Photoelectric Measurement of Lunar Occultations. II Instrumentation
Abstract
The equipment used for occultation observations is designed around a 400-channel multiscaler. The light from the star and scattered moonlight falls on a cooled 1P21 photocell from which the output signal passes to a voltage-to-frequency converter which converts the signal level to pulses. These are counted into a channel of the multiscaler for a fixed interval of time, usually 1 ms, but for some observational conditions, 2 or 4 ms. The accumulation is advanced to the subsequent channel under the control of 1-kllz pulses from the observatory clock which is compared with WWV signals. The multiscaler is modified so that when the 400th channel is passed, the output is immediately switched to the first channel and substitutes the new record for that already stored in that memory channel. By continuing this cycling process, a continuous digital record is kept of the light level for the preceding 0.4 sec of time, at, normally, 1-ms intervals. The process is stopped by the change of level associated with the occultation whether for disappearance or, by a different switch setting, for reappearance. A time delay, usually of 0.1 sec, is incorporated to permit record- ing of all portions of the light curve required for analysis. Direct digital recording in this form has the marked advantage over any other method of presenting the data in a form adapted to routine analysis. The equip- ment works very well in a completely routine fashion. The limiting magnitude attainable with the 36-inch telescope varies with lunar phase and the intensity of atmospherically scattered light, but stars as faint as 9.4 have been observed. Little purpose would be served by observing timings for still fainter stars since their positions are uncertain and data of astrophysical importance are unlikely to be secured for stars as faint as this.
- Publication:
-
The Astronomical Journal
- Pub Date:
- June 1970
- DOI:
- 10.1086/110992
- Bibcode:
- 1970AJ.....75..583N