Re: ATM Cassegrainian coudé: weird image at focal plane

[Bernard Maugoust <maugoust@free.fr>]

Hi,

In message <34bf2f54b%maugoust@free.fr>, 
 Bernard Maugoust <maugoust@free.fr> wrote:

> 
> Hello all,
> 
> Before I post a detailed request which is bound to be longish,

[snip]

> Failing this, and depending on whether offers for help turn up, I'll send
> details.

Well, here they are:

The overall actual optical design is a classical cassegrainian.

For personal reasons, I've decided to fold the optical path with a tertiary
flat so that the beam comes out of the tube through the altitude axis of an
azimutal mount. Thus, the primary had not to be pierced, and better so, as
it's a Zerodur one.

Optical elememts in the play (all dimensions in mm):

- Primary, F 1075, D 204.5, home-made parabolic, tested with Foucault method
to my own satisfaction.

- Secondary, F 259.5, D 42.5, custom-made hyperbolic by an renowned french
optician (now defunct). Its radius of curvature has been tested with a
home-made spherometer and proved to the spec. Guaranteed to be 1/10 of a
wave, and according to the huperbolic factor needed in this particular
combination.

- Tertiary flat, minor axis 25.4, a quartz piece from Protostar
(http://www.fpi-protostar.com/)

- Eyepiece, a 22mm Leica, no less ;-) (working SO slowly gives time to save
up, I finished the primary in 1986!).

- Resulting focal length of the system: 5156

Mechanical elements:

- Primary cell and helical focuser from DAR Astro
(http://www.oneilphoto.on.ca/), very nice pieces of kit IMHO.

- Spiders for the secondary and tertiary from Protostar, 3 blades. Good
stuff too by all records.

- Tube is cardboard (http://www.essextubes.com/), very friendly people I'd
recommend to UK fellows.

- Mount is a roughly finished Dob, good enough for testing purpose.

- No baffling in place whatsoever. Given the design, I'd like to know
whether it can dramatically affect even such preliminary testing. Remember
I'm not yet fine-tuning or star-testing my collimation...

The combination in hand has been checked over and over on the paper to be
sure that I had not messed up and that the expected focal plane is where it's
been anticipated.

Every optical piece is reckoned to be fit for the job in its *own* right.

I've tested the primary at the Foucault rig, and on a quick assembly
together with the Protostar flat and the Leica eyepiece in a Newtonian setup:
the image is fine at the expected focal plane, real and OK.

The secondary was more difficult to assess on the field, so I changed the
distance between the primary and secondary, so as to move the resulting focal
plane more or less onto the primary. And without the tertiary in place, a
piece of paper taped on the primary showed the image of the full moon... It
had the right size for the resulting F/L of the system. again that was a real
image at the correct place.

It's when I put the tertiary in place that I stumble. Before I go on, I must
add that I have a Ritchey-Chrétien in the making as well (D 204, resulting
F/L 1719). Both mirrors are home-made, the secondary has passed my test, but
the primary (also in Zerodur) is not correctly figured, by a wide margin.
Even though, being so teased by the cassegrain not working, I devised a
*crude* setup to test the Ritchey elements. Worked! despite the sheer
crudeness of the secondary assembly (elastic bands!), the *rough* collimation
and the absence of baffling.

>From this I conclude that my calculations for those 2 telescope designs have
no fault, distances between all optical elements are OK. That the
cassegrainian hyperbolic secondary is not faulty (having shown a real image
of the moon on the primary), that the Protostar flat as a tertiary is OK, and
that proper collimation or baffling is not critical when all is wanted is
finding the location of the focal plane.

Now (at last...), to the core of the matter, what do I see that is *so* weird
when the 3 mirrors are assembled?

- 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.

- eye relief, needless to say, is just a word in the case. I've tested the
eyepiece separately on a Leica Televid refractor, and I can assure it is fine
;-)

- eye position is so critical that 'roll' is the best word I can find to
describe the effect, as if the focal plane was so curved to make me think of
a ball. I cannot guarantee my description is the most accurate though. Until
now I have not been able to bring in any competent fellow to 'share the
experience'.

It's been pointed out to me that a 'Mersenne' design produces interesting
side effects. But I don't think the specs I have detailed above match, by
accident, those a Mersenne scope.

Any help will be heartily welcome. My next step will to try and get some sort
of image recorded by a webcam, well if it can be done at all.

TIA,

Bernard

-- 
Bernard Maugoust 3°59'E 44°28'N