Observing Double Stars

Blackant (Ant) · #1 01-09-2009, 09:30 PM

Hi all,

I've always loved looking at double stars with my bino's, and since getting my 8" Dob it's suddenly opened up a whole new world of them for me

One thing I'm a bit unsure about, is how to know if what I'm looking at is really the double star that I think I'm looking at. I'm using Toshimi Taki's brilliant free Double Star Atlas, and basically once I star hop to the right place I then use the details in his star list that give the Position Angle to see if it matches what I'm looking at. Sometimes it's obvius, but if there is a lot of stars in the view it can be tricky...

For example, I just came in from looking at Beta Scorpio, and according to Taki's charts, the Pos Ang Ep1 is 24 degrees, and the Pos Ang 2 is 25 degrees. Now my maths is a bit dodgy I'm afraid

but when we are talking about an angle like that, where is it measured from? Do I just assume that the top of my view is O degrees? Or am I completely barking up the wrong tree and there is an easier way to figure it out

Just one more quick one two, I was trying to split Antares as well at 218X magni and couldn't, should I be able to do that with an 8" dob?

Thanks in advance,

Ant

seanliddelow (Sean) · #2 01-09-2009, 10:01 PM

I was unable to split Antares with my 4mm EP in my 12" dob. Its really hard to split it.

Blackant (Ant) · #3 01-09-2009, 10:20 PM

Quote:

Originally Posted by seanliddelow

I was unable to split Antares with my 4mm EP in my 12" dob. Its really hard to split it.

Thanks Sean, if it's hard with a 12" and a 4mm ep I can imagine it's impossible with an 8" and a barlowed 11mm

Regards

Ant

seanliddelow (Sean) · #4 01-09-2009, 11:24 PM

Im going to try a barlowed 2.3mm Ep.

Robh (Rob) · #5 01-09-2009, 11:54 PM

Double stars are obviously harder to split when they are closer together.
A separation of 10 arcseconds is fairly easy at 100 magnification but 4 arcseconds requires 150 magnification to be seen easily.
However, this assumes the stars are around similar visual magnitude. Where one star is much brighter than the other, the brighter star's glare can drown out the secondary. In the case of Antares, the red primary is of visual magnitude one but the blue-white secondary is of magnitude 5.5. The separation is only about 2.9 arcseconds, so the primary overwhelms its companion. Difficult to see even in larger telescopes as the primary's brightness is enhanced even more.

I find the magnitudes and separations for multiple stars useful but the position angles are not much use for just two stars (binary). The PA of the secondary is measured counter-clockwise from the primary and a line to celestial north. The problem is that in a high power view you don't know where the direction of the NCP is and constellations are continually on the move. Consequently, establishing your orientation is difficult except on lower powers where you can establish the position of a clearly defined nearby star. However, where there are more than two stars in a system, the separations, magnitudes, and position angles can all be used to try to establish which stars are which relative to each other.

Regards, Rob

Dennis · #6 02-09-2009, 08:10 AM

Hello,

I’ve split Antares with my Vixen 4” refractor and a Tak 5mm LE eyepiece on a night of excellent seeing when Antares was almost overhead.

It does require better than average seeing with Antares above say, 50 degrees altitude to minimise the effects of atmospheric crud!

Cheers

Dennis

seanliddelow (Sean) · #7 02-09-2009, 08:21 AM

I will have to try in clearer skies

Dennis · #8 02-09-2009, 02:49 PM

Hi Sean

Just to expand on my post, I have only been able to split Antares less than 3 or 4 times a year; that is how rare that good seeing coincides with Antares being over head with me outside doing visual stuff.

I have been far more successful with imaging, as I can then adjust the capture settings and processing, to suppress the glare from Antares and tease out the fainter companion.

Cheers

Dennis

Octane (Humayun) · #9 02-09-2009, 03:13 PM

I've seen it once. Through Zane Hammond's 25" Dobzilla. I can't remember what eyepiece we were using, but, it was easily split and the colour could be made out, too. It was a rather beautiful sight.

Interestingly, using the same telescope, we were able to see faint colour in the Orion Nebula and easily see the Running Man Nebula. Top instrument, that!

Regards,
Humayun

Blackant (Ant) · #10 02-09-2009, 08:19 PM

Quote:

Originally Posted by Robh

I find the magnitudes and separations for multiple stars useful but the position angles are not much use for just two stars (binary). The PA of the secondary is measured counter-clockwise from the primary and a line to celestial north. The problem is that in a high power view you don't know where the direction of the NCP is and constellations are continually on the move. Consequently, establishing your orientation is difficult except on lower powers where you can establish the position of a clearly defined nearby star. However, where there are more than two stars in a system, the separations, magnitudes, and position angles can all be used to try to establish which stars are which relative to each other.

Regards, Rob

Thanks Rob, I thought it would be a bit more tricky then just up in my telescope, I was just hoping

Good advice on the magnitudes and separations, I'll try and rely mostly on those.

I've been having so much fun with the new scope just touring around that I haven't spent any real time yet trying to build my foundation skills of estimating the magnitudes of stars and distances etc. I'll start now though, the further I get into this hobby, the more I want to understand and get further into it

Uh oh, I feel my wallet starting to hurt

Thanks for the hints on Antares too everyone. Dobzilla has to be the coolest name for a telescope I've ever heard, and very appropriate for a 25" one

Kind regards

Ant

GeoffW1 (Geoff) · #11 02-09-2009, 08:25 PM

Quote:

Originally Posted by Octane

Dobzilla.

Humayun

Miaplacidus (Brian) · #12 02-09-2009, 10:13 PM

I've heard that if you know where Antares' secondary star is in relation to the primary you can have some success utilizing the diffraction spikes from the spider holding the secondary mirror to enhance the odds of catching the faint companion in between. Also, a good cheat, although a fairly rare opportunity, is to use a favourable lunar occultation. Gotta be quick, but once you've seen it, you've seen it.

Or just buy yourself a 25 inch dob...

Satchmo · #13 03-09-2009, 10:47 AM

Quote:

Originally Posted by Miaplacidus

Or just buy yourself a 25 inch dob...

A 6" ED or Apo refractor is just about ideal for splitting Antares.

astro744 · #14 03-09-2009, 12:00 PM

Try to split Antares at dusk. I did it with a 10" and it was easy with a bit of power! Mind you it was about to be occulted by the Moon and that made it easy to find too.

The secondary is within the glow of the primary under dark skies but twilight dims the primary enough for the secondary to stand out.