Re: 150mm refractor diy

From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 03/16/05 

Date: Wed, 16 Mar 2005 15:04:59 GMT

".::SuperBLUE::." <superblue@dontusefakkevip.hr> wrote in message
news:d19ev1$vhn$1@ls219.htnet.hr...
>> You will be wanting to obtain a free copy of OSLO-LT from Sinclair
>> Optics
>> and then learn the program and design a 4 (probably enough elements)
> element
>> lens.
> I have an optalix demo www.optenso.de but I will try to use this too.
>
>> I'll note that the faster the lens, the more difficult the job is to do
>> so
>> without excess color aberration..
> Why?
>
> The violet end of the color spectrum is
>> the worst end so you will probably be using a violet minus filter at
>> all
>> times.
> UV filter, right?
>
> You'll also be spending a fair bit of money on some of the glass
>> types that will be needed in that lens as you will need to reduce the
> color
>> as much as possible in the lens.
> I am looking at BK7 170mm diameter lens from China, www.lzos.ru options,
> Schott glass for grinding :(
> What is the price of a glass like ED glass and where can I find thicks
> round
> blanks or finished ones?
> Smaller elements are not a problem (Edmund Scientific,... )
>
>> One thing that you do have going for you is that film won't respond to
>> a
>> spot size of less than about 1/2 of a thousandth of an inch as that is
> about
>> the grain size. Asttonomical telescopes are usually done to
>> diffraction
>> limited level as we are very interested in getting the best resolution
>> performance from the lens which is why so many telescopes are done as
>> reflectors instead - there is no color with a fully reflective
>> telescope.
>> It is also a lot cheaper to build a reflective telescope as the glass
>> is a
>> lot cheaper than optically pure glass is.
> So you are saying that I can end up with a monochromatic one? That is
> not
> the intention.
'Colour' here is referring to the creation of spurious colour from
chromatic aberration. The faster a scope is, the more extreme the lenses
have to be, and the more different colours are spread. You can make a
simple one element lens, and at F/40, will probably see no perceptable
false colour in normal use. The same lens at F/10, will be displaying very
significant chromatic aberration. At F/5, the effect will be appalling. A
BK7 lens will be of this sort. Adding a second element, and creating an
achromat, allows quite acceptable results at perhaps F/10 for many uses. A
reflective telescope, inherently has no chromatic aberration (hence no
false 'colour'). However it is still able to image every colour in the
spectrum fine. With an achromat, you work by having a lens that is
stronger than you want, made out of a glass with a relatively low
dispersion, and then a second 'negative' lens made out of a glass with a
much higher dispersion, which only lengthens the focal length a little,
but undoes much of the spreading. The key is getting the curves for all
the lens surfaces right, to match the ratios of dispersion of the glasses
to minimise the overall spreading. By chosing multiple elements in
different glasses, chromatic aberration can be corrected at a number of
wavelengths, and other aberrations as well. This is the basis of the
'apochromat'. As the system focal length gets shorter, the thicknesses
required for the lenses get higher, and the cost of the glasses and work
rise. Think of BK7, as effectively high quality window glass. 'ED', is not
a glass name, but a generic 'description' of a number of low dispersion
glasses. FPL-53, is the commonest 'ED' glass, and a search on this from
the glass suppliers should find some blanks. Don't be suprised at how much
even this glass costs, while a flourite blank will make your eyes water...
A reflecting telescope avoids the problems of chromatic aberration, but
without subsidiary optics, or complex shapes on the mirror, will display
other problems.

> I have noticed a few things on froogle from Scopetronix, but those are
> not
> for refractors :(

Best Wishes

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