1. ## SCT design

Just a question I've wondered about recently. While browsing .66X
reducers for SCTs,
I keep seeing the quote that Celestron doesn't produce a f/6.6 SCT
because the design
would require too large a central obstruction.

Why would this be so? Why couldn't they produce it with the same f/2
primary, and
make the secondary 3.3 magnifying instead of 5X, and keep it the same
size?

I know there must be a fatal flaw in my logic, but I don't know what it
is. Any help?

2. ## SCT design

> Why would this be so? Why couldn't they produce it with the same f/2

Doing that would mean the light cone would remain fairly
steep. Therefore the focal point would be inside the tube,
rather than behind it where you can put a diagonal and eyepiece.

Clear Skies

Chuck Taylor
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http://groups.yahoo.com/group/lunar-observing/

If you enjoy optics, try
http://groups.yahoo.com/group/ATM_Optics_Software/
*********************************************

3. ## SCT design

<tedkord@excite.com> wrote in message
Think about the geometry for a moment. The light cone is tapering at the
final focal ratio, forward from the focal point (which typically is
perhaps 8" or more behind the primary), all the way to the secondary. The
secondary has to be large enough to 'cover' this cone, and provide any
extra size needed for the fully illuminated field. If you take F/10, and
have the primary at about F/2, then the secondary can be about Primary
Diameter*1.5 from the primary, giving a cone of light that is 25% of the
diameter of the primary at this point. If you then have the secondary at
perhaps 33% of the diameter of the primary, you have the required margin
to give an illuminated field. If the cone then tapers at F/10 from the
secondary, it will focus at about one mirror diameter behind the primary
(D*2.5 from the secondary). Now change the ratio of the secondary, so it
gives F/6.6. The cone will now reach focus D*1.65 from the secondary. This
puts the focal point just behind the secondary. No good. If you redesign
to give the same distance to the focus point as the F/10 unit, the
secondary has to grow to 37.8%, _just to give an illuminated dot in the
centre field_. Assuming that you need the secondary to give some field
beyond this, it'll typically need to be about 5 to 8% larger than this,
taking it up to about 45%...
The big minus, is the size of secondary needed. The big plus is that you
get a flatter focal field.
On all scopes of this sort, there is a relationship between the
illuminated field, the final focal ratio, the distance that the focal
plane is behind the primary, and the size of secondary required. You can
get scopes with very small secondaries, built to have tiny fully
illuminated fields, or with the design focal point close to the primary,
again giving smaller secondaries for a given focal ratio.

Best Wishes

4. ## SCT design

tedkord@excite.com wrote in news:1132001348.204341.85850

The reason for the size of the secondaries in all reflecting scopes is to
get a well illuminated field of view - generally for scopes that have
primarily visual use you can get away with only a small fully illuminated
field and therefore a correspondingly smaller secondary - this is seen with
a lot of Newts and Mak-Newts where they have a very small central
obstruction to optimise contrast at the expense of field illumination. For
a scope used for photography, you want the entire field of the sensor or
film to be well illuminated which means a larger secondary obstruction.

For a faster scope (lower f ratio), you generally need a larger central
obstruction to get the same illumination as you would for a slower scope of
otherwise the same design. SCTs tend to have fairly large obstructions
anyway but are even larger than you could get away with because they are
usually designed as general purpose visual/photographic scopes.

Klazmon

5. ## SCT design

I wounder about that too, Meade used to sell the classic LX200 with a choice
of ratio's f/10 and f/6 so it is possible

<tedkord@excite.com> wrote in message

6. ## SCT design

On 14 Nov 2005 12:49:08 -0800, tedkord@excite.com wrote:

The flaw is that a secondary that magnifies 3.3 times has to be 1.5
times larger than one that magnifies 5 times. You can't keep it the
same size and still catch all the light from the primary..

7. ## SCT design

On or about 14 Nov 2005 12:49:08 -0800 did tedkord@excite.com dribble
thusly:

Because you'd move the focal plane inside the tube (or close to it).

No matter how you go about it, the secondary will need to be larger to
get an equivalently illuminated field as the focal ratio goes down.
--
- Mike

Ignore the Python in me to send e-mail.

8. ## SCT design

Phil wrote:

Several of their early LX200GPS models were available as f/6.3 too, but
they've been discontinued for awhile (same as the last production of
the
LX200 7" Maks has just been completed; I believe the last run was just
50 or so).

9. ## SCT design

Right, but the 6.3 had a larger secondary....

"Phil" <pjsr@bigpond.com> wrote in message
news:Lt8ef.17686\$Hj2.4855@news-server.bigpond.net.au...