Why Is Seeing Control So Important

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With the goal of extracting the highest resolution possible in our CCD images, seeing plays a much bigger part than is at first realized. Longer exposures do not mean you can detect an object if the seeing is poor. At the 2004 Riverside Telescope Makers Expo, Jim McGaha, an advanced CCD imager specializing in Near Earth Object (NEO) detection, gave a talk centered on detecting objects fainter than 20th magnitude. Using information presented in his talk, Table 8.1 reveals how a much fainter star can be detected with any given aperture of telescope as the seeing improves.

Table 8.1. Effect of Seeing on Detection Magnitude

Seeing in arcsecond

Improvement in Detection

Aperture Required to Achieve

(FWHM)

Magnitude (using 4 arcsecond

Magnitude Increase (using a

seeing as baseline)

6" aperture scope as

baseline) (inches)

4.0

0

6

3.5

0.7

9

3.0

1.4

12

2.5

2.1

16

2.0

2.8

20

1.5

3.5

Figure 8.5. Iris Nebula, NGC7023. LRGB image of 120:30:30:45 minutes total exposures, respectively (3-minute individual exposures) using a non-IR-blocked luminance image and Optec RGB filters. Image calibration and color combination using MAXIM DL, image registration using MAXIM DL, AIP for Lucy Richardson deconvolution on the luminance, luminance layered in Photoshop for final color and star shaping processing. Equipment: Homemade 20-inch f/5 Newtonian astrograph and Finger Lakes IMG6303E CCD camera with all images acquired in 2 x 2 bin mode for an image scale of 1.48 arcsecond/pixel in 3.2 arcsecond seeing and magnitude 4.9 suburban/rural skies.

What this means is that telescopes in the 6- to 8-inch range, when used optimally in good seeing conditions, will perform as well for deep-sky imaging as much larger scopes under average to poor seeing (see Figures 8.5 and 8.6). Seeing levels the playing field for most of us. This is very encouraging to imagers with telescopes in the 4- to 8-inch range. They can capture truly stunning images when the macro seeing conditions are favorable, providing they have taken the proper steps to manage their local "micro" seeing environments. The bottom line is that you can economically improve imaging performance by controlling your local seeing environment without spending large sums on more aperture!

It is common practice now for many imagers living under generally turbulent skies to only take luminance images during nights of good seeing and then take their color data on an average seeing night. This is due to the fact that the luminance image provides most of the resolution data in the final color composition.

Figure 8.6. Iris Nebula, NGC7023. Comparison image using 6-inch scope This remarkable image by Cord Scholz of NGC 7023 compared to the one taken by the author with his 20-inch in similar seeing conditions shows what is achievable with modest aperture scopes used skillfully for deep-sky color imaging. LRGB image of 210:30:30:50 minutes total exposures, respectively, with 5-minute individual exposures in L and10-minute in RGB. Image calibration and color combination using MAXIM DL, image registration using MAXIM DL, RC Console for sigma combine and pixel cleanup, AIP for Lucy Richardson deconvolution on the luminance, LAB combined in Photoshop for final color and star processing. Equipment: 6-inch f/6 Intes Maksutov with SBIG ST-10XME and CFW8 filter wheel in 3 arcsecond average seeing.

Figure 8.6. Iris Nebula, NGC7023. Comparison image using 6-inch scope This remarkable image by Cord Scholz of NGC 7023 compared to the one taken by the author with his 20-inch in similar seeing conditions shows what is achievable with modest aperture scopes used skillfully for deep-sky color imaging. LRGB image of 210:30:30:50 minutes total exposures, respectively, with 5-minute individual exposures in L and10-minute in RGB. Image calibration and color combination using MAXIM DL, image registration using MAXIM DL, RC Console for sigma combine and pixel cleanup, AIP for Lucy Richardson deconvolution on the luminance, LAB combined in Photoshop for final color and star processing. Equipment: 6-inch f/6 Intes Maksutov with SBIG ST-10XME and CFW8 filter wheel in 3 arcsecond average seeing.

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