AN AMATEUR  NO  SLIT

                       SPECTROSCOPE

                    by  Fulvio Mete

After having built some instruments for solar spectroscopy, I tried to expand my interests to stellar spectroscopy too, by means of no slit spectroscopes, in order to  use the point- like stellar images to get a spectra at the focus of a telescope. Anyway ,such  spectras were  not well visible and  the lines not  sharp.Moreover, my intention was to accumulate in the same instrument both capabilities   of solar and stellar observing and imaging,  realizing a simple, compact, but multi purpose device.

Another point of interest was the weight, to maintain low  for the most  of  optical tubes and equatorial mounts to deal with no problems.

The goal was to obtain, in the case of stellar spectroscopy, normal spectras without use of a slit, for the well known difficulty of amateur mounts to keep a star image onto a slit a few tenths of millimetre wide.

I remembered, then, of having seen some time ago, in a monocromator ,  a cylinder lens  set at the end of the exit slit,in order to intensify the spectral image formed by grating, and also remembered that such lens have the property to transform, at their focus, any light point source in a line, star images included.

In the Edmund Scientifics' catalogue were various types of those lens, from 25 to 75 mm of focus lenght, at a good price.I choosed the one of 75 mm, to let enough distance left to the grating position.

The cylinder lens, which had to do both functions of collimating lens and slit, was placed in a 42 X 1 male tube assembly, to be inserted in a pvc box  with  two threaded 42 X 1 female holes at 90° from eachother.In this box was placed a 30 x30 mm  Edmund 1200 l/mm diffraction grating blazed for 500 nm.

As observation lens I used an enlarger objective Kodak of  75 mm of focus lenght, very good for the job for its full plane field (fig 2).On the right side of the box , shown in fig. 1, is visible the grating rotating device, a very simple one, consisting in a knob , threaded to the grating axial  rod, that pushes on a spring interposed among it and the side of the box, in order to obtain sufficient pressure for a smooth motion.An additional micrometric motion has been obtained with a threaded female hole at the bottom of the box, in which is a threaded bar that pulls up the grating to the desired position.In fig 1 is shown the general appearance of the instrument, put at the focus of a Celestron 8 with a 3,3 focal reducer.

The former part of the work seemed then complete, with a system devoted to the star spectras observation and imaging.I was, anyway, concerned with the problem of  using such device with sun spectra in daylight.With the sun, obviously, the amount of incoming light on the cylinder lens had to be cut, to avoid a serious damage to the instrument and the observer’s eyes.The matter was to make the  sunlight to became   point like, by means of a proper system: then, what better than a stenopeic hole put on the front of a telescope,in order to use the focus length of this to permit the use of a not critical dimension hole, say about 0,5 – 1 mm.My first experiment with this configuration was to put in front of the corrector lens of a 8” Schmidt – Cassegrain telescope a cardboard mask on which was applied, off axis, a piece of aluminium  foil with a hole of ¾ mm realized by a sew needle.The  hole in the aluminium foil was applied  on another 2 mm hole made off axis on the cardboard mask, to minimize the risk, in case of break of aluminium , for observer’s eyes and the telescope.(fig. 3)

   After having aimed the scope to the sun and focused, both on the scope focuser and on the 75 mm objective, the Fraunhofer lines of solar spectra  seemed to jump out of the eyepiece, so I decided to record the spectra by a Philips Vesta webcam, put into the eyepiece holder .The result is shown in Fig 4, which is a compositation of various shots in different spectral wavelengths., with  a clear registration of most important lines (Ha, Na, Mg, etc).

Such a result was noticeable, far better the best I’ve seen with a low resolution, home made instrument, so I managed to increase the resolution power of the system.This was obtained  changing the 75 mm  Kodak observing lens with a small Chinese refractor D 50 F 350, of good optical quality, that was light enough for the structure to bear its weight without problems.(Fig 5).

Once having put  a Philips Vesta webcam in the eyepiece holder, I imaged the sodium doublet to verify the resolution level reached.

As  shown in Fig 6, in the inner part of the doublet are registered the nickel line, that’s the most strong  and evident, and 4 other more lines, for a resolution of about 0,3 A /Pixel , the  top, I believe, obtainable with such an instrument without compromise its structure and dimensions.

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    The instrument’s test had, anyway, no longer gone to  the end, and the most difficult part had still to begin, I mean the imaging of stellar spectras, so I inserted in the spectroscope eyepiece holder a Starlight Xpress MX 716 CCD Camera, whose spectral response and high sensitivity is well known, and put the instrument at the prime focus of the 8” SC with a F 3,3 focal reducer, in order to maximize the incoming light on the focal plane.

The first shot, with a 15 sec exposure, aimed to Schedar, Alfa Cassiopea, didn’t show anything, for it was evidently underexposed, while the second, with 30 secs exposure showed clearly the lines of the  green – orange part of the spectra. (Fig.7)

To observe the red part of the  star spectra I had to  refocuse the device, for the use of  simple acromats  produces a serious shifting of the focus point in the different spectral  zones.

Since my first try, I’ve been shooting many other  stellar spectra images, as the one of  Mirfak (Alfa Persei) spectra, at the focus of an 8” SC, (Fig.8) that confirmed the noticeable performances of such a device, considering that the showed output spectras are raw images, with no kind of  processing, and this notobstanding the unusual  angle of observation (order 2^nd).

I also used the spectroscope in more powerful configurations, by coupling it with my old Celestron 11 SC telescope, with an f 3.3 focal reducer to obtain the maximum light grasp.In this powerful mode I imaged  Capella, (Alfa Aurigae), M1 class star, by  Mx 716 CCD camera, (Fig.9) whose lines in the  blue -green  region of the spectra were  compared with those of the sun,.

      I also imaged one of most interesting spectras, the one of Syrius class A0, kwnown  for the strong appearance of Hydrogen Balmer series lines, in which is particularly evident the Hb line.The shot  was taken at the prime focus of C11 at f 3.3, with a focal reducer (Fig. 10)

The spectroscope can also be used in a “stand alone” configuration, using the cylindrical lens as both objective, slit and collimating lens, considering that the modest diameter and optical quality of this lens permits  to shot spectras of medium and high intensity light sources only.In fig .11 is showed the spectra of a neon lamp obtained in this configuration.

At last, the instrument I conceived, also considering the limitations of its home made realization, (to permit anyone to make it) seems to perform exceptionally well .

One  has also to consider that the optical quality of the cylinder lens I used is quite low. May be that the sharpness of spectral images is due to the  optical transformation of a point of light in a line, with no intermediation of mechanical parts, as a slit.Moreover, the photon ‘s package is , before entering the cylinder lens, smoothed by telescope’s optics.

The  overall cost of the instrument  has been defined in about 300$ (200 E at the current  change ratio ), almost the cost of a good eyepiece, with the substantial difference that it is intended not to enlarge any astronomical objects, but to discover a new world for the amateur astronomer, a world that is the focal point of professional astronomy.