M42 Threaded Filter Mount (M42 OL7.5)

M42 (OL7.5)

Name: IDAS M42 (OL7.5) Low-Profile Threaded Filters
Brand: IDAS
SKU: IDAS-MOUNT-M42
Availability: InStock

M42 OL7.5 Filter Mount (7.5mm Optical Path / APS-C Threaded)

*IDAS M42 OL7.5 Filter appearance. 7.5mm optical path (8.1mm physical), 37mm aperture precision frame.

Dedicated Thin Filter Mount for M42 Threaded Systems

The IDAS M42 OL7.5 filter is precision-engineered for astronomical imaging with an optical path length of 7.5mm (physical 8.1mm) and full APS-C sensor coverage without vignetting. Perfectly matches the Celestron RASA 8 (with standard M42 camera adapter: 25.0mm required backfocus) combined with a 17.5mm FB camera (7.5mm + 17.5mm = 25.0mm). Fully compatible with general M42 x P0.75 threaded optical systems.

7.5mm Optical Path Standard — Seamless RASA 8 Compatibility

RASA 8 Example (25.0mm) ＝ Filter (7.5mm) ＋ Camera (17.5mm)

Keeps optical consumption to only 7.5mm across various correctors and optical trains. Specifically on the RASA 8 (25.0mm backfocus requirement), pairing with a standard 17.5mm FB cooled camera achieves the exact 25.0mm reference backfocus.

💡 Physical Dimension (8.1mm) vs Effective Optical Path (OL 7.5mm): The mechanical flange-to-flange length is 8.1mm. Because the 2.0mm thick filter glass refracts light and pushes the focal plane back by ~0.6mm, the net optical path consumption is exactly 7.5mm.

APS-C Coverage & Reduced Vignetting — Optimized M42 Aperture

Engineered with a maximum possible 37mm clear aperture within the physical M42 thread envelope. Significantly reduces peripheral vignetting on APS-C and Four Thirds format sensors. (*For full-frame or Duo guiding cameras, large-aperture M54 or M48 models are recommended).

Direct M42 & Third-Party Adapter Versatility

Standard M42 P0.75 threading on both camera and telescope sides. Mounts directly to M42 cooled CMOS cameras (17.5mm FB), and easily adapts to M48 or M54 cameras using standard step-up rings.

3 Simple Connection Steps (Example with RASA 8)

STEP 1 Attach the standard M42 camera adapter included with your RASA 8 telescope.

STEP 2 Thread the IDAS M42 filter (7.5mm optical path) onto the adapter.

STEP 3 Thread your cooled camera (17.5mm FB) onto the filter. Completed! Exactly matches 25.0mm backfocus.

💡 Technical Advice for Ultra-Fast Optics (F2 Systems)

1. Depth of Focus & Fine Backfocus Shimming F2 ultra-fast optical systems feature an extremely shallow depth of focus (tens of micrometers). Slight mechanical tolerances from telescope mounts or camera bodies can subtly affect optimal backfocus. When tuning corner star performance, fine adjustments with thin shim rings are recommended.

2. Sensor Tilt & Collimation Troubleshooting Symptoms such as star elongation on one side or uneven focus across corners are typically caused by camera sensor tilt or telescope collimation issues, not the filter itself. These cannot be resolved by optical path changes. We advise using a camera tilt adjuster or consulting your telescope (e.g. Celestron) and camera manufacturers.

Mounting Example (Direct M42)

*Direct connection to RASA 8 M42 adapter and 17.5mm FB camera. Exactly 25.0mm total backfocus.

Mounting Example (M48/M54 Camera)

M42 OL7.5 with M48/M54 Step-Up Adapter Example

*Easily adapts to M48 or M54 cameras using standard step-up adapter rings.

Optical Ray-Tracing Diagram

*M42 OL7.5 optical ray tracing. Factoring ~0.6mm glass refraction shift for 7.5mm net path length.

Specifications & Compatibility

Thread Standard	M42 P0.75 (Male Camera / Female Scope)
Optical Path (OL)	7.5mm (Factoring in Glass Refraction)
Physical Path	8.1mm (Scope Flange to Camera Flange)
Clear Aperture (CA)	37mm (Reduced Vignetting for APS-C & 4/3)
Glass Thickness	2.0mm
Compatible Systems	Celestron RASA 8 (w/ standard M42 adapter + 17.5mm FB camera), general M42xP0.75 threaded systems
Production	Made in Japan (ICAS Enterprises Optics Division)