pstinson@pbs.org (08/02/90)
Although I use three telescopes, a 3" Maksutov, an 8" Schmidt/Cass. and a 6" Newtonian, I may not fully understand mirror design. I just use them. According to an E-mail, design has no effect on spectral response - only coating does. Do other telescope experts out there agree with this? I was under the impression Infrared, for example, does not focus at the same point that visible light does and the thing that effects the focal point the most is the curvature of the mirror. I don't see how coating alone is going to solve this problem of more than one focal point. If the mirror is going to be used primarily for infrared work, wouldn't its shape be a bit different? I do know widefield and planetary scopes, dedicated to those purposes vary significantly in the degree of curvature.
mmm@cup.portal.com (Mark Robert Thorson) (08/06/90)
I'm not an expert on infrared astronomy, but as a photography hobbyist I am aware that photographers who use infrared film generally use the same lenses as for visible light work. Most lenses have a focusing ring, which you turn to get the image in focus. On a single lens reflex or rangefinder camera, you focus by turning the ring and looking through the eyepiece without looking at the little numbers engraved on the focusing ring. These numbers tell you the distance the lens is focussed for. When using infrared film, you focus the lens, then take your eye away from the eyepiece and look at the focussing ring. On the lens body next to the focusing ring will be a mark indicating which engraved number on the focussing ring correspond to the focus distance. On a serious lens for photography, there will be a second mark, usually indicated in red, for infrared. Once you've focussed the lens for the visible light distance, you then turn the ring so that the same distance is lined up with the red mark. I don't see how infrared optics would be any different from visible light optics, except for the coating used on mirrors and the material used for lenses. After all, parabolic reflectors are used for both optical Newtonian telescopes and radar antennas. BTW, I've heard that infrared mirrors use gold as a coating.
francis@hydracs.ua.oz.au (Francis Vaughan) (08/06/90)
In article <32494@cup.portal.com>, mmm@cup.portal.com (Mark Robert Thorson) writes: |> When using infrared film, you focus the lens, then take your eye away from |> the eyepiece and look at the focussing ring. On the lens body next to the |> focusing ring will be a mark indicating which engraved number on the |> focussing ring correspond to the focus distance. On a serious lens for |> photography, there will be a second mark, usually indicated in red, for |> infrared. Once you've focussed the lens for the visible light distance, |> you then turn the ring so that the same distance is lined up with the red mark. |> |> I don't see how infrared optics would be any different from visible light |> optics, except for the coating used on mirrors and the material used for lenses. |> After all, parabolic reflectors are used for both optical Newtonian telescopes |> and radar antennas. BTW, I've heard that infrared mirrors use gold as a |> coating. For the most part this is correct, but only when talking about conventional refracting lenses. The different focusing points is due to the fact that most materials refractive index is dependant upon wavelength. This property is called dispersion. It is this property that causes both chromatic aberration and rainbows. A refracting telescope will (if it is any good at all) have a two element "achromat" lens that is made in two parts, each with a different refractive index, and a design that attempts to correct the dispersion of visible light and bring all wavelengths to focus at the same point. (This is of course impossible, but a reasonable compromise is made.) There do exist special low dispersion glasses, Nikon's ED (Extra-low Dispersion) lens range use them. They are noticably sharper than lenses using conventional glasses because of an almost total lack of chromatic aberration. They have the interesting property that they have no special infra-red index mark. IR focusses at the same point as visible wavelengths. They are also unbelivablely expensive. (Like over $10k for some monsters.) So much for lens systems. For reflectors there is no such thing as dispersion. A reflecting telescope has NO chromatic aberration and so long as the reflecting coatings reflect the light the image will as good in any wavelength observed as another. (Well almost true, if the optics are REALLY diffraction limited - the HST was supposed to be, and we hope will eventually be so - the resolution drops in proportion to the increase in wavelength.) The reason for coating mirrors with gold and such is that reflectivity at visible wavelengths may have no bearing on reflectivity at other wavelengths. Silver is transparent to UV (I think, its been a while since I checked). Many of the amateur telescopes use special coatings on the mirrors and corrector plates (taking schmidt-cassegrains here), these would be worse than useless if one wanted to do work in far wavelengths. Of course the corrector plate being ordinary glass would absorb many interesting wavelengths anyway. Schmidt-cassegrains do not suffer from chromatic aberration in the way refractors do because the light is bent such a small amount when passing through the corrector and the corrector is so thin that the aberration induced is undetectable. Francis Vaughan
tholen@uhccux.uhcc.Hawaii.Edu (David Tholen) (08/06/90)
In article <32494@cup.portal.com>, mmm@cup.portal.com (Mark Robert Thorson) writes: > BTW, I've heard that infrared mirrors use gold as a coating. Some do, usually the secondary mirror. It's not the reflectivity that makes gold so desirable. At infrared wavelengths, aluminim has perhaps 98% reflectivity and gold has about 99%. So why go through the expense of using gold if you only gain 1% more light? Well, what doesn't get reflected gets absorbed and then reradiated at thermal wavelengths. So by going with gold, you're cutting the emissivity in half -- a big gain if you're working at thermal wavelengths!
rclark@lpl.arizona.edu (Richard Clark x4971) (08/11/90)
In Message-ID: <9776.26b81e79@pbs.org> >Although I use three telescopes, a 3" Maksutov, an 8" Schmidt/Cass. and a 6" >Newtonian, I may not fully understand mirror design. I just use them. >According to an E-mail, design has no effect on spectral response - only coating >does. Do other telescope experts out there agree with this? I was under the >impression Infrared, for example, does not focus at the same point that visible >light does and the thing that effects the focal point the most is the curvature >of the mirror. I don't see how coating alone is going to solve this problem of >more than one focal point. If the mirror is going to be used primarily for >infrared work, wouldn't its shape be a bit different? I do know widefield and >planetary scopes, dedicated to those purposes vary significantly in the >degree of curvature. With reflective optics the shape, spacing, and tilt of the surfaces are the only factors controlling the quality of image formation. In the case of IR the relatively long wavelength relaxes the tolerances to which the surfaces must be produced for a given image quality. (Quarter wave, 1/10th wave etc). Although this wave specification refers to the wavefront at the image, not the actual mirror surface. The coating is not equally reflective at all wavelengths so that will be a factor in the telescope's efficiency in collecting light. With transmissive optics (lenses, corrector plates) the refractive index is a function of wavelength so these elements introduce the shift in focus with wavelength. For these elements the same factors as above (surface figure, element spacing, tilt) and the change in refractive across each surface (glas-air, flint-crown etc) also effect the power of the surface. Hence chromatic aberations can be introduced into catadioptric (hybrid systems with both lenses and mirrors) systems although usually the refractive element in these is a corrector plate designed to control spherical aberation (such as introduced by the all spherical surfaces in the maksutov) and has very little focusing power. So cat systems essentially free from chromatic aberations. In other words, the focusing is done by mirrors and the lenses do the correcting. The achromatization of refractor systems is accomplished by using elements of different refractive index and dispersion. Dispersion being the rate at which index changes with wavelength. With the familiar refractor doublet objective it is possible to have 2 wavelengths focus at the same point. These wavelengths can be chosen to bracket the wavelength range that the ccd, film, eye, etc is sensitive to. The other degrees of freedom (4 surfaces, 2 thicknesses, and one separation) can be used to control the non chromatic aberatic aberations like spherical, coma, or distortion. Hope the questions I answered are close to the ones you were asking.