Quote:
Originally Posted by robin_astro
Hi Steven,
Dither and drizzle is a legitimate way of improving resolution in undersampled images even in scientific imaging but are your images actually undersampled ? (ie is your star image FWHM less than 2 pixels?)
Aligning and stacking (as typically used in planetary imaging) is also a legitimate tool to maximise resolution when using the Star Analyser. I am wondering if that is what we are seeing here. The drizzle algorithm may align them as part of the process. When you normally combined your images did you align them? If not can you try a simple align and stack to see if that has a similar effect?
Robin
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Hello Robin,
Given seeing conditions of 2 arcseconds and my setup providing a scale of 1.1 arcseconds/pixel "ordinary" images are definitely not undersampled and almost meet the Nyquist requirement for sampling.
Given in the combined images the spectra are in focus and the stars are out of focus, I not even sure what constitutes an undersampled or oversampled image in this case.
Data was collected over two days and both sets of data were therefore dithered.
The software for normally and drizzle combined images required the individual images to be aligned prior to combining otherwise registration errors would be apparent.
If one examines the ratio of the peak height of the drizzle combined over the normally combined, for the H-Gamma, H-Beta and H-Alpha peaks as a function of "enhancement", the ratio declines as wavelength increases.
This could be consistent with the idea that since the FWHM of the Airy disc increases with increasing wavelength, the spectral image becomes progressively more "oversampled" at longer wavelengths and the effectiveness of drizzling decreases. (Pure speculation).
In the attachment are cropped images for the drizzle and normally combined raw spectra. The left hand drizzle combined attachment looks more like it has been contrast enhanced (note the burnt out stellar images).
Regards
Steven