Decision to use Light Pollution Filter

[jtheios]

I've been thinking about using a light pollution filter (perhaps the Astronomik or LDAS LPR?) in my imaging train and was hoping for some input from those who might be using one. My primary concern is about the difficulty of creating the proper color balance. In looking at the transmission curves for these filters, they seem to block nearly the entire spectrum for my green filter (I use Astronomik LRGB filters). I'm wondering if I'd need to take significantly more green images, than the other colors? At the moment I do a simple ChannelCombination as the first step in my work flow after calibration, but I'd be willing to change if necessary. BTW, I've pretty much given up taking Luminance frames in favor of just RGB, since I have a fixed observatory in my backyard.

I live in a suburban area of No. California that's in the "orange" section of the ClearDarkSky light pollution charts (bordering on yellow), and I've calculated my optimum sub-exposure at about 5 minutes (for the area with the darkest skies, which for me is east). I suppose this is okay, but I'd certainly like to go deeper with longer exposures (and have better results when imaging to the south and west which is the direction of light dome of the town I live in)-- but maybe not at the expense of a more complicated work flow. I can certainly just image the same object for a number of nights to make up for the lack of dark skies-- but obviously there is a limit to what I can accomplish with my location and gear.

Any thoughts would be much appreciated. Thanks.

John

[Nocturnal]

Hi John,

learn to use the Dynamic Background Extraction (DBE), Background Neutralization and Color Calibration processes. You'll never worry about color balance again.

[pfile]

im in the oakland/berkeley area and almost certainly the LP is worse here. i use a CLS filter; if you look at a sub or integrated frame from the CLS, there is a strong blue cast. however, just letting the STF autohistogram run on independent channels is enough to restore mostly the correct color balance if you just want to view a sub while imaging. sander is totally correct; since i've been using pixinsight i just do not worry at all about color balancing since i am using the tools he mentions.

the only thing i can say is this - the CLS filter definitely attenuates some blue wavelengths and any DSO with lots of blue (M45, veil nebula) has the blue shifted more toward teal.

[jtheios]

Thanks for the input from both of you. It certainly helps. I think I'll give it a shot.

I must say, I'm really loving PixInsight. What a fantastic software package.

[RobF2]

It makes life tougher and requires a bit more colour cal, but I regularly use my IDAS LPS from the 'burbs and can get quite decent data.  I'd go insane if I had to wait for dark sky trips just to get some data.  Definitely a useful purchase for city dwellers.  Many people nowadays suggest a Ha filter for mono work first if your camera is sensitive enough in that region (but I like to be able collect RGB data when I want to too).

[RBA]

I live in a suburban area of No. California [...] but I'd certainly like to go deeper with longer exposures

If you're not too far from the Bay Area, join TAC (observers.org) and keep an eye on where people are going during New Moon periods.
There's plenty of decent dark sites in Northern California, from moderately dark to brutally dark.
Of course that'd mean making your setup portable, and if it isn't now, it takes some effort and money, but it's SO worth it IMHO.
Better yet, join the imaging mailing list over there.

[RBA]

im in the oakland/berkeley area [...]

So you're also hiding in your backyard?
Ay ay ay....

PS: I'm in Sunnyvale - imaging from home is not an option.

[Nigel Ball]

John

I have used both the CLS and IDAS LPS filters. Of the two I prefer the Hutech IDAS by a mile.

I have recently installed it in my imaging train and have achieved some decent results

Nigel

[pfile]

im in the oakland/berkeley area [...]

So you're also hiding in your backyard?
Ay ay ay....

PS: I'm in Sunnyvale - imaging from home is not an option.

it's worse - i'm hiding on the balcony outside my bedroom, between the roof and a tree. it's true that my time would be better spent going to a dark site, but boy am i lazy. flipside though is 10+hr integrations to get anything reasonable and the LP gradients are ridiculous.

i'll have to look into the IDAS filter.

[martin_magnan]

Hi

I'm currently using a IDAS-LPS filter like this one http://www.sciencecenter.net/hutech/idas/mfa/mfa.pdf

I'm in a moderate to highly polluted area (orange zone here http://cleardarksky.com/lp/SThmsCxtn1QClp.html?Mn=CCD)

I find that when I take long exposures (10 minutes) the filter is producing a strong vignetting in the green layer (unfiltered Canon Xsi). I've ruled out flat problem because the red and blue layers shows no vignetting. Since the filter is cutting light in the green wavelength. I think differential thickness is causing this.

The filter is very good and selective but long exposures need to be precessed intensively with DBE or cropped.

MArtin

[jtheios]

Martin,
Interesting. However, by looking at your image I see that you have a much larger FOV than I do, so I don't think I'd see the vignetting you are. I'm using an HX916 with a 770mm Mak-Newt, so I have a fairly small FOV compared to many others.

John

[jtheios]

If you're not too far from the Bay Area, join TAC (observers.org) and keep an eye on where people are going during New Moon periods.
There's plenty of decent dark sites in Northern California, from moderately dark to brutally dark.

Thanks for the tip on TAC, I'll check it out. I could be portable with a little effort--mostly I'd just need some sort of larger portable power system. I live in Napa, but would willing to drive a reasonable distance to get somewhere much darker.

So you're also hiding in your backyard? Wink
Ay ay ay....

Yes, that's what my wife tells me  and threatens to kick me out into my "astro-shed" if i misbehave! She also doesn't know my astro budget! And I better keep it that way. Little did she know what would happen when she told my daughter and I (who's been my partner-in-crime in amateur astronomy and is now a freshman at UCLA majoring in Astrophysics) to stop spending so much money on Global-Rent-A-Scope...

[RBA]

Thanks for the tip on TAC, I'll check it out. I could be portable with a little effort--mostly I'd just need some sort of larger portable power system. I live in Napa, but would willing to drive a reasonable distance to get somewhere much darker.

Ah ok. I'm i the South Bay so the sites I usually go are, well.. south of the bay   

A site I've never been but some folks in the North Bay go quite often is Lake Sonoma.
Anyway, join the TAC mailing list if you like (or the TAC-Imaging list) and check there... Lots of folks happy to help!