Re: ATM Re: ATM Cassegrainian coudé: weird im age at focal plane

["Dwight K. Elvey" <dwightk.elvey@amd.com>]

Hi Bernard
 One thing you can try, take one of those mini-maglight type
flashlights. Remove the reflector so it is just the bulb.
Put this light where you think the focal point should be.
Work in a darkened room so you can see the beam on a wall,
about 10 or so feet away. You should see two spots on the
wall, one is from the light that passes by your secondary
because you don't have the baffle on it yet. The other would
be the beam from the primary. You should be able to move
the flashlight bulb so that the diameter of the beam from the
primary matches the diameter of the primary. This will be
the correct focal point for your system.
 Looking at it this way, you can also determine the effects
of the placement of your tertiary mirror, secondary baffling
and such.
 A problem using a magnifying glass is that the eye relief
distance is quite long and it is really hard to find the right
place to put you head and glass as you look for the image.
Later
Dwight


>From: "Bernard Maugoust" <maugoust@free.fr>
>
>In message <5.1.0.14.0.20030523100509.00ab9ec0@popd.ix.netcom.com>, 
> Michael Peck <mpeck1@ix.netcom.com> wrote:
>
>> 
>> At 10:26 23-05-03 +0200, Bernard Maugoust wrote:
>> 
>> 
>> >- with a simple magnifying glass acting as an eyepiece I cannot easily 
locate
>> >the focal plane, as one would expect in that sort of test.
>> >
>> >- a smoothed piece of glass would not 'stop' and show the image either,
>> >hinting at some abnormality with it.
>> >
>> >- the best way to describe what my eyes painstakingly saw is that the image
>> >seemed to be contained in a tiny area, like the ball of a pen on which my 
eye
>> >had to 'roll'. I was most often aiming at a convenient TV mast, of which I
>> >could see just as much as my anticipated field of view, that is 15' of arc.
>> >This apparently correctly sized field is worth noting IMO.
>> >
>
>> The first thing you should do is verify the spacing and alignment of all 
>> the components. Primary-secondary separation is especially critical.
>
>It has been checked. I knew the effect of the primary-secondary separation
>before I started.
>
>> Another possibility if you've only tested in daytime with no baffling is 
>> that you're just being overwhelmed by stray light. That might cause you to 
>> have just a small central area of the field where focussed light 
>> "outshines" stray light.
>
>This I have considered. When I next get on with another go at testing, I'll
>try on the full moon. Stray light while present should be less of a problem.
>I hope this will be part of the answer.
>
>> You should also verify that the tertiary isn't a source of vignetting. You 
>> should be able to see the whole secondary when you place your eye at the 
>> position of the focal plane.
>
>It's OK.
>
>> Field curvature is not going to cause the effect you describe and it 
>> doesn't sound like any other optical aberration.
>
>And that's what really baffles me. Many thanks for your suggestions Michael.
>The funny thing is that a similarly 'rough' setup for my Ritchey didn't
>produce the same visual effect.
>
>Bernard
>
>-- 
>Bernard Maugoust 3°59'E 44°28'N
>
>
>