Do I need a Light pollution filter for a CCD?

[Droogie 2001]

I am in the early stages of planning to buy a mono CCD.

One point I need to consider is whether I need a light pollution filter for the mono CCD. I live in an orange zone so I use a CLS filter with my modded Canon. From what I have read it is not as much as an issue as RGB filters tend to let less LP through anyway. However what about no filters and just taking mono? I do of course want to take RGB and Narrowband going forward but it did strike me as something I need to at least consider.

Thanks.

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[Physopto]

Any light pollution will affect your images. If you use a CLS now I would definitely say you will need one with a CCD. Longer exposures and it will record more of the light pollution.

Narrowband will most likely be OK without a CLS but RGB will need one possibly. It all depends upon the type of light, amount and proximity.

A CCD (not one shot colour) is mono anyway, so it will record all light wavelengths it is capable of  without a filter.  Some CCD cameras that are equipped with Fused Silica windows can record ultra violet and far into the infra red spectrum. But that only happens ( for ultra violet)  if there is no lenses in the scope, e.g. it is purely a mirror type.

Derek

[Droogie 2001]

Thanks Derek. So do these tend to be mounted within the imaging train as opposed to being mounted within a filter wheel?

I guess this is obvious but I need to ask.

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[Droogie 2001]

Upon further investigation it looks like it can be used as a luminance channel instead?

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[Physopto]

I used a CLS instead of a Luminance filter for several years. Just gets rid of the unwanted street light wavelengths. You do loose a bit of useful info but it would have been swamped anyway by the pollution. Some imagers use a larger CLS filter diameter in front of their other filters at all times. It is just left in the light train ahead of the CCD and filters.  The luminance filter is really only there to keep the light path equal whilst recording the clear signal for definition in your finished image. A Luminance  filter  does reject some unwanted IR frequencies, as apposed to a clear filter which does not.

A lot of filter manufacturers claim that their filters are AFOCAL  but as you will become aware here on SGL that is very much debated and disbelieved. To a great extent afocal abilities of filters are down to higher f ratio scopes rather than the lower ones, say below f 5 ish. I have never had a problem between filters but my scope is f7. If I went for a f4 scope I would expect to have to focus between every image for different filters.

[swag72]

I use a Hutech IDAS as my luminance filter - Of course with it being a different brand than the RGB and NB filters it;s not parfocal, but that's what auto focusers are for I reckon!

[Physopto]

I have heard  a bit about them and they are regarded as very good. Do you use it in the filter wheel, sounds like you do. I do know of one or two here on SGL who use one separately in the light train.

[johnrt]

I use an IDAS stacked in front of all my filters LRGB & Ha / OIII.

[swag72]

I have heard  a bit about them and they are regarded as very good. Do you use it in the filter wheel, sounds like you do. I do know of one or two here on SGL who use one separately in the light train.

Yes I use mine in the filter wheel instead of a luminance filter - Seems to work OK, but I don't very often use it

[Droogie 2001]

Okay so using one as a luminance sounds like a better option. But that does mean that other filters such as RGB with still suffer with the full effects of LP. Does luminance then help with the LP in post processing? I have the ability to place a filter in the imaging train but I seem to prefer the option to chose when to use it.

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[Physopto]

I am sure it will be a personal choice. You need to try both options. A  Hutech IDAS or LPS filter can be used  for both Luminance imaging or for use before all filters. As I said you will lose some data when using these filters. But in a light polluted area you will not notice the loss but will notice the gain in less light pollution noise. I use the word noise but it is a real signal just not one you want.

You should start reading all associated posts from the  past. There are numerous discussions about these filters here on SGL.

There is no substitute for research. You will learn more by reading past posts.

Derek

[Droogie 2001]

Thanks again Derek. It does look like a personal choice from what I have read so far. What I do know from experience in LPS filters is purchase the best one I can afford. I had a cheap Skywatcher visual LPS which I doubled up as a filter for imaging. Put it this way after one evening of using this I went out a purchased a dedicated Astronomik CLS filter. A lot of research to be done I think. Still better now than after I have purchased a bunch of filters which do suit my requirements. With this weather I think I will have plenty of time to carry out this research!

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