RGB and IDAS filters

[AlistairW]

Hello,

Never tried RGB (Only narrowband so far). I have a filter wheel also. My question is, - I know you 'double up' filters so that you can have a Red and an IDAS light pollution filter, - but where do you place the IDAS filter. Do you screw it directly on to the RGB filters ? If the last question is 'yes', then do you need to add the IDAS filter each time you change the RGB filters (or have 3 IDAS filters) ?

(I am guessing that the space in the filter wheel can accommodate this 'doubling up') ?

Thanks

Alistair.

 

[ronin]

Only wavelengths that are common in transmission to both filters would get through, so unless the idas filter passes the same wavelength as the Red filter would anything get to the sensor.

Isn't RGB = Narrow band anyway ? Or at least the full band passed may be wider depending on the filter I suppose.

Not sure what it is you are expecting to accomplish, filters are not a good area to explain. People think of then as additive but they are subtractive. I would half expect on of 2 results:

The idas to block the wavelengths that the Red transmits, so nothing, or, if it doesn't then the use of the idas is irrelevant, since then the Red is blocking the non-Red wavelengths that the idas would pass and block similar LP ones.

Can you specify the filters in question as the transmission curves are likely to be required so give a better idea of any result.

[AlistairW]

Perhaps I have misunderstood :-) I was thinking a Baader Red filter for example, could be coupled to a IDAS light pollution filter. This combination would then combat the light pollution whist letting the Red through.

[Thalestris24]

Hi Alistair

I think the Baader red filter is relatively narrowband anyway. Whether you'd gain much in putting a lp filter in front will depend on 1) the particular nature/wavelengths of lp that you are subjected to and 2) the particular lp filter and which wavelengths it transmits or blocks in the region that the red filter passes. Best way to ascertain that is to overlay the lp filter and red filter transmission spectra.

Hth

Louise

[AlistairW]

Interesting. I have not looked a the transmission spectra - that is something I will do now. I think my (incorrect) assumption was that a LP filter would only help the situation for RGB (me suffering from a degree of light pollution) - (I know there is no need for Narrowband). So perhaps I will ask - when would you use an IDAS LP filter, - would that be for Luminescence ?

[Thalestris24]

2 minutes ago, AlistairW said:

Interesting. I have not looked a the transmission spectra - that is something I will do now. I think my (incorrect) assumption was that a LP filter would only help the situation for RGB (me suffering from a degree of light pollution) - (I know there is no need for Narrowband). So perhaps I will ask - when would you use an IDAS LP filter, - would that be for Luminescence ?

Yes, certainly for Lum and for whichever of r, g or b you might have a problem with. In fact, if you need an lp filter with any one of them then I guess you might as well use it with all of them. You could, for example, have a 2" lp filter permanently screwed to the front of your coma corrector/flattener/reducer or as a clip filter with a dslr. It shouldn't normally affect r,g, b or nb imaging. 

Louise

[AlistairW]

Thanks Louise. I guess if I screw an IDAS in to the FF/FR then I am going to have to correct backfocus. I shall have to take a look at that.

Alistair

[kirkster501]

A timely question.  I have been wondering this very thing.  I have the IDAS on the front nose-piece of the camera as it inserts into the focuser and in front of the filter wheel.  As you say, since the filters are narrow band anyway, could i remove the IDAS and gain more data for the same exposure length?  I am going to explore this possibility.  Additionally, they have gone LED lights around where I live anyway so not sure the IDAS is doing much...

[Davey-T]

Asked the same question a while ago and the general consensus was it wouldn't make any difference, I have an IDAS filter permanently screwed into the front of my flattener / reducer.

May affect something if it's in the light path for LRGB and not NB.

Dave

[sloz1664]

3 hours ago, kirkster501 said:

A timely question.  I have been wondering this very thing.  I have the IDAS on the front nose-piece of the camera as it inserts into the focuser and in front of the filter wheel.  As you say, since the filters are narrow band anyway, could i remove the IDAS and gain more data for the same exposure length?  I am going to explore this possibility.  Additionally, they have gone LED lights around where I live anyway so not sure the IDAS is doing much...

Sorry this is "off piste". Steve are the LED lights better for LP in R on T.

Steve

[kirkster501]

2 hours ago, sloz1664 said:

Sorry this is "off piste". Steve are the LED lights better for LP in R on T.

Steve

Yes I think they are Steve.  Of course that can't eliminate the sky glow from Nottingham but I do not image in that direction.

Steve

[johnrt]

Leaving the IDAS in place for the RGB makes no difference, nor does it for narrowband. I leave mine in with every filter, even Ha & OIII.

it is a godsend for luminance, meant I could double my exposure times before the curse of lp began to make things unworkable. 

[michaelmorris]

Adding a red filter in addition to an IDAS light pollution filter is in fact an accepted method of getting a 'poor man's Ha' filter when using a an astro-modified DSLR.

 

[kirkster501]

I'm a firm believer in the "If you don't need it, remove it" theory.....  More stuff to go wrong, more surface area for dust bunnies etc....