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Photometric I or R..Only one.

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pablotwa's picture
Photometric I or R..Only one.

Hi and thank you for reading this

I have room for one more photometric filter beyond the B,V and CBB (Astrodon Exoplanet) already installed which one is more useful the R or I?

Thank you!

Pablo Lewin

CTX's picture
Ic Maybe the best Option

From Arne's previous remarks on three filter options I gathered that the B-V and V-Ic are the two color indexes that most of the pros use.  They seem to give the most range as a function of the temperature of the targets while V-Rc has a much smaller range, which can effect your transformed data.  Also Ic is not as affected by dust and extinction as is Rc and a bit easier to use when observing Red Stars.  I think Arne only had about one advantage with Rc but can not remember what the circumstances were... just remember that he favored Ic over Rc if those were the choices... maybe he can chime in on when Rc may have an advantage.

Tim Crawford,CTX

Sequence Team

pablotwa's picture
Ir Ip camera in remote Observatory

Thanks Tim, my only concern with the I filter is that I have an IR IP camera in my remote observatory which I use to keep an eye on the position of the scope. The scope does have a substantial dew shield though.


Eric Dose
Eric Dose's picture
V-I does generally beat V-R

V-I does generally beat V-R as a color index, and I-filter data will generally give higher-value information than R-filter data if you're already measuring in V filters. But for red stars it's not so simple as "I is easier to use when observing red stars".

At minimum, some Miras have V-I = 7 to 8, which means you will often have more trouble getting reasonable exposure times in both V and I (not to mention B) than in both V and R. And it often requires more effort to get useful comp stars for both V and I for Mira stars on the same night--something to watch out for.

At maximum, some Miras outshine all other stars within a reasonable field of view by 5-8 magnitudes in I filter, so that CCD relative photometry in I simply isn't practical for that star on that night. In R there may be some chance of success.

I would prefer I filter to R filter too, and recommend it. But photometry on red stars through an I filter requires more careful planning than one might expect.

HQA's picture

Hi Pablo,

Many of the network webcams have IR illuminators for night vision.  The Foscam variety have commands to turn these illuminators on/off, so that you can remove their impact to any imaging.

The typical webcams use either 850nm (the Foscam) or 940nm LEDs.  The 850nm LEDs do fall within the bandpass of the Cousins Ic filter, and so it is important to control that light.  With the Foscams, I just shut off the illumination when not needed.  With other cameras, I rotate them so that they face away from the telescope when not needed.

(V-Ic) is very similar to (B-V) for most stars and so Ic is a good choice for your third filter.  There are many times when the object is too faint at B or even V, but visible at Ic.  That said, there are some circumstances when Rc is preferred.  For example, if you want to go as faint as possible, most CCD sensors have a peak in their efficiency in the Rc bandpass, so you get the maximum throughput.  If there is a question whether H-alpha emission is present, the Rc filter will tell you since H-alpha falls within its bandpass.  If you need to continuously watch your telescope with your IR illuminator, then Rc would be a better choice than ic.  However, Ic is preferred for novae as it doesn't contain any strong emission lines and gives you a better feeling for how the continuum of the nova is fading with time.

I think most professionals would recommend Ic over Rc if a choice had to be made.  Both are useful, so you won't go wrong getting either one.


pablotwa's picture
Thank you Arne and everybody who contributed to this thread.


I do have a Foscam and I can remotely turn off the IR camera light when I go to sleep I that’s the way to go then, I will get a Photometric I filter next then.

Thank you so much for your help!


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