[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
V-band vs. I-band
Jamie brings up the point:
> if there are star images in the I frame and not present in the V frame,
> which is the case in the frames i analyzed, then a lot of information is
> lost . . if the exposure in the V band is increased to compensate then you
> have more stars available for analysis, which, i think, is the raison
True enough. It will turn out, I believe, that if we use the
same exposure time in both V and I, the I-band camera will usually
detect more stars; there are many more red stars then blue ones.
So, what are the options?
1. take shorter exposure in I-band than V-band
This means that the I-band camera sits idle, gathering
no photons, for a portion of the night. Does this
really give us more information overall?
2. increase the V-band aperture
Um, can't do that without new lenses
3. increase the V-band chip sensitivity
Nope, would need a new chip
4. _decrease_ the I-band aperture or sensitivity
Easily done (with a small mask, or a neutral density filter),
but it again raises the question: do we want to throw away
We can make the number of stars detected in each camera more equal,
but only at the cost of detecting fewer stars in the I-band than
we would otherwise.
At the moment, if we use identical exposure times, I believe that the
ratio of stars detected in I-band to V-band is around 1.2 or 1.3;
perhaps Jamie can correct me. If the ratio were 2 (so twice as many
stars appeared in the I-band frames) or more, then I think we might
have to make some modification. But the current ratio is (I think)
close enough to 1 that it still makes sense to use the same exposure
times for each.
Just my opinion.