andrea tasselli:
Tim Hawkes:
*** Thanks Andrea. I agree and would do the same in clear and dark enough skies. Intuitively the broader filters should provide the truer picture. Id just love to know if there are spectral data on any of this. Tim
*I would doubt there is much or anything at all other than emission lines, as continuum signal is usually subtracted and I can't see what useful information can be extracted than wouldn't otherwise available through photometry. Some interesting spectra here: Nebulae spectra (astrosurf.com)
Obviously nothing beats doing the test yourself, try IC59 & IC63, or even NGC7380.
Thanks so much Andrea for that link. Yes indeed - the spectra therein make it clear that it is impossible to integrate broad underlying peaks and especially with such poor signal to noise so the spectra really can't be used to calculate what proportion of the total visible light (UHC bands) from the nebulae is HA + OIII narrow band and how much is broadband
I have tried now two simple photometry experiment/ calculations of my own using some data sets on IC5070, the Pelican Nebula and on the elephants trunk IC396 region. I took 3 sets of data from each using a UHC filter and a ASI294 camera and either a 7 nm HA filter or a 3.5 nm O3 filter with a ASI294MM camera. The UHC ASI294 MC integration was a CFA x1 drizzled integration image so there was no creation of false colour pixel information via Bayer deconvolution. The UHC x1 CFA drizzle integration - not corrected in any way - was extracted into its RED, BLUE and GREEN channels. All 5 images were aligned and then exactly the same patches of each image carefully selected to represent ca 5000 pixel patches of a) a dark region with no nebula in to define as the 'no nebula' background b) a first region of high nebula signal with no stars and c) a second patch of lower signal nebulosity also with no stars.
Then - using the PixInsight Statistics process - measurements were made of the signal and stddevs in the two homogenous (indicated by small values of Stdev) patches (signal - background) and compared across the the 5 images.
The idea was to quantify how much of the total 'nebula'- specific signal that is detected using the broader UHC filter (i.e within the two ca. 470-510 nm and 630-700 IR cut - windows) i.e. defined as nebula signal versus a null ackground region - can be accounted for as purely the narrow band signal contribution.
With the following assumptions..
1) sky quality/ signal was reasonably similar on the different nights on which the data were obtained (this only really affects SNR) -- measured at between 19.6 to 20.6 mags/ sq arcsec
2) H beta peak signal is 1/2.8 x the H alpha line
3) OIII 495.9 nm peak is 1/2.95 the primary line at 500.7 nm (the 3.5nm O3 filter only detected the primary line)
4) Relative overall quantum efficiencies of the B, R and G dye pixels plus UHC filter/ MC camera at 500 nm and at 656 nm and of the OIII and HA filters were about 50% and 60 % , respectively versus ~ 90% for the MM camera plus either cognate NB filters
and after normalising the data for
gain, exposure time, filter/ camera efficiency,, the H beta and 496.4 nm OIII lines only being detectable in the UHC filter but not the --- etc.
The overall conclusions were ...
1) the AS1294 MM camera/ narrow band filters appeared to detect ~ 1.5X (27.6/ 16.9 = 1.63; 59.52/ 41.76 = 1.42 ) more HII nebula-specific red photons than the OSC/ UHC system in unit time at equivalent gain. These data accord more or less perfectly with the 1.5X difference differences in overall QE. Thus more less all of the nebula specific red photons passing through the UHC filter in the 620 to 700 nm window can be accounted for by the single HA line. Thus no significant contribution to the nebula light in this region from either broadband light or other NB light (SII for example )
2) It was a different story with the blue green 470-510 nm band. Even calculating the H beta based on the HA NB detection and also allowing for the missing OIII band, the NB detection of OIII and HA was insufficient to account for all of the blue + green nebula specific light passing through the UHC filter . After allowing for the relative camera QE's the shortfall was about 40%
So - probably - and based on these initial results the blue/ green nebula-specific light does comprise a significant broad band contribution - likely from the scattered and reflected light from the nearby bright stars. It seems then that the ideal representation of type II nebula colour would include broadband and not just NB contributions ....However of course the practical difficulty would lie in finding dark enough skies and being able to image for long enough to get sufficient SNR and particularly using an OSC
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As a postscript ...I also wonder how much the overlapping colour response of the IMX sensor used in the OSC - i.e with as much of 10% of NB 656 nm red light being detected as green by green pixels ... inherently means that OSC images will render colour of emission n ebulae differently -- more orangy --than colour images constructed from mono NB and even RGB filters??
there are fun DIY options to build out there