5- Tuning

Tuning the Czerny-Turner is not easy, in fact this is actually the greatest difficulty for anyone who wants to embark on the UVEX adventure. The problem lies in the off-axis use of the mirrors, the large number of degrees of freedom, and the co-dependence of the effects. To illustrate this last point, an alignment defect on the mirror M1 or on the mirror M2 can have similar effects.  In this case, how do you find the correct mirror to adjust? Or, an angle error on the grating can be compensated by the orientation of the mirror M2.  It is not easy to tell when the optics are well adjusted, because aberrations can appear out of the blue. That’s the difficulty.

Do not be discouraged, because with patience we will always get it set up correctly, and finally we will be well rewarded. Here is a possible adjustment procedure…

The rule of thumb is to avoid as much as possible making settings on the telescope arranged on its mount and at night. You will exhaust yourself with little chance of a result! The correct procedure is to make all the adjustments on a table using your observing telescope (if it can be removed from its mount), or by using an auxiliary optical tube, such as a small telescope you can carry around. This bench setup is the basic work tool.  Below, a 65mm f/6.5 telescope is used as the test medium, while the spectrograph will be used on a 250mm f/8 Richey-Chrétien. This difference in size is not a problem. For optimal tuning, however, it is preferable that the test telescope was much fasty as the observation telescope.

Another essential element for the tuning is the light source. By day you can use the light from the sky, or ambient light (a white wall…) which gives for free a beautiful Fraunhofer lines spectrum of a G2V type star. A compact fluorescent lamp is also a very good companion thanks to the presence of intense mercury lines in the ultraviolet (see an atlas of lines later). For red light, it is necessary to use a neon night light. In the image opposite, we have a red night light for a child’s room in front of the objective of the telescope (not to be confused with LED sources, which emit no line)! If you find one of these lights in a store, buy it!

1. Post-tuning

The first precaution to be taken is of course to align the optical elements (M1, M2, the grating) by eye during the assembly with respect to the marks engraved in the floor of the case UV01. In this way, you should get a first spectrum on your sensor without further problems, which is a good start. But it is guaranteed not to be a very good spectrum. The orientation accuracy of the elements by eye is about 1 °, while we must achieve angle errors not exceeding 0.1 to 0.2 °.

It is recommended to use the 300 lines/mm grating to start the adjustments (the mark in the box is planned for this grating and so that the center of the visible spectrum falls in the middle of the detector when the spectrograph is well tuned, more precisely the wavelength 510-nm). You can also adjust with another grating, for example the 1200 lines/mm, but the mark will no longer be valid. For example, at the opposite, the characteristic orientation for the 1200 lines/mm grating.

Note that normally the grating change does not require a new adjustment of the entire spectrograph (mirrors M1 and M2). The operation is relatively simple: remove the UV12 support, change the grating, position the grating holder on the UVEX box, and finally, look for the orientation of the grating that makes the detector reach the part of the spectrum that we want to study. You can also decide to make a support for each diffraction grating at your disposal; it’s even faster.

2. Focus the acquisition camera

The first spectrum recorded by the electronic camera will probably be very blurred because of poor focus.  Move the camera backwards and forwards in the camera mount to make the spectrum more or less sharp (do not seek to refine it too much at this point, as the focus will be reset later after you adjust other elements of the equipment).  Ideally, the camera should be able to slide with a gentle force. If it’s too stiff, use an emery cloth on the inside of the camera mount. If the mount is too wide, attach metal tape around the body of the camera (aluminum sealing tape, try DIY stores). With practice, you will achieve a sensitivity of the order of 0.1 mm by hand adjustment. Note that a locking screw (with nut) is provided for in parts UV07 and UV08.

3. Orient the spectrum image

Next, try to align the dispersion direction and the the spectral lines with respect of the detector axis (the pixels grid). Considering the images below, the idea is to go from the top picture to the bottom picture (UVEX spectra made on a table in daylight):

Before
After

To achieve this, the camera must first be rotated in its housing in order to bring the dispersion axis parallel to the sensor lines (in the example, the presence of dust in the slit causes the accidental horizontal line, the “transversalium”, but in this case it helps to orient the camera!). It is then necessary to adjust the orientation of the slit (rotation of the piece UV04 in the UV03 housing).

