## What focal reducer I need for ETX-125????

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• Ok folks I have the ETX-125 and the DSI, and i dont know what focal reducer I need for DSO s. I am bite confused on how focal reducers work (well i now it
Message 1 of 4 , Jun 5, 2006
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Ok folks I have the ETX-125 and the DSI, and i dont know what focal
reducer I need for DSO's. I am bite confused on how focal reducers
work (well i now it reduces the focal lenght so exposition times are
faster and wider)..but i dont know the math behind it..like example:

ETX-125 it's a F/15 scope...there are 2 types of focal reducers I have
seen, the 3.3 and the 6.3....so if it's a F/15...with the 3.3 focal
reducer what should the new focal lenght of the etx would be???...same
with the 6.3.....

F/15 - 3.3 =F?????
F/15 - 6.3 =F?????

What focal reducer my ETX125 + DSI need's???..both?..just the 3.3 or
the 6.3??...

Thanx for all the help...
• Multiply the original F number (or focal length) by the focal reducer s number expressed as a percentage: F15 * 0.63 = F9.45 F15 * 0.33 = F4.95 The 0.33
Message 2 of 4 , Jun 6, 2006
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Multiply the original F number (or focal length) by the focal
reducer's number expressed as a percentage:
F15 * 0.63 = F9.45
F15 * 0.33 = F4.95
The 0.33 reducer is for CCD camera detector format only, not for
visual.
A positive focal length lens in the reducer will bend the light into
a steeper cone of focus than normal from the primary optic (lens or
mirror). If there is enough back focal length (adjusted by moving
the focus control of the main mirror of the ETX), there will be a
new focal plane, a new, shorter focal ratio, and a wider field of
view. Exposure time for extended objects will be decreased by the
same ratio.
This effect is the exact opposite of a a Barlow or negative focal
length lens that acts as a magnification multiplier.
Choosing a focal reducer for DSI camera imaging depends upon several
factors:
imaging chip physical size;
effective focal length of the optical system (which determines plate
scale on the imaging chip);
The above two limit the angular size of the object(s) to be imaged
in one shot.
The plate scale times the chip size shows how much of angle can be
imaged in one shot, and that will show how much sky will be in one
picture. Many shots can be combined as a mosaic with software.
Deep sky objects vary in angular size, due to either distance or
actual physical size. These can range from a few arc seconds to
many degrees across. Image magnification or plate scale depends
upon effective focal length. Small objects like planetary nebula
need high magnification (a few arcsecs per mm or pixel). Large
objects like extended gas clouds or nearby galaxies need lower
magnification (many arcsecs per mm or pixel) to fit onto the imaging
chip in one shot.

--- In ETXASTRO@yahoogroups.com, "Alejandro" <alex_dj2004@...> wrote:
>
> Ok folks I have the ETX-125 and the DSI, and i dont know what
focal
> reducer I need for DSO's. I am bite confused on how focal reducers
> work (well i now it reduces the focal lenght so exposition times
are
> faster and wider)..but i dont know the math behind it..like
example:
>
> ETX-125 it's a F/15 scope...there are 2 types of focal reducers I
have
> seen, the 3.3 and the 6.3....so if it's a F/15...with the 3.3
focal
> reducer what should the new focal lenght of the etx would
be???...same
> with the 6.3.....
>
> F/15 - 3.3 =F?????
> F/15 - 6.3 =F?????
>
> What focal reducer my ETX125 + DSI need's???..both?..just the 3.3
or
> the 6.3??...
>
> Thanx for all the help...
>
• OK thanx for the answer i now understand, now what focal reducer should I choose or be the optimum for ETX-125 + DSI-C combo ?????? In general... I have seen
Message 3 of 4 , Jun 9, 2006
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OK thanx for the answer i now understand, now what focal reducer
should I choose or be the optimum for ETX-125 + DSI-C combo ?????? In
general...

I have seen the meade's one( 6.3, 3.3), the celestron one's (same
like meade).. one made from scopetronix (only ccd not visual that put
the scope to F/7), another from optcorp (.5 and .3 , too only for
ccd )...

What should be the best one for this comboo (giving the scope and the
chip size of the DSI-C)????????

You can suggest another focal reducer if i didnt mention the optimum
one for this....

