and 360x.

Seeing was all over the map, but when the air steadied for brief moments, it was slightly elongated.
The pale golden color of the star was pretty apparent.


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ziridava
super member


Reged: 08/17/12

Loc: Arad,Romania,Eastern Europe
Re: eta Crb & gammaCrb new [Re: Clive Gibbons]
      #5871230 - 05/19/13 12:05 PM

Two years ago ,sometime in the Autumn,I saw in my 8 inch F/6 Dobsonian the Airy disks in an eight shape of both Eta CrB and Zeta Boo at 200x/Radian 6mm and 400x/Radian 6mm+2x Barlow.

Last night I tried Eta CrB in the same 8 inch Dobsonian.The magnification was again 200x/Radian 6mm.
The timing was after midnight,the seeing was 5 to 6 on the Pickering scale,the 65% waxing gibbous Moon was low toward Western horizon.
This time I was able to see not only the Airy disks but ,from time to time,also black space between the two stars of Eta Cr B.
Increasing the magnification to 320x/Celestron ''orange'' 7.5 mm Plossl+Japanese 2x Barlow,the full split was beyound any doubts and the black space between the two stars of Eta CrB was open and stable.Of course ,the whole image was shivering and trembling as always do at this powers.

I will not mention here other double stars I split last night because they are not for the 8 incher league.Except the two ''debillissima stars'' located between the pairs of Epsilon 1-2 ''Double-Double''.I hardly saw that two stars in my 125mm F/7 Dobsonian years ago and only using the 8mm Solid Ramsden eyepiece, a kind of modern day variation of Tolles oculars,made by my friend Silviu.Now at 200x ,the two ''debillissima stars'' were clearly and easy visible.

I hope you will forgive this bit of off-topic.Before going to Eta Cr B, I checked the focusing on Saturn and M13.
On Saturn,the Cassini division was outrageously visible,three Saturnian satellites where aligned very close to the Western ansa and on the disc I believe I saw more details than just the shadow of the ring.
M13 was resolved to stars and chains of stars except the very center of the star-cluster.

My next station was Eta CrB where I spent at least one hour in awe.

Mircea

Edited by ziridava (05/19/13 12:08 PM)


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7331Peg
Sirius Observer
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Reged: 09/01/08

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Re: eta Crb & gammaCrb new [Re: ziridava]
      #5871627 - 05/19/13 02:36 PM

Pretty impressive results for an eight inch at Dob at 320x, Mircea! That little black and orange Celestron 7.5mm Plössl is a great double star eyepiece, though.

I almost had a split of Eta CrB a few weeks ago in my six inch f/10 refractor at 608x, so close I could almost taste it. Current separation is about 0.67" for it.

More on that frustrating experience can be found here.


John


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: eta Crb & gammaCrb new [Re: Astrojensen]
      #5873107 - 05/20/13 05:25 AM

Quote:

Hi Magnus

Sissy Haas had an article about a mathematical formula invented by Chris Lord several years ago in S&T. Does the limit mentioned in her book come from that formula? If so, I can tell you it's flawed. I have tested it and can best it by quite a large margin.
...
Thomas, Denmark




Thomas, this is a crass misunderstanding of the meaning of the results of Chris Lord's algorithm - this is a Rule of Thumb based on statistical analysis of actual observations giving an average value for a required aperture for resolving a specific double. "Beating" this value does not mean falsification of this algorithm but only being successful under conditions allowing resolution with a probability smaller than 50%.
The implementation of Lord's algorithm in AstroPlanner expresses this with non numerical classification for given parameters from "Easy" over "Difficult" to "eXeedingly difficult" to "Unresolvable".
For example Eta CrB with current WDS data of 0.6"sep +5.64/5.95mag gets an "XD" rating for a 125mm refractor under perfect seeing conditions. This means a probability of less than 5% for a successful resolution meaning in this case certainly no clear split with dark space between the spurious disks but a rod pointing into the correct position.
This result seems plausible as I observed Eta CrB begin of August last year with my 140mm refractor with moderate good seeing and got an egg pointing into the correct position of the companion.
For Gam CrB with 0.6"sep +4.04/5.6mag you would already need a 170mm refractor for the "XD" rating according to Lord.
My own statistical analysis of limit observations (based mostly on observations with variable aperture by using an iris diaphragm) indicate for a Newton with central obstruction of 0,3 (as used by Magnus, CO assumed) a required aperture for a 50% chance under reasonable fair seeing for resolving of 200mm for Gam CrB and 176mm for Eta CrB - both values should be easily to beat under perfect seeing conditions with apertures more than 14% smaller.
Wilfried
PS: But there is at least one serious flaw in Lord's algorithm as it does not consider the effects of overall faintness of doubles giving for example for a +3/6mag double the same result as for a +8/11mag double - and this is certainly wrong


