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Minimum power to split Porrima.

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#1 Kon Dealer

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Posted 27 March 2019 - 03:08 PM

Ed Ting reckons x47 with the new Astrophysics 90mm.

Me, I’m struggling to get clear split at x60 with my very good TS90 triplet and a 12mm Xcel-LX.

I’m also at 51degrees North, so Porrima is not very high.

Anyone else in Ed’s ballpark and with what?



#2 flt158

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Posted 29 March 2019 - 01:19 PM

As I have nothing between 40X and 112X, it's my 10 mm Pentax that gives the latter magnification and a lovely split of Porrima over the last number of years, Kon Dealer. 

My scope is a William Optics 158 mm f/7 apo at 53 degrees north.

I will check out this marvellous binary soon. 

And I greatly admire the way it changes its separation and PA year after year.

That's my little piece. Just thought to chip in with my opinion. Thank you for raising your interest, Kon. 

 

But I'm sure 60X or a little more is sufficient to split Porrima If I had such an eyepiece. 

 

Aubrey. 


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#3 Al-Fazari

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Posted 02 April 2019 - 01:33 AM

Did this last Saturday night, 8" reflector, f5.9, with a 8-24mm zoom. Definitely there at 8mm, which is 150x, and somewhere in between, since at 24mm it is only 50x. Nice double.

#4 Kon Dealer

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Posted 02 April 2019 - 07:45 AM

Thanks for responses. Had another go, with my TS90 triplet, on Sunday night and realised in my first post I was in error. 

Got a good clear split with my 7mm (x85) and just about with my 9mm (x67). That’s as low as I could get. 12mm (x50) was a narrow-waisted “peanut”.

Most attractive split was with 5mm (x120).

Then replcated all these splits with my Kson 102 ED f/6. Seemed a touch brighter, but no closer to splitting at x50.


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#5 dUbeni

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Posted 04 April 2019 - 01:36 PM

I was a bit surprised with your first post because I have no difficulty splitting Porrima with my 80mm ED f9, and a couple weeks ago I did split it with a hair of black between them with a 55mm f5.5 telescope.

A post with a sketch of the observation on the sketching Forum:

https://www.cloudyni...mall-telescope/

 

CS

Bernardo 


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#6 Jon Isaacs

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Posted 05 April 2019 - 07:35 AM

According to both Sky Safari 6 Pro and Sky Tools 3, Porrima is a 2.84 arc-second split.  Under stable skies, it should be splittible at 47x or even less.  Porrima should transit for you at about 35 degrees. 

 

I suspect seeing. 

 

Jon 



#7 JimFR

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Posted 09 April 2019 - 09:27 AM

I stumbled on Porrima last night.  What a pretty little double that is, identical twins.



#8 Thomas Marshall

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Posted 09 April 2019 - 07:40 PM

This thread convinced me to look at this star, and Sunday I put it on my list for that nights session with my AT102ED. I used a Baader 8-24 zoom, and it clearly showed double before maxing out at approx. 89X, - But putting in a 3X Barlow, and going 89X thru 267X was the fun part, - clear and sharp, with plenty of dark between them all the way to the Max.  Doubles are fun, but I don't know enough of them, - I'll put together a personal target list of them one of these days. This scope has given me great views of the Dbl./Dbl., Rigel, and now Porima, and some others also, so a session devoted to doubles is worth a "to do" list. 


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#9 Jon Isaacs

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Posted 14 April 2019 - 07:59 AM

Last night I was enjoying some doubles with my 120mm F/8.3 achromat. About 10:30, I remembered this thread.  Porrima was at about 40 degrees and clear of the trees so I gave her a try.

 

In the 120mm, it was a beautiful split but a low magnifications, under 80x, there was no way I was getting a clean split.  The achro optics combined with the large exit pupil and my old eyes meant I needed some magnification.  Next time I will try it with my 80mm apo and see what I can do. 

