On Feb 10,  5:43pm, Rui Malho wrote:
> Subject: Enhanced PMTs

> my first question. If the new and old PMTs are apparently equal (the RSI is
> virtually 1 meaning the photons/pixels are the same in new and old tubes)
> why the gain settings had to be more than duplicated to obtain the same
> signal ? Furthermore, and under exactly the same settings, "accumulate to
> peak" with new PMT1 needed 6 frames while old PMT1 needed only 2 frames. Why
> this difference ? Finally, if one has a pair of enhanced PMTs with a RSI=2
> (or more) can we really expect to obtain a similar signal if the gain
> settings are decreased by a similar factor ?
>
My comments are based on my experience with the MRC-1024, but I think that
these comments apply to your situation.

The gain setting controls the PMT voltage. Because of variation between PMTs,
you cannot make an absolute comparison of tube response as a function of gain.
The tube gain is also a nonlinear function of applied voltage, so you may have
similar ultimate sensitivity, even if at low gain one PMT needs a higher bias
voltage. Therefore, it does not follow that you can reduce the gain by a factor
of two with an RSI of 2. The real question is whether your signal to noise
ratio improved.

Concerning accumulate to peak, the number of frames required depends a lot on
the kind of noise you have. Are the images obtained after 2 passes with the old
PMT are as good as the images obtained after 6 passes with the new PMT?

--aryeh