# Unsure how to obtain galactic roll?

I am having some issues with understanding what my adviser means by "galactic roll," but I have not been able to find anything online regarding this.

My project involves generating an array of az/el coordinates, converting them to ra/dec, then converting them to galactic lon/lat. I then use these coordinates to scan a Healpy map (all in Python).

She did mention starting off by setting horizontal roll = 0, and then converting horizontal coordinates to galactic coordinates. The galactic roll should then take on nonzero values, which is important to the data I am analyzing.

I've read about Euler angles and quaternions and I think I'm on the right path. She provided some code for me to reference, and that uses quaternions and Euler angles. Does galactic roll refer to one of the Euler angles (perhaps $\theta$)? Another reference code she provided mentions parallactic angle; could that be it? Any help or reference material would be appreciated. Thank you!

(And before you mention this, yes I will ask her. I just want to give this an honest attempt first.)

• You should primarily ask your supervisor. Nobody expects students to know everything from the start. The learning process is part of science and nobody should feel embarassed to ask. – AtmosphericPrisonEscape Mar 23 '18 at 22:05
• @AtmosphericPrisonEscape I'm not embarrassed to ask her at all (I've already asked dozens of questions, what's one more?) This is more of a personal challenge than anything. – Rose R Mar 23 '18 at 22:10
• I think this is "roll" in the sense of "roll, pitch, yaw", the three coordinates used to define an airplane's rotation. It's an unusual way of giving galactic coordinates. – barrycarter Mar 24 '18 at 14:13
• I keep reading this question and thinking "Well, I would look for it in my local supermarket, next to the arctic roll." – Mick Mar 24 '18 at 15:14