Progress towards a high resolution deep colour image of M3
Posted: Tue May 30, 2023 10:14 pm
Bortle 7 skies (partly astronomical twilight)
VX12 300 mm F4 Newtonian
Ioptron CEM70 mount
AS1294 MM camera. ZWO RGB filters. RGB captured with 4.63 uM pixels (0.81 arcsec/pixel). Luminance with unlocked 2.315 uM pixels (0.405 arcsec/ pixel)
Unsteady skies (PHD2 guiding no better than 1.3 to 2 arcsec so switched off) and breezes meant that imaging at home during the latter part of May was limited to a strategy of selecting from many short exposures. This is probably more feasible at D = 300 mm and F 4.0 than with smaller longer scopes.
About 100 x 10s gain 124 red, green and blue (ZWO colour filters) frames at -5C totalling 50min exposure – selected over several hours using SharpCap 4 live -stacking and FWHM + brightness filters - were integrated and combined into an RGB image at an image scale of 0.81 arcsec/ pixel using PixInsight. The FWHM values of the linear integrations of each channel were about 2.5 arcsec - use of Blur Xt (deconvolution) star shape correction and slight reduction improved these average values down to about 2.2. After stretching and processing these data produced a fairly sharp and colour vibrant image of the M3 globular cluster.
The star resolution was much improved – resolving M3 more or less to the core – when this initial image at 0.81 arcsec/ pixel was aligned and the luminance transferred from a luminance filter image integrating 1100 x 3s frames at a scale of 0.405 arcsec/ pixel. The linear 1100 x 3s integration was at an FWHM of around 1.65 arcsec (from about 1.9) after sharpening with BlurXt. However although adding the finer scale luminance data improved resolution it also had the effect of somewhat weakening and diluting the colour depth.
So - as always - room for improvement with more data.
(One thing to note of possible interest. Under such unsteady skies one of the biggest apparent problems was star-shape distortion - measurable as high Eccentricity values. It was notable not only that use of RC BlurXt star-shape correction was pretty effective at recovering improved star shapes but also that this process worked best when it was applied to integrations comprising the largest total numbers of frames notwithstanding some of them being poor. In particular pre-removing (and thus reducing total frame numbers by a half or more) by using PI subframe selector to omit the more blurred and eccentric frames actually led to a poorer rather than a better final result in most cases.)
Tim
VX12 300 mm F4 Newtonian
Ioptron CEM70 mount
AS1294 MM camera. ZWO RGB filters. RGB captured with 4.63 uM pixels (0.81 arcsec/pixel). Luminance with unlocked 2.315 uM pixels (0.405 arcsec/ pixel)
Unsteady skies (PHD2 guiding no better than 1.3 to 2 arcsec so switched off) and breezes meant that imaging at home during the latter part of May was limited to a strategy of selecting from many short exposures. This is probably more feasible at D = 300 mm and F 4.0 than with smaller longer scopes.
About 100 x 10s gain 124 red, green and blue (ZWO colour filters) frames at -5C totalling 50min exposure – selected over several hours using SharpCap 4 live -stacking and FWHM + brightness filters - were integrated and combined into an RGB image at an image scale of 0.81 arcsec/ pixel using PixInsight. The FWHM values of the linear integrations of each channel were about 2.5 arcsec - use of Blur Xt (deconvolution) star shape correction and slight reduction improved these average values down to about 2.2. After stretching and processing these data produced a fairly sharp and colour vibrant image of the M3 globular cluster.
The star resolution was much improved – resolving M3 more or less to the core – when this initial image at 0.81 arcsec/ pixel was aligned and the luminance transferred from a luminance filter image integrating 1100 x 3s frames at a scale of 0.405 arcsec/ pixel. The linear 1100 x 3s integration was at an FWHM of around 1.65 arcsec (from about 1.9) after sharpening with BlurXt. However although adding the finer scale luminance data improved resolution it also had the effect of somewhat weakening and diluting the colour depth.
So - as always - room for improvement with more data.
(One thing to note of possible interest. Under such unsteady skies one of the biggest apparent problems was star-shape distortion - measurable as high Eccentricity values. It was notable not only that use of RC BlurXt star-shape correction was pretty effective at recovering improved star shapes but also that this process worked best when it was applied to integrations comprising the largest total numbers of frames notwithstanding some of them being poor. In particular pre-removing (and thus reducing total frame numbers by a half or more) by using PI subframe selector to omit the more blurred and eccentric frames actually led to a poorer rather than a better final result in most cases.)
Tim