Re: [ATM] More on aperature stops

["Mel Bartels" <mbartels@bbastrodesigns.com>]

>I saw the same effect (though not as bad) looking at Mars, and concluded
that my scope needs some more anti-scatter measures. I would have expected
the same effect to happen when watching the moon, but the terminator and
edge appear razorsharp, using exactly the same setup.
This puzzles me, what else than scatter could cause the halo? I would expect
the scatter to be proportional to the amount of light, so no non-linearity
issue. Maybe it is the decreased eyepupil due to the greater intensity of
the moonlight?


You're confusing a number of factors

1. Time of observations: maybe the scope had reached some degree of
equilibrium by the lunar observation

2. Magnification: maybe the lunar observation was too low of power to show
the effect

3. Eyepiece: same, different?

4. Position in FOV: if moon positioned to cover the thermal smear effect,
then it won't be seen

5. Angle in FOV: if moon rotated to cover the thermal smear effect, then it
won't be seen

6. Total brightness: lunar limb across its length has more total brightness,
and might obscure the thermal smear effect (better to use Venus to compare)

7. Different positioning angle in the sky: if thermal effect caused locally,
aiming in other direction may entirely avoid it

8.9.10...et al  Try to control the factors, otherwise you'll never figure it
out, or worse, attribute it to the wrong reason(s)

Mel Bartels


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