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Selecting Eyepieces and related information : Image quality of telescope to a great extent also depends on the quality of eyepiece. In majority of cases, telescope optics remain good but nobody that much cares for having good eyepieces and when with lower quality eyepieces, the image is not good, they blame it to the quality of telescope optics. For telescope purpose the better choice is - Kellner (3 lenses, 2 optical glass elements,1 achromat - for around 4 or
500/-) Last choice may be Ramsden and Huygenian (2 simple glass elements - 100/- or 150/-) which are mainly used in Microscopes and have small OD of 23 - 24.5mm (0.965"). Telescope eyepieces are always bigger 31.75mm (1.25") OD than Microscope eps of 23/24.5mm (.965") OD. That eyepiece is much better wherein optical glasses and Anti-Reflection (AR) coating is there. Optical glasses either of Indian or foreign-ones are always better than the opthalmic and sheet glasses. Optical glass is free of such defects like striae, strain and airbubbles etc. which may be present in cheap glasses and to some extent in opthalmic glasses too. In India you will only find cheaper (around Rs. 75/- to 100/-) Microscope eyepieces (23/24.5mm OD) of Ramsdens, Hugenians and a bit costlier WF which are called wide field (around Rs.300/-). WF with Anti-reflection coated may cost around Rs.350/- onwards. In all these Microscope eyepieces (Ramsden, Huygenian) sheet glass is used - in some better-ones good sheet glass, still better-ones opthalmic glasses (used in spectacles) and in the best-ones optical glasses are used. So the cost increases according to the glass used. As the quantity of glass used in Ramsden and Huygenian (both of .965 or 23mm OD tube) is so small, that the defects are often times too small and generally ignored. Further more, there is not that much advantage conforming to the cost in using optical glasses in Ramsden and Huygenian of .965" OD. For planetary observation or in long focal length (slow) telescopes Ramsdens and Huygenians also perform well. Often, manufacturers put the lenses of simple Ramsden/Huygenian Microscope eps in bigger OD tube and sell as telescope eyepieces in slow telescopes. Here also the design is such that if you don't get high quality eps, these would perform well with your telescope. Thus you can observe, there are different quality of glasses used; and quality and cost both increase depending upon the used glasses. As the telescope eps are bigger - 31.75mm (or 1.25") OD so optical glass consumed is of big diameter and quantity. Minimum 3 to several different optical glass elements of specific RI and Vd are needed, hence using optical glasses free of defects becomes necessity in good eyepieces. So, eyepieces for telescopes like Kellner, Plossl etc. using optical glasses and designed for telescopes are costlier than 23mm OD-ones (used in Microscopes) but their performance is certainly very good and these should be given preference over Ramsden and Huygenian eyepieces containing two simple plano-convex lenses. For the above reason, I tried to learn the art of designing and making eyepieces and Barlow-lenses. I made some Kellner eps of 1.25" OD using the optical glasses from 8.5mm to 25mmfl and clearly saw the increase in image sharpness in comparison to Ramsden and Huygenian eps. Normally you need eyepieces giving you power of 6x (for wide view), 12.7x (for overall good sharp view) and 30x ( maximum attainable in normal conditions, though with superb optics and ideal sky conditions (both are rare) theoretically it can go upto 60x per inch of aperture. Commercial people mostly advertise 60x per inch of magnification even though their telescope may not perform well even at 30x per inch. As mentioned above, for your Mirror of 4.25", you need eps which can give you 6x4.25 = 25x; 12.7x4.25 = 54x and 30x4.25 = 127x i.e of 40mm, 18.5mm and 8.0mm fl. But here you will find Ramsden and Huygenien Microscope eps of 25mm(termed10x), 16.7mm(15x)and 12.5mm fl(20x) from which you will get magnification of 1000/25 = 40x and similarly 60x and 80x. Expert astronomers say that the best, stunning images are seen between 12.5x to 25x per inch of aperture. Hence if the telescope made by you gives you good images at 25x4.25 = around 105x to 110x, your telescope is good and you need not care for maximum theoretical magnification Some people prefer having two eyepieces and one 2x-Barlow so that they can have four combinations sufficient for all purposes. So you can also choose eps accordingly.
The Barlow with less 'x' factor are costlier than bigger 'x'. Like Barlow of '1.5x' will be more costlier than '2x'; '2x' would be more costlier than '3x' and so on. It is because of the reason that bigger-x factor Barlow lenses are easy to make than the less-factor Barlows. Among these also the Barlows having achromat and apochromat lenses are better than having simple lenses. But these are very costlier. Apochromat-Barlows are more costlier than achromat-Barlows. Variable factor Barlows don't give that much sharp image than that of fixed factor Barlows.
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