Stacking filters?

[TheThing]

Is there any benefit in stacking filters like UHC, CLR or ,CLS with an IR cut filter for imaging Saturn?  I'm about 15 miles from Central London so pretty poor skies normally.

[ollypenrice]

You could copy the passband graphs for both and put one above the other to see which wavelengths get through both.

Olly

[vlaiv]

No, yes, depends

Olly is right - if you want to get stacked passband graph - just "multiply" respective graphs. Treat graphs like 0-100% and multiply like numbers 0-1.

If both graphs are at 100% - resulting value is 1 x 1 = 1 = 100%

If both graphs are at 50% - resulting value is 0.5 x 0.5 = 0.25 = 25%

if one of the graphs is at 0% - resulting value is 0 x whatever_number = 0 = 0%  - combination blocks where either of two filters block (two or more).

In that sense, combination of filters is always more restrictive on light then either of individual filters. Color will be distorted and there will be a loss of light. So that is NO part.

However, using filters if you don't want to capture color can be beneficial to planetary imaging in several ways.

Especially good are narrowband filters for this purpose - but you must find a balance.

- Narrower the band of filter - less seeing disturbance there will be. Seeing acts differently at different wavelengths (see below) and when you narrow down wavelengths - you "choose" only one way seeing act. On the other side - narrower bandwidth means less light so longer exposure is needed and seeing affects you more because of this (it is harder to freeze the seeing with longer exposures)

- Wavelength of narrowband filter is also a balance. Detail that you can capture depends on wavelength of light. Longer the wavelength - less detail. You can literally improve resolution x2 if you use 400nm light instead of 800nm (detail depends linearly on wavelength used). On the other hand - atmosphere bends shorter wavelengths more then longer (think rainbow and prism thing:

blue light / shorter wavelengths are bent more then red light / longer wavelengths)

For this reason, seeing is much worse at shorter wavelengths as there is more distortion.

Again - trade off - will you go for longer wavelengths with better seeing but loose detail due to inability to capture detail at longer wavelengths or will you go for shorter wavelengths to be able to capture detail but risk atmosphere ruining it with seeing distortion.

Like I sad - tradeoff

[TheThing]

Wow, vlaiv you really know your stuff.  Thanks to you and olly for your responses.

I bought all my filters 2nd hand so I don't have the graphs to hand, but I'm going to hit Ecosia to find them ( and make the planet greener!)

I think the answer will be that there may be too much loss of detail for my liking, so it looks like I'm going down the one at a time route, but I will investigate further.

Much appreciated gents.

Edited September 4, 2023 by TheThing
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[vlaiv]

Well, UHC and CLS are not particularly well suited for planetary.

They both have similar response but CLS is broader then UHC. They both try to capture OIII and Ha/Hb lines

That is sort of counter productive for planetary as you have two almost opposing sides of spectrum together - so both short and long wavelengths - which makes it hard to optimize for either side.