Introduction

This page contains some notes on the construction of a reflection grating spectrograph that has auto-guiding capability. The basic mounting is a hybrid design of spectrographs published on the Web recently by amateurs such as Christian Buil, the  ARAS group 'Barèges' Spectrograph, Ernst Pollmann and others.

The parameters critical to the correct choice of focal length, size and focal ratio of the various optical components were calculated using Christian's Excel spreadsheet available on his site. This useful little spreadsheet enables those of us doing amateur spectroscopy access to a quick and simple tool to explore different spectrograph design parameters and to examine the effects of component changes on the final telescope-spectrograph combination.  (My thanks to Christian for making this available to all amateurs).

The following components are used in my design:

• Adjustable slit, capable of full closure and to several mm in width.

• Collimator -  150mm focal length achromatic doublet lens (25mm diam.). Available from Edmund Industrial Optics

• Focusing lens - Pentax 50mm f/1.4 SLR camera objective.

• Reflection grating, 50 mm square, 600 or 1200 lpm.

• Off-axis guiding system - 90/10 % beam splitter to send 10% of light to the guide CCD camera.

• Main Detector - Starlight Xpress SXV-H9 camera with 6.45 micron pixels (unbinned).

Go to this link to see the full technical characteristics when the above parameters are used with a D = 305mm f/10 SCT.

The Basic Components

To keep the weight of the spectrograph as low as possible for this prototype version, I wanted to avoid materials such as sheet metal and aluminium. Acrylic plastics such as Plexiglas (Perspex), although attractive and not as heavy as metal, can crack and chip when complex shapes need to be machined.  In the end, I finally decided to make the body of the spectrograph in MDF (Medium Density Fibreboard). MDF is a composite material composed of a phenolic resin and fine wood fibres and particles pressed under high temperature and pressure.  It is almost as strong as ordinary wood for a thickness above 8mm, but is less expensive than plywood and about 30%  lighter in weight. The material can be drilled, sawed and sanded in the normal way and does not splinter as compared to the more coarser particle board ("chipboard"). It does however tend to be somewhat fragile when unpainted and can easily be dented if knocked or dropped, so avoid dropping the pieces!  The images below show some of the basic components of the spectrograph at different stages during construction:

	The base of the spectrograph body. The angle between the main optical axis and the position of the CCD is 38 degrees (refer to calculations on C. Buil site)
	Spectrograph showing some of the optical components and mounts. In this picture I am just playing around with some of the components to check positioning and fit. In order to reduce weight the 200 mm telephoto was replaced by a simple achromat lens (shown later) and the collimator focal length reduced to 150 mm. The fixed slit indicated in this image was eventually replaced by a variable slit (more heavier but much more flexible in use).
	The assembled body without components. One of my holes was drilled incorrectly - a warning to be careful with your measurements.
	Slit seen from the telescope side. The metal plate was retrieved from my junk box (conclusion:  never throw anything away!) and is used to attach the slit to the rear wall of the spectrograph. The corner cut-outs are to allow for attachment screws to pass to fix the spectrograph to the telescope.
	Slit seen from the "grating" side.  An adjustable slit such as this is an important and very useful component, but can be a difficult item to find second hand. This one was obtained from an old and obsolete laboratory monochromator, and then cleaned up and renovated. Note that a slit such as this can be purchased from specialist suppliers (e.g. Edmund Industrial Optics) but is very expensive - more than the cost of the whole spectrograph itself (with the exception of the CCD)! If you cannot find one yourself, a much better idea is to make one from workshop items - link to the Barège Spectrograph to see how to make a slit with pencil sharpener blades!
	The slit, on its mounting plate, can now be seen fixed to the wooden rear side of the spectrograph during this test. Normally the rear side is permanently screwed and glued to the spectrograph body but is shown here separately for clarity. The large black circular plate comes from a disused CCD filter wheel that I dismantled. It has the right type of adapter to connect to my LX200 telescope.
	Same image as above but seen from the telescope side.
	The mounting of the micrometer slit in the spectrograph usually means that a large hole of some sort must be cut into the side of the body, which can cause some unwanted scattered light to enter the spectrograph. I therefore used this aperture stop, mounted in a wooden frame, to block off any stray light. The diameter of the aperture can be varied and is usually stopped down to a width just slightly larger than the length of the slit as seen from the grating position.
	The lens mount for the f = 150 mm, 25mm diameter achromat that collimates the f/10 divergent beam coming from the slit.
	The beam splitter temporarily attached to its mount with tape. This is to be inserted between the collimator and grating to allow an off-axis beam to be available for guiding. Since I want to test the automatic guiding capabilities of my system, my design calls for a portion of the collimated beam to pass through a small video camera lens (adjusted to infinity) that focuses the off-axis beam onto the guide CCD. The guiding system is to be installed in the Mark II version of the spectrograph.
	The support mount for the reflection grating. This is a 1.5 inch square mount from Edmund Industrial Optics (Ref No. 33497) that I attached to a base plate. The mount is also available in a 90 degree version (Ref. No 33499) that can be easily fixed to the spectrograph. In my design a 1/4"-20 thread screw passes from the mount through the base of the spectrograph to allow for grating rotation from underneath. The adjustment screws on the back of the grating mount permit fine corrections if the principal optical axis is not in perfect alignment (which is probable with a home-built unit such as this).

Putting it all Together: 

	In this, rather strange image, I used all of the coupling rings I could find to simulate an f/10 light cone focused directly on the spectrograph slit. A small lens at the end of the long black 'tube', positioned the correct distance from the slit, produces this beam. The collimator lens is then moved closer or further away from the slit until the slit image is in sharp focus. In this way, focused light from the slit is then rendered parallel by the collimator when the spectrograph is attached to the telescope which is used mainly at f/10.  (My son's school books were put to good use here !!)
	The image of the slit (shown here out of focus) can be seen through the 45º prism placed just after the collimator. An focusing ring on the collimator lens barrel allows fine adjustments to be made and can be locked in position with a set screw.
	The left image shows the slit as seen through the access hole that was drilled behind the grating mount at the rear end of the spectrograph. The grating is not in position (obviously!) but the mount is in position. Fortunately, the Edmund mount has a central 8mm hole that allowed me to check the alignment between the grating position and the slit position - my calculations and measurements were OK!  The right image is basically the same but taken through a small finder scope (hence the crosshairs). The jaws of the slit can be clearly seen.
The spectrograph attached to the LX200 SCT. At this stage the off-axis guiding capability has not yet been installed.	Overall size of spectrograph compared to the telescope. Although I made efforts to keep the design as low in weight as possible, the instrument still weighs around 2.6 kg including the CCD camera, so some counterbalancing of the system may be required.

First Light & Initial Results: 

Fluorescent Lamp -

The spectrum was mirror reversed in the x direction and the spectral profile below was obtained using Visual Spec by Valerie Desnoux.

Sun (From reflected cloud cover) -

This image shows that the dispersion axis is not perfectly aligned with the x-axis of the CCD. Pre-processing of the image was performed using Christian Buil's free IRIS software (the ROTATION command) to orient correctly the dispersion axis, before importing into Visual Spec. This is the first non-artificial light source and also the easiest - blue sky or clouds. The spectrum is not radiometrically corrected nor calibrated in wavelength. Most of the absorption lines in this deep red region are due to atmospheric H₂O and O₂.