Query about intro to ST

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Query about intro to ST

Postby Lawrence » Fri Apr 21, 2017 3:44 pm

I am puzzled about the following sentence that appears in the introduction to ST.

"For example, StarTools enables you to keep M42's colour constant throughout, even in its bright core. No fiddling with different exposure times, masked stretching or saturation curves needed... "

If one takes an image of M42 with suitable equipment (almost any amateur camera and telescope) within a few seconds the core stars are saturated. Once saturation is reached, surely you cannot retrieve any useful information for subsequent image processing? I generally use 5 minute subs. In these, the core simply saturates. The suggestion above appears to imply that any exposure can be processed without loss of information at the bright core.

Lawrence Harris
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Re: Query about intro to ST

Postby admin » Sun Apr 30, 2017 4:31 am

Lawrence wrote:I am puzzled about the following sentence that appears in the introduction to ST.

"For example, StarTools enables you to keep M42's colour constant throughout, even in its bright core. No fiddling with different exposure times, masked stretching or saturation curves needed... "

If one takes an image of M42 with suitable equipment (almost any amateur camera and telescope) within a few seconds the core stars are saturated. Once saturation is reached, surely you cannot retrieve any useful information for subsequent image processing? I generally use 5 minute subs. In these, the core simply saturates. The suggestion above appears to imply that any exposure can be processed without loss of information at the bright core.

Lawrence Harris


Hi Lawrence,

You are right of course that data which is not recoverable cannot be made... recoverable. The statement pertains to colour and the way modifying brightness (for example by stretching) influences colouring - StarTools' "color constancy" makes sure that brightness does not influence colouring. Ergo, an M42 core with very bright recoverable detail will retain colour and will not appear white just because it is bright (the latter is a mistake all too often made and conceals important visual information from the viewer). In fact, in StarTools, its bright core will be rendered in a comparable colour (teal green due to O III emissions) to its much darker surroundings because its surroundings are of similar composition (just not as bright). Does this make sense?
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Re: Query about intro to ST

Postby Lawrence » Fri May 05, 2017 1:24 pm

Hello Ivo

I understand your explanation so I assume that the conclusion is that to obtain the best 'final' picture, some initial tests should be done on any bright object (such as M42) to determine the maximum exposure that does not saturate the core, and then produce an image from adding multiple sub frames of that exposure?

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Re: Query about intro to ST

Postby admin » Sat May 06, 2017 4:50 am

Lawrence wrote:Hello Ivo

I understand your explanation so I assume that the conclusion is that to obtain the best 'final' picture, some initial tests should be done on any bright object (such as M42) to determine the maximum exposure that does not saturate the core, and then produce an image from adding multiple sub frames of that exposure?

Lawrence


That's correct! Fortunately, overexposure is very rare. In fact the only problematic object I can think of off the top of my head is precisely M42. You can go down the HDR combining rabbit hole to work around the overexposure though.
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