Confused with how to read PA on Astrometric Eyepiece

[michaelmorris]

I’m currently working on a talk on measuring double stars for my local astronomical society.  The bulk of the talk is about using an astrometric eyepiece.  I think I’ve got the technique for calibration and measuring separation sorted.  However, I’m a little bit stumped on one aspect using an astrometric eyepiece for measuring position angle.

I understand the technique of centring the primary on the dead centre of the linear scale then letting the star drift until it the centre of the star is exactly between the two rings of the outer protractor.  The problem I have is reading off the angle.

Let me go back a step first.  I presume that position of a star in the field of view is affected by the optical arrangement of the telescope. (i.e. is it a refractor of reflector?, Is there a diagonal? Is the diagonal a mirror or a prism?)  Sorting out east from west is easy as this is already known from calibrating the eyepiece and/or measuring the position angle.  However whether north is clockwise and anti-clockwise from west is, I presume, affected by the scope’s optical arrangement and is independent of the eyepiece. 

Q1 - Is this correct?

Q2 - If the Q1 is correct, then would one then also be correct in assuming that the scale on the outer protractor in the astrometric eyepiece is only correct for one particular optical arrangement?

Q3 - If the Q2 is correct, which arrangement is this?

Q4 – If the Q2 is correct, what corrections will need to be made for the different possible optical arrangements of scope and diagonal?

Q5 – The Celestron/Baader Astrometric eyepiece has two diametrically opposite scales on the outer protractor.  Which scale should one use for the different possible optical arrangements of scope and diagonal?

Thanks
 

[Stu]

Hi Michael,

Not sure if I can directly help, but perhaps something in these three documents might?

Stu

Double Measurements.pdf

Double Measuring.pdf

Microguide.pdf

[Paz]

1 yes 2 yes 3 outer scale is for mirror diagonal set ups that flip the image left/right 4 use the inner angle scale for scopes with raci diagonals and scopes with no diagonals e.g.  Newtonians 5 use the outer scale for set ups that flip the image left/right use the inner scale for set ups that don't flip the image, I.e. as per the answer to q4.

The thing I have to keep reminding myself of is that  newtonians rotate the image 180 degrees like spinning a piece of paper around whilst it stays  flat on a table,  they don't flip the image over which would be like picking a piece of paper up and putting it back down own on its other side like when you turn the page of a book.

[michaelmorris]

12 hours ago, Paz said:

1 yes 2 yes 3 outer scale is for mirror diagonal set ups that flip the image left/right 4 use the inner angle scale for scopes with raci diagonals and scopes with no diagonals e.g.  Newtonians 5 use the outer scale for set ups that flip the image left/right use the inner scale for set ups that don't flip the image, I.e. as per the answer to q4.

The thing I have to keep reminding myself of is that  newtonians rotate the image 180 degrees like spinning a piece of paper around whilst it stays  flat on a table,  they don't flip the image over which would be like picking a piece of paper up and putting it back down own on its other side like when you turn the page of a book.

Thanks, as with most things in life, the answer to one question raises some more!  I now have some questions regarding the starting position for measuring position angle

1 - From my understanding you start by placing both stars on the linear scale with the primary dead centre along the line - Is this correct?

2 - Obviously, this alignment of the eyepiece has two possible arrangements, each of which would produce position angles 180 deg apart. - How does one deal with this?

Thanks

[michaelmorris]

1 hour ago, michaelmorris said:

Thanks, as with most things in life, the answer to one question raises some more!  I now have some questions regarding the starting position for measuring position angle

1 - From my understanding you start by placing both stars on the linear scale with the primary dead centre along the line - Is this correct?

2 - Obviously, this alignment of the eyepiece has two possible arrangements, each of which would produce position angles 180 deg apart. - How does one deal with this?

Thanks

Okay, I've managed to answer my own question.  The secondary star should pointing towards the 'Zero' end of the linear scale.

http://www.jdso.org/volume6/number1/frey.pdf