Baader Planetarium TZ-4S 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.

Achromatic 4x Magnification Across a 36mm Image Circle

The TZ-4S extends the focal length by 4x, giving f/30 on telescopes with f/7.5. It has a four-lens optical design with BBAR (Broad-Band Anti-Reflection) coating and is also suitable for white light and planetary photography.

The optical system is designed to fully illuminate a 36mm image circle. The clear aperture is 36 mm on the camera side, and the free aperture, comparable to about 35 mm, does not fully illuminate the largest SolarSpectrum filters with 46 mm filter diameter. An anti-reflective thread and blackened lens edges work against reflections.

97 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 nominal working distance of 97 mm from the base of the T-2 thread. The eye relief of the eyepieces used does not change either, and the focus position moves only 3 mm outward when using the telecentric's 2" nosepiece, compared with a 2" eyepiece.

With the SunDancer II, the ideal working distance becomes 70 mm from the base of the T-2 thread of the H-alpha filter, instead of 65 mm with the original TZ-3S.

Use with the SunDancer II: Moving the Blocking Filter

The SunDancer II comes only with the TZ-3S, which is optimal at f/10, recommended up to about f/8 and usable up to about f/6.5. The TZ-4S replaces it so that the filter can be used at about f/30 on telescopes of f/7 to f/8. The TZ-4S is only available without the blocking filter, so the 2 nm blocking filter pre-installed in the front of the TZ-3S must be moved into the TZ-4S; it is not available separately.

The 2 nm block filter is mandatory for operating the SunDancer II H-alpha filter with both the TZ-3S and the TZ-4S: "If you work without this important protective filter, you would literally 'fry' the large etalon (a crystal) as well as all polarizers in the SunDancer H-alpha filter." The inevitable total repair would not be covered by any warranty, so before each use of the SunDancer II make sure that you are indeed using the telecentric with the 2 nm block filter.

A small 3D-printed tool comes with the TZ-4S for loosening the filter ring in the TZ-3S and securing the blocking filter in the TZ-4S. Baader's steps:

1. Unscrew the TZ-3S from the upper housing of the SunDancer II, which contains the etalon (recognisable by the micrometer screw), at the T-2 connection, and place the upper part so no dust can get in from below. The T-2 reducer ring should stay in the H-alpha filter; if it comes loose, screw it back in.
2. Unscrew the ring that holds the filter in the 1.25" nosepiece of the TZ-3S, and likewise the ring in the TZ-4S, by placing the small tool from above into the kerfs of the ring so that it does not touch the blocking filter.
3. Place the two rings and the blocking filter on a soft surface, such as paper or a soft tissue.
4. Put the blocking filter into the recess in the TZ-4S; either side may face up. If it sits at an angle, poke it gently with a cotton bud or toothpick, without force, because a stuck filter may fracture.
5. Place the holding ring on the filter with its small kerfs visible and tighten it hand-tight with the tool, gripping the tool by its smaller shaft. Do not overtighten.
6. Screw the TZ-4S onto the filter unit of the SunDancer II and put the dust caps on the TZ-3S.

Use in Place of a Barlow Lens

The TZ-4S, like the TZ-3S, can be used instead of a Barlow lens, with better contrast than the common methods for higher magnification, simple Barlow lenses or eyepiece projection. With the 4x telecentric, high magnifications for observing the solar system can be reached even with long focal length orthoscopic eyepieces, and the eye relief of the eyepieces does not change.

Dual 1.25"/2" Nosepiece and Rigid T-2 Thread

The telescope side has a dual 1.25"/2" nosepiece with a 1.25" filter thread and Safety-Kerfs, which prevent tilting due to an incorrectly positioned safety groove and at the same time give a secure hold. The camera side carries the filter cell for the SunDancer II blocking filter.

On the camera side, the TZ-4S features a standard male M42x0.75 (T-2) thread. This gives a tilt-proof, fixed connection even to heavy cameras or to T-2 eyepiece clamps, and adapter rings are available for almost all common connection systems, such as M48, 2" or M68.

TZ-4S and TZ-3S

The two SunDancer telecentrics compare as follows.

• Baader TZ-3S: 3x, f/30 at f/10, 27.5 mm clear aperture, 96 mm working distance; supplied with the SunDancer II, where it holds the blocking filter.
• Baader TZ-4S (This Product): 4x, f/30 at f/7.5, 36 mm clear aperture, 97 mm working distance; for the SunDancer II on telescopes of f/7 to f/8, with the blocking filter moved over.