Comparisons of M13 with different filters

Copyright Benjamin Gomes-Casseres 2009

The images below were taken on July 10, 12, and 19, 2009 with a home-made 8" Newtonian F/6 at prime focus, Canon 20D (unmodified), on Takahashi EM 200 guided with a 900mm FL Unitron refractor and ST-4. Conditions on both nights were similar; clear with lim mag around 4.5; gibbous moon rising at the end of the sessions. The site is in suburban Boston away from direct lights, but with substantial light pollution. Permanent mount. The object was close to Zenith.Tracking was fair (max of A3 on the ST-4).

Images were acquired with IP 3.8 and processed in IP with the following: Calibrated (about 20 darks, flats, bias images), stacked, combined. Neutralize background, correct white balance, digital development, medium sharpening, levels adjustment, save as TIFF. Reduced to 8-bit and saved as JPEG in CS2.

The number of subs for each image and the density of the subs are similar. The same applies to the darks, flats, and bias files used. The main difference between the images is that the second one uses a Lumicon LPF and the first one does not. As a result, the subs in the first image had strong orange cast, and those in the second image had strong cyan cast. IP did a fair job at removing these casts.

I am sure that more can be done to balance the colors, but I am now questioning whether the use of the LP is even worth it: the subs are longer, leading to slightly higher driving-errors (rotation). Beyond that, I am not sure if one or the other cast is easier or harder to remove with other techniques.

M13 without LPF (save to local disk or increase browser zoom for full res image)

Exposure: 21 x 240s, 2 x 60s = 86min at 400 ASA

M13 with LPF (save to local disk or increase browser zoom for full res image)

Exposure: 22 x 360s, 2 x 240s = 140min at 800 ASA (because of the LPF, the histograms of the subs were similar in overall density to those above, except for overweight in blue)

M13 without LPF, with MPCC coma corrector (save to local disk or increase browser zoom for full res image)

Exposure: 20 x 240s, 3 x 60s, 3 x 120s = 89min at 400 ASA

M57 without LPF, with MPCC coma corrector (save to local disk or increase browser zoom for full res image)

Exposure: 20 x 240s = 80min at 400 ASA

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