This is right out of the manual:

3. Inspecting the Optics: Note on the "Flashlight" Test
If a flashlight or other high-intensity light source is pointed down the
main telescope tube, you may at first be shocked at the appearance of the
optics. To the uninitiated, the view (depending on your line of sight and
the angle the light is coming from) may reveal what would appear to be
scratches, dark or bright spots, or just generally uneven coatings, giving
the appearance of poor surface quality. These effects are only seen when a
high intensity light is transmitted through lenses or reflected off the
mirrors, and can be seen on any high quality optical system, including the
giant research telescopes in use today. It should be pointed out, however,
that optical quality cannot be judged by this grossly misleading "test," but
through careful star testing. The Flashlight Test causes even the very best
optics to look "terrible."
As the high intensity light passes through the Schmidt corrector plate, most
(about 98%+) of it is transmitted through while the rest of the light
scatters through the glass. As the light hits the mirrored surfaces, most
(about 94%) of it is reflected back while the rest of it scatters across the
coatings. The total amount of scattered light will be significant, and its
effects allow you to see microscopic details that are normally invisible to
the unaided eye. These anomalous details are real, but their combined
effects will in no way impose limits on the optical performance, even under
the most demanding observing or imaging criteria.

David C. Johnston

-----Original Message-----
Bill Slemer
Sent: Wednesday, May 16, 2001 8:13 PM
Subject: Re: [M]: Stain on the primary mirror

On Wednesday 16 May 2001 10:32, you wrote:
> > David Gimenez wrote:
> >
> > I have found a strange stain covering most of the primary mirror surface
> > (75-80%) of my SCT showing a random and discontinuous shape. Using a
> > small flashlight the reflected light ...

> Sounds like a bad coating and ...

I seem to recall a telescope FAQ that referred to this as "the flashlight
test" and that the "discoloration" is normal. If I use a flashlight on mine
(an 8" lx200), I too see a "discoloration" across the primary mirror and
had I not read that particular FAQ I would have been concerned too.

Maybe someone else can confirm this? I'll see if I can locate the FAQ
and post quote if no one else responds.

--
Bill Slemer

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