Magnifying Visors

How do you assess the quality of telescope optics?

I have Barlow 3x, 50x, 250x optics pieces, and it seems that the 50x is great, the 250x is good, the Barlow + 50x is good, too, but the Barlow + 250x is not that good. I want to determine which optics would be the best to upgrade, or if the problem is the limitations of the scope quality. I wonder if the Barlow's quality is being amplified by the other two optics pieces, so that it really comes out in the 250x, or if I'm simply approaching the limits of the scope itself. How would I assess the quality of their constructions? 5" Celestron reflector scope, FYI Some people talk about poor quality optics that come with the new telescopes. What effect does that have on the observing? Chromatic aberation and edge blurring, I suppose, but are there any other effects? Also, how do you calculate the required aperture for a desired magnification. I saw Jupiter and her moons last night, but I could NOT magnify her clearly and large enough to see the Red Spot. Of course, I hadn't considered that the Red Spot might not always be pointing toward the earth. Obviously, I could have waited for Jupiter to be closer to earth, but I'm a little impulsive and impatient when it comes to viewing things. I was hoping better optics would allow seeing Jupiter's Spot in any clear, dark sky.

Public Comments

1. The problem isn't quality. The problem is that you're overdoing it. A Barlow doubler or tripler is going to increase the magnification of the objective lens, but using it with a 250x lens is going to raise the magnification to a point where not even the best optics will show anything other than a jittery, grainy mess. This will happen with even the best telescope. The highest useful magnification for a 5" scope is about 300x. If you want to get higher than that, you'll need something with a bigger aperture.
2. Wow, 750x is too much. Even with any telescope over 8" you usually can't use more than 400x because of the seeing.
3. With any telescope, as you put on more magnification, the sharpness of the image deteriorates. It's simply a byproduct of the wave nature of light. I owned a scope similar to yours, and found that the magnification topped out at around 180x. Anything above that was just magnifying the aberrations in the optics and atmosphere. Although the traditional way of testing telescopes is by splitting close double stars, I find a far more valuable test is to see how they resolve low contrast detail on the Moon and planets. The main problem with this is that it requires a lot of knowledge and experience on the part of the tester. I have literally logged days of high magnification observation of Jupiter with extremely high quality telescopes which I own. So I know what to expect. Mass produced optics, like your Celestron, can give very nice views, but they lack the high contrast and resolution of really first class custom optics. Looking for the Red Spot on Jupiter is not a very good test. Part of the problem, as you've realized, is that about two-thirds of the time it isn't visible because it's on the far side of the planet or rising or setting on the planet's limb. Also, the Red Spot is currently a very subtle object, only visible under the best of conditions. What I usually look for is resolution of the festoons along the south edge of the North Equatorial Belt. These should be visible at any time, so long as the seeing is decent. I agree with the others that you're using way too much magnification. Most Barlow lenses supplied with telescopes are low quality, and damage the already low quality of the supplied eyepieces. I'm not meaning to be critical of your equipment; that's what most people start out with, and it takes experience to know when to upgrade and with what. Usually you have to double the price to get a 10% improvement in quality, so it's not worth while for most people.
4. So you have a 3x barlow and two eyepieces, one that gives 50x and then one that gives 250x? Well let's just start from the beginning. You have a 5" Celestron. This is probably a fairly good telescope. In fact Celestrons usually have excellent optics. You can test the quality and collimation of the telescope by doing what's called a star test. As a beginner, you will have a hard time interpreting what you see with this, and you do not want to collimate your telescope (a tricky task) unless you absolutely have to so we'll assume your telescope is collimated. The maximum practical magnification you can get out of a telescope can be approximated as 50 multiplied by the aperture in inches. So you should theoretically be able to use 250x magnification. However, in reality, due to things such as atmospheric turbulence, you will seldom be able to use this magnification. To use such a high magnification, the atmosphere must be calm and clear, the telescope must be properly collimated, and it must have been allowed to sit out long enough such that it's at thermal equilibrium with the surrounding environment. This is why high powered planetary viewing is challenging. Next lets talk about your eyepieces. Most reputable eyepieces are labeled by their focal length, not magnification, because the magnification is dependant on the particular telescope's focal length in combination with the eyepiece. Specifically.... magnification = (telescope focal length)/(eyepiece focal length) in millimeters. The smaller the focal length of the eyepiece, the higher power it will provide for any given telescope. I suspect your eyepieces are low quality ones. It pays to invest in some good eyepieces, and a good barlow. I wouldn't use any barlow other than a 2x barlow. Your telescope doesn't have any chromatic aberration because it's a reflector, not a refractor. Your eyeieces probably have some degree of this though. Higher quality eyepieces typically minimize or eliminate this problem, and it's something you would want to invest in if you want to primarily view planets.