The archive contains 12917 plates of full-disk CaII K and Hα spectroheliograms acquired during 5042 observing days at the ”G.B. Donati” tower of the Arcetri Astrophysical Observatory (Florence, Italy) from 1926 to 1974 (Godoli and Righini, 1950).In particular, the archive contains 5976 and 6941 CaII K and Hα observations, respectively. At the time of observations the instrument had a grating of 600 lines/mm and a ruled area of 100 mm × 100 mm, with a dispersion of 0.33 nm/°A at 3934 °A.
The plates stored in the archive have different dimensions and contents. In particular, the ones acquired until 1936 measure 9×18 cm, have two images of the Sun with a diameter of 6.5 cm (13% of the whole archive); on the other hand, the later plates measure 9×12 cm, have just one image of the Sun with a diameter of 6.5 cm (87% of the whole archive). 5% of the data is stored on acetate negatives. The size of the solar disk image on most of the plates is about 6.5 cm, which corresponds to an image scale of about 0.033 mm/arcsec.It is worth noting that several instrumental changes occurred during the approximately fifty years that the spectroheliograph was utilized.
These include the use of additional lenses and changes of the slits positions, which improved the image definition and monocromaticy, and decreased the stray-light level. For instance, observation log-books notice that on 23 Jul. 1938 started the testing of an additional field lens to improve the image definition. This lens has been definitely utilized starting from 20 Jan. 1939. In the same period also started the utilization of filters set upon the first slit. An UG2 filter was utilized for the K2,3 observations from 15 Oct. to 30 Dec.1938, then replaced with an UG3 filter. A RG2 filter was utilized for the Hα observations from 15 Oct. to 29 Jul. 1939. Moreover, on 25 May 1953 the position of the 2nd slit was modified, decreasing its position respect to the plate from 1.3 cm to 2 mm. We note down these instrument changes because they mark a few discontinuities on the data collection. Notice that the instrumentation used to acquire these observations is no longer available. There is an average of about 100 CaII K and 120 Hα plates per year during the whole period, while the number of plates quadruples from 1956 to 1961.
The Arcetri solar tower and spectroheliograph showed from the very beginning limitations for the study of the solar surface details (Gasperini, Mazzoni and Righini 2004, Righini 2003), being the spatial scale of observations degraded by the local seeing on average to about 5 arcsec. Nevertheless, the Arcetri archive is unique respect to the other historical series. In fact, more than 65% of the plates in the archive contains the step-wedge exposures for the calibration of the non-linear response of the photographic emulsion to the flux of the incident radiation. Observation log-books notice that 36 different photographic supports from 8 different manufactures were used along the whole period of observations. The spectroheliograms obtained since 22 Feb. 1938 carry calibration exposures obtained through a Zeiss K 58 step filter. At first, only one photometric mark was stored on the plates, by placing the Zeiss filter upon the first slit and gathering the radiation emitted from the solar disk center. Starting from Sept. 1947, two photometric marks were stored on the plates by changing the width of the first slit. From Aug. 1948, there have been three photometric marks stored on each plate: the first obtained with the radiation emitted in the CaII K2,3 or Hα from the solar disk center, the second one in the radiation of the near-continuum to the line at issue, and the third one always in the same continuum, but by doubling the width of the first slit.
Table 1 gives the transmission factors for the step-wedge exposures recorded in the Arcetri log-books. These factors are used
for the conversion of the plate blackening into plate exposure, as described in §4. The log-books also notice that the velocity of driving motors which translate the photographic plate during the wedge exposition was equal to that utilized during the solar image exposition. Whether more observations were obtained during the same day, the calibration wedge was stored only on the latest plate exposed. However, being all the plates obtained in the same day photographically processed at the same time, a sole calibration wedge can be used for all the specthroheliograms of the same day and photometric measurements can be performed as well through all the plates obtained on the same day.
The about 13000 plates of the Arcetri solar archive are permanently stored in plastic envelops housed in three eight-drawer metal racks at the Arcetri Astrophysical Observatory. A MOU signed between the Arcetri and Rome Observatories allowed the progressive moving of the plates to the Rome Observatory were the digitization work took place.
Table 1. Transmission values of the Zeiss K 58 step filter from the observation log-books of the Arcetri Observatory.
6000°A 100 72 45 28 19 11
4000°A 100 68 42 29 20 14