Telecentric systems are mainly used in H-alpha filters for solar observation, where the 1/100 lambda plane-parallel etalon in the H-alpha filter requires a perfectly parallel beam of light over the entire image. These telecentric systems deliver very sharp images, but are usually only optimised for the H-alpha wavelength.
This achromatic SunDancer 4x telecentric is designed for the entire visible spectrum and can therefore not only be used on H-alpha systems, but this TZ-4S as well as the Baader SunDancer II Telecentric System TZ-3S can also be used instead of a Barlow lens. Due to the parallel beam of light, it is very good-natured with regard to the working distance: While the magnification factor of a Barlow lens changes with the distance to eyepiece or camera (zoom eyepieces are also based on this principle), the magnification factor and image quality of the TZ-4 do not change, even if the camera or eyepiece is mounted well behind the nominal working distance (97 mm from the base of the T-2-thread).
The achromatic SunDancer II TZ-4S illuminates an image circle of 36 mm at a distance of 97 mm from the base of ther T-2-thread. The contrast is better than with the common methods used for higher magnifications: simple barlow lenses or eyepiece projection. With the 4x telecentric system, high magnifications for observing the solar system can be achieved even with long focal length orthoscopic eyepieces. The eye relief of the eyepieces used does not change, too! The focus position changes only 3 mm outward when using the 2″ nosepiece of the telecentric, compared to a 2″ eyepiece.
The connection on the telescope side is made via a dual 1.25″/2″ nosepiece with safety kerfs (these prevent tilting due to an incorrectly positioned safety groove and at the same time provide a secure hold). On the camera side, the T-2 thread provides a tilt-proof, fixed connection even to heavy cameras or to T-2 eyepiece clamps. For the T-2 thread, adapter rings are available for almost all common connection systems (e.g. M48 / 2″/ M68).
The TZ-4S in use with H-alpha-Filters
The TZ-4S was specially designed for the Baader SunDancer II H-alpha Filter (#1363056) and replaces the original 3x telecentric. This allows to use the SunDancer II H-alpha filter also on telescopes with f/7-f/8 with the optimal focal ratio of about f/30. Please note: For this, the 12 mm 2 nm H-alpha blocking filter, which is pre-installed in the front of the TZ-3S, must be installed in the TZ-4S. This blocking filter is not available separately. The ideal working distance when using the SunDancer II is then 70 mm from the base of the T-2 thread of the SunDancer II H-alpha filter (instead of 65 mm with the original TZ-3S).
For H-alpha filters with up to 36 mm diameter with their own block and energy protection filter (e.g. SolarSpectrum filter), the TZ-4S can be used instead of the long-building Telecentric System TZ-4 (4x focal length) (#2459256) .
The SunDancer II Telecentrics
The Baader SunDancer II Telecentric System TZ-3S (#1363070) with gold anodized label: Achromatic 3x telecentric with 95 mm working distance from the base of the T-2 thread and 27.5mm clear aperture. Provides the desired focal ratio of f/30 on telescopes with f/10. This telecentric is also available without the SunDancer II H-alpha filter, but then only without the 2 nm block filter of the H-alpha filter. For clear identification, this telecentric has a gold-colored anodized banderole.
The 3x-Telecentric TZ-3S without block filter, recognizable by the gold-coloured ring
TZ-3S with 2nm block filter and red anodized label: Achromatic 3x telecentric of the Baader SunDancer II H-alpha Filter (#1363056) is equivalent to the Baader SunDancer II 3x Telecentric TZ-3S, but additionally contains a 2nm (FWHM) block filter. It shimmers golden when you look into the SunDancer from the telescope side. This block filter is mandatory to protect the actual H-alpha filter from solar radiation. For clear identification, this telecentric has a red anodized band, otherwise identical to the Baader SunDancer II Telecentric System TZ-3S (#1363070) . Please note: We deliver the SunDancer II only with TZ-3S. An alternative cover with TZ-4S is unfortunately not possible.
The 3x-Telecentric with integrated block filter is only available together with the SunDancer II H-alpha filter. Here the block filter of the SunDancer II is pre-installed, the version is recognizable by the red ring.
Baader SunDancer II Telecentric System TZ-4S (#1363080): with violet anodized label: Achromatic 4x telecentric with 97 mm working distance from the base of the T-2 thread and 36mm clear aperture. Provides the desired focal ratio of f/30 on telescopes with f/7.5. For clear identification, this telecentric has a violet anodized label. This telecentric does not include the 2 nm blocking filter of the H-alpha filter; for use with the SunDancer II, the 2 nm block filter must be converted from the TZ-3S of the SunDancer II filter to the TZ-4S. The use with large SolarSpectrum H-alpha filters with already integrated block filter and additional energy protection filter is possible without this additional 2 nm block filter.
The blocking filter reflects the sunlight just before the etalon and the telecentric system. This protects the etalon from excessive heat. In addition, an etalon also transmits wavelengths that are a multiple of the desired transmission line; these are also stopped by the blocking filter, too. The blocking filter or the TZ-3S with pre-installed blocking filter are not available separately.
The 4x-Telecentric TZ-4S without block filter, recognizable by the violet ring
TZ-optical systems are essential for undeteriorated performance of heated SolarSpectrum filters: Please note that for H-alpha observations below 1 Ang halfbandwidth (FWHM) a telecentric lens system is absolutely mandatory, in order to achieve the recommended focal ratio of f/30 to (preferably) f/40! Because only with a telecentric approach you can create a perfectly parallel optical path which is essential for the unrestricted function of such a complicated etalon.
A TZ-lens-system is an ESSENTIAL prerequisite. One could also bring a telescope to a focal ratio of f/30, but this does not mean at all that a parallel optical path is created. And mind you, a still slightly conical optical path with f/30 sent into an etalon with 0.5 Ang FWHM would only provide the contrast as if the filter had a FWHM of 0.7 Ang or much worse.
The main difference between TZ-4S and the older TZ-4 are adaptation to the telescope ( the TZ-4 uses T-2-threads for a stable connection – perfect for the long SolarSpectrum filters) and their working distance. The free apertureof the TZ-4S is comparable with about 35 mm and does not fully illuminate the largest SolarSpectrum filters with 46 mm filter diameter. But it doesn’t always have to be a TZ-4. The 2″ Research Grade TZ-3 Telecentric System (3x focal length) (#2459257) with 46mm clear aperture is much better in terms of optical calculation than the 20 year older TZ-4. So it makes sense for the user to start with a TZ-3 RG and rather stop down the telescope aperture to get to f/30. This will bring out the real contrast of the chosen SolarSpectrum-filter much better than with a poorly matched system where, for example, the older TZ-4 has a smaller clear aperture than the Solar Spectrum-Etalon would require. In the long run, a recalculated, large RG TZ-4 is in preparation – it is worth to wait for it and to get familiar with H-alpha observation with the TZ-3 first. Later, one will surely need both systems (RG-TZ-3 and RG-TZ-4) and will have to stop down the telescope aperture as needed.
Only AFTER the H-alpha etalon you can extend the field size by reducing the f-ratio with adding the new ResearchGrade telecompressor 0.4 into the optical train.
Basically – a Barlow lens has no place in an H-alpha system!