Double star separation estimate

[Sunshine]

Hi! when looking at double stars how do i estimate separately without looking it up? i know arc seconds are about 1/3500th of a degree but that doesn’t help without a visual example at the eyepiece of what an arc second separation looks like. Having said that, how does magnification change this and how do I account for it in the formula? lets say Theta Aurigae appears to be separated bu x distance at 100x and X distance at 200x. Lets say I’m observing it at 200x, how do I convey it’s apparent separation in such a way that can be visualized by you, the reader.

Because right now I am temped to say that Theta Aurigae appears to be separated from its companion by twice its diameter at 175x in my 4” refractor, but this is not true “amateur astronomer” talk, which is an oxymoron if there ever was one 🤣

Edited November 18, 2023 by Sunshine

[Franklin]

A Meade Astrometric reticle eyepiece or even better a Microguide (Baader/Celestron). You calibrate the scale for a given optical setup by timing transits with a stopwatch, Once this is done you can use the scale to measure separations and also use the protractor on the reticle to measure position angles. 

Edited November 18, 2023 by Franklin

[Sunshine]

Thanks, sounds like an Einstein project but I’ll look into it 😅

[josefk]

Hi @Sunshine - i found this to be a good read: https://www.handprint.com/ASTRO/bineye1.html#standard

Except the anomalous behaviour of the eye/scope combo at "diffraction magnification" (where the visual separation doesn't change even if you magnify it - something i understand in theory but don't know how to apply in practise) it's basically good sound advice to understand your EP FOV in arc minutes then go from there in estimating fractions.

With wide angle EPs i'm not too accurate visually dividing the FOV up to estimate a narrow separation but with narrower EPs (e.g.a FOV ~15') you can be surprisingly accurate estimating even at sub arc minute fractions and when i get my eye in even at +/- ~5" (e.g. <5", ~5", 5-10", 10-20", etc.) - especially when you are doing a few doubles in an evening. You sort of get calibrated as it were.

I tried an etched reticle (simpler than the one @Franklin shows) and it sharpened up my estimation for position angle (drifting stars from the centre of the EP out of the edge and using the guidelines on the reticle to do this precisely) but i can't get it magnified enough to usefully use the scale for "measuring" separation rather than just being a useful help to more accurate estimations.

Cheers

[Franklin]

The rate of sidereal drift is 15 arc seconds per second of time, so by timing the E-W drift of a star across a scaled line you can work out the size of the scale for a given magnification. Once you know the size of each mark on the scale, in arc seconds, you can use it to measure separations, just like using a ruler. The method is to set the scale in line with the E-W drift and with the star at one end, turn off the drive and time the drift of the star across the whole length of the scale. Do this many times and then take the average, then multiply this average by 15 to convert to arc seconds and then divide the result by the number of marks on the scale. If you change scopes or use a barlow then you must re-do the timings for the different optical configuration. There are also more accurate methods which take into account the cosine of the declination of the star but if you make your calibration timings with a star close to the celestial equator the simple method works fine.

[Franklin]

53 minutes ago, josefk said:

i found this to be a good read: https://www.handprint.com/ASTRO/bineye1.html#standard

Interesting article, thanks for posting.

[Mr Spock]

1 hour ago, josefk said:

i found this to be a good read: https://www.handprint.com/ASTRO/bineye1.html#standard

Excellent resource - thank you 👍

[Franklin]

Here's another good explanation from a fellow SGL member.

[josefk]

26 minutes ago, Franklin said:

The rate of sidereal drift is 15 arc seconds per second of time, so by timing the E-W drift of a star across a scaled line you can work out the size of the scale for a given magnification. Once you know the size of each mark on the scale, in arc seconds, you can use it to measure separations, just like using a ruler. The method is to set the scale in line with the E-W drift and with the star at one end, turn off the drive and time the drift of the star across the whole length of the scale. Do this many times and then take the average, then multiply this average by 15 to convert to arc seconds and then divide the result by the number of marks on the scale. If you change scopes or use a barlow then you must re-do the timings for the different optical configuration. There are also more accurate methods which take into account the cosine of the declination of the star but if you make your calibration timings with a star close to the celestial equator the simple method works fine.

My problem is I can’t get a magnification high enough to make the marks on the scale measure a meaningfully small number of arc seconds and I don’t fancy buying a x5 Barlow or similar especially to do it. 

I should say I use the Baader Polaris and it isn’t as finely graduated as the one you showed. 
 

Edited November 18, 2023 by josefk

[josefk]

10 minutes ago, Franklin said:

Here's another good explanation from a fellow SGL member.

That’s another great post to digest!

[Franklin]

These illuminated reticle eyepieces have a 12mm and 12.5mm focal length and it is recommended to use them with a minimum 2000mm focal length scope, so you may need to use a barlow to get the image scale large enough to take the measurements and this means that the drift will literally fly across the field of view. The book "Observing and Measuring Double Stars" ed. by R.W. Argyle is the definitive resource on all this stuff and is highly recommended, though it does get quite technical in places.

[Franklin]

18 minutes ago, josefk said:

I should say I use the Baader Polaris and it isn’t as finely graduated as the one you showed. 
 

Yes, the Baader Polaris is 25mm so you would need a 5x barlow at least to use it for this purpose.

The Meade Astrometric is still in production but the Baader/Celestron MicroGuide would have to be found used.

Edited November 18, 2023 by Franklin

[josefk]

@Franklini can get it down to 21” per graduation with a x2 PM in a 2.4m focal length scope but this set-up isn’t preferred and I really would need x5 Barlow in other scopes.  Good fun though and educational already in its own way. 

[Franklin]

The Meade Astrometric with your 2.4m scope and x2PM would get you up to 400x, which is the kind of silly high power you need to be using.

[The60mmKid]

1 hour ago, Franklin said:

Yes, the Baader Polaris is 25mm so you would need a 5x barlow at least to use it for this purpose.

The Meade Astrometric is still in production but the Baader/Celestron MicroGuide would have to be found used.

Are you sure the Meade is still in production? I was under the impression that it isn't. I just pulled mine out of storage, so this eyepiece has been on my mind.

[Franklin]

1 hour ago, The60mmKid said:

Are you sure the Meade is still in production?

Still listed on RVO but doesn't say in stock, I got one from Opticstar (Meade UK distributor) a couple of years back but not listed there now, so it looks like the used market for both. I did read on the Baader website that they are taking orders for the Microguide from interested parties and if there are enough they will be producing some more.

[lunator]

I have the celestron eyepiece and have used it for measuring doubles. The Bob Argyle books says you need a focal length of 2-4m for this to be useable for measurement. I use a 2x Barlow on my scope which gives a focal length of 4.5m (x360 mag).

I have measured wider doubles by using the ring micrometer and chronometric methods. Both covered in Bob's book and possible with a phone stopwatch -Ring micrometer method and. Stopwatch/illuminated eyepiece for the chronometric method.