 

While I was lined up with Porrima, I decided to give it a try in the finder.  It's a 70mm Orion Multi-Use finder.  The original objective was pretty poor so I replaced it with a 70mm F/4.5 air spaced Carton objective from Sheldon Faworski.  It's much better.  

 

At F/4.5 there's a fair amount of false color though it tends to be redder than the typical purple one sees in most scopes.  One of the difficulties is that there is no focuser, it's slip-slide focusing and at F/4.5, that is pretty tricky.  My first try was with the 4mm TMB Planetary for 79x and I saw elongation and a possible split, with the 6mm TMB and a 2x Barlow for 105x, I saw a clean split.  

 

I think Porrima is at 2.85 arc-seconds, an easy split in a good 70mm with a real focuser but with this finder and no focuser, I was darn happy.  It's not what one would call a double-star scope and the color was way off but I did make the split and it's the closest split I have made with this particular scope.

 

In light of the other threads current in this forum, What is the Best Double Star Scope and Aesthetics versus Aperture, this scope proves that any scope can be a double star scope and there are challenging, doubles for any scope. 

 

Compared to the beautiful white pair in the 120mm, this messy red image was not aesthetically pleasing but I just making the split was beautiful. 

 

Jon


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#10 John Fitzgerald

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Posted 14 April 2019 - 10:28 PM

Porrima has widened nicely again in the last several years.  It was an easy split in the 1970's-80's-early 90's.  Then it rapidly closed.  I couldn't split Porrima at all for a year or so around 2004-5.


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#11 Jon Isaacs

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Posted 20 April 2019 - 07:42 AM

Porrima has widened nicely again in the last several years.  It was an easy split in the 1970's-80's-early 90's.  Then it rapidly closed.  I couldn't split Porrima at all for a year or so around 2004-5.

 

:waytogo:

 

This time of the year in 2004, Sky Tools 3 says Porrima was 0.64",   I have enjoyed watching it slowly widen.  There was one season when it began right at the Dawes limit for an 80mm and widened enough that it was noticeably easier, I think 1.51" towards the fall.

 

As far as splitting Porrima at 47x, I haven't managed it but I suspect it's my eyes.  They are not as sharp as they once were.  10 years ago, I could convince myself that I had split the double-double at around 40x but that ain't happening now.  

 

The eye doc says that my observing eye has no correctio and he could call it 20-20 but says that's not real.  I liken my eyes to a substandard telescope, they come to focus and there's no astigmatism but there's all sorts of other issues.. 

 

Jon


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#12 fred1871

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Posted 20 April 2019 - 08:34 AM

Jon, Porrima got to be a bit closer than 0.65" at minimum. Looking at the listing of measures using speckle interferometry, 0.65" is the separation for January 2004; for 2004.3 (matches this time of year) 0.58"; for 2005.3 somewhat closer, at 0.38", and that was near enough the minimum, although there are a couple of 0.37" measures toward mid-2005.

 

By 2006.3 it was back to 0.47" (widening happening), and by 2007.3 a much easier 0.75". For 2008.3 it was 1.01", and 2009.3 an easier 1.23". Closer to now, the 2014.3 measure gives 2.15". So, by 2014, even a 60mm scope would suggest two stars, with the separation by then wider than Dawes and approaching Rayleigh. Small 'scope observers could have tracked the widening over those years - first re-appearance as a non-single object would depend on aperture. 

 

Regarding eyesight - that's going to be the determining factor for minimum magnification, for an optically good scope of reasonable aperture. Some observers find that a magnification giving the equivalent of 160" is needed for seeing a bright pair resolved; others with above average eyesight might manage with 120" subtended. Better and worse than these numbers also happen. So if we take your figure of 2.85" for current separation, 160" eyesight will need 56x, and 120" eyesight will need 42x. Simple arithmetic. Find what power works for you, that gives your eyesight resolution for this pair with the scope you're using. Decent seeing conditions assumed.


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#13 Frisky

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Posted 21 April 2019 - 02:16 PM

I found and split Porrima last night. I used my zoom and split it at 8MM. I backed off to about 9 and 10mm, and it remained split. Really a striking little double! 

