From: Alan Friedman <alan@greatarrow.com>

Date: February 15, 2010 7:36:19 AM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] about focus and seeing...

Hi Antonio,

Good questions... In terms of color calibration, I don't have a particular method other than to expose the capture streams as carefully as possible using the histogram in Astro IIDC. Once I have stacked and processed the R,G and B images, I usually find the color result from my RGB channel compilation in Photoshop very close. Fortunately, we have quite a few space missions sending back pictures of our planetary subjects so we have some very good images to use for reference. I do very little levels adjustment before combining the RGB images, though I will move the white end slider in a little if there is a lot of unused range on this end... preferring to wait until the color composite is made before making adjustments with levels or curves. I usually prepare a separate luminance layer from the RG and B images and use this for adjustment of the levels and sharpness in the final image.

Seeing is a fascinating topic. The Pickering scale estimates are a good place to start, but because we are looking through the air in milliseconds, the particular character (and there are quite a few of these) of the turbulence has a lot to do with whether the individual frames can be stacked successfully or not. A lengthy discussion of these different characters of seeing would be interesting. I can tell a lot about the quality of the night by comparing the RGB and IR captures. On the best nights, the images through the green filter have the highest resolution. On an average night, the red appears the most detailed. I assume a night where the blue captures are sharpest would be the best scenario, but I have not experienced this from my location. I will often capture an IR exposure for comparison, but I don't use these in my RGB images. On all except the poorest nights, IR is noticeably weaker in resolution. It also focuses at quite a different position so requires a lot of time-consuming fiddling with the focuser as the planets rotate.

My personal feeling is that life is too short to spend time collimating telescopes - all the scopes that I own are permanently collimated. But everyone using an SCT seems to agree that precise collimation is critical when using this type of telescope.

Looking forward to seeing more of your images!

steady skies,

Alan

On Feb 15, 2010, at 3:37 AM, aa27100 wrote:

I have one more question for all you experienced imagers: how do you judge one night is worthy spending time on imaging?
Sometime the planets occasionally show exciting details visually, but the vision is all but steady, and I hurry setting up everything only for later disappointment when image processing does not yield the desired results.
My own experience is that if I am unable to say the scope is perfectly collimated with visual star-test (because of turbulence-distorted, indiscernible diffraction pattern) imaging is a loss of time. Maybe this is only due to my own incapacity of finding with certainty the focal plane, anyway I am nearly convinced that if I am frankly unable to say the scope is collimated, it cannot deliver diffraction-limited images...
When I captured these Mars images the star-test Airy pattern was reasonably clear, confirming the scope alignment, and Mars focusing at the laptop's screen was snap-on even at f/36 (both by eye and the sharpness estimator).
What is your thought and your experience?
Thanks!

Antonio
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