IDAS OL7.5 Low-Profile Filter for Celestron RASA 8 (M42 / M54)

The 7.5mm Optical Path Solution for RASA 8

The Celestron RASA 8 demands a precise 25.0mm backfocus distance from its M42 camera adapter to the sensor. Because the OL7.5 frame consumes exactly 7.5mm of optical path, it pairs with a standard cooled camera that has a 17.5mm flange back to give the exact 25.0mm reference backfocus.

Installation takes three steps:

1. Attach the standard M42 camera adapter included with your RASA 8 (with the M54 version, first thread the M42-M54 adapter ring onto it).
2. Thread the IDAS OL7.5 filter onto the adapter.
3. Thread your 17.5mm flange-back cooled camera onto the filter. The total is exactly 25.0mm.

8.1mm Mechanical vs. 7.5mm Optical: How 2.0mm Glass Refraction Works

While the physical, flange-to-flange measurement of the OL7.5 filter holder is 8.1mm, its effect on your optical train is only 7.5mm. This difference is due to the refractive properties of the 2.0mm thick filter glass. The glass refracts light and pushes the focal plane back by about 0.6mm.

IDAS designed the frame around this shift, so the net optical path consumption is exactly 7.5mm.

M42 OL7.5 Mount: A 37mm Aperture for APS-C Sensors

The M42 OL7.5 version is for APS-C and Four Thirds sensors. It has M42 x P0.75 threads on both sides (male camera side, female scope side) and a 37mm clear aperture, the largest possible within the M42 thread. This significantly reduces peripheral vignetting on APS-C and Four Thirds sensors. It mounts directly to M42 cooled cameras with a 17.5mm flange back, and adapts to M48 or M54 cameras with standard step-up rings (not included). For full-frame or Duo guiding cameras, IDAS recommends the M54 version.

M54 OL7.5 Mount: A 47mm Aperture for Full-Frame & Duo Cameras

For full-frame sensors and Duo cameras with a built-in guide sensor (such as the ZWO ASI2600MC Duo), choose the M54 OL7.5. It has M54 x P0.75 threads (male camera side, female scope side) and a 47mm clear aperture that accommodates the steep light cones of ultra-fast optics such as F/2. Compared with M42 or M48 threads, it suppresses aperture clipping and significantly reduces peripheral vignetting on APS-C and full-frame sensors, and it prevents frame clipping on the outer guide sensor of Duo cameras.

The M42-M54 Adapter Option (0mm Optical Path)

To use the M54 filter on telescopes that end in an M42 male thread, such as the RASA 8 with its standard M42 camera adapter, IDAS makes a dedicated M42-M54 adapter ring. It sits completely flush inside the filter's front threads, so it adds 0.0mm of optical backfocus and your 7.5mm + 17.5mm calculation stays the same.

Choose M54 OL7.5 + M42-M54 adapter for a RASA 8 with its standard M42 adapter, or if this is your first M54 OL7.5. Choose M54 OL7.5 alone if your telescope already ends in a native M54 male thread, or if you already own the adapter and are adding a second filter.

Choosing Your IDAS Filter: From NBZ-II to Clear

Each filter also has its own IDAS product page on our store with its transmission curve and full details. NB1 and DTD come in the M42 mount only, and NBZ-II in the M54 mount only.

• NBZ-II: Dual-band H-alpha / OIII nebula booster.
• DTD: Next-generation natural nightglow and light pollution cut color booster for comets, galaxies, the Milky Way and nebulae.
• NGS1: Night glow suppression light pollution filter.
• GNB: Galaxy and nebula booster with a high near-infrared passband.
• HEUIB-II: H-alpha enhanced UV/IR block filter.
• NB1: Triple-band nebula booster (OIII, H-alpha, NII).
• Clear: Optical clear protection filter (ODW in the M54 mount).

A Note on F/2 Optics: Sensor Tilt, Collimation, and Shimming

Ultra-fast F/2 systems like the RASA 8 have an extremely shallow depth of focus, in the tens of micrometers. Small mechanical tolerances in the telescope and camera bodies can move the best backfocus slightly, so fine adjustment with thin shim rings is recommended when tuning corner stars. Stars elongated on one side, or uneven focus across the corners, are typically caused by camera sensor tilt or telescope collimation, not the filter.

Tilt and collimation cannot be fixed by changing spacing. Use a camera tilt adjuster, or contact your telescope or camera manufacturer.