LP filter in or out?

[Astronutjob]

Hello iv just recently started getting guided images, iv used a clip in lp filter when imaging, which cuts out a lot of the light pollution but means i need to expose longer for the same data. Do you think if i remove the lpf My images wil be too washed out to process, should i leave the filter out and process the images to remove the light pollution from them?? Was looking for a heads up on this incase i waste an imaging session?

[Stephen]

The LPF should only block the sodium/nasty wavelengths that you are trying to avoid, you shouldn't need to expose longer to get the same result should you?

[dph1nm]

The LPF should only block the sodium/nasty wavelengths that you are trying to avoid, you shouldn't need to expose longer to get the same result should you?

If only! Unfortunately LP filters knock out a fair amount of light from galaxies and stars (emission line objects are a different kettle of fish), so it is not entirely obvious whether you need a longer exposure or not to get the same s/n. I once had an LP filter which required at least 10x times the exposure!! Needless to say I no longer use it.

NigelM

[Martin-Devon]

I think Astronutjob has raised an important question here - many folks new to imaging (like myself) are uncertain whether to include a light pollution filter or not in the light train to the CCD. At the moment I have my standard Baader Neodymium light pollution filter that I use for visual work screwed into my flattner/reducer, so in front of my Atik 314 CCD, but I must admit I just don't know if this is of any benefit or not?

[RobH]

I have had a Hutech IDAS filter for a good while now, but only used it a few times due to the amount of extra exposure time needed.

These days I get rid of any LP during processing by sampling the exact colour of the LP, making a new image the same size as the original, filling it with the LP colour and then subtracting that colour from the original image. This way you keep your star colour accurate....if you simply equalise your background colour to make it the same in each colour channel, you'll end up getting dodgy star colours.

Rob.

[dph1nm]

This way you keep your star colour accurate

Although this is technically the correct procedure, doesn't it mess up saturated stars? They are initially white, but end up coloured when you subtract the LP colour.

NigelM

[bamus]

I am still testing whether LP filter gives you any advantage comparing to imaging without any filter, I guess this has got a lot to do how bad your LP is.

In my relatively bad LP in Southend area, I have noticed that I need to exposure twice as long with LP filter in order to get same amount of signal comparing to images without filter. Noise is greater as well but contrast is better too with LP filter. Pain is post processing, as my EOS clip filter is changing RGB balance a lot.

Need to do more testing, but as today I would say that I am disappointed with my filter performance.

[RobH]

Although this is technically the correct procedure, doesn't it mess up saturated stars? They are initially white, but end up coloured when you subtract the LP colour.

NigelM

Yes, but you should avoid saturating your stars. If you have a star that you can't avoid saturating, i.e. in the middle of a nebula, it's easy to select this individually and then correct the colour.

Rob.