Hi All,

For illustration, here is a photograph of the "minimum tilt Schiefspiegler":
http://bhs.broo.k12.wv.us/homepage/alumni/dstevick/lampert/Schieffr.jpg

The incoming bundle of light JUST passed over the secondary, which can be seen
in part on the left side of the open "tube". Close examination will reveal a
small patch of black cardboard pasted to the upper part of the entrance stop,
which acts as the effective system stop, to reduce washout.

Yes, it worked, but I changed that in a later version of the instrument.

The 2" clearance provided by Roger in his design is generous.

Regards,

Guntram

>
> Ed,
>
> yes, the "Dob" form is excellent when it comes to baffling and tilt angle. All
right here.
>
> You guessed it, it was a Schief, an anastigmatic one, to be precise.
>
> My goal had been to squeeze out the last ounce of performance, so I used the
smallest possible tilt angle.
>
> Baffling was complete only on axis, and in the part of the field closer to the
primary. The opposite part suffered from some sky flooding.
> Not a real problem at night, but a lesson learned the hard way. Avoid
extremes.
>
> Moreover, and this is a more subtle point, one can into trouble when tweaking
out aberrations caused by inaccurate construction of the OTA using the star
test.
>
> In some cases, this will require tilting the primary even less than the
blueprint called for. As a consequence, the secondary will slightly obscure the
primary at its periphery. This I observed one time, too.
> Although this will be of little significance in the image, it is nice to have
some breathing room available.
>
> Regarding Yolos, Swiss Yolo enthusiasts equip their instruments with front
baffle tubes of about 1.5x to 2x the primary diameter in length for
no-compromise baffling. I have uploaded a photograph of such an instrument,
which I could use for a few looks. The baffling looked excellent, no sky
flooding.
>
> Guntram
>
>
>
>
> > Guntram,
> > The Chief in the "Dob" form has both minimum tilt and excellent baffeling.
It is only when you want to do a longer fold does the issue of baffeling become
an issue. What sort of experience have you had? Schiefs?
> >
> > Ed
> >
> >
> > --- In spiderless@yahoogroups.com, "Guntram Lampert" <glampert@> wrote:
> > >
> > > Hi All,
> > >
> > > Just a few thoughts regarding stray light suppression.
> > >
> > > Having designed and built various TCTs over the years, my personal
experience is, that minimizing tilt angles has two sides.
> > >
> > > Small tilt angles are most welcome in simple designs that offer few
degrees of freedom for aberration control. Generally, field correction and focal
plane tilt benefits from reducing the tilt of the elements to the minimum.
> > >
> > > On the other hand, on can run into mechanical, thermal and optical trouble
if the tilts are reduced to the bare minimum.
> > > Designs that look good on the computer screen may later require
compromises in the mechanical execution of the cells and placement of the
baffles.
> > >
> > > I speak from experience.
> > >
> > > This has forced me to abandon the "minimize the tilt" mantra and use
designs that offer robust baffling without the need to take measures that
increase the instrument length or cause vignetting in the field. Even if the
spot diagrams look a trace worse. It goes without saying that optical quality is
not to be sacrificed for comfortable baffling.
> > >
> > > It is most satisfying to look into the focuser tube of a finished, well
baffled instrument and see nothing but darkness surrounding the entrance
aperture when the scope is aimed at the daytime sky, knowing that a prerequisite
for contrasty views has been met.
> > >
> > > So, whatever kind of telescope you decide to build out there, make sure
from the outset that proper baffling is possible!
> > >
> > >
> > > Regards,
> > >
> > > Guntram Lampert
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > > Hi, Ed.
> > > >
> > > > But if you increase the primary tilt a little so that there's some space
between the edge of the flat and the incoming beam, then complete baffling
becomes possible. My folded refractor had no stray light at all in the focal
plane. What happens to the aberrations from the extra tilt in the Chief, I
haven't investigated. Obviously the present arrangement would only be of
interest for those who want a slow beam (high f/ratio) but a compact form. My
folded refractor gave wonderful planetary and deepsky images, despite being
"just" an achromat. And no stray light. The trade-off is a smaller field since
the image scale is larger than in your basic Chief form. The basic form will
certainly be better for most people, most of the time. But for a few who want a
refractor-like OTA and large image scale, the folded-refractor-like form can
certainly be of value.
> > > >
> > > > Cheers,
> > > > Roger