Re: Ground Smog?

[Richard Miles <rmiles.btee@btinternet.com>]

Tom wrote:
> At this time of the year I notice that the V images of tom1 do not
> reduce very well.  Early morning the images look out of fucus even
> though I am pretty sure the lens is focused properly.  tom1 looks
> relatively close to the horizon and so sees more of the smog.  The I
> lens sees through the smog and gets good images.  Any one familiar with
> this?

Tom,

As I observe simultaneously in V and I filters, I too certainly benefit when 
working in 'I' closer to the horizon.  The main advantage is usually the 
lower extinction (e.g. 0.11 mag/atm cf. 0.20 mag/atm).  For example I have 
been able to image and get acceptable astrometry at an airmass of 25 or so 
(1.5 degrees above the geometric horizon using the I filter.  At this 
airmass, you lose about a factor of 10 more light in V than in I: hence the 
advantage of I.

However, I do notice that if there are any water clouds about then they show 
up more clearly (with more contrast) in the I-band image.  There are some 
strong H2O absorption bands in the NIR spectrum but there are also gaps 
between these so overall the I filter might win over the V filter even with 
fog or smog in the air.

Bets wishes,
Richard Miles