M97 and the perils of wasting narrowband time
Posted: Sun Mar 07, 2021 12:32 am
Not a great image below but thought it instructive to post? As mainly just very hot plasma emitting only at the narrow wavelengths corresponding to the fusion products of dying stars planetary nebula must be the most ideal of targets for narrow band and without the need for any RGB since there isn't a lot else creating light.
But I should have read Oopfan's December 2020 thread on "A scientific approach to narrowband imaging" - which I recommend - under the Tips forum on this website first. I didn't realize just how much the owl, for example, is dominated by just a single green OIII emission line.
Here collected 90 min of OIII alongside 90 min of H alpha (at a slightly lower gain) only to later understand during processing that it would have been better to have collected perhaps only 10 minutes or so of OIII and spent the rest of the time (or ideally more time) with the HA filter and at a higher gain. The problem is that the HA (and NII - which probably predominates ?) signal is relatively very weak and so when you stretch it up to combine proportionately with the OIII it just ends up adding noise back into the combined image. I have been having the same problem but the other way around with emission nebulae where most times the OIII signal is relatively very weak and to avoid having the OIII adding in noise and stray light artifacts takes an awful lot of high quality lights - not achieved yet by me. I suppose it could be argued that beefing up the weak signals in this way creates misleading images --- but I think that is OK if it is understood that for example the red fringe that we are used to seeing on for example M97 is just a qualitative way of mapping where the NII and HII are located.
A pleasing point about the image below is that while it looks nothing like an owl it does faintly show the outer as well as inner shells of the nebula.
8 inch Skywatcher Newtonian, Baader flattener, F 5.0, CEM 70 Ioptron mount, PHD2 guiding via ASI 120MM camera and 9 x 50 finder
Optolong 7 nm HA filter, Astronomik 6 nm OIII filter
ZWO ASI 294 MM PRO
28 x 3 min OIII lights at gain 151 to B and G channels and 45 x 2 min HA lights gain 124 to the R channel, preprocessing and processing in PixInsight using Pixmath to combine the aligned images and then MLT smoothing, HDRM and curves to process and stretch
TimH
But I should have read Oopfan's December 2020 thread on "A scientific approach to narrowband imaging" - which I recommend - under the Tips forum on this website first. I didn't realize just how much the owl, for example, is dominated by just a single green OIII emission line.
Here collected 90 min of OIII alongside 90 min of H alpha (at a slightly lower gain) only to later understand during processing that it would have been better to have collected perhaps only 10 minutes or so of OIII and spent the rest of the time (or ideally more time) with the HA filter and at a higher gain. The problem is that the HA (and NII - which probably predominates ?) signal is relatively very weak and so when you stretch it up to combine proportionately with the OIII it just ends up adding noise back into the combined image. I have been having the same problem but the other way around with emission nebulae where most times the OIII signal is relatively very weak and to avoid having the OIII adding in noise and stray light artifacts takes an awful lot of high quality lights - not achieved yet by me. I suppose it could be argued that beefing up the weak signals in this way creates misleading images --- but I think that is OK if it is understood that for example the red fringe that we are used to seeing on for example M97 is just a qualitative way of mapping where the NII and HII are located.
A pleasing point about the image below is that while it looks nothing like an owl it does faintly show the outer as well as inner shells of the nebula.
8 inch Skywatcher Newtonian, Baader flattener, F 5.0, CEM 70 Ioptron mount, PHD2 guiding via ASI 120MM camera and 9 x 50 finder
Optolong 7 nm HA filter, Astronomik 6 nm OIII filter
ZWO ASI 294 MM PRO
28 x 3 min OIII lights at gain 151 to B and G channels and 45 x 2 min HA lights gain 124 to the R channel, preprocessing and processing in PixInsight using Pixmath to combine the aligned images and then MLT smoothing, HDRM and curves to process and stretch
TimH
