Cloudy Nights ask me to explain what the spherical aberrations undercorrected and overcorrected mean and how to tell what you have as you almost always have at least one. Even high end APOs have this condition most of the time but to what degree and over or under makes a big difference.
First lets deal with astigmatism which can be seen very easy unless it is so slight then it will not be of importance. To test for astigmatism which is the same as the human eye problem where the focus is stretched vertical on one side of focus and horizontal on the other side of focus. When testing for astigmatism DO NOT use a wide angle EP as it will almost always show astigmatism, which may not be there. Use a moderate to high power EP around 150X for a smaller scope 80 to 90mm and 250X or near that for a 5" or 6" scope. and I am referring to refractors but the same rule for SCTs and Maks and DOBs.
Bring a star or an artificial star to sharp focus. Seeing must be good. Then defocus inside and outside and look for the star to pinch like an egg. Not round and on one side it will be elongated vertical and the other side horizontal then as you go more defocus it will snap to a perfect symmetrical circle. When it is not round but elongated that is astigmatism. If you have just a tad it will go from pinpoint focus to an egg shape just for a very small touch of focus then go round. That is a small amount of astigmatism, which will not be a problem for good views. But if it stays elongated for significant amount of defocus then you have allot of astigmatism and the scope will loose performance more so on high power viewing. The scope is a good low power deep sky and that's all
Now for the spherical errors there are two of importance they are called the LSA 3rd and the HSA 5th, which is over and under corrected. When you defocus outside if the outer ring is brighter and sharper than the inside focus you are overcorrected and a ronchi test will confirm this. If the Inside focus is brighter and sharper on the outer ring then you are undercorrected. If both sides look the same then you have good optics with good correction. NOTE there can be misleading problems that will give false test as poor collimation and cheap EPs or focuser out of alignment. The test are a subjective test to get a good estimation of the optics and are not 100% as there are many factors that can make the test weak or outright wrong. Good seeing and collimation are the most important also using the Ronchi test will show more precise figures. But this simple eyeball test works pretty well.
As for turned edge that is more problematic with mirrors and not so much with refractors but refractors can have it too. A turned edge is when the mirror or lens is not ground straight at the edge following the curve of the mirror or lens. In other words the edge is taking a dive down or up not crisp like a knife-edge. This will look like a frosty or degrading sharpness at the edge of field where the light is defused off axis to the correction of the lens or mirror resulting in poor performance. A small mask of black paper or cardboard about 1/8 inch around the mirror edge will fix this but you loose a tad of aperture but performance increases. DO NOT GLUE OR STICK mask to mirror use tape on outer side of glass to affix the mask and if there is no improvement remove it. If you put tape or glue on the surface you will destroy the coating or make it a mess. Never touch the coating on a mirror or lens if possible. The salt and sweat or oil from your skin will cause premature coating degrading.
Back to spherical errors lower errors are worse than higher errors Lower being the 3rd and higher being the 5th .Hence the name LSA and HSA Now LSA will cause a washing out of detail and a blurring of image depending on how much error is present.HSA is not as bad and not so destructive to image unless very high HSA error.
The last error of mention is that horrible no one one like to see is ZONES. Zones are the worst in my opinion unless very slight. Zones looks like colored rings or blobs where there shouldn't be anything but nice even pattern out to the edge except for the dot in center or secondary shadow on SCTs MAKs and DOBs. Looks like something on the lens or chunk of dirt on the EP. If it's bad the scope is not of much use except low power. Most of the errors mentioned are very common but the big question is how much is there. A tad or loads. Both of my scopes the 120mm and 150mm Refractors are undercorrected LSA but just a tad on the 120 and a tad of a tad on the 150mm which has very good correction for a SYNTA only about 20% are this well corrected and my 9.25 is so good I cant tell without a Ronchi test or interferometer.
Start testing those scope lenses and mirrors many of the errors can be fixed or made better with proper adjustment of the entire optical axis. You may Email me for help or advice as best as I can give as I don't know everything but will share what I know and what tricks that only I have found and devised myself. I have spent countless hours doing research on optics and was into photography and film developing for a few years while stationed in Germany and even talked to some very nice engineers last week that make optics for NASA and the Government and they showed me some very interesting facts and physics of optics. HAPPY PHOTON busting!!作者: skystar 时间: 2007-6-22 20:47
转道去看就是麻烦呀兄弟:)作者: 夜猫子 时间: 2007-6-24 16:10
呵呵，我爱祖祖这是在给自己的博客拉点击呀:lol