High Resolution page
The Grange Obs. is increasing its expertise in the digital processing of electronic imaging; the current field of application
is the enhancing of resolution on Moon pictures imaged with a Philips SPC900NC webcam coupled with the Vixen 140 mm refractor telescope recently purchased.
The main telescope at the observatory (300 mm Newton reflector) would develop a better optical resolution, depending on the objective diameter D [mm] (Daves formula yields 120/D arcseconds), but can be rarely exploited due to the air movement
(inside the instrument or over the depth of the atmosphere, the latest called seeing) and the fact reflector telescopes have an obstruction (a secondary mirror), actually lowering the above theoretical resolution.
Refractor telescopes (with lenses) have no optical obstruction and a sealed tube, generally performing better than open reflectors.
The high resolution technique development is based on Registax 5 processing of MPEG footages at given frame rates, whose results are post-processed with the IRIS program.
The Philips SPC900NC webcam equipped with its Sony ICX098 1/4" CCD can produce footages of up to 90 frames per second (fps), depending on the subject illumination (ultimately on chip sampling); the webcam control software allows to use the manual or the automatic setting of imaging parameters.
Actually the higher is the frames rate, the better the seeing affecting the image quality can be theoretically frozen.
The Registax processing is based on the selection of the best frames of the footage, which are stacked together to maximise
the signal-to-noise (S/N) ratio; the software does a selection of control points on the image for the subsequent frames
equalization and stacking; moreover indicates a percentage of resolution increasing and can do wavelets processing.
A footage imaged with a good seeing shall have more frames selected and stacked, being the S/N higher than a typical bad seeing result.
IRIS post-processes the Registax results by applying selected filters, the more powerful for the resolution increasing is the
unsharp masking.
The CCD sampling [arcseconds/pixel] can be calculated by multiplying the pixel dimensions [mm] by 206265 (i.e. the number of
arcseconds in a radian) and dividing the result by the telescope focal length F [mm]; if an eyepiece of focal Fo [mm] is used to increase the telescope focal length, the equivalent focal [mm] will be Feq = F*(T/Fo-1), where T [mm] is the distance between the webcam CCD chip and the eyepiece's lens closer to it.
The CCD optimum sampling applying the Nyquist law can be calculated dividing 37 by the scope diameter D [mm]; webcam
footages could be over-sampled (for enlarging the particulars), but the problem is the image illumination and consequently
the S/N ratio decrease.
The following Moon picture was obtained with the webcam on January 16th 2011 using a Kellner 18 mm eyepiece projection, giving a
0.6 arcseconds/pixel sampling; the 20 s footage at 45 fps yielded 900 frames, of which one is shown here below:
Registax selected and stacked the best 300 frames, obtaining the following result:
The Registax result shown herein was cropped and processed with IRIS:
The following Moon picture was obtained with the webcam on April 10th 2011 using the same Kellner 18 mm eyepiece projection, but giving a CCD sampling (0.28) closer to the 37/D value, since the T parameter was increased; the 20 s footage at 30 fps yielded 600 frames (as can be seen the seeing was poor); Registax selected and stacked the best 120 frames,
afterwards processed by IRIS:
The theoretical resolution of the Grange Obs. 140 mm refractor shall permit to do webcam imaging with Moon features
of about 500 m.
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