Measuring positional angles

[peterbolson]

I've recently become interested in double stars & would like to try measuring PAs. Would the Meade MA 12 mm Astrometric Eyepiece be suitable for this ( less than half the price of the Baader ) or are there are other cheaper ways of doing it ?

Thanks

Peter

[DarkerSky]

The Meade eyepiece would be a good choice. I don't have it but I know of it and it'll do the job for sure. Alternatively, but slightly less accurately, you could use a 360-degree marked circle pasted onto cardboard with a central hole cut out so it fits onto your eyepiece barrel. You'll need a way to secure it, and you'll also need to laminate it or similar to make it waterproof. Basically, you can use it just like the astrometric eyepiece, only the degree-marking that you're taking measurements off is exterior to your eyepiece view. You're therefore having to follow the 'imaginary' line to read your PA that bit further, and by taking your eye further back from the eyepiece - all of which introduces a bit of error. With practice you can read off PA's to within 1-2 degrees accuracy.

[peterbolson]

Thanks Seb

I had already made one of these cardboard circles that you describe but was wondering whether I should go for something a bit more high tech. I have just come across this fascinating article which descibes how to use the Meade ep

http://www.perezmedia.net/beltofvenus/archives/000778.html

I think my best plan is to get some practice with my bit of cardboard before I start spending any money ! If I can achieve 1-2 degrees of accuracy that sounds pretty good to me. It might be worth making the modified protractor that Jeremy Perez describes in the above link & using that instead of the cardboard.

Peter

[astrofox]

Scaled eyepieces cost a fortune.

If you have a goto mount could I suggest another way. If you have an eyepiece with simple cross hairs centre on one star and read alt/az from the handset. then centre on the other star and repeat. The differances in the reading are two side of a right angled triangle. Simple pythagorous will give the hyp. or angular seperaration

[peterbolson]

Hi Frederick

About £92 for the Meade.

Your idea sounds a good one but I don't have goto.

I'm going to stick with my cardboard protractor for the moment & try & get the hang of looking at it & the eyepiece at the same time ! I find watching for where the double drifts to the edge of the ep field to determine west is the most difficult bit to do accurately. As I have a Newtonian I have to reset it every time I change constellation.

[astrofox]

Hi Peter,

Well if it works for you great. I just re read the link you posted Its quite a complex process to set up but also an elegant solution.

Dont think reading the setting circles would be accurate enough on a non goto. Well guess its back to the grey skys of Oxfordshire

[lunator]

Hi Peter

I have been measuring doubles using the celestron eyepiece. The Meade will work well.

I recommend getting Bob Argyles book on doubles. It will propvide you with all the information you need.

I have used the chronometric method fro measurements and you can get pretty good results.

Cheers

Ian