关于地平式反射望远镜的CCD平场

摘要

The CCD flat fielding for the majority(if not a11)of existing normal designed reflectors suffers from remains of scattered light.For equatorial mounted reflectors,the very high precision flat fielding is not necessary in differential time series CCD photometry,and the key point is the targets being measured are always placed at the same pixels Oil the CCD chip.However,the method mentioned above is less effective for altazimuth mounted reflectors,because an altazimuth telescope must be with a field de-rotator to counter the effects of the rotating Earth,and the orientation of the CCD camera relative to the telescope always changes during tracking.Therefore.to eliminate the influence of scattered light in CCD flat fielding for altazimuth telescope is more important than that for equatorial one.In other cases the night sky exposure is necessary if a 0.1%-0.3%precision flat fielding is required for equatorial reflectors.For altazimuth ones whether a 0.1%-O.3%precision can be obtained by night sky exposure should be investigated.In our opinion,the following condition should be satisfied:Cij=c(r,θ)≡C(r),which implies that the center of rotater must be coincided with the optical center of the reflector. The EOS 1-m altazimuth reflector of NAOC at Xinglong 0bservatory is a typical example whidl can be used to show the influence.Though this 1-m telescope is mounted additional device by NAOC to eliminate scattered light,unfortunately the CCD flat fields so obtained still show 2%-3%differences for all the B,V,R and I filters at different rotater positions(primarily as a gradient).Further improvements are suggested.The solution for this problem will have general meaning for all altazimuth mounted reflectors.%现有标准设计的反光望远镜在拍摄CCD平场时,绝大多数都受残留散射光的影响.但是赤道式与地平式反光镜所受影响是不同的.在赤道式上做时间序列较差测光时,只要待测星永远位于CCD的固定像元上,不太准确的CCD平场也能得到高精度的测光结果.当需要0.1%～0.3%精度的平场时,则可以采用夜天平场.地平式的特点是,它的CCD相机必须置于旋转器上,在跟踪天体时不停地旋转,以抵消地球自转的影响.上述用于赤道式的方法失效,因此,在CCD平场时,消除散射光的影响比赤道式更为重要.一个典型的地平式反光镜的例子是NAOC兴隆天文台的EOS 1米镜.虽然该台已附加了防散射光的装置,但是对所有B、V、R、I滤光片,在不同旋转器位置拍摄的CCD平场,仍然有2%～3%的差别(主要是梯度).该文给出了改进的建议,必须满足下面两个条件:Cij=C(r);旋转器的中心与反光镜的光学中心重合.此问题的解决对所有地平式反光望远镜都有普遍意义.