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Posts Tagged 'rasa'

NBX – the new dual nebula filter from IDAS

October 19 2020, Jan Ströher

A new, high quality dual band filter has appeared on the market, in the form of the NBX nebula filter from Japanese company, IDAS.  It is especially efficient for nebula photography using fast optics, like the Celestron RASA, for example. Like the existing IDAS (Astro Hutech) nebula filters, the new NBX is of impeccable quality and workmanship.

The IGAD (Ion-Gun Assist Deposition) coating technology which has been specially applied to astronomical filters by the Japanese manufacturer, Astro Hutech, was originally developed for optical communication, where long-term stability (> 25 years) is required in rough field conditions.

IDAS NBX 48mm nebula filter

This leads to filters with robust coatings and durable spectral stability – even at extreme temperatures and humidity levels. This stability is especially important for bandpass curves with steep sides, such as H-alpha, LPS filters and other narrow band filters. Filters with IGAD coatings almost completely eliminate bandpass shifts of +/- 3 or 4nm, which are typical with standard filters.

The NBX is a dual filter which specifically focuses on OIII and H-alpha lines. The NBX‘s transmission curves in the H-alpha and OIII ranges are very clearly illustrated in these two figures:

Transmission curves in H-alpha


Transmission curves in OIII


You can see that the IDAS NBX filter primarily realises its full potential when used with very fast optical systems and astrographs, such as a Celestron RASA, especially fast Newtonian reflectors with f-numbers between f/2 and f/4 and special devices, such as the Officina Stellare Veloce RH 200 Mark II-AT (Riccardi-Honders).

The NGC 281 “Pacman” Nebula in the constellation Cassiopeia

This dramatically increases the contrast within emission nebulae, which predominantly consist of oxygen (OIII) or hydrogen compounds (H-alpha). Additionally, longer exposure times are no longer required, predominantly due to the use of the above-mentioned, extremely fast optics, but also due to the NBX filter’s special contrast effects. The filter is ideal for use with ‘one shot’ colour cameras and monochromatic CMOS cameras. An IR blocking filter is then no longer required. Additionally, the NBX is parfocal with other IDAS LPS nebula filters.

Celestron RASA Astrographs: Telescopes for Astrophotography

September 20 2019, Stefan Taube

The RASA range of telescopes were developed specifically and exclusively for astrophotography. They are very fast; with a focal length of f/2 they take pictures twenty times faster than a Schmidt-Cassegrain lens at f/10! Because of this a tracking control is not required, and generally fewer demands are placed on the mount!

The RASA 1100 model with a 279 millimetre aperture has already been on the market for some time, and Celestron is now also offering a smaller version, the RASA 800, with a 203 millimetre aperture.

Celestron Astrograph S 203/400 RASA 800

Celestron Astrograph S 203/400 RASA 800

This optical system is also within reach of smaller budgets with the RASA 800, particularly since it can be used with a smaller mount. The illustration above shows the RASA 800 with the AVX mount. This astrograph is optimised for astrophotography cameras with a 22 millimetre field diagonal. The larger RASA 1100 is the better choice for those who prefer to use a DLSR for their photography.

The abbreviation RASA stands for the developers’ names: Rowe-Ackermann-Schmidt-Astrograph. It is basically a normal, routinely-produced Celestron Schmidt-Cassegrain instrument; that is with a spherical primary mirror in front of which is a corrector plate invented by Bernhard Schmidt. Instead of using a secondary mirror to direct the light path towards the Cassegrain focus at the end of the telescope, the camera sensor is incorporated in the prime focus. A corrector assembly comprising four lenses made of especially high-quality glass ensures optimal illumination.

Die RASA-Optik von Celestron

The RASA telescope from Celestron

As the term astrograph implies, RASA telescopes are designed exclusively for photography. There is no option to use an eyepiece. But if you are an amateur astronomer who specialises in astrophotography, you will not see this as a disadvantage, instead you will quickly learn to appreciate the advantages of the RASA telescopes.

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