Home Page       Mon Dec 1 02:11:21 JST 1997

Field flattening

At first I show you how images could be flattened in the way mentioned in this article. The following images show the distribution of positive/negative value comparing to the sky field. Green means the value is greater than the sky field, red means less than the sky field. In the middle image the sky field is a simple average of all pixel values without field flattening. The bottom image is after restoration of level correction in the way mentioned in this article. When noises appear equally all over the image in the bottom image, it means the experiment succeeded.

• Nova Cas 1995
  Photo: Seiichi Yoshida
  Jan. 9, 1996, 19:01:30 JST (1 min)
  Fujishiro Town, Ibaraki Pref. Japan
  25-cm f/6.3 Schmidt-Cassegrain
  Konica color GX3200
  

This image is originally a printed picture and scanned. Although the center of the image is brighter, it is flattened well and noises appear scattering in the bottom image. There are curves in left and right side because of the flatfield function and the sky field are resembled by a quadratic function.

The uneven sensitivity is actually irregular as the image below shows. The center image shows the standard deviations of each 10x10 block and the bottom image shows the obtained flatfield function resembled by a quadratic function. As it shows, there are still some errors with this method.

• Noise map of Nova Cas 1995

• Comet Hale-Bopp
  Photo: Atsuo Kuboniwa
  05:05:22 - 05:12:20 JST (6 min 58 sec)
  Kukizaki Town, Ibaraki Pref.
  BORG 125ED F4 (f=500mm), Takahashi EM-200
  Fujichrome 400 Provia
  

This image is also originally a printed picture and scanned. But the flattening has failed in this case. The variance of background brightness does not reflect to the variance of noises. Actually, the map of standard deviations of noises represents an adverse aspect, it is smaller in the center.

• Noise map of Comet Hale-Bopp

• Comet 116P/Wild 4
  Photo: Akimasa Nakamura
  May 31, 1997
  

This is a CCD image after level correction. Because it is flat originally, an evident effect did not appear as a result. The object in the center of the image is comet 116P/Wild 4. Although the original image is after level correction so that the comet becomes clear, you can see that a diffused object like a comet is much wider actually.

By the way, this is the result of a JPEG image but the original is in FITS format. However, the result of the original FITS image is quite different. The ellipse in the bottom image is because the current system resemble functions as quadratic. I do not know why they are so different.

• Comet 116P/Wild 4 (FITS)