Post by dobsonian on Oct 8, 2006 13:41:58 GMT
Hello,
Recently when examining formations on the Lunar limb,I got the impression that the image was not as sharp as it should be. I was using a 250mm dob at 170 power . I then dragged out my 200mm F6 dob and compared the images. The 200mm dob was undeniably sharper. This led me to check the collimation on the 250mm, which was very good. I then inserted a 40mm Meade and did a star test. ( usually you do this with a high power ) This low power test revealed a small amount of astigmatism, which was invisible at higher powers. The Ronchi screen test also revealed no such errors. This problem took some time to reveal itself, as the error on the mirror is quite small. Nevertheless it is enough to reduce the detail seen at high powers, and since most optics today originate from the vast Chinese factories, I am sure a few sub standard items slip through. Every telescope owner should get to know how to test their optics, or perhaps not as the discovery that they are not top flight might cause dissapointment.
John.
Recently when examining formations on the Lunar limb,I got the impression that the image was not as sharp as it should be. I was using a 250mm dob at 170 power . I then dragged out my 200mm F6 dob and compared the images. The 200mm dob was undeniably sharper. This led me to check the collimation on the 250mm, which was very good. I then inserted a 40mm Meade and did a star test. ( usually you do this with a high power ) This low power test revealed a small amount of astigmatism, which was invisible at higher powers. The Ronchi screen test also revealed no such errors. This problem took some time to reveal itself, as the error on the mirror is quite small. Nevertheless it is enough to reduce the detail seen at high powers, and since most optics today originate from the vast Chinese factories, I am sure a few sub standard items slip through. Every telescope owner should get to know how to test their optics, or perhaps not as the discovery that they are not top flight might cause dissapointment.
John.