 

Joe


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#14 Redbetter

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Posted 22 April 2019 - 04:34 AM

I don't think that optical quality has that much to do with splitting doubles at lower power, unless the optics have some sort of substantial defect that noticeably distorts the airy disk pattern.  This is the level of defect that renders an optic a reject to my eye.  Low power and larger exit  pupil also make minor levels of astigmatism more of a factor, which will ironically favor a smaller aperture for the same magnification (due to smaller exit pupil), as long as it is sufficient aperture to easily resolve the airy disks. Seeing also plays a part in requiring more magnification for the same split and is a problem for brighter stars.

 

I was using Porrima as a test subject tonight for drift testing some eyepieces as well as checking some objectives.  Seeing was poor, about 3/10 in the 8" SCT and 5/10 in an 80mm refractor--several arc seconds with substantial twinkle...and this was a good night compared to the prior two which were nearly useless windy affairs.  I was testing out a 60 f/15 objective tonight, and it was having trouble with the split although the 60mm showed it, distorted on one axis but apparent, and resolved at 100x although not pretty or what one might expect.  The primary problem was with the optics, probably not fixable, didn't feel like tweaking it tonight.  Meanwhile the 8" SCT was also having trouble at ~100x because the seeing was blurring the diffraction pattern so badly that it was a moving mess spilling on to one another (thermals were not an issue as conditions were stable and very mild.)  I could see the two stars, but they were bloated.  Higher power did resolve them more clearly, but demonstrated how badly the seeing was impacting the diffraction pattern.  The other problem with an 8" on a tight and bright pair is that 100x is a 2mm exit pupil.  By the time I reach 3mm, my rather low level of astigmatism becomes a factor limiting acuity, while at higher powers it is irrelevant. 

 

Next up was the 80 f/5 (Meade version of the ST80)...this particular sample has very good optics for a short achro, and has actually put up credible planetary images.  It proved to be a "Goldilocks" solution for conditions, my eye's minor level of astigmatism, and inherent brightness of the pair.  I could see a notched peanut at 67x despite the poor seeing.  At 80x the two airy disks were separate but appeared to be nearly touching.  Beyond that they were clearly resolved, although the first rings and spurious disks were dancing about...illustrating the limit to resolution this night. 

 

On even a mediocre night all of these scopes would perform better and my eye/exit pupil combination would likely become the limiting factor.


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#15 Redbetter

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Posted 23 April 2019 - 03:34 AM

Slightly better conditions tonight, more drift timing, so I turned to Porrima.  I could see the two components at 85x nominal in the 8" SCT with 2.4mm exit pupil, the pair is a bit bright for this. 

 

I brought out the AT60ED and was able to see the double lobe at 60x, beginning to clearly resolve at 72x, and fully resolved by 90x. 

 

Out of curiosity I again used the Meade 80 f/5 achro and could see some indications of the split at 57x and it was definitely beginning to resolve at 67x and cleanly resolved beyond that. But what I really wanted to test was stopping it down, so I put the lens cover on and removed the central cap to achieve an effective aperture of 42mm.  This was showing elongation at only 67x, and showing a double lobe at 80x.  At 100x the impression was of two overlapping disks.  Each alternately appears a little brighter/in front of the other, and this contact binary appearance remains at 133x


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#16 Robert Zebahl

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Posted 24 May 2019 - 03:41 AM

I visited Porrima several times this year. My smallest used telescope is a 55/500mm refractor (achromat). My notes with this refractor:

 

At 38x slightly elongated. At 56x touching diffraction disks. At 71x clearly separated with very small gap.


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#17 CharlieB

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Posted 24 May 2019 - 10:35 PM

I got a full, decent split tonight at 100x with my 50mm Unitron (700mm f/l) and a 7mm ortho, but it was beautiful at 140x with a 5mm ortho with a good sized gap and nice diffraction rings.  I wanted to get the 50mm Nippon Kogaku (750mm f/l) out to compare, but Porrima was already in the trees and the mosquitoes were hungry.


