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Telescope Questions - Varied
My research and your guidance have lead to another couple of
questions. These are not really related to my research into buying a telescope. They are more curiosities I came across in my research and haven't been able to figure out yet. Also, if there are any members of the SDAA out there, look for me tomorrow night at your event at Mission Trails. I'll be the overweight man with the bright yellow binoculars (hey, I did it so I wouldn't misplace them as often). The questions: 1. I notice that most of the GT scopes on fork mounts have both 3 star and 2 star alignment modes. The ones on a German Equatorial mount only have 2 star alignment modes. After thinking about this, I think I know what the difference is. The two star modes assume that you have already polar aligned the mount. This way, they only need the 2 stars to determine where everything else is. Am I correct? 2. One disadvantage to longer focal length scopes seems to be the difficulty in achieving LOW magnifications for deep sky observing. For example, an 8 inch telescope with a 2550mm focal lengh and a 40mm eyepiece is going to have a magnification in the low 60's while the same aperature but a 1000mm focal lengh and a 20mm eyepiece is going to give a 50x magnification. It is also harder to find the longer focal lengh eyepieces (at least where I've been looking). People keep talking about focal length reducers but I can't picture how this works optically (I understand extenders). Can you point me to an explaination of the reducers? 3. You want to let your optics reach thermal equilibrium to get the clearest image. However, refractors and SCT's sometimes (often?) need protection against dew and one way of doing this is to slightly warm the front lens. Doesn't this degrade the optical quality by destroying the thermal equilibrium? 4. Planisphere's seem to have east and west reversed. Now this bothers me because I'm used to glancing down and then up and have to perform some mental gymnastics to reverse things in my head. Are they all designed to be held above your head? Are any designed to be looked down at (ok, this one is hardly new, just something that has bothered me on and off since I was a kid)? Thanks . Mark |
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Hi Edward,
1. I notice that most of the GT scopes on fork mounts have both 3 star and 2 star alignment modes. The ones on a German Equatorial mount only have 2 star alignment modes. After thinking about this, I think I know what the difference is. The two star modes assume that you have already polar aligned the mount. This way, they only need the 2 stars to determine where everything else is. Am I correct? You are correct. Actually, if you have good polar alignment, you can do one star alignment with many of them. 2. One disadvantage to longer focal length scopes seems to be the difficulty in achieving LOW magnifications for deep sky observing. For example, an 8 inch telescope with a 2550mm focal lengh and a 40mm eyepiece is going to have a magnification in the low 60's while the same aperature but a 1000mm focal lengh and a 20mm eyepiece is going to give a 50x magnification. It is also harder to find the longer focal lengh eyepieces (at least where I've been looking). People keep talking about focal length reducers but I can't picture how this works optically (I understand extenders). Can you point me to an explaination of the reducers? A reducer is a positive lens placed before the focal point. Often these are used to flatten the field as well. It is different from the eyepiece in it's placement. The objective forms an image of the object being observed. The eyepiece is then used as a powerful magnifying glass to inspect this image. With the reducer, it is placed well before the focal point. 3. You want to let your optics reach thermal equilibrium to get the clearest image. However, refractors and SCT's sometimes (often?) need protection against dew and one way of doing this is to slightly warm the front lens. Doesn't this degrade the optical quality by destroying the thermal equilibrium? Somewhat, but not as much as the dew obliterates the image! :-) What you want is to have the front lens or corrector just a tiny bit warmer than the night air. 4. Planisphere's seem to have east and west reversed. Now this bothers me because I'm used to glancing down and then up and have to perform some mental gymnastics to reverse things in my head. Are they all designed to be held above your head? Are any designed to be looked down at (ok, this one is hardly new, just something that has bothered me on and off since I was a kid)? See http://skyandtelescope.com/howto/vis...ticle_75_1.asp for a good explanation. Thanks . You're welcome. Hope this helps. Chuck Taylor Do you observe the moon? Try the Lunar Observing Group http://groups.yahoo.com/group/lunar-observing/ ************************************************** ********** Mark |
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