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7 Sisters

Started by Protious 1, 12/20/2012 07:25AM
Posted 12/20/2012 07:25AM | Edited 12/20/2012 07:30AM Opening Post
This is my second attempt at shooting the Plaides, I think it turned out better than my first. But I'm not sure I had it completely in focus! Any suggestions on how to get a good focus looking through your camera view finder would be greatly appreciated. I've tried picturing Jupiter, but just can't get a good focus on it. I'm using a 6" refractor and when I try to use an eyepiece adapter I just can't seem to get a good focus. This picture was taken on the 19 of Dec. about 2000 Hrs. Prime focus with Nikon D5000. Thanks Steve

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steve dejarnette smile
Posted 12/20/2012 02:26PM #1

Looking at your other posts, it appears that that you are using a Celestron C6-RGT, this is an achromat. Unfortunately, I believe this is the cause of your difficulties.

An crown-flint two lens achromat does a much better job of bringing all the colors to a common focus than a single lens. A single lens works very much like prism, the colors are never in focus at the same time. Achromats were developed in the late 1700's and represented a major improvement in the color correction of refractors. They use two elements, one is typically crown, the other flint that are designed so the one lens corrects for the other and the colors are brought to focus at nearly the same distance from the objective.

But it is not perfect, larger apertures and faster focal ratios result in more chromatic aberration. A 6 inch F/8 shows a quite a bit of chomatic aberration or CA. Visually, you will see it on brighter stars, on the planets.

When it comes to photography, the situation is worse. Achromats are designed so that the light in the center of the spectrum where the eye is most sensitive are focused, this is they yellow and the green. The ends of the spectrum, the violet and the red, are out of focus but the eye is not very sensitive to red and violet so CA less visible than one would expect.

Cameras are sensitive to a much broader spectrum of colors and so they capture the defocused blue and red light, I believe that is what you are seeing as the blue rings around the bright stars in your photo. There is no easy way to focus the image because those colors are simply out of focus.

To avoid chromatic aberration one can either use a Newtonian or SCT/Mak, these use mirrors. Or one can use an apochromat, the next generation of refractors which use Fluorite based glasses to dramatically improve the color correction. Unfortunately a 6 inch apochromat is the far side of $6000, one can take amazing photos with an apochromat.

Typically what people do is use small apo's, an ED-80 which shows very little false color can be purchased for around $300 used. One can gain something using filters and/or photoshop, I am not familiar with the techniques they use.

Jon Isaacs