- 08 Ara 2010, 01:49
Bugun, 12,5 cass. teleskopun ikincil aynasini yapmaya basladim. 98 mm capinda ve 25 mm kalinligindaki camlari sagolsun sevgili Ugur bey temin edip gondermisti (Kendisine tekrar tesekkur ediyorum);
4 adet olmasina gelince, (Dick Parker'in yontemi) bir tanesi ayna, bir tanesi tool, bir tanesi ayna icin cilalama aleti, bir tanesi de tool icin cilalama aleti. Tool'u cilama nedeni (burada newtonian ayna gibi icbukey olmasindan dolayi) dogru odak uzunlugunu foucault ile olcmek (normalde disbukey yuzeyin odak uzunlugunu bu yontemlerle olcemedigimiz icin) Zaten tool'u mukemmel cilalamayacagiz, sadece ronchi cizgilerini gorebilecek kadar ya da foucault ile olcme yapmamiza yetecek kadar cilalama yapacagiz.
Bu konuda bildiklerim buraya kadar. Bundan sonrasini, yani disbukey aynayi cilalama ve bicimlandirme surecini onumuzdeki gunlerde (sevgili Jerry ile birlikte) hep beraber yapip ogrenecegiz umarim.
Cassegrain teleskoplarin onune konulan "corrector" merceklerin islevini sevgili Jerry'e sordum. Ben ayrica "field flattener" ile bunlarin ayni sey olabilecegini dusunmus ve butun bunlarin coma ile ilgisini de merak etmistim. Sevgili Jerry'nin anlattiklarindan anladigima gore, bir kere field flattener ile corrector lens'ler aynı sey degil. Field flattener, odaga konulan kucuk bir mercek (bizim de yapabilecegimizi soyluyor Jerry). Ama hepsi de alan, sacilma ve coma ile ilgili. Sevgili Jerry diyor ki; bir optik yuzey size sadece bir sacilma alani ile ilgili sans verir (on-axis). Ama yaptiginiz dizaynda birden fazla optik yuzey varsa sadece merkezdeki degil diger alanlardaki sacilmayi da kontrol etme imkanina sahipsiniz. Mercekli teleskoplarda buna renksel sapinclar da dahil oluyor. Dolayisi ile schmidt veya maksutov'larda kullanilan corrector'larin her biri dizayna gore degisik bir amaca sahip. Benim anladigim yanlis ya da farki olabilir, yanlis
anlasilmalara sebebiyet vermek istemedigimden Jerry'nin mesajinin ilgili bolumunu oldugu gibi buraya kopyaliyorum;
Saygi ve sevgilerimle,
PS: Umarım sevgili Onur askere gitmeden önce bu mesaji okur (Bu arada nereye gideceği belli oldu mu?) Ve böylece bizlerin de bu konuda aydınlanmamıza yardımı olur.
>As for my question; I have not seen yet a cassegrain telescope except telescope >pictures. The "field flattener" >or "corrector" that we have seen on the front of schmidt or maksutov cassegrain telescope, what does it do?
"The lens on the front of Schmidt and Maksutov Cassegrains are used to allow more “degrees of freedom” to correct aberrations.
The parabola of the Newtonian corrects spherical aberration perfectly on axis (the very center of the field of view). But the parabola has coma and astigmatism off axis. This reduces the quality of the image off axis. The single optical surface of the Newtonian allows only one degree of freedom in the design. That degree of freedom allows fixing only one aberration. To use a different conic curve (oblate ellipses, sphere, prolate ellipses, parabola, and hyperbolas) on the Newtonian mirror might improve one aberration, but it will degrade the others. The Coma could be made better but the spherical aberration will be worse. The secondary mirror is a second surface, but it must be flat, so it does not allow a second degree of freedom.
Your Cassegrain has a curved secondary. This allows another degree of freedom in design. This allows correction of another aberration. By using different conic curves on the primary and secondary correction can be made on two aberrations.
Actually you have a third degree of freedom… the focal ratio of the primary. You could have the same Cassegrain focal length with different primary focal lengths. This allows you to control another aberration, such as field curvature. The Ritchey-Chretien design uses different conic curves (a degree of freedom) than the classical Cassegrain to reduce or eliminate coma.
The Schmidt corrector and the Maksutov correctors have front and back surfaces and thicknesses that are degrees of freedom. Also there is the spacing of the corrector from the primary mirror. That is another degree of freedom.
The basic reason for the corrector is to eliminate spherical aberration with a mirror that is not a parabola. There are numerous variations of the designs that use different conic curves on the mirrors and spacing of the corrector and mirrors. These numerous designs have features that may be chosen to suit the particular use of the telescope or the ease of making the optical elements.
The thickness of the Maksutov corrector is used to control chromatic aberration of the corrector lens. The thickness of the Schmidt is not used as a degree of freedom, a suitable thickness is chosen to be mechanically practical. But the Schmidt corrector curves have a profile that can be designed to place the thinnest part of the lens at a chosen radius from the center of the lens. This allow chromatic aberration to be reduced to a minimum. So the zone location of the lens that is thinnest is a degree of freedom.
Refractors have multiple surfaces, usually 2 for each element. I say usually because if two lenses are cemented there is only one curve between them. If you change the radius of one you must change the other to match. A triplet lens has more degrees of freedom in design than a doublet. A doublet has more than a singlet lens. A single lens of a particular glass type has 3 degrees of freedom. The curve of the first surface, the curve of the second surface, and the thickness of the lens. The type of glass used is another degree of freedom. A doublet lens has these degrees of freedom for each element and also the spacing between the lenses.
Having more degrees of freedom in designing the optics does not necessarily mean that you can bring all the aberrations to zero. It gives you more control to lessen more aberrations. Some aberrations can be brought to zero at some location in the field of view, but not over the entire field. These are the sorts of things that the designer considers in designing the system. How large do you want the usable field to be, how much aberration can you allow in the center of the field in order to improve the aberrations in the outer parts of the field? How much chromatic aberration can you allow while improving coma or astigmatism or spherical aberration?
There is much more all this than I can say here J J.
Needless to say… This is what keeps the mathematicians busy, and why there is no perfect optic. But as with your Cassegrain, we who are less capable with numbers use what the mathematicians have already calculated to design our telescopes.
The field flattener for the Cassegrain is a smaller lens near the focus. It would need to be designed for the particular telescope. A field flattener designed for one Cassegrain design (Ritchey-Chretien or Classical or other design type, primary diameter and focus, secondary power, back focus and so on, would not work well on a different Cassegrain design. Measure carefully the curves of both mirrors when finished, and spacings after the telescope is properly aligned, then we can work on designing a field flattener. If you make a good design (I might be able to help J) the lens will be small and should be relatively easy to make."