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fred1871
scholastic sledgehammer


Reged: 03/22/09

Loc: Australia
Re: eta Crb & gammaCrb new [Re: ziridava]
      #5875015 - 05/21/13 12:05 AM

Mircea, nice to see this thread revived after two years.

I'm impressed that you can see a clean split on Eta CrB at only 200x - with the current separation, ~0.67", that indicates good eyesight, as well as good optics and air steadiness. Doubles that close I usually find easier for "black space between stars" at 300x or so. Eta CrB is within the Dawes Limit for an 8-inch, and has nearly even stars (delta-m is 0.3).

I looked at it recently, but air steadiness was only good, not great at the time (modest altitude for Eta CrB at my southerly latitude). I was using my 140mm refractor, so I didn't expect a clean split. It was however elongated, with hints of notching in the best moments, at 400x, so I was quite pleased.

Wilfried, I take your points re Thomas's comments, but I too have some issues with Chris Lord's analysis. You're right of course, as discussed previously, that Lord's algorithm "does not consider the effects of overall faintness of doubles".

But, as well, your comments a while back are correct in suggesting that Lord's database is inadequate - not enough pairs, and even fewer that are "near-limit" examples for the telescope used.

As well, I'll remark again that not taking CO effects into account with uneven pairs is unhelpful. From my own observing, long-term with both obstructed and unobstructed telescopes, CO is definitely a factor for the uneven pairs. I'm presently observing some of the same doubles with 140mm refractor and 235mm SCT for comparisons. I'm also attempting pairs that I hope will be "near-limit" for the SCT. More on that when I've got more data.

Your figure for aperture needed for Gamma CrB, compared to Eta CrB, is interesting (50% chance). Where Eta has a small delta-m (0.3m) Gamma has a noticeably larger one (1.6m), and is slightly closer at present (probably 0.54" vs 0.67"). The separation difference by itself might suggest nearly 25% aperture increase, even before adding a Dm factor.

So I'd agree on 176mm for Eta CrB - effectively, the Dawes Limit, with so little brightness difference. Therefore, elongated with somewhat smaller apertures, as I found with 140mm. But Gamma CrB has a DL that matches 215mm (6th orbit catalog ephemeris), so I'd expect, with noticeable Dm (1.6m), that 215mm would have a harder time getting a neat DL split on Gamma than 176mm on Eta.

Returning to Thomas - he gives, I think, quite a good list of factors involved with uneven pairs. And he notes what I've commented on before, the "small telescope effect", splitting pairs that might be thought beyond small telescopes (when the optics are excellent, and the double fairly bright, as with Theta Aurigae). My RoT on that one would suggest 80mm, with a small chance (5%) at around 50-55mm. Thomas mentions 42mm!!! Ditto, astonishing claims re Epsilon Bootis (Izar), detected (rather than split, I'd expect) with 30-35mm.

On the other hand, the claim regarding Arguelles and another even detecting a 0.35" uneven double with a 4-inch (102mm) refractor I do not find credible. SW Burnham with a 6-inch (152mm) I can believe; anyone with a 102mm, no. (Unless it was the "blue filter trick" noted here 6 months ago - on a very bright double with smallish Dm - maybe, maybe - and that takes us into different territory).

On Thomas's suggestion of very high powers, such as 100x per inch for some doubles, I'm in agreement. Although 400x (73x/inch)is my standard power on the 140mm for the tightest pairs, I sometimes use 570x (103x/inch). For significantly uneven pairs I find less power (160x-285x) generally more useful unless the secondary star is fairly bright, say mag 8 or brighter.


