Of the many diatribes in AP world one of the most recurrent is the role that "fast" optics plays in achieving a given result or more generally speaking whether being "fast" is better than being "large". And here is the comparison:
In one corner stands the champ, an 20" AG iDK running at around f/6.8 and sporting the best in prosumer CCD tech, a FLI PL16803. It is located in the mountains of the Namib (at ~1600m a.s.l.), with average seeing below 1.5" and with a sky the would melt the most hardened APer's heart, at B1 (I would spend hours just watching the whole-sky camera some nights!).
In the opposite corner is the challenger, a 2nd hand Nikon Nikkor 300mm AFS-ED (got it relatively cheap because some minor fungus) sporting a blazing fast f/2.8 optics and shooting from my dreadful backyard, with conspicuous light pollution (courtesy of the nearby industrial estate) to my S and W, with a zenith Bortle scale of B7 (18.35 m per square asec) on a good day and rather worse than that when the sky is affected by haze and thin cirrus clouds which is more often than not. It is also sitting at the vertiginous height of 23m a.s.l. The lens is fitted with a IMX571 OSC sensor and at least this last piece of hardware is fairly modern.
The object: Abell 31 (PK219+31.1), a large faint PN located in Cancer at Declination 8deg 48' N
The kitting: The iDK is running OIII and Ha Astrodon 6nm filters, equally split+ integrations of 600s in bin 2x2 while the Nikon is running an ALP-T 5nm DB (Ha and OIII) filter with 2/3 of the exposures of 180s and the remainder using 300s integrations.
The results:
The total integrated light for the iDK has been scaled to match the image scale of the Nikon and both framed in the same way (within few pixels). In both cases no processing was done other than standard CC and auto-stretching. The IMX571 image has been CFA drizzled for this presentation's purposes so it is somewhat of lower SNR than the un-drizzled counterpart (which is the one actually used for the final image).
And here is the relevant parameters' table to make the comparison both meaningful and worthwhile:
The last column gives the equivalent exposure of the Nikon normalized to that of the iDK and as can be seen is significantly lower than that. Unaccounted for in the calculations carried out in the table above is the relative effect of the altitude of the two locations, which isn't trivial but hard to work out in actual hard numbers. Also, the different climates isn't accounted for (as in air transparency /moisture) which is expected to be better (like a lot) in the dry scrublands of the Namib w.r.t to the "anything goes as long isn't really cloudy", which is the standard operating mode for my backyard observations.
The effect of the different Bortle classes (which might be contentious to some) has been carried assuming each increase in Bortle class carries a penalty of 12.5% more imaging time for same results when using NB filters (of reasonably close pass-bands). With 6 classes of difference (and that at the zenith folks!) this results in a factor of (1+ 0.125)^6 = 2.03. Also, the efficiency loss of using an OSC camera opposed to a monochromatic one has been assumed to be 0.9 across the board. Also, no accounting for the difference in QE between the two cameras was considered but the 16803 sensor is a top notch CCD sensor with very high overall QE, comparable to that of the IMX571 sensor. Also, the ALP-T filter isn't the High-Speed version but the standard version, with a drop of efficiency of around 20% in OIII band and somewhat less for the Ha band. This hasn't been accounted for in the calculations. In both cases the effect of the Moon and her phase is assumed to be trivial.
Passing to comparing the actual results four things seem to be clear, at least to me:
1. The iDK image has a much more pleasing background noise, very uniform without clustering while the Nikon's show some clustering which I think is a direct result of the poorer sky conditions globally.
2. The OIII signal is somewhat stronger in the iDK image w.r.t. the Nikon's (could be more than few things playing here, discounting exposure times, notably absorptions at the shorter wavelength)
3. The Ha signal is signally stronger for the Nikon that is for the iDK and while not shown clearly in the image, there is a very low brightness Ha area to the south of the PN which is picked by the Nikon but not in the iDK's.
4. Resolution is way better for the iDK, but that doesn't bring much joy in the Ha outer shell since the signal there is weak.
