M57. Working to improve image sharpness
Posted: Sun Jun 20, 2021 12:54 am
An update on trying to get 'lucky imaging' to deliver improved resolution. Here under the Heathrow flight path sky seeing never gets better than FWHM ~ 2.5-3 arc sec.
Image 1 is an integration of 183 x 5s exposures at gain 200 - carefully selected from about 270 to eliminate those with FWHM > 3.3 and high Eccentricity etc. It represents about the best I typically get using conventional methods.
SW Synscan 250PX Dobsonian with 0.9X SW flattener reducer (F 4.16 and f = 1050mm). SW electric focuser.
ZWO AS1294MC PRO camera BIN1 with 4.3 um pixels and 14 bit ADU. Image scale 0.9 arcsec/ pixel at -10C.
The idea here was to improve on this image by transferring the luminance from a higher resolution image obtained by lucky imaging.
Pilot observations of the double double Epsilon Lyra were first carried out in order to scope out suitable conditions for lucky imaging. The two star pairs, E1 and E2 are separated by 2.3 and 2.4 arcsec respectively.
Images 2a and 2b are individual 'good' frames chosen from about 200 of the E2 pair. 2a is a 5ms frame sampled at an image scale of 0.9 arcsec pixel and 2b the same but at an image scale of 0.45. About 5% of the frames were of similar quality Not surprisingly the finer 0.45 scale gives the better resolution.
Image 2c is an example of a 'good' 300ms frame sampled at 0.45 arcsec/pixel and Image 2d of a 3s frame. Both are much dimmer than the 5ms frames because I had to cut down the light by interposing an SII filter in order in order to avoid saturating the 12 bit ADU. Surprisingly perhaps - given that the time is so much longer than air turbulance changes - the 300ms frame still shows quite good resolution and even the 3s frame still shows some enhanced resolution.
For lucky imaging of the ring nebula the conditions were as follows
i) Images collected by Sharpcap as ~ 15 min .SER files
ii) Exposure time 300ms and gain 285
iii) Frame size 1800 x 1200 pixels (810 x 540 arcsec) - must be big enough to contain sufficient starts for alignment
v) Sampled at 0.45 arcsec/ pixel
SW Synscan 250PX Dobsonian with 0.9X SW flattener reducer (F 4.16 and f = 1050mm). SW electric focuser.
ZWO AS1294MM camera in BIN1 46Mb mode with unbinned 2.315 um pixels and 12 bit ADU (0.45 arcsec/ pixel)
Rejection and selection of frames for stacking is a critical part of lucky imaging. SER files were converted to thousands of FIT files using the PIPP freeware. The PIPP contrast-based quality algorithm was used to select the top 70%. Then subframe selector in PixInsight was used to select only those frames showing more than a certain number of stars (bright enough) that were well defined (FWHM < ~ 1.7 and Eccentricity < 0.65). Pass rates varied from night to night and within nights between as low as zero up to as high as 10%. Visual inspection using BLINK may be superior but is laborious for large numbers of frames.
Image 3 shows the result of stacking about 2000 x 0.3s -(total ~ 10min) - of selected frames.
Image 4 shows the final colour result obtained by replacing the luminance of image 1 with that of of image 3
The overall process looks to have substantially improved the resolution fron an FWHM of about 3.2 arc sec to about half of that. This is illustrated by the successful resolution of a (line of sight) triple star close to M57 with components of differing brightness that are all within 2 arc sec of each other (see image 3).
Undoubtedly resolution could be improved with shorter exposures. However this would come at the cost of more noise (including accumulated raed noise) and for each target object a careful compromise needs be struck between resolution and achieving a workable level of quality in each frame.
TimH
Image 1 is an integration of 183 x 5s exposures at gain 200 - carefully selected from about 270 to eliminate those with FWHM > 3.3 and high Eccentricity etc. It represents about the best I typically get using conventional methods.
SW Synscan 250PX Dobsonian with 0.9X SW flattener reducer (F 4.16 and f = 1050mm). SW electric focuser.
ZWO AS1294MC PRO camera BIN1 with 4.3 um pixels and 14 bit ADU. Image scale 0.9 arcsec/ pixel at -10C.
The idea here was to improve on this image by transferring the luminance from a higher resolution image obtained by lucky imaging.
Pilot observations of the double double Epsilon Lyra were first carried out in order to scope out suitable conditions for lucky imaging. The two star pairs, E1 and E2 are separated by 2.3 and 2.4 arcsec respectively.
Images 2a and 2b are individual 'good' frames chosen from about 200 of the E2 pair. 2a is a 5ms frame sampled at an image scale of 0.9 arcsec pixel and 2b the same but at an image scale of 0.45. About 5% of the frames were of similar quality Not surprisingly the finer 0.45 scale gives the better resolution.
Image 2c is an example of a 'good' 300ms frame sampled at 0.45 arcsec/pixel and Image 2d of a 3s frame. Both are much dimmer than the 5ms frames because I had to cut down the light by interposing an SII filter in order in order to avoid saturating the 12 bit ADU. Surprisingly perhaps - given that the time is so much longer than air turbulance changes - the 300ms frame still shows quite good resolution and even the 3s frame still shows some enhanced resolution.
For lucky imaging of the ring nebula the conditions were as follows
i) Images collected by Sharpcap as ~ 15 min .SER files
ii) Exposure time 300ms and gain 285
iii) Frame size 1800 x 1200 pixels (810 x 540 arcsec) - must be big enough to contain sufficient starts for alignment
v) Sampled at 0.45 arcsec/ pixel
SW Synscan 250PX Dobsonian with 0.9X SW flattener reducer (F 4.16 and f = 1050mm). SW electric focuser.
ZWO AS1294MM camera in BIN1 46Mb mode with unbinned 2.315 um pixels and 12 bit ADU (0.45 arcsec/ pixel)
Rejection and selection of frames for stacking is a critical part of lucky imaging. SER files were converted to thousands of FIT files using the PIPP freeware. The PIPP contrast-based quality algorithm was used to select the top 70%. Then subframe selector in PixInsight was used to select only those frames showing more than a certain number of stars (bright enough) that were well defined (FWHM < ~ 1.7 and Eccentricity < 0.65). Pass rates varied from night to night and within nights between as low as zero up to as high as 10%. Visual inspection using BLINK may be superior but is laborious for large numbers of frames.
Image 3 shows the result of stacking about 2000 x 0.3s -(total ~ 10min) - of selected frames.
Image 4 shows the final colour result obtained by replacing the luminance of image 1 with that of of image 3
The overall process looks to have substantially improved the resolution fron an FWHM of about 3.2 arc sec to about half of that. This is illustrated by the successful resolution of a (line of sight) triple star close to M57 with components of differing brightness that are all within 2 arc sec of each other (see image 3).
Undoubtedly resolution could be improved with shorter exposures. However this would come at the cost of more noise (including accumulated raed noise) and for each target object a careful compromise needs be struck between resolution and achieving a workable level of quality in each frame.
TimH