Baader Planetarium TZ-3S Telecentric System T-2 for SunDancer II and more

Why H-alpha Etalons Need a Telecentric

The 1/100 lambda plane-parallel etalon in an H-alpha filter requires a perfectly parallelised beam of light over the entire image, and for H-alpha observation below 1 Ångström half-bandwidth (FWHM) a telecentric lens system is absolutely mandatory to reach the recommended focal ratio of f/30 to (preferably) f/40. A Barlow lens can also bring a telescope to f/30, but that does not create a parallel optical path: a still slightly conical beam at f/30 sent into an etalon with 0.5 Å FWHM would only give the contrast of a filter with 0.7 Å FWHM or much worse; put plainly, a Barlow lens has no place in an H-alpha system.

Baader adds that many telecentric systems deliver very sharp images but are usually optimised only for the H-alpha wavelength, whereas this achromatic SunDancer telecentric is designed for the entire visible spectrum. For use with other H-alpha filters, Baader offers the TZ-3S without the blocking filter.

Achromatic Design and 27.5mm Image Circle

The TZ-3S is designed for the entire visible spectrum and is also suitable for white light and planetary photography. It has a four-lens optical design with BBAR (Broad-Band Anti-Reflection) coating, an anti-reflective thread and blackened lens edges against reflections and for maximum contrast. Its contrast is better than with simple Barlows or the eyepiece projection often used for high magnifications.

The optical system is designed to fully illuminate an image circle of 27.5 mm. This is ideal for the small sensors of modern, fast planetary cameras that need high magnifications for the Moon and planets. For visual use, the 3x telecentric lets you reach the necessary high magnifications with long focal length orthoscopic eyepieces. The clear aperture on the camera side is 27.5 mm.

96 mm Working Distance

Because of the parallel beam, the telecentric is very good-natured with regard to working distance: while the magnification factor of a Barlow lens changes with the distance to the eyepiece or camera, the magnification factor and image quality of the telecentric do not change, even if the eyepiece is placed well beyond the design working distance. The interpupillary distance of the eyepieces used does not change either. The design working distance is 96 mm from the base of the T-2 thread without an additional H-alpha filter; in the SunDancer II, because of the H-alpha filter's glass path, it is 65 mm.

Baader compared the TZ-3S with a Baader Q-Barlow brought to a triple magnification factor with spacer sleeves, photographing a church steeple with an APS-C DSLR at the same camera settings, to show that the telecentric keeps the working distance, image quality and image scale while giving a high-contrast image.

Using the TZ-3S with the SunDancer II H-alpha Filter

The telecentric of the SunDancer II H-alpha filter (#1363056) is the same as this TZ-3S but additionally contains a 2 nm (FWHM) blocking filter, which shimmers golden when you look into the SunDancer from the telescope side; for identification it has a red anodized band, while this standalone TZ-3S has a gold-coloured one. The blocking filter is absolutely necessary to protect the actual H-alpha filter from solar radiation. The SunDancer II manual is explicit: "Only when the original block filter is installed in the designated position in front of a SunDancer TZ-3S (scope of delivery) or TZ-4S, safe solar observation is possible. Without this pre-filter, the etalon will be destroyed immediately, as will your eyes."

The blocking filter reflects the sunlight just before the etalon and the telecentric system, which protects the etalon from excessive heat; it also stops the wavelengths at multiples of the desired transmission line that an etalon also passes. The blocking filter, and the TZ-3S with the blocking filter pre-installed, are not available separately. The SunDancer II comes only with the TZ-3S; for faster telescopes of about f/7.5 there is the TZ-4S (#1363080), into which the blocking filter of the SunDancer II is moved. The SunDancer II manual states that the H-alpha filter must only be used with a SunDancer telecentric system and the blocking filter which is pre-installed in the TZ-3S; "otherwise, the filter will be destroyed, and there is no protection for your eyes."

Mechanical Interface: Dual 1.25"/2" Nosepiece and T-2 Thread

The telescope side has a dual 1.25"/2" nosepiece with Safety-Kerfs, which prevent tilting due to an incorrectly positioned safety groove and at the same time give a secure hold. On the camera side the T-2 thread gives a tilt-proof, fixed connection even to heavy cameras or to T-2 eyepiece clamps; adapter rings are available for almost all common connection systems, such as M48, 2" or M68.

• Telescope Connection: Dual 1.25" (31.75 mm) and 2" (50.8 mm) nosepiece with safety kerfs.
• Camera Connection: M42 x 0.75 (T-2) thread.
• Filter Thread: The 1.25" nosepiece includes an internal filter thread for standard 1.25" filters.

TZ-3S and TZ-4S

There are two telecentric systems to match different telescope focal ratios to the required f/30 for H-alpha filters. In the SunDancer II, the 3x telecentric is optimal for telescopes with f/10, recommended up to about f/8 and usable up to about f/6.5; on optics faster than f/10 the FWHM increases, which is why the TZ-4S exists for telescopes of about f/7.5.

• TZ-3S (3x Magnification): Identified by a gold anodized ring, the TZ-3S is designed for telescopes with a native focal ratio of f/10. Working distance 96 mm, clear aperture 27.5 mm.
• TZ-4S (4x Magnification): Identified by a violet anodized ring, the TZ-4S is designed for faster telescopes around f/7.5. Working distance 97 mm, clear aperture 36 mm; it does not include the blocking filter.