Resolving Porrima, vs Antlia spiral

[sculptor]

Anyone got some recent data on Porrima (gamma virginis)? According to Hartung it is supposed to be a Romp-It-In 3" arc. Last night I had a go at resolving it with a York Optical 6" refractor (focal length 900 mm). Nothing like 3". I added a Tele Vue x4 and a 13 mm eyepiece, which effortlessly resolves eta orionis at 1.4" arc with room to park a bar-fridge in between the stars. Nothing. Switched to an 8mm eyepiece (450x), which is theoretically beyond the ability of the instrument. Now Porrima was clearly resolved (ie not just elongated), with a position angle of around 45 or 235 deg (the two stars are equally bright). I am guessing the separation is around 0.6" arc.

A look round the web shows Porrima was closest round 2005 at 0.3" arc.

Interestingly, the 11" Celestron SCT had no hope of resolving it. "Economical" light buckets seem good at photographing faint galaxies (see NGC 2997 attached: Mag 9.7, size 10' x 7' arc, SB 13.5 or so) not resolving doubles.

[sculptor]

Come on all you guys with magnificent kit, pop outside, have a look at Gamma Virginis, eyeball the position and separation, and let me know your verdict!

All I know is that I can just barely resolve it. :) But what is it that I'm just barely resolving? There is just no up-to-date info on the web that I can find, and it is a famous binary that is changing fast. Even that is a little bit interesting.

Heroes needed!

[ngcles]

Hi Sculptor,

This:

http://articles.adsabs.harvard.edu/full/1990JBAA..100..277H

and for general info on the star:


http://www.astro.uiuc.edu/~kaler/sow/porrima.html


may be of some use. The BAA Journal indicates that the (revised) periastron should have occurred in 2005 (not 2008 as predicted long ago) with the components now rapidly widening -- according to the ephemeris, the sep should now be approx 1.1 arc seconds with PA at about 25 degrees, reducing pretty quickly (in the scheme of things).

However the 6th catalog of orbits of visual binaries indicates here:

http://ad.usno.navy.mil/wds/orb6/orb6ephem.html

a slightly different prediction for Porrima (STF 1670AB) that the separation should currently be 0.924 arc seconds in PA 41. The same catalogue indicates that by 2009 it is out to almost 1.2 arc-seconds in PA 30, and that periastron occurred in 2006 at about 0.40 arc-seconds

It seems the orbit has not been well known and so the predictions -v- observations have had a bit of scatter in them. Observations of this periastron will probably refine the elements somewhat for the future and reduce the uncertainty.

Very well done in splitting it! Your observed PA is pretty/very close to the prediction:thumbsup: but your estimate of the sep is a bit off according to either predictions.

I'll take a peek if I remember when the 'scope is out next (but with Sydney's weather recently ???) (but shoud I really bother trying at all:: I've only got a "light-bucket" -- woe is me!):sadeyes:

Sculptor wrote:

"Interestingly, the 11" Celestron SCT had no hope of resolving it. "Economical" light buckets seem good at photographing faint galaxies (see NGC 2997 attached: Mag 9.7, size 10' x 7' arc, SB 13.5 or so) not resolving doubles."

Hmmm ... can't see why a (good) C-11 wouldn't do the trick. As for "economical light buckets ... not resolving doubles" -- Pah! My old-faithful 10" F/6 on a Samson GEM mount once resolved Gamma Sextantis in 1994 which was at the time I think 0.51 arc-seconds.

Try that with your fancy-schmantsy 6" refractor!:P

Nice Pic BTW.

Best,

Les D
Contributing Editor
AS&T

[glenc]

Guide 8 gives 1.03" @ pa 33.6. In a years time it gives 1.27" @ 25.1. In 2 yrs time it is 1.48" @ 19.2.
http://www.projectpluto.com/

[sculptor]

Thanks guys! If Porrima is around 1 sec arc, it's no wonder I could resolve it in the 6", and I'll stop patting myself on the back. But it's a puzzle that it was a mess in the 11".

Rutten (Telescope Optics, p221) gives a detailed discussion arguing that a large central obstruction (SCT) can make a small improvement in resolving power on bright doubles, but a huge worsening on low-contrast planetary detail. Berry (Star testing astronomical telescopes) makes the same point, and casually throws in (p289) that "seeing must be superb before the double star resolution test yields interesting results".

The good old Cambridge Encyclopaedia of Astronomy says "an optical telescope of 10 cm diameter has a resolution of about 1 arc sec. If that diameter were increased to 1 metre, the scope would have a theoretical resolving power of 0.1 arc sec, but ... is spread over an angle seldom less than 0.5 sec arc by fluctuations in the earth's atmosphere". In other words, even good seeing limits the effective diameter of the scope to 20 cm. I have a dim recollection of having read that elsewhere, but can't put my hand on it.

So on a superb night from a desert mountaintop, a superb 11" SCT should actually be better than a 6" refractor. But on a good night from a back garden at sea level, the SCT should be no better than an 8", and perhaps the other night, my very ordinary SCT looking through very ordinary to turbulent air, was much worse than the 6" refractor mainly because of the fatter column of turbulent air. Attached are some images of some random doubles on random nights through the 11". It can clearly manage 1.2 sec arc on a typical night.

I'm not trying to pontificate here - I really don't know - I like galaxies and planetary nebulae and gas and dust. They're more than a pixel across, and they have personality. For me, doubles and variables and planets and asteroids and open clusters are what you do when it's windy or the moon's up. I'm out on a limb.

I really hope this discussion excites some of you to go out and look at gamma virginis, or other doubles like eta orionis (1.5), gamma CrA (1.3), xi sco (0.6), pi lup (1.4), zet boo (0.8), - with a variety of instruments, on typical and brilliant nights, and let us know what you see.

The open questions for mugs like me are:

- what can you resolve with a 6 to 12 inch scope on a typical night from sea level, vs a brilliant night from Wirruna ?
- does a central obstruction make an important difference for us?
- what is the effect of the price tag? Should I buy a 14" ACOS RC, or stick to trying to photograph with my STL-11000M through a pair of hand-held binoculars because on a larger better instrument the seeing will get me?

Let's go out and look! :) :) :)

Meanwhile, does anyone have any good quantitative info on the effect of poor to median (typical) to good (one night in 10) to superb seeing on resolution in seconds of arc for a large instrument?

Cheers,
Mike