Internal Refraction in a USNO Meridian Circle

A new type of systematic error affecting meridian observations of declinations has been found in observations of horizontal flexure with the U. S. Naval Observatory 6 in. meridian circle. The effect is called internal refraction, INR, or tube refraction, since it is, most probably, due to refraction caused by a vertical temperature gradient inside the tube. This gradient typically amounts to 1.5 K/m and is caused by the change of temperature and slow conduction of heat between the inner air and the outside. The total flexure, including INR, will be approximately proportional to the sin (z), similar to the differential mechanical flexure of the telescope tube, but the day and night flexures differ by 0."25. The flexure is, furthermore, variable, decreasing by about O."14 during day observations and O."2 during the night because the rate of change of the temperature decreases during the day observation and during the night. Systematic corrections of declination observations for INR can perhaps be made. Furthermore, the effect may be eliminated in existing instruments by installation of tangential ventilation of the tube. This is expected to give considerable improvement of absolute observations of declinations since the remaining, purely mechanical, flexure has about the same size as the present INR component at the USNO instrument. Two consecutive periods of observation, W550 and W650, of a few years each were analyzed. It appeared that both the mechanical flexure and the INR were almost negligible during the first period and were significant during the second. The reason for the difference is presently unknown, but the difference may apply for the flexure measurements only and not for the star observations.