Re: [ATM] Hartmann Mask

["Bob May" <rmay@nethere.com>]

When you use a Hartmann mask, you are finding the best focus
point for that particular zone (as identified by the mask holes)
on your mirror.  This isn't necessarily the best place to be
measuring the focal length!
When you focus in reality with the whole mirror as the source of
the light, you are averaging the l9ight into the smallest bundle
that you can.  This can be something wildly different with a
typical mirror by one of the commercial makeers.
Note that the site does say that the focus for a PERFECT mirror
will be the same for the Hartmann mask and the whole scope.  The
thing here is that you can't get a perfect mirror, only something
that approximates one to a certain value.
As a result, you're going to be setting the focus with the
Hartmann Mask to something that the whole mirror isn't at, as an
average value.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Mircea Pteancu <mirceapte@yahoo.com>
To: <atm@atmlist.net>
Sent: Sunday, April 27, 2008 10:53 AM
Subject: [ATM] Hartmann Mask


> Jason Hissong's astronomical pictures are beautifull ,proving
he is giving a good use to his equipment. Reading Jason's
article about making a Hartmann Mask ,some question arise to my
mind.
>   I learn from a site that I love
>
>   http://www.oldham-optical.co.uk/
>   at ''Optical standards'' : ''Minimum spot size and minimum
Wavefront error are actually mutually exclusive''.This is
supported by my experience.For years I was focusing the scope on
a close double star but than I had to refocus for Moon and
planets.My questions are :
>   -Is this peculiar to my eyes or others have the same
experience ?
>   -If the Oldham statement is right,and I believe it is ,why
use a Hartmann mask?
>   -How can we find the focus ensuring the scope is operating at
minimum wavefront error?Focusing on stars it is a rough
focusing,in my understanding now,the focus position showing the
best planetary details is closest to the optimum.But if ,at the
moment of observation no planets are visible?''Observe more
,worry less'' ?Mircea
>
>
> ---------------------------------
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