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#18 gdjsky01

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Posted 26 May 2019 - 06:44 PM

I split Porrima last night (May 26th 05:00 UTC) clean with a Sears 80mm f/15 1970's refactor at 96x. It was not split at 45x. Took note of the angle with reference to the westward drift of the equal white pair, and checked when I got inside. Bingo.

 

I am 15 miles south of LAX and 9/10ths of a mile from the Pacific. Very light breeze.


Edited by gdjsky01, 26 May 2019 - 06:56 PM.

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#19 MP173

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Posted 28 May 2019 - 01:13 PM

May 26th

ETX90RA:

26mm - 48x - no

16mm - 78x - split, possible touching.

16mm 2 x barlow - 156x - WOW!  Diffraction rings, great view.

 

Similar star, I call it Porrima lite is STF 1669 which is south of M104.  5.9/5.9 mag @ 5.3" separation.  Another beauty for small scopes.  Easy split at 26mm (48x).

 

Ed


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#20 Nucleophile

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Posted 28 May 2019 - 10:37 PM

I have a data point, but I'm not sure how valid it is considering the instrument I had available to me while vacationing (see pic below).  Some of the more experienced folks on here may recognize this orange 60mm f/11.7 refractor--it is pretty old.

 

RescuedRefractor.jpg

 

This refractor has a single 20mm 0.965" EP remaining in its arsenal along with a 2x barlow--this gave a whopping 60x magnification, which is what I used to see Porrima as elongated with an occasional hint of notch.  I feel better optics would have shown it as resolved to two discs, but maybe not split (?)

 

It is amazing I even got the object in the FOV considering the finder was broken and I had to sight the star using the end of the optical tube.  It certainly helped the locale is very dark.  The mount was not too bad--it responded well to nudging to keep Porrima in view.   At least on Polaris no nudging was needed. smile.gif

 

So that was some double star fun I wasn't expecting to have.


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#21 Uwe Pilz

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Posted 30 May 2019 - 10:12 AM

I remember when I observed Porrima at periastron. I used my 12 inch Dobson at 430x, ant this was not an easy observation. Now it is rather easy im my 4 inch refractor. It was not a problem to split it with around 100x.


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#22 SeaBee1

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Posted 02 June 2019 - 09:39 AM

On May12, this double was on my observing list. Using my new to me SW 120ED, I had no expectations about splitting Porrima, and I had never viewed it with any of my other scopes. This would be my first observation. I could not split the pair under 180X. As many of the other observers here report, I should be able to split the pair at a much lesser magnification. My notes indicate the viewing conditions were not optimal, and I am thinking this would have been the reason I needed such extreme magnification. I will have to try this one again when the opportunity arises, and hopefully under better conditions.

 

Keep looking up!

 

CB



#23 dUbeni

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Posted 02 June 2019 - 06:02 PM

Yes SeaBee1, please try again, it is a wonderful pair, and I did split it this year with a 55mm, f5, telescope, at 121x, hopefully you want regret it.

 

CS

Bernardo


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#24 SeaBee1

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Posted 03 June 2019 - 09:30 AM

Hi Bernardo! I do intend to try again, but clear nights aligning with my work schedule have been rare recently (I work nights...) but I am sure the opportunity will come up again, just not sure when. I certainly agree this is a wonderful pair, and could easily become a favorite! It would certainly be interesting to observe this pair over time and note the changes in separation and position angle as it moves through its orbital period.

 

Clear skies Sir!

 

CB


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#25 CharlieB

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Posted 03 June 2019 - 09:57 PM

Finally got the 50/750 Nippon Kogaku out for a few minutes before Porrima went behind the trees.  Again, a very clean split at 109x with a 7mm ortho.   It looks like 100x is about what one would need for a clean split with a good 50mm scope - maybe a bit less.  That large Airy disk needs some extra power to separate, but the stability of the image is worth it.


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