Imaging with multiple filters.

[jacko61]

I've had a dig through the search buttons but couldn't find a specific answer to this one so...   Is it possible to combine multiple filters  during a single imaging plan e.g. UV/IR and a light pollution filter.  I realise it is 'possible' as they'll just screw together and then into the  reducer, I guess I mean advisable, or will this give strange results? Scope will be a WO ZS73 with FLAT73R and camera is a ZWO ASI533MC pro.

Graeme

 

[AstroMuni]

8 minutes ago, jacko61 said:

Is it possible to combine multiple filters  during a single imaging plan e.g. UV/IR and a light pollution filter. 

It should be possible to combine some, but not all. So your question needs to be a bit more specific I guess. I am sure someone who uses filters extensively can give you a comprehensive answer

[wimvb]

You have to look at filters as light blocking devices. Whatever light a filter blocks, will never make it to the sensor. But also, if you stack filters, you risk introducing reflections. Personally I would avoid it if possible.

Edited September 30, 2021 by wimvb

[DaveS]

If you are using a colour camera then i would use either a UV/IR cut if your LP is low, or else a LP filter, but see little point in combining the two. If you were running LRGB with a mono camera and your LP is poor then it might be worth putting a LP filter ahead in the train.

[wimvb]

… or skip the luminance and only collect RGB.

[vlaiv]

Simply put - take both filters response curve graphs and multiply them together and that will be resulting filter response curve.

This means that some combinations of filters simply won't work. Take for example - blue filter stacked with Ha filter - you'll get complete block over whole spectrum as blue passes only 400-500nm while Ha passes around 656nm.

Some combinations will work like only single filter - take UHC type filter that passes both OIII and Ha and stack it with Ha filter for example - result will be the same as using Ha filter alone.

Sometimes combination of filters makes sense - you can take for example Astronomik L3 filter and combine it with R, and B filters to remove unfocused parts of spectrum if you have residual CA (say fast FPL51 doublet scope or something like that).

As already pointed above - there will be increased possibility of reflection artifacts with interference filters.

18 minutes ago, wimvb said:

… or skip the luminance and only collect RGB.

Or skip B and collect only LRG

[jacko61]

Thanks folks. Some useful things to think about. The point about internal reflections puts me off the idea straightaway  The question was mostly brought about after I did some imaging of M31 a few weeks ago. WO ZS73 plus flat73R and ZWO ASI533MC pro.  I had my UV/IR filter in place as that's recommended for CMOS cameras as standard I believe. I'm in a bortle 4 zone but the night I was doing my imaging, when I stretched the stacked files the dark sky lights up just as much as the galaxy. From recollection there may have been some very light transparent cloud cover that's added to the light being captured. If I ever get another cloudless night I'll have another go just with the UV/IR cut again and see what results I get.  ( since the start of september, my little slice of scotland has had 2 cloudless nights and I've been working for both of them  )

Graeme

 

 

[vlaiv]

5 minutes ago, jacko61 said:

I'm in a bortle 4 zone but the night I was doing my imaging, when I stretched the stacked files the dark sky lights up just as much as the galaxy

That is sort of "normal".

Difference between signal in galaxy and sky signal (even darkest skies have some signal to them) is rather small. Point of stretching is to differentiate the two, but it can easily happen that simple stretch will not do that.

[jacko61]

Thanks Vlaiv. I still have a huge amount to learn about post-capture processing. Currently I'm doing it all in GIMP but there seem to be many other options. I might try the free download of startools on the data I already have to see if it makes a difference.

graeme 

[DaveS]

If you have Bortle 4 skies then you may not need a LP filter. My skies now are the dark end of 4 nearly Bortle 3 and I don't need one. I did though when I still lived in West London under the horrid orange glow.