RE: [APML] 2 new images

["Chris Schur" <cschur@therim.com>]

An awesome image, I did a deep shot like this last year with my Schmidt
camera from the darkest high altitude skies of Arizona, and you can compare
it:

http://www.psiaz.com/schur/astro/filmimagepages/m45.html

Like your shot, I found an extensive field of blue nebulosity around the
main cluster.

Chris Schur

-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Wei-Hao Wang
Sent: Thursday, February 26, 2004 6:10 PM
To: astro-photo@seds.org
Subject: [APML] 2 new images


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
______________________________________________________________________



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