It’s actually quite hard to know where to start on this one. We’re at the beginning of a new imaging season and I thought I would take a good look at past data in an attempt to see what the best sub-exposures were for the various bits of kit, with and without duo-narrowband filters, from my Bortle 4.5 location. Whilst I’m in the middle of doing this I come across this YouTube video which I am only half-watching at the time https://www.youtube.com/watch?v=eXdv86sH6co
I was going to immediately reply to the bloke that I thought what he was saying was a pile of rubbish when something in the subconscious said don’t be so quick and check out what he’s saying against your own data, after all you’re looking at it right now for similar reasons. I am so glad I didn’t write him a snotty reply as of course he’s quite right and this has completely upset my plans for this season’s imaging. But at least they’ve been upset for the better.
So what does this guy say that I immediately poo-pooed as rubbish? He’s basically saying that narrowband filters are not magic nebula revealers and that the same, if not better data is present in the UV/IR cut filtered full RGB images. Sounds like nonsense at first, but it is very easily checked with data I already have, in spades.
Looking at the images above, the first one is a full RGB image (UV/IR cut filter) of the Crescent nebula region, 20 subs at 15-minutes per sub, and let’s be honest, the nebulosity is pretty crap. Now take a look at the next image taken with an Optolong dual-band L-Enhance filter 11 subs at 20 minutes per sub. Now that’s much more like it, clear nebulosity looking way better than the nebulosity in the full RGB image – or is it? Take a really close look at the filtered image. Does the nebulosity only appear to be better than the unfiltered image because all the detracting stars have been suppressed? Well that’s dead easy to check. Take the full RGB image and reduce the stars using Russ Croman’s StarXterminator with Noel Carboni’s Photoshop action which gives you a slider to control the amount of star reduction. The result of simple star reduction on the full RGB image is seen in the third image down, Oh-oh. What’s this? The brighter nebulosity actually appears a bit better in the star reduced image than it does in the narrowband filtered image. The narrowband filtered image better shows some of the faint nebulosity, but that’s simply because the star reducing process has taken out that faint nebulosity, and better processing on my part would have retained it. Oh dear, what do we make of this?
Well for me it has completely put the kaibosh on the plans I had for the kit over this season, and I need to think it all out again. Fortunately it changes nothing on my plans for the Hyperstar as it doesn’t use any narrowband filters.
I should point out that the filtered data was 11 subs at 20-minutes and the full RGB data was 20 subs at 15-minutes so that’s not like for like in that we have 220-minutes total imaging time versus 300-minutes, but I know from experience that the difference is not that hugely significant in this case where I am trying to prove there is no real advantage in using the duo-band filters. If pushed I would say that the slightly better bright nebulosity in the star reduced full RGB data might well be down to the longer exposure time, but if I had used similar exposure times for both I expect the narrowband filtered data to be no better than the full RGB data, and that’s the real point to be made here. What a turn up for the books.
Well, the final nail in the coffin, as far as I’m concerned, would be to take a full RGB image, with a long total exposure time, that I was unhappy with due to poor nebulosity, and knock the stars right back and see what happens. That is the purpose of the 4th image above. Here we have 32 subs at 20-minutes per sub on the IC410/405 region in Auriga. That is a huge amount of data totalling nearly 11 hours!!! My standard processing gave me a result I was very dissapointed in given the amount of time I had spent on the thing. But just now I really whacked the image with the star reducing app, and this is the result. I am much more happy with that, and although it could still do with another 18 subs (at least) for a very nice image, it has at least shown me the way forward.
Thank you Sky Story on YouTube for supplying a most amazing piece of astro-imaging work.
