LRGB imaging and LP filters

[lukebl]

Just starting out with RGB imaging and I'm sure this has been answered before, but I can't find it.

When taking RGB images on a mono CCD, is it OK or necessary to add an LP filter? I am using the basic Skywatcher one, and Baader filters which include IR cut.

[AndyUK]

(Ditto Luke - I'd like to know the answer to this one too... I only have an Ha filter at present!)

[Psychobilly]

I guess it depend on which LRGB set your using an in particular the transmission spectrum of the filters...

The cutt-offs of the R and green filters in the Astronomik type IIc set are such that the Sodium emission wavelength fall in the cutt-off reigon of both

http://www.gerdneumann.net/v2/english/eng_astronomik_lrgb_type2.html

With Narrow band then you should be fine...

[lukebl]

I know that narrowband probably won't need it. It's just I have the cheapo RGB filters which don't have the IR cut built-in, so I'll end up with three filters in a row (LP + IR cut + either R,G or . All that glass must cause artifacts.

[Psychobilly]

Have you got the transmission curves of the filters?

[johnrt]

I have seen lots of talk of imagers doing this, although I don't know which RGB filters they used. A CLS filter with ir cut then colour filter should be ok?

[AndyUK]

Luke - Please forgive me for a slight short-term hijack, but I think this is still on topic...

Peter - I'm in a slightly different position from Luke as I currently don't yet have LRGB filters at all, but when I do, I intend buying the "posh" Baader filters - The transmission curves are similar to the Astronomik set you've mentioned (ie a slight gap between R&G to cut out Na and Hg spectrum). So for RGB, "normal" LP wouldn appear to be taken care of...

However, what about when taking luminance? As I understand it (please correct as necessary!) the standard L filter is basically a UV/IR block, so L frame data content (and exposure times) would therefore be effected by the background LP(?), which would seem to imply that an LP filter would be necessary... so maybe, where LP is an issue, the L filter should be replaced with an LP filter(?)

(Or am I missing something obvious here...? )

[roundycat]

best to get filters with IR cut, at least it will delay your suicide attempt. There is a lot of IR out there and you basically do not want it. Have a look at the prices of Astrodon filters, around £2000 for a set of 2". It puts the cheap one in perspective. Something about a free lunch.

Luminance can be over-ridden by more colour. Instead of shooting hours of L which will, in the end, dilute your colour why not take twice as much RGB?

I have some light pollution but do not use LP filters. What gradients there are can be cured in Photoshop.

Dennis

[AndyUK]

Cheers again Dennis - I think it'll be a case of trying it out and then seeing what the effect is. I do like the logic of using more RGB's to supply the luminance though as the spectrum gap between R&G should hopefully strip it out nicely...

[Uranium235]

Here is the transmission curve for the Baader RGB filters, you can see there is a gap between red and green to account for some (but not all) emission lines of LP.

I havent used an LP filter for the past two months because of its affect on exposure times. But if youre using the non-IR blocked RGB filters then there is no choice but to get an additional UV/IR blocker... or do as suggested earlier and use the astronomik CCD CLS filter (which arent exactly cheap).