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ziridava
super member


Reged: 08/17/12

Loc: Arad,Romania,Eastern Europe
Re: eta Crb & gammaCrb new [Re: fred1871]
      #5875613 - 05/21/13 11:32 AM

John
Thank you for your kind comments.I agree,the 7.5mm ''orange'' Celestron Plossl is a very good eyepiece.This was a gift from Martin Stangl,my astronomical friend from Austria.
I'm since half a year subscriber to ''Star Splitter''. Please continue to offer trough your very fine blog a lot of inspiration and very good reading.

fred1871
Thank you for your kind comments.I was obsessed by Eta CrB since I saw its touching Airy Disks two years ago.

Mircea


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: eta Crb & gammaCrb new [Re: fred1871]
      #5876370 - 05/21/13 05:08 PM

Quote:

...
I'm impressed that you can see a clean split on Eta CrB at only 200x - with the current separation, ~0.67", that indicates good eyesight, as well as good optics and air steadiness.
...
As well, I'll remark again that not taking CO effects into account with uneven pairs is unhelpful.
...
215mm would have a harder time getting a neat DL split on Gamma than 176mm on Eta. ...



Fred, very good eyesight indeed - this means a visual acuity of 134 arcseconds. Would like to have this too but I think I cannot beat 160 arcseconds.

Regarding CO - Lord's algorithm includes CO if only in 4 steps but ignores the effect of CO on the size of the Airy disk. He argues that this effect is too small to be considered - this may be understandable as he is focused on separation but tranlated into aperture even small effects can mean a whole aperture class difference.

The mentioned 215mm for Gamma may really be a bit optimistic - this is due to the small number or limit observations above 140mm aperture in my data set so far. So the numbers below 140mm dominate the statistical analysis and above all results are more or less linear projections and these are prone for being wrong. I assume larger apertures perform in average not this much better than smaller apertures as the scope related seeing is mandatory worse.
But I hope I can soon add my own limit observations up to 235mm aperture to my data set.
Wilfried


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fred1871
scholastic sledgehammer


Reged: 03/22/09

Loc: Australia
Re: eta Crb & gammaCrb new [Re: WRAK]
      #5876915 - 05/21/13 08:56 PM

Wilfried, I'm also inclined to think the effect of CO on Airy disc size is not of much significance with most telescopes, having CO in the 25% to 36% range. My main concern is with transfer of light from the Airy disc into the diffraction rings, as it affects close uneven doubles. There I can see an effect of enhanced ring brightness and a bit more diffuse light around the brighter star, perhaps from a smearing of the diffraction rings by the atmosphere.

Mirror optics typically have more scatter than lens optics so that's a further factor making comparisons difficult. Even very good mirror optics have in my experience this tendency.

My most recent observations of uneven pairs, comparing 140mm refractor with 235mm SCT, is starting to show some benefits to the larger aperture. I'll keep on with the comparisons as weather allows. Years ago, comparing a C14 (35cm) with an AP apochromat refractor (18cm) I found little benefit to the larger aperture for close uneven pairs. For even pairs, both the brighter very close pairs and the dim close pairs, the C14 was better. But I don't think I pushed the C14 to its limits back then. So now, with the smaller SCT, I'm working on finding the limits of what it can do, beyond the expected improvement of closer even pairs and more light for dim companion stars.

One recent success with the C9.25 was HO 389 in Bootes (1452.1, +2017) a double of magnitudes 7.0 and 10.5 (Delta-m 3.5) at 1.6" separation, where the companion was just visible at 235x and better at 294x. Another night I looked at it with the 140mm refractor but I couldn't see the companion (230x, 285x) - though air steadiness was less good that night, so I'll try again with the refractor under better conditions.


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: eta Crb & gammaCrb new [Re: fred1871]
      #5877497 - 05/22/13 08:07 AM

Quote:

... also inclined to think the effect of CO on Airy disc size is not of much significance with most telescopes, having CO in the 25% to 36% range...



