It has been a while since I’ve made an astronomy post, not because I haven’t had the scope out imaging, but because I haven’t been all that happy with recent results. My images have been showing a nasty central brightening that prevents fainter details being brought out in post processing.
Why is this a problem? Take a look at the image below of M51. I’ve stretched the data a little to emphasise the brightening. The problem is that in order to bring out fainter details in the galaxy or the lower magnitude stars around the edges of the image, the data levels need adjusting. Long before those stars are visible, the bright centre has spread across the entire image and ruined it.
After doing some research, it turns out this is known as vignetting and is a problem caused by the telescope optics. MAPUG-Astronomy describes this in great detail, but the important part is
The image is brightest on axis, at the center of the field of view, and dimmer off axis. This is a classical case of vignetting. Note that this is not a sharp cut off of the image, but a gradual dimming of it. Unfortunately, it is a feature of folded telescope optics which cannot be avoided.
Although nothing can be done to prevent vignetting, it turns out there’s a way to account for the dimming and in turn remove it from the images. By taking a photo of an evenly lit area, the dimming of pixels from vignetting can be isolated and then in turn removed from the light frame.
I’ve built a light box using foamboard to construct the box and holders for the diffusers. Two sheets of opal perspex slot into the holders to diffuse the light (from four LEDs) and present a flatly lit front pane. The box is loosly based on the many designs posted on-line. It won’t win any awards for construction, but it was cheap. About £25 in materials which can likely be sourced for cheaper than that.
I captured CCD images in various rotated positions to ensure the field is evenly lit and that the dimming is due to vignetting and not shoddy light box construction.
After calibrating the images with the flat field frames, here’s the result.
The bright gradient has now been eliminated. All that remains is a bit of signal noise which can be handled by stacking multiple short exposures. Below is the first stacked and calibrated image. I’m still working on alternative ways to process the fits data to improve the image, but a quick processing shows promising results.
After a closer inspection of the combined frames, there is still a slight gradient from the bottom of the image moving upwards towards the centre. Whether this is due to the light box not providing a totally flat field frame or simply light pollution, I’m not yet certain.
A Trio of Galaxies
|Exposure Time||30x120s Avg|
|Date||2009-03-05 03:06 UTC|
|CCD||Starlight XPress MX716|
|Dark Frames||10x120s Avg|
|Flat Frames||10x0.8s Avg|
|Apparent Dimension||11 x 7 arc min|
|Visual Brightness||8.4 mag|
The centre of the image shows two galaxies. The first, larger galaxy is NGC 5194 and just above it is NGC 5195. These two colliding galaxies are better known as M51 the Whirlpool Galaxy.
During processing I noticed a faint object in the lower right corner. According to the reference catalogue of bright galaxies, this is IC 4263, a magnitude 15 galaxy measuring only 2x0.4 arc minutes in size.
The image above has had a high pass filter applied as an attempt to sharpen the image a little. I’m still not sure which of the two I prefer most.
I’ve glossed over all the details of calibrating images, but if you’re interested in knowing more there’s a few sites with more detailed descriptions. Such as AAVSO
There’s still a lot of room for improvement. Still, compared to my previous attempt at M51, I think the light box was worth every penny.