假如用有限距離的人造點光源, 一支well correctod給無限遠source的鏡會呈現over-corrected. 如果真的要用人造光源, 距離最好要有焦距的20倍以上, 不過還是會呈現八分一波長彧以下的over-corrected. 用人造光源的確有一些好處: 1. 不易受大氣擾動影響, 2. 唔洗用摩打赤道儀追著粒星, 3. 唔好天都做到.Anonymous 寫:However, restricted by the geometry of my home, I could not "view" the star with my C8. I will try this setup again. But I would like to have your comments on testing the telescope indoor.
Regards,
啟明
P.S.: I came across an article in S&T that people use a silver metal ball to make the artifical star. The smaller the ball, the stronger the curvature of the surface, and thus the smaller the image of the flashlight. This will make a better "star".
我也試過用距離我幾百米遠反射自水晶的燈光去較光軸, 的確比用恆星方便. 而且真是可以睇到不同顏色的diffraction pattern有不同大細! 不過由於SCT的主鏡跟baffler tube有點虛位, 我發覺把鏡打橫放校好光軸之後, 睇天頂附近的星時光軸會有所改變. 所以如果要非常認真的觀測或拍攝某一目標, 就最好拿目標天區附近的恆星來校光軸.