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David,
welcome to the real world of slit spectroscopy!
One advantage the Spectra-L200 has, it the ability to see the zero order image of the slit.
When calibrated, the grating rotation angle should be set to bring this zero order image to the centre (say along the X axis) of the imaging frame; this will give some control over the central wavelength and the spread of the spectrum recorded.
So far so good.
Now to register and position the target onto the slit .... obviously if the guide system (flip mirror/ guide scope etc) is 100% aligned to the main optics and the CCD chips are exactly square to each other, then a star at x=300, Y=300 in the guide camera will be "registered" and mapped to a point x=500, Y=500 on the main image (assuming chips of 600 x600 and 1000 x 1000 pixel respectively)
This is never easy to achieve in practise...
What happens is that the target is position as close to centre as possible in the guide image, "scanned" across the X-axis until a star image appears in the imaging camera ie the star is visible through the slit. Fine tuning then moves this star image onto the fixed edge of the slit. If you have a rear illuminator fitted then this can allow the rear of the slit opening to be seen in the image with the star superimposed on the slit - it can then be eased up and down until it lies central to the height of the slit. These fine position adjustments are made using the position of the star in the imaging camera - now look at the relative position of the star in the guide image - it may, say be sitting at X= 307, Y=284 this then becomes the "guide" star registration point for future targets. If the reticle is positioned at this coordinate then you can double check, by easing the star up and down the slit the inclination of the slit in the guide image and rotate the reticle to suit. You now have a "virtual" slit superimposed on the guide image which shows exactly where the centre of the slit is located and the direction of the slit. this makes is 100% easier to position the subsequent targets exactly on the slit time after time. Remember the slit will only be a few pixels wide, so there's not a lot of room for error.
Hope this helps,
Ken
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