Submitted 1st double star measurement

[lunator]

Thanks Ian

Next bit then :- Have figured out how to calculated those two expressions, apart from the Sin bits! I can't get from 0.998 to 86.1 - what have I missed / not going??

Daz the sine of 86.5 is 0.998. The answer generated by the equation is 0.998. You apply sin-1 to get 86.5

The rest, I think, makes sense (you'll be glad to hear!!), oh apart from the statement that you add the two values to give you a total of arc seconds, because one star was above the centre line and one was below. Could you just clarify this a bit more.... I mean do you plot the stars on the chart (as per your sketch) for this bit.......

Thanks mate

The difference in declination can be added togeter or substracted.

The equation field radius R x cos Ya or Yb gives the 2 stars distance in arc seconds from the centre of the field.

If the stars pass on opposite sides of the centre line you add the distance in arc seconds from centre of the field together to give the total distance between the stars in arc seconds.

If both stars pass in the north or south part of the field you substract Yb result from the Ya result. This will give you the difference in declination between the stars.

[daz]

Thanks Ian for your patience :lol:

Now I geddit! Took me a few minutes of staring at the Windows calculator of how to do the sin-1, but have sussed that and can work through the calculations

I still have the list of doubles you posted a little while ago to look at, so I'm going to practice on these!!

[lunator]

Daz

If you pan on making some measurements. Start with a pair that are about 100+" apart and also will have a reasonable East/West split.

STF196CD was tricky to do as they are very N/S oriented so enter ans exit the FOV quite close together.

The first star measurment I did was on theta Lyra.

http://www.cloudynights.com/item.php?item_id=1209

Cheers

Ian

[daz]

Hi Ian - I found this a few minutes ago and guessed it was you from other posts you've made about where you live.

A good read and I think I'll try and locate a copy of the Bob Argyle book.

[daz]

One last question!!!

To calculate the FOV radius I need the True Field of View (degrees) * 60 (seconds) * 4.

e.g. A 20mm EP with 52 APOV gives a 1.04o FOV * 60 = 62.4, then * 4 = 249.6

Is this correct?

[GazOC]

Daz,

Thats the 'good enough' equation for FOV, if you want to be more accurate you'll need to use the field stop of the eyepiece.

TFOV = 57.29578*EFSD/TFL

where EFSD is the eyepiece field stop diameter and TFL is the telescope's focal length.

Gaz

[daz]

Ta Gaz

[GazOC]

Theres usually between 1% and 5% difference in the answers given by the two methods. Not much but still there.

[lunator]

Daz

At a basic level the true field of view is the AFOV/Magnification. The focal length of the scope is key to this.

If you also use a Barlow to get a flatter FOV this obviously cuts down the true FOV.

if your scope is 1000mm long the 20mm gives a magnification of x50. The true FOV is 52/50 = 1.04 degrees or 3744 arc seconds (3600 arc seconds in a degree).

I my experience the true FOV is slight smaller than the figure you would get by dividing the AFOV/Mag. I have oput this down to manufacturers 'rounding up' the AFOV number.

The way to accurately work out the True FOV is to pick 2 bright stars that will pass on either side of the field of view.

You time their transits and apply the 2 equations

tan X = 7.5205 (S2-S1)cos ds + 7.5205(N2-N1)cos dn/delta d

Tan Y = 7.5205 (S2-S1)cos ds + 7.5205(N2-N1)cos dn/delta d

The field radius is R = delta d/2 cos X cos Y.

I checked wirh the Simbad database to get 2 suitable stars.

Cheers

Ian

[daz]

Good job I like maths!!

Thanks guys

[GazOC]

The first I'd do is sit down a write a couple of Java apps for the maths!!

So Ian the other FOV method is accurate enough is it?

[lunator]

Gaz

It seems to provide an pretty good level of accuracy.

The ring micrometer method is seen as providing good results for wide doubles. I have measure down to a 50" seperation but below this the time taken to press the button on the stopwatch becomes a significant source of error.

I am in the process of measuring wide doubles that appear to have CPM so may well be gavitationally linked.

Cheers

Ian

[daz]

I timed my 40mm last night on delta Orionis. It took 5mins 52 seconds to transit, and I calculate this as 1' 27".

I have to confess, I have more questions on the last calcs you put up, but I waanted to research a bit before hitting you again, Ian

[lunator]

Daz

Fire away. I'm always happy to answer questions.

What is the AFOV of the 40mm? and what magnification does it porvide?

I got a response from Bob Arygle today asking meet to write a report on the method I used.

I also got a reply from Brian Mason who manages the Washington Double Star catalogue. :lol:

Cheers

Ian

[lunator]

It appears I am the only person in the UK (or the world?) that is measuring double stars this way.

I have had further emails from various Webb Society members asking me to explain the process. They are very interested in the results I have obtained and they are trying to undersrtand the process.

My results are being analysed and should be submitted to the WDS.

I'm feeling quite happy at the monment despite the relentless clouds.

Cheers

Ian

[daz]

That's great news Ian

Many congrats.

I WILL come back some questions, I have been a bit busy so not had a chance to sit down and think properly. But I will !!

:cheers:

[lunator]

Just a brief update.

My submission appears to have raised some interest. I am now in discussion with some fellow astronomers about the measurements as they are using my information to see if they can replicate the results.

During this process it has been discovered that the delphi programme that is used has a small bug in how it calculates the averages.

The current question I have in this discussion is that the C component has a Declination of 20.5954 but this has been alterd to 20.9984 in some of the responses. I am going to ask why this is (it maybe about changing the degrees into base 10?) if any of you know why I'm always interested.

Cheers

Ian