Optimum Ha exposure time???

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Optimum Ha exposure time???

Post by Merlin66 » Mon Jul 13, 2015 6:25 am

While working on the SHG book I came across an interesting situation which may be of interest to the members.
When we observe/ image in Ha as you know we are actually looking "into" the absorption line centred on the Ha core with it's extended wings (red and blue wings).
Relative to the surrounding continuum the depth of the Ha core is about 16% (1/5th) the intensity of the continuum. This infers that a "good" white light exposure would have to be multiplied by 5 to reach the detail visible in the extreme core of the line.

You probably also know that the detail visible in Ha varies between the wings and the core, the core showing Ha features at a much higher regions of the chromosphere.
To record the maximum Ha core detail we'd therefore need a long enough exposure to see down deep into the absorption line - especially when narrow band (double stacked around 0.3A) or a SHG is used. Short exposures will lack contrast.
This leads me to the issue at hand....
We all want maximum frame rates - to freeze the seeing BUT faster frame rates also mean that the individual exposures will be shorter these may not record the extreme core detail. It could be that there is a minimum stacking requirement to "build" up the image (and improve SNR) beyond the usual quality criteria.
Thoughts/ comments.
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