Focusing this proved much more challenging than the larger lens. The main reason is, at this combination of pixel size and focal length, the stars are very under-sampled. As such, it proved impossible to use a Bahtinov mask to focus. Even adjusting based on a star profile was challenging.
To capture the images during focusing, I centered a bright star and used LiveView to repeatedly capture a sequence of 5sec frames with 1x1 binning and 1/8 region of interest. To maximize image detail, I focused using a luminance filter which is par-focal to the H-alpha filter eventually used to capture the light frames.
To analyze the focus quality, I alternated between these two tools in Astro Photography Tool:
- Focusing Aid - The focusing aid computes star-profile metrics which are shown both numerically and graphically. I used this to get an objective confirmation of minimal star size.
- Magnifier - Shows a highly zoomed-in view of the selected star. I used this as a sanity check for way-out-of-focus stars that showed up as doughnuts. Only a little tweak of the focusing ring can send things way out of wack when focusing manually.
Update 2014-09-08 - After sharing this difficulty with the author, looks like the next version of APT will allow the Magnifier view to track the active region of the Focusing Aid.
The tests I performed were at 35mm. I had planned on trying 18mm but, since I was imaging from the observatory, most of the field-of-view was obscured by the dome slit! The following image represents 7 frames of H-alpha data, each 900 seconds. The star quality on the edges of this lens are very elongated, but this is what I expected from previous uses.
|QSI-540ws, Canon EF-S 18-55mm IS - 7x900s, Halpha|