A prospective customer emailed me the other day wanting me to do a particular kind of photometric test using an artificial star to determine the evenness of illumination of our XL panels. I was intrigued by the method, and began testing. Essentially, the method requires a photometrically stable night, and a series of images of a star near the zenith with the scope slewed so that the star moves diagonally across the detector in subsequent sets of images. The star’s brightness is compared to that of an artificial star that is added to the image.
Of course, every telescope/detector set up has some vignetting, so when you go to see how the star brightness changes along the transect, an unflatted series shows that vignetting and the star’s brightness falls off as you get farther from the center of the detector. We apply flat field correction to eliminate the vignetting, and if the flat field illumination is even, the star’s brightness should be even across the detector when compared to the artificial star.
Well, when I went to analyze my results, I noticed that the star had strange brightness changes after being flatted, and I wondered what might be the cause. I’m sure that the XL does not exhibit radial changes to its brightness from other testing, So I started hunting around for other causes.
Here’s the master flat field image that I used to flat the star images. Note the strange bright ring around the center so that the brightness doesn’t fall of evenly from the center to the edges. This bright ring would explain why, when my images were flatted with this flat field image, there was an under-correction under the bright ring, which led to the star being dimmer than it should have been at that position.
The master flat field image. Note the ringlike structure.
To give you some background, I heard a talk by Michael Barber at NEAIC last year about the effect of internal reflections on image quality. Essentially, he showed us how an internal surface that is not well coated with anti-reflective paint can really mess up an image by scattering light onto the detector. He showed us some pictures of flat field images with very bright central peaks that he said might be due to this sort of scattered light.
I began to wonder about this bright ring. To get a better picture of what was going on, I imported the flat image into Mathematica and made this 3D contour plot:
3D contour plot of the flat field image
This shows the general falloff in light being recorded at the detector as you go away from the center, but you can see a ridge of brightness in a ring around the center. I made another contour plot with a zero plane slicing through the contours at the mean value of the pixel brightnesses that I think emphasizes the circular ridge.
The circular bright ring shows up here as a semicircle above the mean value plane
Today I went out to the observatory and dismounted the camera and sure enough, there was a bright ring visible around the secondary mirror image coming from one of the constituents of the image train. I haven’t identified exactly where it is coming from, but here’s what it looks like when you peer in the eyepiece holder up towards the secondary. You can clearly see the bright ring around the secondary mirror image that is coming from a metal surface inside the primary mirror baffle.
Now I need to go in and try to black out the reflection and then see if the bright ring disappears from the flat. Stay tuned!
The view up the eyepiece tube. Note the reflection of the secondary (the circle with the black dot in the middle. The big problem here is the next thin bright ring. Thats coming from the inside of the primary baffle, I think.
