Re: [APML] Help Needed -- ST-4 and Sky Background

[Aplanatic@aol.com]

Rick,

>  On pg 5 of the ST-4 manual, Table 1, they state the nominal value (0-99
>  range) of 60 for a 60 mm refractor given  a mag 5 star, 1 sec exposure.
>  
>  A 60 mm scope has 4.38 sq in aperature.  A 5 mag star is 100 times less
>  strong than a zero mag.
>  
>  Thus, for a 1 sq in aperature and zero mag star I get
>  
>  (60/100)*255*100/4.38=3493 counts,  vs 12000 counts.
>  
>  Can you point me to your reference?   I'm not trying to knit pick, just
>  understand.

I am happy that you are looking into this in more detail.  Your 
comments are most welcome.

I also noted this "inconsistency" in the SBIG manual.  Here's 
what I think about it:

When used as an autoguider, the ST-4 reports, on the display 
box, the maximum value seen over the array.  Because the light 
of the star is spread over several pixels, this maximum value will 
be significantly less than if all of the light was recorded on a single 
pixel.  In fact, the peak (maximum) to total (integrated) ratio should 
be pretty close to the discrepancy that you noted above 
(12000/3493 = 3.4).  In other words, the pixel recording the 
maximum value contains only about 1/3 of the total light from 
the star, the other light falls on the surrounding pixels.  In addition, 
there are some losses in the optics which must reduce the total 
(integrated) light from the star.  Perhaps 10% or so for a refractor.

When I image with the ST-4 through my concentric Schmidt 
Cassegrain, I see this effect quite clearly.  The integrated intensity 
of a star is always a lot higher than the maximum pixel value in 
the star.  Even a dim star is rarely, if ever, a single pixel.

So, we must take SBIG's calibration factor on face value, and not
use the emperical numbers stated in their Table 1, because these
numbers must be based, I think, on starlight being spread over 
several pixels in the array.  In light of this, I don't thing that there is
any serious inconsistency in the ST-4 manual.

Dave Rowe


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