Based on diffraction theory there should be a rather significant effect for tight equal bright doubles like Eta CrB amounting to a 1" smaller aperture with a CO of 0,3 (7" instead of 8") for a 50% chance - but I still have to check this calculation with own observations.
Please keep me informed on your further results with the C925. The current collimation state of my C925 seems now not this bad anymore and the iris is now ready mounted on a metal dew shield. All I need now are some clear nights still very rare lately in my location.
Wilfried


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ziridava
super member


Reged: 08/17/12

Loc: Arad,Romania,Eastern Europe
Re: eta Crb & gammaCrb new [Re: WRAK]
      #5877691 - 05/22/13 10:17 AM

I don't know if this is of any help,the CO of my 8 inch Dobsonian is of 22.7% ,the size of the secondary mirror being 46mm.
Other parameters seem to be more important,my smaller Dobsonian of 125mm F/7 is providing much nicer images even if CO is of 28%.
Since I'm in the Double Star Realm ,I was unable to go deeper than 1'' with the smaller,tough!
Mircea


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WRAK
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Re: eta Crb & gammaCrb new [Re: ziridava]
      #5879533 - 05/23/13 02:38 AM

With a wavelength of 550nm the size of the Airy disk would be 1.38 arcsec. The CO of 0.227 would reduce this to 1.3 arcsec. The size of the spurious disk for a ~+5.5mag star should be about 60% of the Airy disk - this gives 0.78 arcsec for the diameter of the spurious disk or a radius of close to 0.4 arcsec. With a separation of 0.6" for Eta CrB it would seem impossible to see dark space between the spurious disks as they have to overlap according to this calculation.
Mirceo - while I do not doubt your observation this means that there is need for explanation. Maybe my calculation or the advertised separation is wrong.
Wilfried


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fred1871
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Re: eta Crb & gammaCrb new [Re: WRAK]
      #5879591 - 05/23/13 04:56 AM

Wilfried, I think Mircea was claiming a split (with dark sky between components) on Eta CrB with the 8-inch (203mm) not the 5-inch (127mm) telescope he has. With the 8-inch I don't think there's a problem as the ephemeris gives about 0.67" for Eta CrB at present, and the Dawes limit for an 8-inch scope is 0.57". The components are near-enough of equal brightness.

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Re: eta Crb & gammaCrb new [Re: fred1871]
      #5880481 - 05/23/13 03:14 PM

Fred, don't see your point - Dawes or even Rayleigh means overlapping spurious disks.
Wilfried


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fred1871
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Re: eta Crb & gammaCrb new [Re: WRAK]
      #5880985 - 05/23/13 07:55 PM

Dawes or Rayleigh mean overlapping diffraction discs, but because the discs fade towards the edges the observer doesn't see it that way - the spurious discs look smaller. There's a neat little diagram in Sidgwick's Amateur Astronomer's Handbook illustrating the point; it relates also to why dimmer stars appear to have smaller diffraction discs, and is a phenomenon that George Airy long ago commented on in his study of the diffraction image.

EdZ has remarked here some time ago that with higher magnifications one can see something of this effect, of overlapping discs; whereas at moderate magnification, one sees an apparently clean split. Based on my own observing I'm not sure how much of this effect is from scattered light, including diffuse spreading of light due to various factors (atmosphere, optical issues), and how much might be what EdZ takes it to be.

I do know that my recent observation of Zeta Bootis did not appear to have overlapping discs - given the large CO of my SCT (36%) that would reduce the diffraction discs' size a little, so the effect I saw, at what would be the Dawes Limit for an unobstructed telescope, was a good fit for what I expected - two discs almost touching. As I had reached 470x to get this view I didn't try higher powers to check for an appearance of disc overlap.

So I'm inclined to think the suggestion that we don't usually see the outer parts of the diffraction disc because it falls below the eye's detection threshold is correct, which is why we see an appearance of separate discs with pairs closer than optical theory allows. The theory is correct in describing what's there; but our eyes don't see the whole story. Spurious discs look smaller than the true diffraction discs.


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fred1871
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Re: eta Crb & gammaCrb new [Re: fred1871]
      #5880995 - 05/23/13 08:03 PM

I've now found the quote from Airy in 1835:

The rapid decrease of light in the successive rings will sufficiently explain the visibility of two or three rings with a very bright star and the non-visibility of rings with a faint star. The difference of the diameters of the central spots (or spurious disks) of different stars ... is also fully explained. Thus the radius of the spurious disk of a faint star, where light of less than half the intensity of the central light makes no impression on the eye, is determined by [s = 1.17/a], whereas the radius of the spurious disk of a bright star, where light of 1/10 the intensity of the central light is sensible, is determined by [s=1.97/a].

I'm suggesting that a similar threshold effect applies to all diffraction discs in terms of how we see them. There is this differentiation between brighter and fainter stars; but even the brighter stars don't allow the full size of the diffraction disc to be seen.


