Re: [APML] 2 new images

["Chris Cook" <ccjd@ix.netcom.com>]

Hi Wei-Hao -

A spectacularly deep image of the M45 region!  Very nice.  My only criticism
of the image is the satellite trail.  It is a big distraction....at least to
me.  It is easily removed in PS.

IC2177 is also very nice!

Being at 13,000ft under some of the darkest skies in the world is a great
advantage.
Thanks for posting!

Chris

----------------------------------
Chris Cook
Astronomical & Nightscape Photography
www.abmedia.com/astro

-----Original Message-----
From: Wei-Hao Wang <wang@ifa.hawaii.edu>
To: astro-photo@seds.org <astro-photo@seds.org>
Date: Thursday, February 26, 2004 8:16 PM
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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