实现Johnson标准UBV系统中的某些问题(英)

ON THE PROBLEMS IN REPRODUCING THE STANDARD JOHNSON UBV SYSTEM

Odewahn等人发表的CCD标准星区NGC7790的目录是挑选过的非密集星目录．在其证认图上都为单星或仅有相对很暗的近星，但在深度露光图上许多星显出有近星，有的貌似单星但实际上是双星或三重星．本文给出这些（光学）伴星的相对距离和星等差，以便测光工作者挑选标准星时参考使用．另外，比对NGC7790的CCD测光与光电和照相测光，发现Sandage的光电内插照相星等有系统偏离，远大于其测光误差．检验Sandage及其合作者的其他类似工作可知Johnson等人的工作（M67， M11），在误差范围内，内插照相值的确和光电值在同一系统．Sandage和Walker的NGC4147，所说的照相光电一致与事实不符．Burbidge和Sandage的NGC7789，在改正了照相值的系统偏离后仍没有全改掉．Sandage和Walker的M92，真正改掉了小的非线性偏离．Sandage和Wildey的NGC7006，由于星暗难测，其照相值不能坚持光电内插的原则，结果不但有明显的颜色方程，而且星等有大的标度误差．CCD测光也应从中吸取经验教训.

Odewahn et al. (1992) improved the accuracy of the standards in the vicinity of NGC7790 selected originally by Chrishan et al. (1985) and extended to fainter magnitudes by aperture photometry. Only uncrowded stars were included in their list. However, nearby faint stars appear for many of the standard stars in deep CCD frames. Some stellar images look like single stars, but each of them consist of two or three stars. The relative distances and differential magnitudes of these nearby stars are given in Table 1 in order to help other CCD users in selecting suitable standard stars. Besides,comparing the CCD photometry of NGC7790 with the photographic and photoelectric photometry, large systematic deviation appears in Sandage's (1958) photographic measurements and this is contradictory to what he said in his paper. Therefore, many other similar works by Sandage and his collaborators have been checked. The statement that 'there is no magnitude or color equation between the photoelectric and photographic results so all the data are on the Johnson's U, B, V system to within the accuracy of the measurementS' is true for the work of M67 (Johnson & Sandage, 1955) and M11 (Johnson, Sandape & Wahlquist, 1956), but not true for the work of NGC4147 (Sandage & Walker, 1955), not true for the work of NGC7789(Burbidge & Sandage, 1958) even with the systematic deviations corrected,true for the work of M92 (Sandage & Walker, 1966) where the small nonlinear deviations were corrected. For the work of NGC7006 (Sandage &Wildey, 1967), due to the difficulty in measuring faint stars, the photoelectric sequence is not good enough so the photographic results show not only large color equation, but also large scale errors in V magnitudes.CCD users should remember the experience and lessons.