5 replies to this topic

[dixiefla]

Which light pollution filters are best in terms of frequencies filtered out to get rid of scattered moonlight without having to revert to narrowband filters?

 

Maybe a dumb question, but I mostly have done mono imaging and of course everybody says just do narrowband during moonlight. I have found often the only useful filter to use during heavy moonlight is a narrowband Ha. However, one does not need a dozen nights of Ha for an image, and this was so limiting I often gave up imaging during heavy moonlight. Ideally, it would be better to be able to image some LRGB during heavy moonlight.

 

I recently tried an Optolong L-eNhance filter and it seemed to reduce scattered moonlight about 50% or more, while letting in LRGB. My targets on screen were clearly visible and the contrast was very good. This was great, as it allowed me to do "normal" imaging during a nearly full moon. (Its specs say it transmits ONLY Ha and OIII frequencies, but I found that it lets in LOTS of RGB also, while reducing moonlight). Perfect!

 

Are there better filters out there for this purpose?

 

And also what about the best filter for typical skyglow, which I understand is typically from blue wavelengths?  UV cut filters?

[imtl]

I'm assuming you are referring to imaging broadband objects with moon light. There are no filters to handle it. It's a broadband "LP". Just pick objects as far away from the moon as possible and extend your integration time a lot. 

[jonnybravo0311]

The filter doesn't let in lots of RGB. It lets the specified bandpasses through only - or you have a defective filter. If you're imaging mono, I assume then you're using the L-eNhance as your luminance filter, then the standard RGB filters. It's the R, G and B filters that are "letting in" the data.

 

Filters aren't magic. So-called light pollution filters are just whacking out swaths of the spectrum. They might have been marginally useful when local light pollution sources were mostly sodium/mercury vapor lamps. However, LED is now far more prevalent.

 

Typical luminance filters will allow wavelengths from about 400nm to 700nm pass (i.e. they are UV/IR cut filters). You can get ones that will block more of the blue - for example, the Astronomik L3 has tighter constraints than their L1. See the attached chart.

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[scanner97]

I recently tried an Optolong L-eNhance filter and it seemed to reduce scattered moonlight about 50% or more, while letting in LRGB. My targets on screen were clearly visible and the contrast was very good. This was great, as it allowed me to do "normal" imaging during a nearly full moon. (Its specs say it transmits ONLY Ha and OIII frequencies, but I found that it lets in LOTS of RGB also, while reducing moonlight). Perfect!

 

I would class the L-eNhance as a "broad, dual band filter".  The bandpasses are centered on Ha and OIII, but they are on the order of 15-35nm compared to 3-12nm for typical narrow band filters.  I imagine it would be a little weak in blue, since the lower bandpass is at OIII.  Something like the Antlia Triband has a third passband between about 425 and 450 so would offer stronger blue, but it seems to have even broader bands, so more of the moonlight will get through.  And you really don't need the extra band down there since you're still using your B I assume.

 

In the end, if the L-enhance works better as your L in moonlight, go for it!

 

And also what about the best filter for typical skyglow, which I understand is typically from blue wavelengths?  UV cut filters?

 

Skyglow typically refers to general LP, although many of the filters marketed that way are older designs that target the Na and Hg wavelengths common in older street lighting - which is not, in fact, blue.  A lot of the skyglow these days, including more recent street lamps is coming from LEDs, which don't have a specific frequency that can be blocked.   So it would probably be true to day that modern sky glow has more blue than it used to, but there's not really anything you can do about it.

Edited by scanner97, 22 May 2025 - 01:00 PM.

[Domdron]

You could try this technique, although it’s rather involved: https://youtu.be/e7v...Bf1pm65HxE2S83J

[dixiefla]

Thanks, guys. Lots of info there.