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WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT PERMANENT BLINDNESS CAN EASILY RESULT FROM SIMPLE MISTAKES.
NEVER LOOK AT THE SUN THROUGH ANY LENS, MIRROR OR INSTRUMENT UNLESS IT HAS BEEN FULLY TESTED AND APPROVED FOR SUCH USE. YOU FOLLOW MY EXAMPLE ENTIRELY AT YOUR OWN PERIL!
WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT PERMANENT BLINDNESS CAN EASILY RESULT FROM SIMPLE MISTAKES.
NEVER LOOK AT THE SUN THROUGH ANY LENS, MIRROR OR INSTRUMENT UNLESS IT HAS BEEN FULLY TESTED AND APPROVED FOR SUCH USE. YOU FOLLOW MY EXAMPLE ENTIRELY AT YOUR OWN PERIL!
The image below shows the rough layout of a solar telescope based on the Celestron CR150HD 150mm f/8 [1200mm f/l] refractor and the Coronado PST. A 90mm Baader D-ERF filter is assumed. As is the standard 20mm diameter PST Etalon and its associated optics.
In the standard instrument the light cone from the 150mm objective is physically reduced in aperture to 138mm by the narrowness of the main tube where it starts some 10cm behind the lens. [Mechanical vignetting] At 10cm behind the lens the tube should have about 145mm minimum internal diameter. So vignetting with the original telescope design is inevitable despite the claim of 150mm aperture . Where do I queue for a refund?
The true, clear aperture is probably somewhere around 142-143mm or very slightly more. This is easily measured by running cords from a 15cm spacing down to the focal plane 120cm away. I used a 20mm pin spacing at the focus. Measuring across the cords shows that the tube obstructs the clear aperture until well behind the objective lens. So the main tube is acting as a stop or undersized baffle. Worse, the light is at grazing incidence where it strikes the inside of the tube. Which must mean light scattering and some loss of contrast compared with a flat, perforated baffle.
The true, clear aperture is probably somewhere around 142-143mm or very slightly more. This is easily measured by running cords from a 15cm spacing down to the focal plane 120cm away. I used a 20mm pin spacing at the focus. Measuring across the cords shows that the tube obstructs the clear aperture until well behind the objective lens. So the main tube is acting as a stop or undersized baffle. Worse, the light is at grazing incidence where it strikes the inside of the tube. Which must mean light scattering and some loss of contrast compared with a flat, perforated baffle.
The effective clear aperture of a solar telescope based on the same instrument is limited by the 20mm optics at the etalon. The PST's etalon must be placed at 20cm from the focus to obtain an f/10 light cone. This is the only position where the PST optics can provide the etalon with parallel light. So the PST Etalon is forcing a 120mm effective aperture on the instrument. Aperture x focal ratio = focal length. 120 x 10 = 1200mm. Or, aperture = focal length divided by focal ratio.
The D-ERF cannot be placed further than 70cm from the focal plane. Or closer than 50cm from the objective. Both dimensions amount to exactly the same thing. The limited 90mm diameter of the filter face fixes its allowed position if one is to avoid vignetting of the light cone.
Much the same situation exists as with the flat mirrors of a folded refractor. The light cone must be contained by the aperture circle of the reflective element. With a little to spare to avoid the very edges of the glass. Which sometimes suffer a slight rounding off [turned edge] from the polishing action.
Only by stepping up to the next size of 110mm [at considerable extra expense] can the D-ERF filter be placed much nearer the objective. The 20mm Etalon would still be the limiting factor on aperture so the extra expense is probably wasted on an instrument of this size.
Thanks to the small aperture of the PST components the instrument has effectively become a 120mm f/10. So one could re-position the D-ERF filter. The effective light cone would start narrower at the objective than before. Allowing the the same sized filter to be placed slightly closer [only a centimeter or two] nearer to the lens without vignetting. I have marked the narrower light cone in red in the image below.
The larger the telescopes aperture the longer the focal length must be to obtain an f/10 focal ratio. This also limits the field of view and forces rather high magnifications. Higher still, if it is desired to fit a bino-viewer and its inevitable extra optics just to be able to reach focus. Such high magnifications demand good seeing conditions which are not always likely with the sun heating the surrounding ground and atmosphere.
A smaller aperture instrument would offer lower magnifications and a larger field of view. So more of the Sun can be seen [or imaged] at one time. However, the smaller aperture means lowers resolution and dimmer images.
Only by stepping up to the next size of 110mm [at considerable extra expense] can the D-ERF filter be placed much nearer the objective. The 20mm Etalon would still be the limiting factor on aperture so the extra expense is probably wasted on an instrument of this size.
The image above, not to scale, shows the lack of room at the end of the main tube for the etalon. There is certainly no room for the shortest focuser which are around 10cm. The etalon cannot be inside the main tube or it would not be available for tuning the precise wavelength for displaying the surface or prominences. The tail end of the main tube can easily be shortened. Then it just needs new holes drilled to hold the cast, focuser tailpiece.
Thanks to the small aperture of the PST components the instrument has effectively become a 120mm f/10. So one could re-position the D-ERF filter. The effective light cone would start narrower at the objective than before. Allowing the the same sized filter to be placed slightly closer [only a centimeter or two] nearer to the lens without vignetting. I have marked the narrower light cone in red in the image below.
The larger the telescopes aperture the longer the focal length must be to obtain an f/10 focal ratio. This also limits the field of view and forces rather high magnifications. Higher still, if it is desired to fit a bino-viewer and its inevitable extra optics just to be able to reach focus. Such high magnifications demand good seeing conditions which are not always likely with the sun heating the surrounding ground and atmosphere.
A smaller aperture instrument would offer lower magnifications and a larger field of view. So more of the Sun can be seen [or imaged] at one time. However, the smaller aperture means lowers resolution and dimmer images.
Click on any image for an enlargement.
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