LP Filters - Rule of Thumb?

[FZ1]

I've been browsing some light pollution filter threads on the forum for some general pointers and came across this comment from a member in a 2013 post:

Put a light pollution filter over your eye and then look with both eyes are the light polluted area and you'll see it at its most effective. It does work (to my eyes). It turns orange haze into a softer blue haze.

The more you increase the magnification the lesser the effect. So at 1x it's good, at 30x its definitely working, at 60x its a bit 'meh' and from 70-80x upwards you probably shouldn't bother. That's my general experience anyway.

Does that "Rule of Thumb" still hold good? (Given that its 5 years ago and things might have moved on?)

Jon

 

[Ben the Ignorant]

Light pollution filters work the same at all powers because light hits them before it hits the eyepiece, so they do their job regardless. The thing is, surface brightness changes when magnification changes, but again, after light has passed the filter, the difference in surface brightness between two eyepieces is in the same ratio as without the filter.

I think the citation is from one particular person for that particular person, and doesn't apply to the majority.

Edit: for those not familiar with surface brightness, say the second eyepiece magnifies twice as much as the first, the image is twice as wide and twice as tall, so it's four times larger in area. Surface brightness is four times lower, and the presence of a filter does not change the math provided it's the same filter in both eyepieces, of course.

Lower surface brightness makes the sky dimmer, adding to the effect of the filter.

[SIDO]

I would say as magnification increases light diminishes so the the light pollution filter becomes more effective with more magnification.

Not mentioned a lot here but some eye pieces are better contrasted than others and eyepiece coatings and the amount of elements also can make a big difference, my old 20mm Meade superplossl from Japan is an awesome eyepiece when the moon is interfering.

[Ricochet]

I have an Astronomik CLS filter that, depending on the target, is useful at pretty much all magnifications I view DSOs at (i.e. >1mm exit pupil). It's probably the only CLS filter I would actually recommend, but beware that as with almost all CLS filters it is aimed at the old style sodium street lights, not new LED lights. The IDAS D2 is designed to cut down LED emission, but as it is a "photographic" filter I don't recall anyone posting a review of how it performs visually.

I also have a neodymium filter that is sold as a light pollution filter, but to my eyes it doesn't improve any DSO compared to the unfiltered view. It does, however, perform excellently as a lunar and planetary filter. 

[FZ1]

Excellent info, thanks all

[Ruud]

I found it. Interesting views in that thread.

 

[FZ1]

Yes, that's the one (sorry, I should have put up a link to it).

[FZ1]

Supplementary question!!

What's an O111 filter used for?  Is it primarily for imaging or visual?

Thanks,

Jon

 

[bomberbaz]

1 hour ago, FZ1 said:

Supplementary question!!

What's an O111 filter used for?  Is it primarily for imaging or visual?

Thanks,

Jon

 

Both is the simple answer.

For visual it improves detail and contrast on some nebula such as orion, the ring and crescent nebula.

More info in this link filter usage

[FZ1]

Thanks for the answer and the link. ?