[ATM] More on aperature stops

[Mike Carambat <mcaramb@rocketmail.com>]

Thanks all for the help on my off-axis design.

Answers to the questions were:

1. Why do Herschelians point the eyepiece at the
return path, but Off-axis newtonians point the
eyepiece at the center of the parent mirror?

Turns out Herschelian scopes use long focal length,
tilted parabaloid mirrors, not a piece of a larger
parent parabaloid mirror like off-axis newts do. Thus
the eyepeice should always remain perpendicular to the
focal plane although tilting it to the optical center
of the returned path doesn't create enough aberations
to be noticeable if you need to to get your head out
of the way in some designs :)

2. Does aperature size and shape matter with aperature
stops?

Aperature size and shape matter in the same way spider
size and shape matters. It can still cause diffraction
artifacting. Simple circular aperature is best, as big
as possible without hitting the edge of the mirror or
including spider vanes or secondary. Ovals seem to do
equally well with no spiking, but you get oval shaped
stars in star tests instead of a nice round dot.

I just posted a picture of why I'm so interested in
the Herschelian (or off-axis newtonian) designs. While
playing around with the aperature stops (the easiest
way to make an off-axis telescope) I have become
completely sold on them and this is why:

http://www.designwizardry.com/aperaturedifference.jpg

NOTE: (These are photoshop artist renditions as I
can't take a photo through my newt since it's on a dob
and is a real bugger for astrophotography at high
magnifcations. But it's very accurate as to what I'm
really seeing)

I'm pretty sure that the image on left is pretty much
what most people see with a 10" f/4.7 newtonian at
7.5mm with questionable atmospheric conditions with no
filters. At least that's how I still explain the
brightness, blurriness, loss of detail and duplication
and ghosting of image. I see this in other scopes as
well at the local Observatory.

If anyone disagrees with me about my comments on the
image at left, please let me know. I've been told it
could be related to front surface thermals on the
mirror, but I'm not seeing the "boiling and roiling"
apparently typical of this in star tests. Just the
regular, slow moving, horizontal "shower glass"
wavering you see at high magnication through the
atmosphere.

Using an aperature stop really seems to work well,
which is why I'm building two scopes with a common
removable primary. One for deepsky viewing and one for
planetary off-axis viewing without need of a secondary
mirror :)

http://www.designwizardry.com/planetaryscope.jpg

Thanks again,

-Mike

Mike Carambat
Design Wizardry - http://www.designwizardry.com

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