Main Page the Spectrograph / Spectroheliograph / Spectroscope

Uploaded 2/23/14

The newest version of my spectrograph for astronomical observation is nearing completion. Here I detail some of its features, and how it may be used to generate solar spectra, drift scan spectroheliograph images, and of course a very nice bench top spectrograph for both viewing and recording images. It still has a long way to go, but I feel it has progressed along far enough to starting putting together some write ups on my progress!

Spectrograph - Version 3.

This spectrograph was designed to work both well on the bench, and with very fast optical systems found in my f/4 astrographs. For solar work, I will be using an f/6 system with a Baader Herschel Wedge for basic spectra and drift scan tests. Lets take a look now at the outside of the box as it sits in my home lab right now. Nothing is painted black yet! This is a bonus for now, as it is FAR easier to photograph the individual pieces when they are not nearly invisible black.

Here it is seen in visual mode, the eyepice extends toward the lower left, entry port for light on the left end. There are many hidden details here. The second focuser on top is of course for an eyepiece to view the input beam before the slit for object centering. It is as you can see just over a foot long and about 5 inches tall.

Lets start with some of the EXTERNAL features so you can get familiar with it. This is a close up of the input port. It is a 1.25 recepticle for normally inserting occulars into. But here, I use it two ways. First, you can see Ive put a 1.25 inch tube into it and now it can be inserted into any telescope (that can hold its weight). The other option is the collimating laser - a very essential item for spectrographs. Everything MUST be absolutely in line to keep the spectrum flat and centered with a minimum of distortion.

Now lets look at that second focuser port. You can see down inside it a small square mirror that reflects the input up here. The eyepiece must of course be able to focus at the same focal plane as the slit, so it may have to be a longer FL occular to work.

Now by turning the brass knob on the outside of the box, we can flip the mirror up and out of the way - so that the light goes to the hole in the flip-slit module, and onto the slit itself. The mirror of course can be locked up!

Now looking down into the hole, we see the mirror is flipped up and locked in place - out of the way.

When the mirror is flipped down, this is the eyepiece in the focuser to view the image through the entry port. This could be the Sun, or any celestial object for that matter.

On the other side of the box is another brass knob. THIS one flips the 25 micron slit up and down so that you can directly see the image of what your shooting with the camera or eyepieces with no slit. Normally, when you take spectra or do a drift scan, it is flipped down. This entire mechanism which I call the "Flip Slit" assembly took me the longest to come up with!

On the back of the spectrograph is a lever and door. The lever, which rotates many times around with your finger sets the angle of the internal diffraction grating allowing you to tune the frequency. A degree scale I have not installed yet is nearby. The door - An access hatch to the screws holding the gratings mount so I can swap them out with other gratings for different amounts of dispersion depending on what you are imaging. I have a 300, 600, 1200 lines per mm grating set, all mounted on adjustable bases that I can swap out.

Example of finger rotation of the tuning lever

With the door swung up by loosening a thumb screw lock, I can reach in to the screws on the bases of the grating mounts. (more on this later)

Whats inside the box:

Both lids open up on brass hinges, and reveal the inner components. The left half is the slit, collimator lens (135 mm f/2.8 camera lens), and the imaging zoom lens with an eyepiece installed for viewing the solar spectrum. The right side is the grating and mount for adjustment.

Lets star with the grating and Flip Slit assembly. It is made of clear lexan, and of course will all be painted flat black inside when were done here. But for now, you can see its workings better. The slit is a home made - under a powerful stereo microscope I may add - set of pencil sharpener blades mounted on a flap of lexan. it is attached to the hollow rod above it and the whole affair can be rotated up with the rod extending to the right here. The rod coming in from the left, that is connected to the brass knob flips the 45 degree mirror. Works like a charm.

Slit and mirror flipped up revealing the half inch hole for the light to pass through for the laser collimation and to be able to actually see the image as a zero order reflection on the grating with the camera. NOTE: the slit is masked here to 3mm, but when I remove the black tape on the ends its a full 10mm. This full length will be used when using this in spectroheliograph mode. The solar image with my Zeiss 80mm APO is about 4.8mm.

The primary collimator lens for the slit is a super high quality lens for 35mm film. It is a Pentax 135mm f/2.8 Super Taukamar, one of the finest film lenses ever made for astro imaging. Here is has a new life. The lever (ok its a big tie wrap) seen in my fingers is to vary the focus of the lens slightly for sharp images from UV to IR.

Here is the grating mount and adjuster. The grating ( 1200) is mounted on an adjustable tilt plate for collimation and rotation, and the whole affair is mounted on the shaft of a potentiometer. This will in the future give out a voltage reading that is proportional to the frequency of the gratings tilt, and I can read it with a microprocessor and read out directly in a digital readout the wavelength. Thats for later. The lever mounted on the back is for tilt, and the outside lever for tuning is a long screw that pushes the lever, and the long spring seen here pulls tension on it.

Top view shows how the external lever adjusts the tilt of the grating in a very precise manner. That long silver tube like thing is a spring that is pulling back to keep tension on the push bolt.

Here is the imaging lens. It is a 12 - 75mm zoom of superb quality that is very fast too. I can get the full spectrum in about two shots with the DMK51 with the 300lpmm grating, and it will take 6 or 7 frames to see the full spectrum with the 1200. This is shown with a 25mm eyepiece on the back, mounted on a threaded plastic cap I found that fits the C mount thread. Hot melt glue guns are great!

Seen here with the DMK 51 camera installed, it comes to perfect focus! Spectral lines are razor sharp.

Until I get the inside painted black, i use this black foam board with a small hole in it to get the solar spectrum in the day time. Light still comes in around the imaging lens, and Im working on a black shroud for this...

Lids open with DMK camera installed.

I can use an UV flashlight which puts out 390-400 nm for testing the calcium K response:

Also a standard white LED flashlight fits right in the input port and is a great source for testing the full range of the spectrum visible at different zoom levels.

Thats what I have right now, it is a work in progress, but is coming together fast. First test images soon!