Total Solar Eclipse 2008:
Flash spectrum with the Dados slit spectrograph

Manfred Rudolf

The EurAstro team planned to record a flash spectrum with the
Dados slit spectrograph during the solar eclipse in China. The spectrograph was mounted on a small travel mount, and a 1000mm f/10 Maksutov lens was used as the objective lens. The image below shows the spectrograph on its mount at the preliminary tests before the eclipse:

The Dados spectrograph was used with its 900 Lines/mm grating, and a scale was attached to the focuser to allow reproducible focus setting. The spectrum was recorded on Ilford SFX200 black and white film which has extended red sensibility (SFX200 Datasheet). The camera was oriented such that the spectrum was diagonally over the film. Thus a wavelength range from ca. 360-710nm could be recorded.

The same setup was also used for recording the flash spectrum in China. The spectrograph was rotated in a way that the slit was tangentially orientated to the expected point of second contact of the eclipse, to record a maximum portion of the chromosphere.

The progress of the eclipse could conveniently be observed through the eyepiece of the Dados, using a ND5 Astrosolar filter. Close to second contact, the slit was positioned just in front of the lunar limb, the filter removed and the spectra recorded.

One frame of the flash spectrum afer developing and scanning the film (ca. 20% of the original size): The chromosphere was kept on the central slit of the Dados spectrograph. Through the adjacent slits above and below, only the most intense lines of elements such as CaII, hydrogen and helium can be seen as small streaks:
(the blue end of the spectrum is to the lower left, the red end to the upper right)

The spectrum obtained with the 25 micron central slit was used for further processing. Numerous emission lines can be seen in the spectrum (original size, blue to te left):

The dark horizontal streaks in the spectrum are caused by traces of last photospheric light from the Sunīs disc peeking through lunar valleys near second contact. These streaks disturb the evaluation of the spectrum, in particular for the weaker lines in the red part of the spectrum, when the columns are averaged, and should therefore be eliminated.

A "background mask" containing only the dark horizontal streaks but no spectral lines has been created from the above image.

The background mask:

The original image was then flattened with the background mask. The dark horizontal streaks are no longer there but have been replaced by "background pixels" and do no longer contribute as a "signal" when the columns are evaluated:

Averaging in the columns of 20% of the darkest pixels resulted in an (artificially looking) spectrum which was used for line identification:

For calibration and line identification, a BASIC program was developed which allows wavelength calibration using multiple reference lines, and line identification following the tables of chromospheric emission lines published by S. A. Mitchell (S.A. Mitchell, The Spectrum of the Chromosphere; ApJ 71 (1), 1-61 (1930)).

The columns have been integrated, and a wavelength scale added. Average resolution is 0.88 Angstrom per pixel.

Alternatively, other freeware programs such as Visual Spec can be used for evaluation.

Line identification

The following Line table has been created automatically, comparing the wavelengths of the peaks with those cited in Mitchell (cf. above) and listing the most likely matches within a tolerance of +/- 2 angstrom.

Corona spectrum

After recording the flash spectrum, the spectrograph was moved to the corona to get a spectrum thereof. The corona spectrum looks essentially like a continuum, with only two lines hardly visible (upper spectrum, original scan):

The same procedure as above (flattening and averaging) has been made with the corona spectrum, however no further clearly distinguishable lines except the abovementioned two lines could be extracted. Thus no useful lines for calibration were found (lower image).

Fortunately, the telluric absorption bands of molecular oxygen (Fraunhofer B band starting at 6867 angstrom) and the bands of water vapour (7160 angstrom) can be discerned as bright bands to the far right in the image, thus allowing a rough wavelength calibration of the spectrum:

The two emission lines are at wavelengths 5303 and 6374 angstrom and can be attributed to highly ionized iron Fe XIV and Fe X.