Obviously this technique only works if the luminance exposures offer sufficient SNR for the object in question (i.e. there is no point whatsoever in achieving a low FWHM value in a linear image that then needs stretching by that much more in order to adequately show an image). The total number of short luminance frames also needs to be kept manageable and not so many as to add too much read noise relative to sky brightness.
General observations were as follows …
1) Surprisingly large improvement in sharpness - FWHM of linear integration of > 100 debayered OSC frames improved from ~ 3.3 down to around 2.2 - with a change of telescope from a SW PDS200 F 5.0 sampling at 1 pixel/ arcsec ~ to a bigger Orion Optics (UK) VX12 300mm Newtonian F 4.0 sampling at 0.8 arcsec/ pixel.
While the VX12 (1/10 wave) is certainly a better quality telescope the scale of and reasons for this scale of improvement elude me. It could simply be that the seeing has been that much better this than last spring or that my focusing alignment skills are much better?
2) Further significant improvement in the sharpness of the final image - from FWHM 2.2 down to 1.8 - after adding in the luminance of 800 or so carefully selected ‘good’ 3s frames to an OSC image of M51.
3) BUT the process of selecting 800/2300 best 3s frames, calibrating, cosmetic correction, alignment and integration was laborious! It took half a day for my computer simply to calculate the integration (M51 being a relatively large object).
4) Directly using the Sharpcap autostacks of the 3s frames (selected using brightness and FWHM filters) for luminance and bypassing the selection of 2) and 3) was far less painful, much quicker and produced nearly as good (FWHM only down to 2.0 but SNR 3x better) an improvement in image sharpness.
So the bottom line message is that - in order to produce very short frame sharp luminance images - it is as well simply to make use of what Sharpcap already offers. Set the FWHM and brightness filters to only select ‘good’ frames , make a further stack of the autostacks and use this as luminance. It saves an awful lot of raw file handling - that in the end might make only a marginal improvement
The one thing that ideally I would add to Sharpcap autostacking in order to perfect it for this use (message to Robin
) would be to add an ellipticity filter for taking out the frames with oddly shaped flared stars - that still pass the FWHM filter.---------------------------------------------------------------------------------------
Details were as follows.
VX12 Orion Optics (UK) Newtonian (f = 1200mm, F4.0), SW parracor type coma corrector, CEM70 Ioptron mount, Baader steeltrack focuser, Pegasus Cube2 focus controller, PHD2 guiding using an ASI 120 mm guide camera and 80 mm SW startravel refractor at f = 400 mm.
ZWO AS1294 MC OSC camera, 4.63 um pixels, 14 bit ADU at 0.8 arcsec/pixel. Frames taken on 240322 under moonless Bortle 6 skies. All frames pre-selected for quality using the FWHM and brightness filter within Sharpcap, darks and grayscale master flats (no bias) prepared using Sharpcap. Preprocessing, further frame selection (Subframe Selector selecting just 40% based on Stars, Ellipticity and FWHM) and processing (using CFA drizzle x1 integration which produced much superior colour than debayering) in PixInsight.
ZWO AS1294 MM mono camera, 2.15 um pixels, 12 bit ADU at 0.4 arcsec/pixel. Frames taken on 270222 and 26-270322 under moonless Bortle 6 skies. All frames pre-selected for quality using the FWHM and brightness filter within Sharpcap, darks and grayscale master flats (no bias) prepared using Sharpcap. Preprocessing, further frame selection (Subframe Selector selecting just 30% based on Stars, Ellipticity and FWHM) and processing then in PixInsight.
A is from 160421 and a total of 3h imaging under various skies using the AS1294 OSC and PDS200 telescope. The FWHM of the starting linear integration of debayered frames was 3.3. 30 min of HA was added to the red channel.
B is 167 x 40s exposures at gain 124 (near unity) using the VX12. The FWHM of the starting linear integration was ~ 2.2.
C is the same as B but with the RGB luminance replaced with the luminance from an integration(FWHM 2.0) of 5 x Sharpcap autostacks of 3s gain 200 monocamera frames totalling 1950 frames
D is the same as B but with the RGB luminance replaced with the luminance (FWHM 1.8) from an integration of 796x3s gain 200 of monocamera frames itself combined with the RGB luminance in a high dynamic range compilation (PI HDRC process). The effect of this is that the bright parts of the centre of the galaxy reflect the mono luminance and the fainter outer regions, the original RGB luminance.
I am pretty happy with how much more detailed the centre of M51 now appears - although the original 2021 PDS200 image was actually a much better image in terms of showing galaxy extent because it was partly taken under darker Bortle4 skies it is rather blurred at the centre. Handling a thousand or so individual fairly large short frames is laborious and just directly using the Sharpcap autostacks a much easier compromise.
