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.
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.