Spectrograph Build

As promised, here’s how I put the spectrograph together.  I’ve been using this latest toy to see how my filters transmit and block light.  I went mainly with  a design from makezine.com with minor design changes.

Instead of plumbing PVC, I used some left over central vac pvc which was lighter.  I already had purchased some 1000 line per mm diffraction grating film.  To cut the film and its frame and the disk with a slit where the light enters, I used the compass cutter (already making itself useful after the last project I bought it for!)Gluing the diffraction grating onto the frame.  Impatient, so I used super glue.I cut the slit for the distal end of the spectrograph using two razor blades with a piece of card stock in between.

 

Diffraction grating in place!  Actually, in the wrong place.  You’re supposed to view it at an angle, not put the diffraction grating off the light path.  Nothing is glued in place, so easy enough to move things around until I figure out what’s going on…

 

Here’s one of the filters in front of the slit at the end.  Not having the slit fully covered by the filter allows for you to see the full spectrum of the light source in thin lines above and below the filtered spectrum (see example below).  Not that I planned it that way.

 

Here it is in action, with a bright Compact Fluorescent lighting up some paper in front of it.  CFLs have nice emission lines to calibrate with and you can even see them on the camera LCD screen in this picture.  The CFL does not have strong wavelengths in the range of the Oiii filter I’m testing, so I’m shining an LED spotlight on the paper as well.

I recently bought a 1.25″ Celestron Oiii filter.  Amazon has them for a really good price.  Celestron says: “The 1-1/4″ OIII narrowband filter isolates just the two doubly-ionized oxygen lines (496 and 501nm lines) emitted by planetary and emission nebulae, while blocking the rest of the overall spectrum of light.”    Let’s see!

This thumbnail is a bit squashed, but if you click on it you can see it correctly.  The spectra at the top and bottom are coming through the slit unfiltered.  The blue green block on the left in the middle is what the Oiii is passing.  But does it pass 496 and 501?   For that, we need some software.  After trying an online app I didn’t like, I checked out RSpec.  I found RSpec to be easy to use and very flexible.  I’m on the 30 day free trial but I think it’s worth the full price of $99.  Maybe this post will go viral and they’ll comp me,  hmm?  Yeah, doubtful.

I calibrated my spectrograph with RSpec using the CFL. Then I brought in the Oiii filter spectrum.

In addition to generating the transmission curve, the software allows me to do neat things like label important wavelengths, and figure out Full Width at Half Max to show how narrow the filter is.  I haven’t figured out what the Y axis units are yet.

Anyway, it looks like it gets 501nm OK but barely transmits 496.  It does a great job blocking everything else out, but this filter really functions more like a line filter for 501 than how its described.  Of course, my setup could be calibrated wrong.  But I’ve read chatter that Baader makes Celestron’s filters and that Baader Oiiis have this characteristic (Baader does not claim their filter transmits both lines, that I could find).  I have an email into Celestron asking about this but they haven’t gotten back to me after a week.

[-Update:  I heard back from Celestron, as follows-

Yes, this transmits just the 496 and 501nm lines, but it’s not a double line bandpass.  The filter bandpass is broad enough to cover both lines as his graph shows. Also, the measurement would be very sensitive to tilt which would push the bandpass towards the red if the filter glass is not parallel to the spectrograph.

Yes, these filters are supplied by Baader, and the filter case also has Baader’s name on it.

So there you have it.  Guess I should have checked the packaging.  Others have noted these characteristics about the Baader Oiii, such as this web page which describes the curves nicely and this well designed analysis on Cloudy Nights.  I guess we could all be tilting our filters. 8^]

Next post:  Shooting through the Celestron Oiii and Meade Nebular Narrowband filters.  Which nebula to try out with the recently acquired EF 100mm f/2.0?