American Association of Variable Star Observers (AAVSO)
Sat, 02/11/2023 - 23:11


Running the risk of addressing an old topic (couldn't find this issue in past messages), may I ask that someone comment on the usefulness of J-C Ic filters?

I know that Rs and V are the most used ones. What about Ic?



Ic usefulness

Hi Ari,

The two most common J/C filters are B and V.  After that, I usually recommend Ic.

Most astrophysics relies on a star's color, and that is best represented by either (B-V) or (V-Ic).   Usually, (V-Ic) is a bit larger and easier to measure, and this is especially true when dealing with red stars like LPVs.  There, it is often difficult to get good signal to noise at B without very long exposures, while Ic is easy.

That said, while you often get more signal through the Ic filter, the sky is not as transparent at Ic.  That means the sky background is usually brighter, and the photometry usually has more error as cirrus-like transparency variations take place on minute timescales.  So expect a bit less accuracy at Ic than the star flux would suggest.

So if you only want to buy two filters, either B/V or V/Ic will work.  For three filters, I recommend B,V,Ic.  For five filters, I recommend B,V,SG,SR,SI as was done for APASS, as including those 3 Sloan filters will satisfy most professional astronomer's requests.


American Association of Variable Star Observers (AAVSO)
Ic filtering

     To add to Arne's comments:  in re star color-indices, one usually doesn't get anything from the R filter (i.e. the V-R color-index) that you do not get better from V-I, and the color-baseline leverage you get form V-I in terms of star temperatures (and changes) is quite a bit better than from V-R.

     Arne mentions the increased sky brightness and variability (from the night-airglow) at I.  But one wonders if folks observing at urban/suburban sites might well find that the sky is darker at I simply because of the reduced light-pollution --- or for that matter, when the Moon is bright.  Richard Schmidt has worked this angle observing from Georgetown in the middle of the Washington DC urban jungle.  The thing do, again, would be to use the same (portable) camera system in the city and again at a 'true dark' site and measure the difference.