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Re: eta Crb & gammaCrb new [Re: fred1871]
      #5881620 - 05/24/13 04:24 AM

Fred, I am speaking about the spurious disk which is the part of the Airy disk we can actually see as it is above the visual threshold. For brighter stars up to +6mag we can according to for example EdZ as well as other sources expect the spurious disk to be about 50-60% of the Airy disk (maximum to first minimum). Even if we take the lower value of 50% we can expect kissing disks at Rayleigh and overlapping disks at Dawes.
Wilfried


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ziridava
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Re: eta Crb & gammaCrb new [Re: WRAK]
      #5881643 - 05/24/13 05:49 AM

As pointed out by fred1871 ,I was succesful in splitting Eta CrB with the 8 inch F/6 Dobsonian,having a 22.7% CO.
The black space between the stars was really thin and fluctuating but it was there at 320x ,without any doubts.
With my smaller Dobsonian of 125mm F/7, I was not able to go deeper than 1''.This have 28% CO but generally provide much nicer images on double stars.
Mircea


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fred1871
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Re: eta Crb & gammaCrb new [Re: ziridava]
      #5881846 - 05/24/13 09:41 AM

My viewing of Zeta Bootis was similar in appearance to Mircea's of Eta CrB - I think a factor here is a contrast effect, that masks the visibility of the outer parts of the diffraction disc, even when that area might be expected to be visible. Because the outer disc areas are much less visible than the central parts the eye usually does not see light there - rather a contrast effect of darkness between the discs. The theory is solid but the eye frequently doesn't see it that way.

I'll experiment with some Dawes Limit pairs, and try much higher powers to find out if I can see more of the diffraction disc by using more than the minimum power needed, as some have suggested. Distinguishing that from scattered light from atmosphere effects and optical limitations may not prove easy.


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Re: eta Crb & gammaCrb new [Re: fred1871]
      #5882233 - 05/24/13 12:44 PM

Quote:

... I'll experiment with some Dawes Limit pairs...




Would be of interest as it could very well be that a +6mag spurious disk is less than 40% of the Airy disk because this would explain such observations.
Wilfried


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Re: eta Crb & gammaCrb new [Re: WRAK]
      #5882888 - 05/24/13 06:47 PM

Fred, the calculation for your observation of Zeta Boo gives the following results:
Airy disk for 235mm without CO = 1.18"
Airy disk for 235mm with 0.36 CO = 1.04"
Spurious disk 60% assumed = 0.624"
With a separation of 0.6" this would mean a minor overlap and therefore no dark space between the spurious disks but
- the size of the spurious disk should be a bit smaller due to the fact that CO not only reduces the size of the Airy disk but also the amount of energy in the Airy disk and therefore in the spurious disk. Same visual theshold with less energy should result in a smaller spurious disk
- separation 0.6" does not sound very precise and could mean 0.56" to 0.64" - if the latter ...
- may be the spurious disk size for +4.5mag stars is not 60% but 55% of Airy disk. Have not yet found a precise algorithm here.
The first factor alone makes your observation of Zeta Boo with dark space between the spurious disks plausible according to the rules of diffraction theory.

Mircea, again the calculation for your observation of Eta CrB with some modifications:
Airy disk for 200mm without CO = 1.38"
Airy disk for 200mm with 0.227 CO = 1.3"
Spurious disk assumed (to be on the save side with only) 50% = 0.65"
With a separation of 0.6" this would mean a minor overlap and therefore no dark space between the spurious disks but as Fred mentioned the separation may be rather 0.67". Together with the argument that the energy loss due to CO may reduce the size of the spurious disk a bit more makes your observation plausible within the rules of the diffraction theory.

