Globular cluster M 13 with a small 65mm Refractor – September 24, 2014

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The following images are part of some LRGB (Luminance, Red, Green, Blue channels) image acquisition tests that I’ve started a while ago in order to establish a workflow for my future deep-sky images.

This is why I’ve only used my very small Refractor, a TS APO 65Q with 420mm focal length, the ASI120MM mono camera and Baader RGB CCD filters for colors and a UV/IR cut Baader filter for the Luminance channel. They were all mounted onto the EQ6 SW equatorial mount with the “standard” three-star alignment procedure. That was one of the reasons why I’ve acquired 20 seconds frames instead of longer exposures (the full “technical” details would also include: Gain setting: 50, Gamma setting: 90. Only one dark frame was acquired during the entire session). The sky at my location was clear but the limiting visual magnitude next to M 13 was around +5.2, so rather poor conditions for serious astrophotography.

The main LRGB image below is made out of 136 frames for the Luminance channel, and 30 frames for each of the color channels. That would put the entire imaging session at 75 minutes, but the “true” exposure time was actually only 45 minutes. That seems to be enough to be able to capture the magnitude +16 (!) galaxy in the right part of the image.

So, the color version of the image:

M13 color

And a Black-and-White version:

M13 BW

A comparison of the above images, enlarged and cropped to better distinguish the details of the cluster:

M13 detail

For now it seems that the LRGB workflow needs more practicing, but what really surprised me was the amount of detail and limiting magnitude possible with such a small refractor on +5 magnitude skies. I think I will get the small instrument outside again very soon.


(September 25, 2014)

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