CCD Imaging with the Celestron CR-150 6" f/8 Achromatic Refractor

Note: to view all of the detail in these images, you may need to adjust the brightness and contrast controls on your monitor! See my monitor adjustment page for detailed suggestions.
I recently purchased a Celestron CR-150 6" Achromatic refractor to test it for CCD imaging. I had seen some fairly nice images taken in black and white on the web (posted by Tom Davis in the alt.binaries.pictures.astro newsgroup). I have a color filter wheel, and wanted to analyze how this refractor handled focus of the visible colors.

The stack of small images at right shows how the refractor stacks up in this analysis. From top to bottom, the images are in red, green, blue, and unfiltered light. Typically, achromatic refractors are corrected best for green light, and this one is no exception. The stars in the green image are well-defined, with a sharp border. In addition, the two small stars at upper right of the green image are brighter and more distinct. These are all signs of better focus.

The red, blue, and white-light images, on the other hand, show a halo around the stars; the edges are not distinct. In addition, the two dim stars at upper right of each image are barely visible. All of these are signs of not quite being in focus.

Of course, I could have re-focused the setup before each color shot, but that would have defeated the purpose of the experiment.

Conclusion: for successful imaging, it will be best to focus for green light, and to filter out red and blue colors.

The images of M13 at right were taken through the CR-150. The top image was taken with a Yellow and an IR Blocking filter in place. The lower image was taken with no filters; it shows the halo effect. 

Without appropriate filtering, star images are fairly large, though the image is actually quite nice looking. With the filters, the image quality is quite good.
The image at right was taken through an APO refractor (Takahashi FC-125, a 5" refractor). Note that the stars are smaller (no halo effect). The CR150 can't reach this level of perfection, but with careful filtering, you could use it to take decent images if your mount is steady enough to handle the weight of the CR150.
Here's another comparison. The image of M101 at right was taken through the CR150. Note that, due to different colors focusing differently, fine detail is missing. In addition, star images are somewhat bloated. No filters were used; the image was taken in white light.
This image of M101 was taken through a Takahashi FC-125. Note that fine detail is preserved very effectively.
Once it was clear that I was getting differential focusing points for different colors (red, green, and blue), I used the red filter to focus, and took the image at right. This is red light only, focused for red light. Note that the star images are reasonably sharp. What you are seeing here is the sharpness of the refractor itself, since only a single color has been used for the image.
Without changing the focus point, I switched to the green filter and took the image at right. Note that the stars are very soft, with many of the dim stars missing entirely. This illustrates the main problem with imaging using the CR150: you can't bring multiple colors to focus at the same time. No one color is ideal -- some nebulae are red, for example, while others are blue! Shooting something like the Trifid Nebula, which contains blue and red, would be out of the question for a single shot. One part or the other would be out of focus. Stars, of course, come in a range of colors. If a color filter is used, then stars will not reflect their actual relative brightness.

Two further tests suggest themselves, but I did not perform them before I sold the CR150:

  • Try using a yellow filter, which covers a wider portion of the color spectrum. I got my best results with the green filter, and I can't help wondering how well the wider bandwidth of a yellow filter would work. On the one hand, it would capture a wider spectrum, which is good; on the other, it might have some minor focus problems. Only way to know is to try it.

  • Try using an RGB filter set to take images, carefully focusing for each filter, and then combining. This holds the best hope, since it allows you to focus for each color separately. What I don't know is how well the images will fit back together; since they are at different focus points, the image scale may vary a little between images, causing star images not to align perfectly. A product like Registar would allow you to compensate for any such problems, and the latest revision of Maxim DL also does better matching.