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Filter confusion!


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Hello everyone

With the assistance of the forum I bought my wife her first scope for Christmas (Celestron Nexstar 130 SLT) with a Barlow, moon filter and Skywatcher LP filter. We have enjoyed some cracking views of the moon so far although the cold, gales, rain and snow have limited our chance to spend any real time with it yet.

We have been reading through Turn Left at Orion which is excellent, but it often recommends the use of a Nebula Filter for viewing DSOs. From trawling the forum I believe the Skywatcher UHC 1.25" filter is what I am after, although I am struggling to understand what the UHC filter will do that the Light Pollution filter does not.

Could someone please explain the difference and let me know if you think it is worth investing in the UHC filter?

Huge thanks in advance

Tom

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An LP filter simply tries to block the wavelengths which deliver the bulk of the light pollution. They aim to be transparent to the rest. Nebular filters aim to be transparent only to the wavelengths in which emission nebulae shine with their own light, while blocking all of the rest. So a nebular filter will block reflection nebulosity, for instance, when an LP filter won't.

How successfully nebular filters work depends largely on the object. Personally I use them very little but on some targets that 'very little' makes a lot of difference. The biggest beneficiary is the Veil Nebula. Don't expect them to perform miracles though. They can only enhance contrast, they cannot brighten any object. Indeed they invitably dim them a little but the idea is to dim the background sky by much more than the target.

Olly

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First a disclaimer:
I think the Skywatcher UHC is produced by Baader simply because I cannot locate the filter curve of a Skywatcher but can for a Baader.

The UHC seems to effectively cut out wavelengths from 520 to 650 nM

What they do in transmission terms is pass the Hydrogen beta, OIII doublet, Hydrogen alpha and Sulpher II.

These are the "common" wavelengths from Nebula.

However this means a large amount of light is blocked, so visually this are dimmed.

A Light Pollution filter is/was aimed at the Sodium lines from street lights. As these 2 lines fall in the blocked bit of a nebula filter they are blocked in both filters.

Slight warning is that there are several nebula filters and they are all subtly different.

I have  a link to filter curves and there are 7 UHC filters listed, all different, and Lumicom seems to make 3 of them that are different to each other. One Baader is not listed.

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In my (mostly) dodgy London skies, it's only thanks to the UHC filter that I made out the Dumbbell Nebula, and it does bring out detail in M42 (Orion Neb) and makes M57 (Ring Neb) easier to view...so based on these three I like the UHC.

Haven't tried for the Veil yet, but looking forward to it.

A UHC seems to work pretty well in my 6"/F5 and I would say (from a pretty limited experience-base it has to be said, and bearing in mid I've not much idea how good Keighley skies are, better than mine I'd guess) it's worth trying. 

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