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image m13-500mm-90d-rnclark-c09.03.2021-IMG_0910-16-av5.f-c3-0.67xs.jpg is Copyrighted by Roger N. Clark,

M13, The Great Globular Cluster in Hercules

Messier 13 is a globular star cluster in the constellation Hercules. M13 contains several hundred thousand stars, mostly white, yellow and red, and only a few blue stars. M13 is about 25,100 light-years from Earth and is about 145 light-years in diameter.

In the image, to the upper right is NGC 6207, a 12th magnitude edge-on galaxy. About half way between M13 and NGC 6207, just below the line connecting these two objects is the small galaxy IC 4617. North is to the right in the image. There is also a faint galaxy in the lower right corner.

The image shows the view similar to that in a large amateur telescope, about 12-inches aperture. In amateur telescopes, the tens-of thousands of faint stars in the cluster form a background glow of unresolved stars. In large telescopes the red giant stars provide a beautiful sight. In a 31-inch aperture telescope, I have seen these stars appear blood red due to their contrast with the hotter yellow to white stars in the background.

Technical. Stock Canon 90D digital camera, Canon 500 mm f/4 lens at f/4, ISO 1600. Five 32-second exposures (160 seconds total, 2.67 minutes). No dark frame subtraction, no flat fields. Tracking with a Losmandy G11 equatorial mount with an autoguider. The image shown here is 2/3 of full resolution, 1.98 arc-seconds / pixel and the original data are at 1.32 arc-seconds / pixel. Image from September, 2021.

Post processing: stretched with rnc-color-stretch. Also see Astrophotography Image Processing Basic Work Flow.

This is a natural color image. The high dynamic range of astrophotos must be stretched to bring out the range of details the camera recorded. But the typical image stretch process loses color for brighter subjects (e.g. stars and the brighter parts of deep sky objects become whiter as they are made brighter). This image uses a new algorithm, rnc-color-stretch that does not lose color during the stretch. How do we know the colors are reasonable? The star colors can be checked against stellar photometry. Faint red stars have B-V > 2 (not the orange stars, which have B-V of 1 to about 2). The blue-white stars have B-V in the range of 0 to -0.5. The colors closely follow the color sequence in Table 1 at Color of Stars. Solar-type stars have a B-V of 0.63 and appear close to white (daylight white balance).

The Exposure Factors, CEF, CEFA are measures of the relative amounts of light received from a subject. It can be used to fairly compare wildly different lens/telescope apertures and exposure times. For this image:

Modern DSLRs like the Canon 90D include on sensor dark current suppression and low fixed pattern noise at ISOs around 1600 and higher, making no need for dark frame subtraction. Modern raw converters correct for light fall-off and also correct for hot/dead/stuck pixels. This makes processing low light images easy: simply align and average.

To learn how to obtain stunning images like this, please visit my Extensive Articles on Photography .

Keywords to this image = astrophoto-1 nebula low-light digital_astro canon_90d Messier rnc-color-stretch NEW

Image ID: m13-500mm-90d-rnclark-c09.03.2021-IMG_0910-16-av5.f-c3-0.67xs.jpg

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Last updated September 10, 2021