This web page contains some of what I've learned about extracting information from solar images obtained by a small, inexpensive doublet refractor, using a CCD taken from an inexpensive webcam as a detector.

Inexpensive achromat refractors are unable to simultaneously focus all wavelengths in the visual passband. Usually, the green or yellow regions are optimized by lens designers, leaving the red and blue/violet wavelengths to fall out of focus, causing chromatic aberration. In addition, typical mylar and glass solar filters allow (a safe amount of) infrared radiation to pass. An achromat will fail to focus this IR as well. If a CCD is used as a detector, it will need to be protected from the IR, since typical CCDs are sensitive to infrared. One may also use so-called "minus-violet" filters, which remove the out-of-focus blue/violet photons. Some MV filters, including the one used here, can remove out-of-focus red photons as well, cleaning up the image even further.

When out-of-focus light is filtered out, the image will be somewhat less bright. However, contrast and sharpness will be better, more than offsetting the small loss of light. Here I compare prime-focus solar images of the same field of view obtained under the same conditions with and without IR-blocking and MV filters in order to show the effect each filter has on image sharpness and contrast. At the telescope, focussing was done very carefully. Image processing afterwards (linear intensity stretching and unsharp masking -- nothing fancy) was identical for each case. Hence, the results are as unbiased as I can manage, given the modest equipment used.

Observing hardware for these tests:

• Telescope: 120mm f/8.3 achromat (one of the popular Chinese "Synta" refractors)
• Solar filter: Baader Planetarium solar filter material (a high-strength, high-optical-quality polymer sheet in a homemade cell)
• NIR blocking filter: Sirius Optics near-IR blocking filter. This is an interference filter consisting of dielectric materials vacuum-deposited on a high-quality optical glass substrate. It blocks wavelengths from about 6800 Å to 9300 Å.
• MV filter: an "MV2" from Sirius Optics. (The MV2 is a little more agressive in the blue than the very similar MV1.) This filter is also a dielectric interference filter, passing wavelengths between about 460 nm and 710 nm.
• Camera: a cannibalized Philips Vesta PCVC657K webcam (659×494 pixel CCD with 5.6 µm square pixels), cooled with two small CPU fans.

NOTE: Click on the images for larger versions.

Unfiltered

This image is of NOAA Active Region 9608 (as well as AR9607 and AR9610), taken at prime focus on 9 September, 2001, from my back yard. It is a composite of 6 images that were flat-fielded, registered automatically, and stacked by software that I wrote for this purpose. (AstroStack, a freely available shareware program, also registers and stacks images.) Here, no filters were used at all. The only thing between the telescope objective lens and the CCD was the clear cover plate glued to the CCD itself, and which seems to serve no purpose beyond physical protection of the CCD. The stacked image was stretched and sharpened in Photoshop. Extreme care was taken at the telescope to achieve the best focus possible, by use of a Hartman mask (1-inch holes separated by 3.7 inches). This is not a bad image, but notice the blurriness and low contrast, despite the level of care and processing. This is due mainly to IR contamination; there is also some blurriness due to unfocussed blue/violet light, which we will see below. (Click on the image for a higher-resolution image.)

IR Blocking

Here is the same region taken by the same equipement about 10 minutes later but with a near-IR blocking filter in front of the CCD (again after careful focussing). It is a stack of 12 frames. The stacked image was processed exactly the same as the previous image. Notice the great improvement in both sharpness and contrast, due entirely to the removal of the NIR contamination. (Click on the image for a higher-resolution image.)

IR Blocking + MV Filter

For this image, the MV filter was added in tandem with the NIR blocking filter, thus removing unfocussed blue/violet light in addition to the far red and NIR. This image is also a stack of 12 frames. The stacked image was processed exactly the same as the previous images. (Click on the image for a higher-resolution image. The increases in sharpness of this and the following case are easier to see in the higher-resolution images, which are at the original 640×480 that the gutted webcam firmware puts out.) The result shown here, or rather the difference from the previous case, is pretty close to what you see at the eyepiece: not drastic, but certainly noticeable and easily large enough to be worthwhile.

IR Blocking + MV Filter + Subpixel Registration

The additional sharpness evident in this image (more easily seen in the 640×480 higher-resolution image) is due not to an additional filter but rather due to a difference in the processing. The same 12 frames as the previous image were used for this stacked image, and the processing of the stacked image was very similar. However, before stacking the frames, I first resampled each of them (as well as the flat field frames) at twice the resolution (from 640×480 to 1280×960), using the standard bicubic interpolation algorithm in Photoshop, and then I applied a small amount of unsharp masking to each resampled image. The upsampled, flat-fielded frames were then registered and stacked as before, and the result was stretched, sharpened, then downsampled, again with bicubic interpolation, back to the original resolution of 640×480. The result of this is effectively a subpixel registration of the images, which on solar images can yield a slight (i.e., noticeable but not drastic) increase in sharpness. Note that, if too much sharpening is applied to the upsampled frames prior to stacking, the result after stacking, downsampling, and processing will be too contrasty and unnaturally "brittle". This image is pretty close to what can be seen by eye at my telescope with an MV filter and during moments of good seeing. (Click on the image for a higher-resolution image.)