SaratogaSkies Jim Solomon's Astropics


Latest news

December: Test shots with new scopes/mounts

Dec 21: TMB 80/480 Arrives!

Dec 3: AP1200 Arrives!

Nov 30: TMB 152/1200 Arrives!


M42 (The Great Nebula in Orion)

[Hα+R G B]

Mouse-over to see annotations. (Requires Javascript) Click to see high-res version.

Image Details:

Camera: Mofidied Canon Rebel XT (350D): Hutech Type I Filter Replacement
Mount: Celestron AS-GT
Scope: Orion ED80 (80mm f/7.5 APO Refractor)
Configuration: Focal Reduced
Additional Optics: William Optics 2" APO 0.8x Reducer/Field Flattener
Filter: Hutech Hα Front Filter (HA-FF)
Effective Focal Length: 480mm
Effective Focal Ratio: f/6
Exposure: 31x4m, 11x30s @ ISO 400 (RGB); 21x8m, 10x1m, 10x15s @ ISO 1600 (Hα)
Total Exposure: 5hrs, 10min
Date: 2/20/2006 (RGB); 1/7/2007 (Hα)
Location: Saratoga, CA, USA
Acquisition: DSLRfocus
Focus: DSLRFocus
Dithering: GADFly 1.0.5
Guiding: GuideDog via Philips ToUcam Pro II (840k) through Orion ST80 w/ Celestron 2x "Kit" Barlow


  • IRIS: Dark subtraction, registration, gradient removal, high dynamic range compositing
  • JimP: Flat field, Kappa-Sigma Stacking, White balance, HαRGB combination, ASINH stretching
  • Photoshop: Levels, cropping, JPG conversion
  • Neat Image: Noise reduction

Image Description:

This is an update of my previous M42 image, with 3hrs of Hα exposure added to the existing RGB data. The Hα really helps bring out many of the features in this busy field.

To create the HαRGB composite, I used the original RGB data for Hue and Saturation, and used the equivalent of Photoshop's “Screen” blending mode to combine the luminance of the original RGB data with the (grayscale) Hα data. This Screen-blended Luminance, along with the Hue and Saturation of the original RGB image, was then converted back to RGB. This method seemed to preserve details in the non-Hα-emitting regions, provide excellent detail in the Hα regions, and preserve the colors.

Processing M42 provides a significant challenge because its dynamic range is so enormous. Long exposures are required to bring out the faint wisps of nebulosity, but those long exposures have the core/trapezium region completely blown out. So, short exposures are required to capture the detail in the bright, core/trapezium region. The typical method to combine the short and long exposures is using layer masks in Photoshop, as described well by Jerry Lodriguss. But for this image, I used IRIS's merge_hdr command to automatically merge and blend the various exposures into hdr (High Dynamic Range) images. The technique worked extremely well throughout the entire Hα image, as well as in the core region of the RGB image. However, it struggled to merge some of the clipped stars in the RGB image (away from the field center), perhaps due to registration imperfections. All in all, though, I'm extremely happy with this technique, and kudos to Christian Buil (author of the IRIS software) for this implementation of HDR.

Mouse-over the image to see the Hα image in grayscale. Mouse-off to see the HαRGB composite. A higher-resolution image is also available. North is up.