Re: [APML] E200 Pre-Exposure

[Wei-Hao Wang <whwang@asiaa.sinica.edu.tw>]

Hi Dave,

I tried once on E200.  This is the final image:
http://www.ifa.hawaii.edu/~wang/gallery/picutres/m42-2003.htm

I have a webpage written in Chinese comparing this M42 pictures 
taken with and without pre-exposure:
http://www.asiaa.sinica.edu.tw/~whwang/randomwalk/1-18-2003-pre-exposure.html
Please ignore all the Chinese charactors in that page.
There are two sets of images at the middle of the page.
The left row is with pre-exposure (the one you saw from my English page)
and the right row is without pre-exposure.  From top to bottom are
raw images from a scanner, histogram of the raw images, and randomly 
enhanced images.  The pre-exposed and non\-pre-exposed images are enhanced
in a similar way, so you can compare the difference in the shadow.

This is my summary of pre-exposure.
1. Contrast becomes lower by increasing the brightness of the shadow.
2. Can record more faint nebulaes
3. Because the background brightness increase, it seems more grainy.
4. my zone II1/2 pre-exposure is too strong.  Perhaps just zone I is
   enough.

Frank is right that the light-polluted sky acts like pre-exposure.
If your are already doing sky-limited exposures, perhaps you don't
need to do pre-exposure.  I suspect that pre-exposure is needed
when your focal ratio is large and you don't plan to do long 
exposures.  

Hope this helps.

Merry Christmas,

Wei-Hao

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