A quick question on LP filter without IR block

[Astroboffin]

Hi,

I have taken the plunge into CCD mono imaging, I have bought a full set of Astronomik LRGB and narrowband filters, I am also going to get the matching CLS LP filter.

My first question is that when doing narrowband am I correct in thinking that I will not need to use the LP filter, because of the narrow pass and of the filters in question not letting LP through anyway??

And secondly as my RGB filters have IR block built into them am I also right in thinking that I will not need the CCD version of the CLS filter, the one with IR cut, and will be ok with the cheaper one with out IR cut, and then when doing the Luminance images, I can just screw the L filter to the CLS LP filter for IR cut??

Thus saving the need for the extra £25 needed to buy the version with built in IR cut.

Appreciate any help

Regards

AB

[Psychobilly]

If the LRGB set are parafocal (it's still worth checking until you are happy they are) then  "Stacking"  the  L and CLS filters will requirer a focus shift...

Peter...

[Astroboffin]

Focus shift will not be a problem, I would do that anyway, my main concern is about the IR cut on the LP filter, do I or don't I need it??

Regards

AB

[riklaunim]

On some telescopes (like refractors) it would be good to have long wavelengths cut out when imaging on shorter For pure mirror designs this isn't a problem in general. On low altitude targets dispersion may also make it a bit worse without IR cut out.

[Astroboffin]

On some telescopes (like refractors) it would be good to have long wavelengths cut out when imaging on shorter For pure mirror designs this isn't a problem in general. On low altitude targets dispersion may also make it a bit worse without IR cut out.

Thanks for that, but maybe it's me, but don't see how it answers my two questions, I realise I need IR cut, but my questions are about whether I need it in the LRGB AND the LP filter.

Regards

AB