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 astronomical pictures imaged with a Philips SPC900NC webcam coupled with the Vixen 140 mm refractor telescope, as well as a QHY5V imaging & guide camera.
The main telescope at the observatory, a 300 mm Newton reflector, has a better optical resolution, depending on the objective diameter D in [mm] according with the Daves formula 116/D arcseconds for the 560 nm visual wavelength (it will be 83/D for 397 nm in UV).
But reflectors diameter can be rarely exploited due to the air movement
(inside the instrument or over the depth of the atmosphere, the latest called seeing) and they have an obstruction (a secondary mirror), actually lowering the above theoretical resolution.
Refractor telescopes (with lenses) instead 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
The CCD sampling [arcseconds/pixel] can be calculated by multiplying the pixel dimensions [mm] by 206265 (i.e. the number of
arcseconds in one 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 (in visual wavelength) 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 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 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 diffraction resolution of the Grange Obs. 140 mm refractor shall permit to do webcam imaging with Moon features
of about 1.5 km, but with peculiar atmospheric conditions the limit could reach 1.3 km.
The planet Saturn was imaged using the 140 mm refractor with the Philips SPC900NC color webcam at 5 fps with a CCD sampling of 0.23 arcsecond/pixel, achieved with a vintage Clavé 6 mm eyepiece; the footage was processed using Registax5 (which selected 50 frames of 100) and the result was enhanced with IRIS:
The IRIS Richardson-Lucy deconvolution processing with a simulated diffraction Airy pattern on the image) is shown in B&W, giving some detail on the planet's disk and ring; the NA 140 refractor theoretical resolution is equal to 0.8 arcseconds in visual wavelength, and the Cassini division clearly visible on the ring's edge is about 0.7 arcseconds:
Jupiter was also imaged with the 140 mm refractor at f/33.3 and the QHY5V planetary camera obtianing a resolution of 1.3 arcsecond at 890 nanometers IR wavelength due to the poor seeing; the Ganymede satellite shadow and the Great Spot Hollow around the Jupiter's GRS are clearly visible on the planet's disk here below:
The importance of applying key image processing techniques is evident in this 3 x 5 arcmin central Sun's image in UV obtained stacking 600 out of 1200 frames on a QHY5V camera footage processed by Registax:
First of all, being the image obtained from a focal eyepiece projection, the dust pattern (shown at left) must be removed using the Registax flat fielding option; if that is not done, the image shall look as shown at right:
Once the flat field is done frame by frame using Registax, with the IRIS tool the unsharp masking and the contrast enhancement commands shall be applied for making faint features to become more evident; unfortunately, that process will add random electronic noise.
The IRIS Richardson-Lucy deconvolution algorithm will help to reduce the image noise, as shown from left to right:
The RL IRIS command (using the verbose window) has been applied after convolution with a synthetized Airy disk for the 397 nm wavelength; it must be remembered the algorithm works only for square images with a number of pixels equal to a power of 2 (512x512 or 1024x1024 for example). The sizing of smaller images is possible using the WINDOW3 IRIS command.
The final image resolution is equal to 0.6 arcseconds for the 140 mm astrograph, corresponding to 500 km on the Sun chromosphere, which is sufficient to show solar spiculae.
For achieving a filtering system capable to isolate the 397 nm UV wavelength for solar imaging (Calcium H absorption line), please have a look of the Grange Obs. SPIN-OFF page.
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