Comments on: Is there an Astrophysicist in the house?

Question: Is there an Astrophysicist in the house?

reflecked — Thu, 04 May 2006 16:34:19 -0800

I would like help understanding the CHANDRA xray satellite mission.

I'm having trouble understanding X ray astronomy, specifically the CHANDRA mission. After reading the majority of the information from Harvard's CHANDRA site, MIT's, and Penn State's, as well as numerous data searches in Astronomy journals, I still have huge gaps when trying to visualize what is happening.

Most importantly, I can't figure out what is happening with the High and Low Energy Transmission Gratings as they rotate to reflect the rays. I "get" the Rowland circle pattern that the gratings are mounted in, but can't visualize what is called the "Rowland torus" that occurs when the Low energy waves are manipulated by reflection.

What I'm looking for is someone who can walk me through what happens to an X ray from the time it enters the sun-shaded aperture until a recording is made on the data grid.

:) No, I'm not taking a class. If I was, I'd likely have the maths needed to understand more completely.

By: sergeant sandwich

sergeant sandwich — Thu, 04 May 2006 19:17:30 -0800

does this help?

look at the diagram on the bottom right. basically i think the idea is that the whole thing is not flat like a circle, but curved.

By: sergeant sandwich

sergeant sandwich — Thu, 04 May 2006 19:41:11 -0800

sorry, i guess i missed the part where you wanted an overview - i was only answering the torus bit.

basically this telescope has 2 functions - imaging and spectrometry. imaging is just what it sounds like - you make an image of something very far away, like a regular optical telescope.

spectrometry on the other hand is when you use something like a prism to separate light into different wavelengths. this information (the relative intensity of various wavelengths) allows you to infer all sorts of fantastic information about whatever it is that you're looking at. generally you do this with something called a diffraction grating.

the transmission gratings in chandra are simply diffraction gratings that work by transmission, and there are 2 of them, optimized for two different ranges of wavelengths. light goes through them, and because of interference, different wavelengths go off at different angles.

ok so anyway - light comes in the front of the telescope, and is reflected off the grazing-incidence mirrors. without anything else in the way, it would be focused to a CCD array at the back of the telescope where it forms an image of the thing the scope is pointed at. this is imaging mode.

if you insert the transmission gratings into the mirrors, then instead of getting an image, you'll get a spectrum on the CCD. (i would think) you'd get rings of bright and dark at the image plane, where different radii correspond to different wavelengths. then you can plot intensity vs wavelength and you have a spectrogram.

that's basically it - sorry if this is too basic; i have assumed a general audience.

By: sergeant sandwich

sergeant sandwich — Thu, 04 May 2006 19:41:56 -0800

if you insert the transmission gratings into the mirrors

erm, behind the mirrors!

By: reflecked

reflecked — Fri, 05 May 2006 03:21:34 -0800

sergeant sandwich, bless your heart! I was thinking that my question was badly worded and I'd get no answers. Thank you so much.

Your answer was NOT too basic. I'm downloading the pdf now,

Questions answered beget more questions, as is only right. You've given me enough to go on. I "get it" now. That feels very very good.

Again, thanks.

By: saffron

saffron — Fri, 05 May 2006 04:07:10 -0800

I'm acquainted with someone who designed CHANDRA and I'd be happy to put you in touch. Check your email :)

By: reflecked

reflecked — Fri, 05 May 2006 09:05:15 -0800

Email received and replied to, saffron. What a great opportunity. I really appreciate that.

By: reflecked

reflecked — Sat, 17 Jun 2006 16:38:28 -0800

Follow-up:

What I was doing was giving a presentation on the satellite mission. I belong to a "club": a group of professional women who meet once a month to hear a lecture by one of the members on a complex subject. We all have our areas of expertise, and are assigned topics totally outside that.

I'm a medical type, with a love of chemistry and microbiology, so... I got astrophysics.

sergeant sandwich's explanations helped me understand an aspect that had been completely eluding me. They also helped me to ask good questions when I contacted that Chandra designer that saffron put me in touch with.

I gave a killer presentation. I focussed on the satellite itself and did not stray into the realm of celestial mechanics. (Not so dumb after all.)

Thank you both so much. I appreciate your help; you are both indirectly responsible for me NOT making a fool of myself.