Vignetting of Hutech Hα Front Filter?
Introduction
I recently picked up on Astromart a Hutech, Hα, Front Filter (from herein abbreviated Hα-FF). Before going out under the stars for first light, I decided to play with it “on the bench” to figure out, among other things, what the right Tv (exposure time) and ISO setting should be to properly expose Flat Lights. Properly expose, that is, the red sensors (only) of my Modified 350D, since the green and blue sensors will “see” almost nothing from the Hα-filtered Light Box.
After a couple of experiments, I settled on 10sec @ ISO 800 which gave me a median value of 1700 or so, a bit shy of the target of 2048, the latter of which is the center of the linear range of the 12-bit A/D converter in the 350D. Those 12-bit unsigned values, by definition, fall in the range [0..4095]. I should probably crank the exposure time up a bit, but this is close enough for the purposes of this article.
Test Equipment
All of the analysis was performed and all of the images were collected with:
- Scope: Celestron C8-N (8" f/5 Newtonian)
- Coma Corrector: Baader MPCC
- Camera: Hutech-Modified Canon Digital Rebel XT (350D), with Type I (UV/IR block) filter replacement
- Front Filter (when used): Hutech HA-FF (Hα-pass Front Filter)
- Illumination: Light Box
In all of the example images that follow, I'm showing only the red pixels from the 350D's Color Filter Array, and, for maximum clarity, I'm showing those pixels as a grayscale image. IRIS's convenient split_cfa command was used to extract just the red pixels from a raw, linear, uninterpolated, CFA image.
A Problem Arises
Upon inspecting the exposures, it appeared to me as if there was a
radially symmetric “step function” present in the Flat
Lights, which looked sort of like a bright circle centered in the
image.
Further Analysis
Not yet discouraged, I continued to process the Flat Lights by
sigma-combining a handful of them (five), all of which had been
normalized after calibrating against the Flat Master Dark. As
expected, the general form of the image remained the same, though the
sigma-combining of five exposures helped to reduce the random noise in
the image. The lower random noise makes the ”pattern
noise“ (checkerboard pattern, more evident vertically than
horizontally) all the more evident, however. The result is in
One final way of looking at the problem is to take the Master Flat
with the Hα-FF, treat it as if it was a Light, and divide
it by the Master Flat without the Hα-FF. That
tends to factor out the vignetting inherent in the optical system, and
focus the attention on the difference between the expsoures with and
without the Hα-FF. That result is shown in
(Equals | (Equals |
Conclusion
It's pretty clear from these images that there's a problem
somewhere in the optical path. I'm not sure whether the Hα-FF
is simply causing some nasty vignetting, or whether there are
reflections being caused by the UV/IR replacement filter in the 350D,
the coma corrector, the Hα-FF, or all of the above. Of course,
it's also possible that at the Hα wavelength, the diffusion
layers of the Light Box are such that it really looks like the
image shown in