Its optical design means that UVEX produces a sharp spectrum only over a relatively small slit height. UVEX is specifically designed for observing point objects, like stars (but you can also realize the spectra of small nebulae, galaxy nuclei …). A form of astigmatism occurs outside the sharp zone, which has the effect of widening the spectral lines and thus of losing spectral resolution. You can see this in the spectral image below:

… we are looking at the emission spectrum of a compact fluorescent lamp in blue. The spectrograph is correctly adjusted here, but it can be seen that the mercury lines become fuzzy at both ends of the monochromatic images of the slit. It’s a form of astigmatism. As a result, this image is really exploitable only in its central part (between the two yellow lines). Note that this area of sharpness widens as you work with slower telescope optics: it’s larger with a telescope working at F/10 than with a telescope at F/5.

Another important point to emphasize is the centering of the area of sharpness, indicated in this figure by the vertical position of the red line. Depending on the machining quality of the UV12 support of the grating, or precision positioning angle, the sharp zone can be offset upwards or downwards (or even right out of the physical width of the slit in the worst situations!). In the event of a problem, remove the UV12 support (this operation is simple, just unscrew the two screws of the UV13 lever), try to modify the inclination of the grating (possibly add a shim of paper to fix the tilting), reassemble the support, and see if the situation improves (perfect centering of the sharp area is not strictly necessary)

4. Orient the M1 miror

We now proceed to the adjustment of the mirror M1. It is here that the calm comfortable and warm bench setting makes the difference. A first approach to the adjustment is to orient the mirror M1 so that the optical beam is centered on the surface of the components that will follow (the grating, M2, the detector). For this, it is necessary to use a compact fluorescent lamp, which is successively brought to one edge, then the other, of the objective of the telescope while observing the spectrum, as in the illustrations below:

Note that the movement is on a horizontal plane. For the procedure to work it is necessary that the spectral dispersion plane is also horizontal, which involves orienting the spectrograph as shown below.

Understanding the purpose of this maneuver requires some optical principles. The following diagrams are ray tracings in UVEX belonging to an optical beam operating at F/6, with the length of the detector assumed to be about 12 mm (the grating is 300 lines/mm). First, here is the situation when the spectrograph is well tuned:

The entire the telescope pupil
is illuminated. There is no
optical vignetting (loss of
rays over the optical edges
of the components) . The
rays, however, pass very
close to the mechanical
limits (note the red part of
the spectrum that borders
the edge of the grating when
arriving at the detector).
In practice, it is necessary
to move the lamp in front
of the pupil of the telescope
during the exposure to
obtain an approximately
uniform illumination.

Here only the left edge of
the telescope mirror is
illuminated. The spectrum
is less intense, but of course,
the rays are conveyed
without vignetting from one
end to the other of the
system. Note that the
spectrum appears extremely
sharp because the beam
is very narrow, which greatly
reduces the optical aberrations
(note, this is stigmatic optics,
the physics teacher can use
this to introduce some nice
concepts of optics, and the
mathematics teacher a little
applied formalism and
geometry).  .
Now the rays come
from the only right
edge of the objective
of the telescope,
simply by moving
the light source.












Now let’s see what happens when an angle error affects the M1 mirror. In the simulation below this mirror was accidentally rotated by 1 ° (only), as indicated by the arrow:

Now the most extreme
red part of the spectrum
is vignetted by the mirror
M2 and also cut off by
the mechanical edge of
the grating. For the
observer, this corresponds
to attenuation in intensity
of the red side of the
recorded spectrum.
There is also a shift in
the wavelength range
(“green” rays no longer
reach the center of the
detector).
Illuminating the left
side of the telescope
pupil. The shift of
the spectrum is always
observed.









The case is then made
more interesting by
illuminating the right
edge of the telescope
mirror. This time the
effect of vignetting is
even more marked for
the observer.






The natural reflex in this situation is to recenter the green part of the spectrum on the detector center by orient the grating :

Turning the grating as
indicated by the arrow,
to place the green part
of the spectrum in the
center of the detector
makes everything
seem fine… …






By illuminating the left
edge of the objective,
again, we observe a
fine spectrum and in
the right place.








But the effect is spoiled
when the right edge of the
pupil is illuminated. This
time, the end of the ray
path goes past the edge
of mirror M2, and in the
image, the red part of the
spectrum is abruptly cut off.
This symptom is the sign of
a misalignment, which we
erroneously tried to
compensate for by turning
the grating, while the real
problem is the mirror M1.

