Double Star separation

[Scooot]

In sky safari it lists the separation of the double and it's location from the host star in °, at least I think that's what it is.

So Albireo for example is 34.7" at 55°. I understand this is a separation of 34 arc seconds but I can't understand the 55°, what is it 55° from? I don't understand why 0° Is where it is, or what it represents?

Can anyone explain this for me, many thanks.

[cotterless45]

The separation is given in arc seconds , marked as ".

The position angle is given in degrees. This is the angle from north that the companion makes from the primary, as explained in Sky

"The two measures of a double star are separation and position angle. The directions shown here are for an inverting scope, such as a Newtonian reflector. Other telescopes may have different field orientations.

Sky & Telescope illustration.

For the double-star observer, there is nothing more inviting than a clear and steady night. After polar aligning, the first order of business is to make sure that your reticle eyepiece's position-angle scale is set up correctly. The inevitable jostling that the telescope is subject to between uses warrants that this be checked at the start of every observing session. Fortunately, the procedure is straightforward. I do this by allowing a star near the celestial equator and on the meridian to trail along one of the two parallel lines of the micrometer's linear scale. This is done with the telescope's drive off and without using a Barlow lens. The micrometer is rotated in the eyepiece holder until the star follows the reticle line very accurately. Once this condition is met, I rotate the position-angle protractor until the 90° mark is indicated by the pointer, taking care not to disturb the eyepiece position. This process is repeated with the Barlow. The pointer is now calibrated with the north point at zero and east at 90°.

With the preliminary adjustments out of the way, you're all set to begin making double-star measurements. The pair being measured should be at least 30° above the horizon to reduce differential refraction and well within the magnitude and resolution limits of your telescope. Begin by locating your first pair and boosting the magnification until the stars span as many reticle divisions as practical. The method I find best for measuring separation with the Microguide reticle micrometer is to carefully rotate the eyepiece until the primary and secondary stars are parallel to the linear scale and the primary is dead center on one of the scale's divisions. Estimate the separation of the pair to 1/10 of a division and convert this into arcseconds using the calibration value for the setup. That's all there is to it — you have just measured the star's separation.

Before you start celebrating, keep in mind that there is a second equally important measurement to make: the position angle of the pair. Begin by rotating the eyepiece until one of the reticle lines of the linear scale bisects both the primary and the secondary at the same time. Now is a good time to note the approximate position-angle reading on the degree scale. This will serve as a reality check to ensure that you haven't made a mistake and that your position angle is near the expected value for the stars being measured.

When you are confident the reticle is correctly positioned, take an accurate reading off the degree circle. Then rotate the reticle 180° and repeat the reading. This procedure is done again for a total of four readings. The results are averaged (remember to subtract 180° from the readings obtained by rotating the reticle). Keep in mind that when the primary star is centered in the field of view, the companion is said to have a position angle of 0° if it is directly to the north of the primary, 90° when directly east, 180° when directly south, and 270° when directly west. Be careful to avoid accidentally offsetting your measurement by 90° or 180°. This is where your reality check comes into play. For greatest accuracy, the measurements of separation and position angle should be repeated on four to six nights and the results averaged."

- See more at: http://www.skyandtelescope.com/observing/observing-double-stars-for-fun-and-science/#sthash.aF5o5x57.dpuf

It's not as grim or geeky as it sounds. Mostly the pleasure is finding combinations of colours and separations. In practice the pa is often used to identify the possible position of elusive companions. The separation can be challenging and the faint separations can be just fun.

If you head down the threads you may come across viewing and imaging double stars, where a host of companions are available !

Nick.

[Scooot]

Thanks Nick,

I did wonder whether it was north but couldn't rotate my screen and keep the zoomed double in the view at the same time.

The article is a good read. -

[Qualia]

Richard, a little while back I wrote a little article on measuring doubles. I use a micro- guide, but to be honest, if you just wanted to work out the position angle of a double, a circular protractor cut around an eyepiece will suffice.

Needless to say, ace post Nick

[Scooot]

Richard, a little while back I wrote a little article on measuring doubles. I use a micro- guide, but to be honest, if you just wanted to work out the position angle of a double, a circular protractor cut around an eyepiece will suffice.

Needless to say, ace post Nick

That's a great article Qualia. I must admit I wouldn't mind having a go at some measurements sometime but not sure I want to buy a micro guide just yet, and I'm also not sure the tracking on my EQ platform would be good enough to get any meaningful results. As you say it's a good way of learning more about the stars etc without really trying.

Originally I was probably being a bit dense by not being able to work out from what the position angle was being measured when North should have been obvious. I was trying to use the sky chart on sky safari to understand it and couldn't see its orientation at the same time the double was visible on the screen.