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Re: [APML] 2 new images
Hi Wei-Hao,
Those are very nice. IC2177 is especially beautiful.
Rob Gendler
email: robgendler@att.net
Web Site: http://www.robgendlerastropics.com
----- Original Message -----
From: "Wei-Hao Wang" <wang@ifa.hawaii.edu>
To: <astro-photo@seds.org>
Sent: Thursday, February 26, 2004 8:10 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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