Question for me remains the actual size of the spurious disk in % of the Airy disk depending on magnitude. This 50% or 60% or less or more seems so far not more than (hopefully educated) guessing.
Second for me open question is the potential additional reduction of the size of the spurious disk due to the CO caused energy loss in the Airy disk. Further investigation needed.
Wilfried


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ziridava
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Re: eta Crb & gammaCrb new [Re: WRAK]
      #5884262 - 05/25/13 04:04 PM

Wilfried
Thank you,your explanation is an eye opener for me.
Last night I was trying to duplicate the split of Eta CrB with the 8 inch Dobsonian at 200x/Radian 6mm,but I had not any chance.The turbulence was high and fast moving clouds were crossing the sky,I was trying my luck observing trough sucker holes.
When clouds covered Eta CrB ,I was jumping to clear corners of the sky.This way I revisited
Polaris,Izar,Saturn,Mizar,Albireo and Delta Cyg,all resolved,only Eta CrB remaining just a fuzzball.Then I noticed masive clouds comming from South-West.I made a final try at Eta CrB,seeing only the same annoying fuzzball.I continued to look as clouds become thicker.
The image start to fade and ,from a certain moment, to become clearer until the eight shape of the touching spurious disks become visible.After about two minutes from noticing the eight shape,Eta CrB was no more visible in the telescope.
Mircea


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Nucleophile
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Re: eta Crb & gammaCrb new [Re: magnus]
      #5893544 - 05/31/13 02:01 AM

I have observed these recently with two reflectors as detailed below.

With a 15" f/4.5 reflector fitted with a Paracorr lens on a night of very good seeing (Pickering 7):

Eta: 398x (Pentax 5mmXW): resolved; 498x (UO 4mm): barely a split with a thin black line; 569x (3.5mm Pentax XW): clearly split to two evenly matched light yellow stars--beautiful.

Gamma: [observed using a newly installed Antares Optics 1/30 wave secondary]: 220x (9T6): elongated; 398x: beautifully resolved-----and then a huge fog bank rolled into sight and its been cloudy up until tonight. I need to finish this one for the split--guessing the 4UO may do the trick or the 3.5 Pentax.

Tonight I revisited these with my 8" f/5.9 reflector (also fitted with a Paracorr lens). I wanted to make sure I could observe at high power for extended periods, so my equatorial platform was used for tracking. I really wasn't expecting much but felt the data was important nonetheless. The seeing tonight was excellent--easily an 8 on the Pickering scale. The results were amazing to me:

Eta: 395x (Pentax 3.5mm XW): definitely elongated; 460x (3 mm Radian): classic peanut shape on the border of elongated and resolved; 550x (Pentax 2.5 mm XO): resolved! no question 2 objects were seen.

Gamma: 460x: single star; 550x: clearly elongated indicating two entities.

The Dawe's limit for this 8 inch telescope is computed to be 0.57"---I think I got really close to it tonight.


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ziridava
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Re: eta Crb & gammaCrb new [Re: Nucleophile]
      #5922449 - 06/15/13 03:23 PM

Mark
Congratulations,I'm envious on your equipment and I see you have such a good seeing there too!Those high magnifications are out of question at my location.

Last night I revisited Eta Cr B on a Pickering 6/7 sky.At 200x/Radian 6mm,the two discs were clearly visible,in an eight shape.I spent at least half an hour looking and looking again at that beautiful image.
Next I increased to 320x/Celestron ''orange'' 7.5mm Plossl+2x ''no name'' Japanese Barlow and at times,fluctuating,the black space between the components was there.But the image was not so pretty like at 200x,so after obtainig the proof of split,I returned at lower magnifications and nicer images.

I had no luck with Zeta Boo and Gamma Boo which I courted.

Mircea


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Nucleophile
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Re: eta Crb & gammaCrb new [Re: ziridava]
      #5922826 - 06/15/13 08:42 PM

Quote:

I spent at least half an hour looking and looking again at that beautiful image.





Many of my double star entries contain the word "beautiful". I had no idea what a satisfying endeavor double star observing was when I first forayed into astronomy. ( I am still addicted to faint fuzzies, for what it is what).


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Re: eta Crb & gammaCrb new [Re: Nucleophile]
      #5962429 - 07/09/13 03:58 PM

Yesterday with NEML +3.5mag and medium seeing Pickering ~5-6 I did a session in CrB including Gamma CrB 0.6" +4.04/5.6mag according to WDS. I noticed despite the not this good seeing several times for parts or seconds an elongation in the direction of 2 o'clock - checking afterwards for my time and location confirmed this observation (or with a low probability of less than 5% a lucky hit). This should indicate that the above mentioned ~0.7" separation is probably correct as I don't think such an observation could be in combination with a delta-m of 1.56 possible with a separation of 0.6".
Wilfried


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