My conclusion: F ratio matters
P.S.: Probably more important: Do not waste money on remote telescope time in NB unless resolution is paramount.
In one corner stands the champ, an 20" AG iDK running at around f/6.8 and sporting the best in prosumer CCD tech, a FLI PL16803. It is located in the mountains of the Namib (at ~1600m a.s.l.), with average seeing below 1.5" and with a sky the would melt the most hardened APer's heart, at B1 (I would spend hours just watching the whole-sky camera some nights!).
In the opposite corner is the challenger, a 2nd hand Nikon Nikkor 300mm AFS-ED (got it relatively cheap because some minor fungus) sporting a blazing fast f/2.8 optics and shooting from my dreadful backyard, with conspicuous light pollution (courtesy of the nearby industrial estate) to my S and W, with a zenith Bortle scale of B7 (18.35 m per square asec) on a good day and rather worse than that when the sky is affected by haze and thin cirrus clouds which is more often than not. It is also sitting at the vertiginous height of 23m a.s.l. The lens is fitted with a IMX571 OSC sensor and at least this last piece of hardware is fairly modern.
The object: Abell 31 (PK219+31.1), a large faint PN located in Cancer at Declination 8deg 48' N
The kitting: The iDK is running OIII and Ha Astrodon 6nm filters, equally split+ integrations of 600s in bin 2x2 while the Nikon is running an ALP-T 5nm DB (Ha and OIII) filter with 2/3 of the exposures of 180s and the remainder using 300s integrations.
The results:
The total integrated light for the iDK has been scaled to match the image scale of the Nikon and both framed in the same way (within few pixels). In both cases no processing was done other than standard CC and auto-stretching. The IMX571 image has been CFA drizzled for this presentation's purposes so it is somewhat of lower SNR than the un-drizzled counterpart (which is the one actually used for the final image).
And here is the relevant parameters' table to make the comparison both meaningful and worthwhile:
The last column gives the equivalent exposure of the Nikon normalized to that of the iDK and as can be seen is significantly lower than that. Unaccounted for in the calculations carried out in the table above is the relative effect of the altitude of the two locations, which isn't trivial but hard to work out in actual hard numbers. Also, the different climates isn't accounted for (as in air transparency /moisture) which is expected to be better (like a lot) in the dry scrublands of the Namib w.r.t to the "anything goes as long isn't really cloudy", which is the standard operating mode for my backyard observations.
The effect of the different Bortle classes (which might be contentious to some) has been carried assuming each increase in Bortle class carries a penalty of 12.5% more imaging time for same results when using NB filters (of reasonably close pass-bands). With 6 classes of difference (and that at the zenith folks!) this results in a factor of (1+ 0.125)^6 = 2.03. Also, the efficiency loss of using an OSC camera opposed to a monochromatic one has been assumed to be 0.9 across the board. Also, no accounting for the difference in QE between the two cameras was considered but the 16803 sensor is a top notch CCD sensor with very high overall QE, comparable to that of the IMX571 sensor. Also, the ALP-T filter isn't the High-Speed version but the standard version, with a drop of efficiency of around 20% in OIII band and somewhat less for the Ha band. This hasn't been accounted for in the calculations. In both cases the effect of the Moon and her phase is assumed to be trivial.
Passing to comparing the actual results four things seem to be clear, at least to me:
1. The iDK image has a much more pleasing background noise, very uniform without clustering while the Nikon's show some clustering which I think is a direct result of the poorer sky conditions globally.
2. The OIII signal is somewhat stronger in the iDK image w.r.t. the Nikon's (could be more than few things playing here, discounting exposure times, notably absorptions at the shorter wavelength)
3. The Ha signal is signally stronger for the Nikon that is for the iDK and while not shown clearly in the image, there is a very low brightness Ha area to the south of the PN which is picked by the Nikon but not in the iDK's.
4. Resolution is way better for the iDK, but that doesn't bring much joy in the Ha outer shell since the signal there is weak.
My conclusion: F ratio matters
P.S.: Probably more important: Do not waste money on remote telescope time in NB unless resolution is paramount.
