Low-tech spectrometry: A suggestion for observations of bright near-solar comets. Christopher Taylor Most observers, I imagine, would welcome the chance to use their telescopes for new types of observation that reveal interesting phenomena not accessible to simple direct imaging. It is perhaps surprising then, that there is such a possibility which is very little used despite the fact that it requires only a telescope on a simple altazimuth stand, a stopclock and a few minutes preparation time. There is no need for an equatorial mount, a drive, photography, a CCD, computer or any investment in ancillary instrumentation. Pretentiously it could be called "zero-technology objective-grating spectrometer"(!). Take a piece of plain, unpatterned nylon gauze curtain such as is to be seen in suburban front windows, preferably with a mesh as fine as 1/3 mm and stretch it flat over the main aperture of the telescope, being careful to keep the threads as straight as possible. The result, as is well known1, is a crude and rather inefficient means of producing spectra of celestial objects. What appears to be much less well known is that such a primitive device can actually be used for quantitative spectrometry. It is possible to make absolute measurements of the wavelengths in emission line sources such as planetary nebulae and the heads of near solar comets. All that is necessary is a measurement of the diffraction angle q, an absolute count of the threads in the gauze to establish their precise mean spacing D, and a simple application of the grating formula: nl = D sin (q) Essentially, the theory of the method is this: the 1st-order (n=1) q-value is of order 5-6 arcmin., for which the grating formula may be approximated to better than 1 in 106 by l = Dq (q in radians). D is determined from the aperture of the grating and a direct count of the threads using a strong magnifier. q is obtained as q = 0.5 dq.Dt, where dq = 7.2921 x 10-5 cosd sec-1 is the diurnal rate at declination d and Dt is the time taken by the image, drifting at this rate, to pass over the interval between the two 1st-order spectra. The latter must, therefore, be aligned fairly carefully 'fore and aft' to the preceeding-following directions in the field. One great advantage of a nylon curtain as dispersing element is that we can determine the grating constant D by actually counting the individual threads as stated above, and this determination of wavelength is therefore, an absolute one requiring no use of comparison spectra or interpolation formulae. The nylon used should have a reasonably uniform mesh, without superimposed patterns and should be stretched flat over and fixed to a rigid diaphragm which is placed over the entrance aperture of the telescope in such a way as to be freely rotatable about the optical axis. The results below were obtained with one of 250 mm clear aperture, having 894±2 threads, hence D = 0.2880±0.0006 mm (±0.3%). The transit timings Dt were made with an electronic darkroom stopwatch by watching the two opposite 1st order spectra proceed in tandem across a register mark in the centre of the field; on bringing the object into view and rotating both the grating and the register mark appropriately, one obtains an arrangement of 0th order and four 1st order images which should be thus:- [Fig] It follows that the true field of the eyepiece must be at least 4q, preferably rather more, but one should use the highest power that gives this - a typical comet eyepiece giving 30 - 35 arcmin. at x100 is ideal. It is obvious that another huge advantage of this method is that one looks straight into the eyepiece, at a normal telescopic view with full unrestricted field uncluttered by additional optics - there is, for instance, no awkwardness of keeping a narrow spectroscopic slit on target. At most declinations the transit time Dt generally ranges from 50 to 80 seconds, so it only takes a few minutes to repeat the timing in order to improve accuracy by averaging: I generally take sets of four to six measures. The single most important procedural point is to ensure that the axis of dispersion (see diagram) is square to the register mark and that the latter is square to the p.-f. direction (i.e. N.-S.), both to within a tolerance of say ±3 . The second pair of 1st order spectra will be found very useful in making the first of these adjustments (assuming that the 'mesh' of your nylon is truly rectangular); an offset here will cause a systematic negative bias in the results, while an offset of the p.-f. direction will have the opposite effect. Neither of these systematic errors is significant if the offsets are at the 2 .level but start to become serious at about 4 . Undoubtedly a rather thick and accurately square pair of cross-wires in the focal plane would be a great advantage here but my trials of the method used nothing more than a matchstick glued across the field stop of the eyepiece! Other sources of systematic bias to watch out for, which will not be apparent in the random scatter of the Dt measures themselves, include inaccurate clock rate, wrong D-value (error likely to be ±0.3%), systematic over estimation of Dt due to the angular diameter of the source (likely to be 0 - 1%) and finally the dq rate may need correction for the object's own proper motion in the case of a very fast moving comet. The miracle is that such an absurdly crude 'grating' is at all capable of giving spectra of sufficient purity for any meaningful measurements. I have recently put this to the test by using some bright planetary nebulae as 'standard sources', all having visual spectra completely dominated by the [OIII] emission, which has a centre of intensity at l = 499.5 nm. Among the resultant sets of measures, that with the smallest scatter was also the best: NGC 7027 (visual mag +8.5) gave l = 500.8 ±3.0 nm, allowing for both random and systematic errors, a result only 0.3% above the expected value. The miracle actually works! Other sets of 4 - 6 measures were also taken for each of NGC 6826 (mv = 8.8), 7009 (mv = 8.3), 7662 (mv = 8.3) and 3242 (mv = 7.7) with a worst case result only 2.7% in error, this being a rather rough first attempt.... and all this with only an altazimuth telescope, a piece of old nylon curtain and a darkroom stopwatch! I have no doubt on the basis of these results that more careful use of this primitive instrumentation would be quite capable of ±1 - 2 nm accuracy. Theoretically, the resolving power in 1st order of this piece of nylon is nearly sufficient to split the sodium 'D' lines. Planetary nebulae are rather boring objects in this sense - they always yield the same wavelength, but near solar comets are much more interesting, being essentially dynamic phenomena and displaying a rich variety of emission lines. (Novae would make another interesting field of application for the same reasons.) Comets at solar distances of 0.5 - 1.0 AU usually radiate strongly the molecular bands of C2 at 473.7 nm, 516.5 (often the dominant visual wavelength) and 563.5 (band-head l’s) and of CN at 388 nm. Sun-grazers, by contrast, are exercises in atomic spectra rather than molecular chemistry. An earlier and much cruder attempt in this direction which I made with the same piece of nylon 20-odd years ago yielded a value of l = 523±15 nm for comet Kobayashi-Berger-Milon on 1975 August 24. Subsequently published professional spectra showed the C2 band-head at 516.5 nm as the brightest visual emission at that time. In addition, my further observations over the days following gave at least a strong hint of night by night changes in the spectrum. The nylon grating method clearly has more than sufficient spectral discrimination to reveal very quickly which of the major cometary bands is the dominant emission and with a really bright comet - Hale-Bopp in April and May 1997 ? - we may reasonably expect to be able to see them all separately and so to determine their wavelengths one-by-one, and make rough visual estimates of relative intensities. This certainly has the capability of showing up nightly spectroscopic changes that would be quite beyond the reach of direct imaging with or without colour filters. I would strongly urge members of the Comet Section to add observations of this sort to the more usual direct imaging planned for Hale-Bopp this spring. Given that the comet will, when sufficiently near the sun to produce strong line emission, be at least 6 magnitudes brighter than NGC 7027, there will not be a single telescope user in our Association who will be prevented by lack of equipment from making some potentially very interesting measurements with this truly minimalist method - why not give it a try ? Reference 1. North, G. Advanced amateur astronomy, pp284-6, 1991