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Greetings, I live in the very light polluted county of New Haven, Connecticut, just outside the city. My current imaging setup uses a Skywatcher ED80 Apo, 0.85x reducer/corrector, and an Orion Mono G4 CCD. I use an Orion Skyglow filter along with LRGB filters to help, and it's ok, but the background sky is still very bright and I'm still learning processing techniques. I still have yet to process a complete LRGB image, aside from a few test shots. (I've only had the camera for 3 months.) I'd like to get into narrowband imaging, but I have not yet purchased any filters. I had my eye on the $350 Orion set of 7nm filters, mostly due to cost, to get started with. I read some of the 3nm vs 7nm discussion in another thread here and it got me thinking. Aside from the cost of the 3nm filters, is there a drawback to using them with a small aperture? Is there a balance I need to achieve between aperture and speed in light polluted areas? I'd like something faster, but I'm worried it will brighten the sky even more. Any advice is appreciated. Mike 8) |
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Go with Optolong narrowbanding filters, i think it is slightly better a bit or same as Baader, but they have Ha in 7nm while OIII and SII with 6.5nm. Narrowbanding is different than LRGB, it won't give you a bright background unless you use very high gain and very long exposure, but sure you will set those properly to have good results, i live in a red zone light pollution and narrow banding is fine although not perfect or good enough yet, even with LRGB i was able once to have an image of Orion because it was higher in the sky. |
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Tareq Abdulla: Oh I know this, What I am worried about is whether my aperture is large enough for 3nm filters, or if I should stick with 7nm. |
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Michael Juliano: It is not about aperture if i know that, it is about focal ratio which is "F", the faster you are the better, and 3nm is much narrower not broader, which mean you should get brighter background with 7nm not 3nm, you just thought wrong about it. |
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one random thing is that with the 3nm filters, the Ha filter excludes the NII signal. there's not much there, but it's blocked. another thing is that very fast optics can be hard to use with super-narrow filters but i don't think your ed80 is in that territory. here's the slides from an old presentation that talks about this stuff: http://www.aicccd.com/archive/aic2007/Goldman-AIC2007Talk2.pdf rob |
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very fast optics can be hard to use with super-narrow filters but i don't think your ed80 is in that territory Interesting! That presentation was informative, though some was a little over my head...  I would have thought that faster=better for narrowband. I generally shoot at f/6.4 with my corrector/reducer attached, f/7.5 without it. So speedwise I'm probably ok w/ 7nm filters. I suppose I was worried that I would need extra-long exposures to narrowband with the ED80, and I was worried about bright background that inevitably happens with longer exposures. |
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Michael Juliano:very fast optics can be hard to use with super-narrow filters but i don't think your ed80 is in that territoryInteresting! That presentation was informative, though some was a little over my head... well there's fast and then there's fast - things probably get hairy down around f/3 with very narrow filters. how long your exposures need to be is really about which camera you are using. for me, with 5nm filters, i do 30 minute narrowband exposures with a 8300M CCD and f/5.5 telescope. i think if you are using one of these really low read-noise CMOS cameras you can do much shorter exposures (and then just do a whole boatload of them.) rob |
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| Below f/3, the narrower the bandpass, the more 'choked off' the light cone gets as the center wavelength gets shifted. But this is only an issue if you're using a very fast camera lens, RASA, Hyperstar or the 180 Takahashi Epsilon. Baader makes some 'high speed' narrowband filters down to f/2.0, but they won't specify the bandpass. I've heard rumors that they're just regular 12 nm narrowband filters being sold at a premium (no evidence though). You'll be fine with your setup using anything from 3 to 12 nm - just make sure you get a set that has good reviews. Lower quality narrowband filters (like the ones I have) can leave halos around brighter stars. |
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| I'm in a red zone too and I haven't gone narrowband yet, I'm using hutech lpr filters. I've found I can often do post processing reduction of green and drop the light pollution effect greatly for most nebulae. Galaxies simply need dark skies in my experience. I'd give it a try, I've had some success with it without going to narrowband. If you're using a DSLR, bump up the ISO setting and try reducing green later. It takes some experimentation but I have an M57 shot that looks like it was from dark skies. |
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you can substitute long integration times for dark skies. takes a lot of patience and processing out the LP gradients is a serious PITA but it can be done. https://www.astrobin.com/full/268641/0/ that's M33 from a red zone... 34h total exposure time |
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I second what pfile said. I image from a white zone about 9 miles west of center city Philadelphia. As long as I'm willing to dump 20+ hours into a galaxy, I've gotten some good results (at least results I'm happy with). 24.5 hours at f/3.9 on IC 342: https://www.astrobin.com/375415/ 43.3 at f/8 on NGC 3718: https://www.astrobin.com/338207/ 21 hours at f/3.9 on M33: https://www.astrobin.com/314175/ I've made some passable broadband images with 9.5 to 16 hours, but from now on, I plan on always doing at least 20 hours (at f/3.9) under my skies, since the results are so much better. |
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| Thanks for the suggestions I certainly haven't put that kind of time into the images it sounds like it's worth a try. I've been restricting my galaxy imaging to summer star parties at dark sites but most of the year I use a POD in my yard so I'd like to be able to leverage that more. |
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Here's an image I recently took with my Starfield Gear 80ED using Baader 3.5/4/4 nm filters. I'm in a Bortle 6-7 suburban location. Some of these subs were taken through some wildfire smoke haze. I moved from 7nm filters to the more narrow band filters and am very glad I did - they permit 5 minute subs for each of the filters. The Baader filters are nice - the filter material is very evenly applied in my samples, as shown in the flats.  |
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Here is my 2 cents worth. I have imaged from a Bortle 9 (Red) zone for the past 8-9 years. I used to have 7nm filters, I moved to the current setup with 5nm filters. I am looking to move to 3nm filters in the near future. Narrowband imaging from light polluted areas is much easier than LRGB. With LRGB under my skies, mu subs are typically 15-30 seconds, since the light pollution will swamp my image. That means LOTS of subs to process. With narrowband, I typically have 5 minute subs. I could go 10 minutes with my setup, but I prefer to take more subs, instead of fewer long ones. The main being that I live near an airport and I have invariably to deal with airplane lights. I routinely image with a f/8 and f/10 scopes with no issues at all. As a matter of fact, with LRGB, a fast scope will produce subs that are unreasonably short - in the order of 10 seconds or lower. I have an f/4 newt that I never use for LRGB for that very reason. My integration times right now are a minimum of 10 hours of data combined for Ha / OIII/ SII. So, yes, it can be done fairly easily. |
