What do you need for Double Star Position Angles?

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  • Member since
    February, 2009
What do you need for Double Star Position Angles?
Posted by LoneStar77 on Wednesday, February 04, 2009 5:32 AM

I've never had expensive equipment, but I plan to upgrade soon. One of my areas of interest is double star observations and measurements.

 I have several star catalogs that give double star position angles (PA's) and separations (seps), and for some stars, for two or more years. Combining the readings from several sources, it is relatively easy to approximate a period for some pairs. It is also relatively easy to see that some are merely visual doubles, because the relative motion is completely linear -- the product of proper motion only.

 For many stars, the data is sorely lacking. I'd like to add to the existing data for some of those star systems, but I don't know how to acquire the needed accessory. Well,... I don't even know what such would be called. Protractor? Angular reticule? And is this made for amateur telescopes? If so, where would I acquire one?

LoneStar77 (Carl Martin) www.AncientSuns.com
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  • Member since
    July, 2002
  • From: Texas
Posted by chipdatajeffB on Wednesday, February 04, 2009 9:50 AM

You've touched on a subject near and dear to my heart, but I'll try to be as brief as possible(!).

I'd recommend you start out with a Celestron Microguide or Meade Astrometric eyepiece. These have a built-in position-angle and separation measuring reticle. With a little practice and the right technique, you can stay within 1/2 degree PA and 0.2 arc-seconds separation (depending on your scope and seeing) reasonably well. This is quite good enough for the purposes you mention (verifying pairs infrequently -- if ever -- updated). You can also use these reticles for other purposes, such as measuring the diameters of lunar craters (anything where the distance to the object is known accurately), the length of a comet's tail or the width of its nucleus, etc.

The next Reply may seem a bit involved, but essentially if you use an astrometric reticle eyepiece, it will be like projecting a ruler and a protractor into the sky and aligning the two stars with it to do the measuring. In concept, it's just that simple.

 

The universe is not only stranger than we imagine, it's stranger than we CAN imagine. --- JBS Haldane

Come visit me at Comanche Springs Astronomy Campus (we're on Google Maps) in Texas.

www.3rf.org

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  • Member since
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  • From: Texas
Posted by chipdatajeffB on Wednesday, February 04, 2009 10:17 AM

Bob Argyle's recent book on Observing & Measuring Visual Double Stars should be your first stop (click here for an Amazon link). That's about the best all-in-one reference I'm aware of. It's about equal parts history, technique, and "interesting projects/equpment".

It covers the use of the astrometric eyepieces quite well, though relatively concisely. There is more to it than just slapping the eyepiece in the focuser, reading the angle, and writing it down.

Both the Meade and the Celestron reticles include an outer circular protractor (in the eyepiece field of view, but around its periphery) and a linear scale crossing the center. You use the linear scale for separation and the protractor for PA.

First, you must calibrate the reticle to your scope. Since each scope's focal length is different (at least slightly), and since differences occur from time to time depending on how you seat the eyepiece and whether you use a diagonal, barlows, filters, etc., you must calibrate the reticle for each different setup you use. Some folks calibrate or reverify calibration at each session. Remember, you're going to be using as much power as the seeing will allow when you're measuring very close doubles (anything under about 15 arcseconds separation), so even a tiny difference in the setup will show up.

If you're just into this for grins, an easy way to calibrate is to observe and measure a star pair whose separation is currently known very accurately. This will give you a rough approximation (with about 10%, depending on the currency of the listed separation).

A more accurate way is to time the drift of a star along the linear scale and convert the time and number of scale divisions crossed in that time, into arcseconds. The eyepieces come with an instruction sheet that gives the formula for doing this. Note that the arc traversed by a star in the sky depends on its declination, so that's a factor in the equation (the cosine of DEC).

So, you'll want a scientific calculator if you're going to "do it right" ... and if you plan to do much of this.

Once you know the calibration of your reticle (arcseconds per scale division) at a given magnification, then measuring separation is straightforward. Most of the time you will make multiple measurements, then average them. But, there is more to it if you want to submit your results to organizations like the AAVSO's Double Star group or the Webb Society.

Remember that our atmosphere is denser at lower target altitudes (you're observing through more air), so that will skew the measurements. The technique, then, is to make 5 to 10 measurements of a star twice each night: one set of measures on the East side of the meridian (and preferably at a target altitude of more than 45 degrees),and one set on the West side at an equal target altitude. Average these measures to get the answer for a given date (which, by the way, is reported as a Julian day to three decimal places, if you're submitting results to a journal).

You should also do this at least twice during a given year, separated by a few months (I like to space the measures 6 months apart, to allow for Earth's motion-related parallax).

How you measure position angle with these reticles depends on your scope setup (whether it inverts or reverses the image ... the protactor scales have inner and outer markings, depending on what type of optical setup you're using) and which reticle you have. While at first glance the Meade and Celestron reticles look the same, they are slightly but significantly different.

I use the Celestron eyepiece, which is slightly more complex to use unless you use a refractor for your measurements, viewed straight-through (no diagonal). I just purchased a C-11 SCT to get more "reach" than my 6" refractor allows, so I will have to change my routine.

But, in either case, once you have decided which reticle scale you'll be using, the technique is very simple: You center a star (any star in the pair's field of view) on the center of the linear reticle, turn off your mount's drive, and let the star drift to the protractor reticle, then turn the drive back on. The position of that star on the reticle is the PA. HOW you center the star depends on which eyepiece reticle you have. Read your eyepiece's instruction sheet (or the directions in Argyle's book).

If you want to get a feel for how this all works, without spending any money first, go to the Celestron site and click Support/Downloads. You can download the MicroGuide manual in PDF form there.

Alternatively, you can search online for articles about this type of project. Sky & Telescope published two articles in the 1990s about this (if I remember correctly, both were by Ron Tanguay).

All my double star documentation is currently at an observatory about 15 miles from where I sit right now. I was doing measurements just last night and will do so again tonight. When I get back there after work today I'll check those for additional references and get back to you if you have questions.

Get Argyle's book before you buy an eyepiece (or, more expensively, a filar micrometer!). You'll be glad you did.

The universe is not only stranger than we imagine, it's stranger than we CAN imagine. --- JBS Haldane

Come visit me at Comanche Springs Astronomy Campus (we're on Google Maps) in Texas.

www.3rf.org

  • Member since
    February, 2009
Posted by LoneStar77 on Thursday, February 05, 2009 1:07 AM

ChipdatajeffB, thanks for both of your informative posts. Sounds as though you've been at it awhile. I can hardly wait to get a copy of Argyle's book, as you suggested.

I see you're from the Dallas area. I'm originally from Odessa, Texas (my screenname, of course, gives me away as a former Texan).

Part of my interest is in writing software, like one 3D piece I wrote a few years ago, called "Stars in the NeighborHood" (www.SpaceSoftware.net).

 

LoneStar77 (Carl Martin) www.AncientSuns.com

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