# Diffraction grating for astronomy?

1. Mar 18, 2009

### bina0001

Hi, i am new here, so am not really sure wheather i am asking this qus at the right place. I have a Lab based qus. I just did the atomic spetra analysis using a diffraction grating, analysing hydrogen gas.

But but when i calculate the wavelengths of the lines, its way off from the literature values. I asked my prof, he said somthing about the grating is actually for astronomy, and not really suited for this experiement. I have to find out wats the diffrence.

so whats the diffrence between grating for astronomy and the normal gratings? I tried googling, cant really get any viable results. is there any refeance material I could look at?

2. Mar 18, 2009

### Andy Resnick

Richardson Grating Labs has a nice little book all about gratings, available for free. In it, 'astronomical gratings' are simply very large gratings- there is not inherent unique property about gratings used for that purpose.

Now, there are many types of gratings based on manufacturing method (ruled, holographic, replicated), optical power (planar, concave, grisms, echelles), but none of those obviously accounts for your data. So, my question is:

What have you done to calibrate the grating? Laser lines are ideal, but the sodium line(s) is also a common standard.

3. Mar 18, 2009

### bina0001

Actually according to the prof, the grating was mistakenly bought this experiment. I suppose the original plan was to get a grating that would not require calibrating cos the lab manual never said anything about calibrating the grating. But since we end up with this, prof decides to turn this into a mini research project, and we would only need to discuss qualitatively the discrepancy in the values.

So, if gratings requires calibration, u mean to say not all gradings fit the standard equation of dsin$$\theta$$=n$$\lambda$$? So how do i calibrate it? is it similar to the calibration of a prism? Are there some other forms of grating equations? I am reading through the handbook that u recommended, but it seems like most of the gratings they describe are reflective gratings. However the onces i have read about so far and used in the experiment was something that let light through, then causes interferance on the other side. are the similar conditions?

sorry about the long and wordy reply, i do like to ramble on, and thank for all the help!

4. Mar 18, 2009

### mgb_phys

Yes they fit the equation but - how do you measure the angle for each line?
The normal way is to shine light of a few known wavelengths into the system, note the position on the detector and then fit a function to wavelength/position.