Re: ATM TDE Mask

[Andrew.Martyn@dao.defence.gov.au]

Shane wrote:

>...What is the most direct way for me to "test" this optic to figure if my
hunch is right >(TDE) and determine at what radius it starts?  After that,
is there a proven masking >technique that works?.....

Shane, I also own a 10" f5.6 and have been doing a bit of star testing on
it as I recently got hold of Suiter's Star Testing book. Don't know if you
have heard of this book but it is pretty good. I can make sense of most of
the pictures but have not come to terms with some of the maths yet. Anyway
back to your problem, the most likely problems are either TDE or an
over/under corrected mirror.

To star test pick a really good seeing night and then check the the images
of about a 4rd mag star at a power above x250. You need to check both the
inside and outside focus images at a distance of about 10 wavethingys (or
what ever suiter calls them?) From my memory this means moving the focuser
in or out a distance of 1.4mm for a 10" f5.6. If there is a difference
between the images then you have a problem of some sort. I found that my
scope had a bright outer ring and dimmer inside rings when outside focus
and a bright inner ring when inside focus. From my understanding this is
either TDE or overcorrection. If the results were opposite, then you have
an undercorrected mirror. To check specifically for TDE rack the focuser in
and out 30 wavethingys and check the image. If there is a difference here,
it is most probably due to TDE as Suiter says correction problems are hard
to detect at this large distance from focus. I hope I got this right as I
don't have the book with me at the moment!

As for masks just experiment to see what works. Put different size masks on
just above the mirror and see if it makes any difference to the out of
focus star images and your images of Saturn, but try to do it all on one
night when the seeing remains the same.

Let me know how you got on as I'd be interested to know.

Andy
Australia