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Re: [APML] 2 new images
Wow!
Both pictures are fantastic! Very good argument to encourage lazy guy like me to read your post describing your flat field
technique!
Thanks for sharing!
P.
Philippe Chrétien
http://www.astrophoto.ca
----- 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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