Tuesday, September 2, 2014

QSI with 35mm Lens

Continuing my experiments with using the QSI-540ws and camera lenses, last night I tested my Canon EF-S 18-55mm f/3.5-5.6 IS lens which came as the stock lens with our Canon 450D. Small lenses such as this do not have a tripod collar. Fortunately, the QSI has 1/4"-20 threads on the main body. I attached a Manfrotto RC2 quick release plate to the camera and a Manfrotto 323 RC2 quick release head onto the short Vixen dovetail. I mounted the assembly to the new imaging deck:

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. 
I wish these could be used simultaneously or that the Focusing Aid could also show magnified views.

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

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