Your first task is therefore to balance the vignetting on the red and blue ends of the spectrum by adjusting the orientation of M1 and using the principle of “sub-pupil” lighting. This operation is valid if the other components are at their nominal position, which is not guaranteed.  All of which means that it is then necessary to proceed by iteration, a process that requires being patient and methodical. We start here with the mirror M1 because it is more sensitive to defects.

After this first-order initial adjustment of M1, we have to work more finely by observing the inpulse response of the lines: the spot image when we illuminate the spectrograph input with a monochromatic point source (like a star that produces light only in one wavelength). An efficient way to produce a point source is to opt (possibly temporarily) for a clear slit Shelyak OP0073 or OP0092 that is mounted on the UV06 support (note, the engraved surface must be on the spectrograph side, not the telescope side, because of the presence of a chamfer). The slit system OP0073 is equipped with an isolated hole of 20 microns in diameter, the OP0092 system offers 3 aligned holes of 10, 15 and 20 microns:     

Here is the view of the spectrum of this point when using a compact fluorescent lamp: each of the points corresponds to a monochromatic image of the hole for various wavelengths (UV part of the spectrum). The telescope is working at a focal ratio of F/5:

The goal is to get an image of the point as clean and symmetrical as possible (case A). Incorrect settings of M1 preferentially generate coma (case B in the figure above). It is then necessary to retouch M1 to arrive at the case A (this is adjusted to within a few tenths of a degree).

Case C corresponds to a defect of adjustment of both mirror M1 and mirror M2 (an error on the mirror tends to widen the spectrum trace). In case D, the camera is poorly focused.

When you do not have a source hole, you have to resign yourself to examining the spectral lines, but with a less accurate diagnosis…

In the example above, the 2D spectrum of the top image corresponds to a pretty good adjustment. In the spectrum below, the group of 3 lines on the left is sharp, but the lines on the right are fuzzy, with a characteristic asymmetry of the coma.  Adjust M1 first to deal with this type of problem, while checking that the “vignetting” test has passed.

5. Orient the M2 mirror

If, when observing the source point or a star, you see the view of the spectrum below (variation of the width of the spectrum as a function of the wavelength): :

… it is necessary to rotate the mirror M2 in the correct direction to reach the result: :

6. Mongitudinal setting of the entrance slit

– if when observing the source point you can not obtain both fine spectral lines and a narrow spectrum over its entire wavelength

– if when observing a star with a telescope, the image of the slit is very sharp in the guide camera, as well as the image of the star (it appears punctual), but that the spectrum trace is hopelessly uniformly and unusually wide over its entire length (see the example below on a spectrum extract of the Arcturus star made with UVEX equipped with a network of 1200 lines / mm at the focus of a telescope opened at F / 10):

… it is likely that the planes of sharpness of the star image and focus of the cylindrical lens are not confused. This means that you can observe fine spectral details without getting a narrow spectrum, or vice versa, even when you try to focus as much as possible by moving the camera longitudinally. This is the symptom that the slit carried by the piece UV04 is not at the right distance from the mirror M1 (not respecting the 100 mm gap between the slit and M1 (see the “optical formula” section). In the part devoted to the assembly, I indicated that it was necessary to respect a distance of 1 mm approximately between the contact planes of the shoulder of UV03 and UV04.

If the anomaly of abnormally wide spectrum width appears (a spectrum wider than 15 pixels typically with 2.4 microns pixels for example), it is necessary to adjust this distance, then to redo the point of the spectrum, then finally, to note if the situation improves (in the example of the Arcturus spectrum above, the positioning error was of the order of 2 mm compared to the nominal, which means a gap slit-collimator distance of 98 mm to compare at the nominal value of 100 mm If the situation gets worse, it is necessary to move the slit longitudinally in the opposite direction.

In the opposite example, the slit support is moved so as to move the slit away from the mirror M1. In doing so, the focus point of the entire spectrograph is changed. You have to catch up by moving the camera as indicated by the arrow. This work is iterative and quite tedious because it will also at the end redo the point of the guide camera if it is already in place.

Here is the result on the image of the Arcturus spectrum as we converge towards the good result: fine lines + narrow spectrum. t.  

7. Final control

The spectrograph is correctly adjusted when, after modifying the setting for the wavelength of the spectrum (rotating the grating), the spectral lines remain sharp, see at right.

But of course the final test is on a true stellar object: all is OK if in the same time the slit image, the guide image of the star and the spectrum of the star are sharp. 



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