--- In ETXASTRO@yahoogroups.com, "Gregg" <starryskyn@...> wrote:
>
> Multiply the original F number (or focal length) by the focal
> reducer's number expressed as a percentage:
> F15 * 0.63 = F9.45
> F15 * 0.33 = F4.95
> The 0.33 reducer is for CCD camera detector format only, not for
> visual.
> A positive focal length lens in the reducer will bend the light
into
> a steeper cone of focus than normal from the primary optic (lens or
> mirror). If there is enough back focal length (adjusted by moving
> the focus control of the main mirror of the ETX), there will be a
> new focal plane, a new, shorter focal ratio, and a wider field of
> view. Exposure time for extended objects will be decreased by the
> same ratio.
> This effect is the exact opposite of a a Barlow or negative focal
> length lens that acts as a magnification multiplier.
> Choosing a focal reducer for DSI camera imaging depends upon
several
> factors:
> imaging chip physical size;
> effective focal length of the optical system (which determines
plate
> scale on the imaging chip);
> The above two limit the angular size of the object(s) to be imaged
> in one shot.
> The plate scale times the chip size shows how much of angle can be
> imaged in one shot, and that will show how much sky will be in one
> picture. Many shots can be combined as a mosaic with software.
> Deep sky objects vary in angular size, due to either distance or
> actual physical size. These can range from a few arc seconds to
> many degrees across. Image magnification or plate scale depends
> upon effective focal length. Small objects like planetary nebula
> need high magnification (a few arcsecs per mm or pixel). Large
> objects like extended gas clouds or nearby galaxies need lower
> magnification (many arcsecs per mm or pixel) to fit onto the
imaging
> chip in one shot.
>
>
> --- In ETXASTRO@yahoogroups.com, "Alejandro" <alex_dj2004@> wrote:
> >
> > Ok folks I have the ETX-125 and the DSI, and i dont know what
> focal
> > reducer I need for DSO's. I am bite confused on how focal
reducers
> > work (well i now it reduces the focal lenght so exposition times
> are
> > faster and wider)..but i dont know the math behind it..like
> example:
> >
> > ETX-125 it's a F/15 scope...there are 2 types of focal reducers I
> have
> > seen, the 3.3 and the 6.3....so if it's a F/15...with the 3.3
> focal
> > reducer what should the new focal lenght of the etx would
> be???...same
> > with the 6.3.....
> >
> > F/15 - 3.3 =F?????
> > F/15 - 6.3 =F?????
> >
> > What focal reducer my ETX125 + DSI need's???..both?..just the 3.3
> or
> > the 6.3??...
> >
> > Thanx for all the help...
> >
>
• To answer optimum reducer for ETX-125 and DSI, you must determine the facts that I refered to in the last half of the reply I gave to you: ************begin
Message 4 of 4 , Jun 9, 2006
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To answer "optimum" reducer for ETX-125 and DSI, you must determine
the facts that I refered to in the last half of the reply I gave to
you:
************begin quote*********
imaging chip physical size;
effective focal length of the optical system (which determines
plate scale on the imaging chip);
The above two limit the angular size of the object(s) to be imaged
in one shot.
The plate scale times the chip size shows how much of angle can be
imaged in one shot, and that will show how much sky will be in one
picture.
*****************end quote*********
Your choice will depend upon how you wish to use the reducer:
CCD imaging only, or also visual;
types of deep sky objects to be imaged;
how much of sky surrounding what sized objects above;
whether mosaic or one-shot imaging is to be done;
price and availability.
You might search www.astromart.com and www.surplusshed.com.

--- In ETXASTRO@yahoogroups.com, "Alejandro" <alex_dj2004@...> wrote:
>
> OK thanx for the answer i now understand, now what focal reducer
> should I choose or be the optimum for ETX-125 + DSI-C combo ??????
In
> general...
>
> I have seen the meade's one( 6.3, 3.3), the celestron one's (same
> like meade).. one made from scopetronix (only ccd not visual that
put
> the scope to F/7), another from optcorp (.5 and .3 , too only for
> ccd )...
>
> What should be the best one for this comboo (giving the scope and
the
> chip size of the DSI-C)????????
>
> You can suggest another focal reducer if i didnt mention the
optimum
> one for this....