M17 with Canon EF 200mm f/2.8 L II and Astrotrac

Date: 8/9/2010 – San Bruno, California – near the SFO airport

Transparency: 3/6 – Wind between 5mph and 10mph

Mount: Astrotrac travel system

Camera: Modified Canon XTi (Standard IR filter replaced with an astrodon IR filter by Hap Griffin) with Canon 200mm  f/d 2.8 teleobjective (prime lens) opened at f/d 3.5 – at Iso 400.

22 exposures of 90 seconds with Astronomik CLS CCD clip filter, 10 flats frames, 10 darks, 10 biais.

Full Frame

M17 Area – Skytools v3

Cropped Frame centered on M17


  • MaximDL5:  darks and flats subtraction, alignment and averaging
  • Photoshop CS4: stretching, selective sharpening
  • Noise Ninja:  noise removal

I used a Bahtinov mask as a focus help on a bright star (Antares).

This is important to use a bright star when using the Bahtinov mask, to have the right in-focus diffraction pattern.

Note that at f/d 3.5 the focusing tolerance is +/- 7 Microns!  See Thierry Legault’s site on this topic.

Interestingly, having the canon not perfectly in focus elongates slightly the stars on the frame edges.

I initially thought the lens was the culprit and using it at f/d 3.5 was “too fast”.

In fact, by having an optimized focus the stars are round on the whole frame! See below – pictures are at 100% size.

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