[APML] 2 new images

[Wei-Hao Wang <wang@ifa.hawaii.edu>]

ALOHA,

I processed two of my new images taken this month.  (Others will take 
long long time to process.)

The first one is Subaru Deep Field:
http://www.ifa.hawaii.edu/~wang/gallery/picutres/subaru_deep_field.htm

This one made use of my new flat field technique.  Despite the new 
technique, there is still difficulty in the image processing.  The 
main problem is sky gradient.  In the unprocessed image, the sky
gradient and the vignetting pattern are mixed. I cannot find a
unique solution to decouple these two (perhaps I didn't try hard 
enough).  Therefore, both flat fielding and gradient removal are not 
perfect.  I did a lot "twist" in PhotoShop to correct these effects.

For this reason, I don't trust any brightness change in this image 
that is larger than ~30% of the image size (i.e., large nebulas).  
They may be just remnant of the imperfect sky gradient and vignetting 
removal.  For small-scale brightness changes smaller than ~25% of the 
image size (small nebulas), I'm pretty sure they are real and are not 
artifacts.


IC2177:
http://www.ifa.hawaii.edu/~wang/gallery/picutres/I2177-2004.htm

The new flat field technique failed on this one.  The vignetting 
is corrected using my old method that assumes axisymmetry for the 
vignetting pattern.  The reason why flat fielding failed is unclear.
There seems to be some strange scattered light in all the 3 stacked
images, which doesn't appear in the flat field shots.  Perhaps 
such scattered light is caused by the 1.4x rear converter.  This
rear converter is designed for Pentax 67 lenses but not for the 
telescoep.  (The one for my telescope is discontinued.)  And because
of this scattered light, my old method doesn't work well, either.
Again, I did lot of manual twist in PhotoShop to remove the leftover.

Centuria 800 is quite grainy, comparing to Centuria 400.  Of course,
one hour at F9.4 is too under exposed.  This is part of the reason why 
it's grainy.  Although I stacked three images, the stacked image still 
looks grainy, and noisy.  Surprisingly, stars are extremely sharp and 
have well-defined shape.  (Centuria 400 also produces very sharp stars.)  
The finest stars on the film have diameters about 40 micron, as small 
as I measured on E100S.  Unfortunately, I have to do some smoothing in 
PhotoShop to remove the grains.  This slightly demaged the resolution 
of the image.  I may take 3 or 4 more 1-hr exposures next year to 
improve the S/N so I don't need to smooth the grains and can keep the 
very sharp stars.

Any comments are welcome.

Wei-Hao

______________________________________________________________________
Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

Address:                       Phone: 808-956-9867                  
2680 Woodlawn Drive            Personal Website:
Honolulu, HI 96822             http://www.ifa.hawaii.edu/~wang
______________________________________________________________________



_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo