Reflection from primary to secondary mirror

I have a dobsonian telescope. Last night while I was observing the
moon,I put my hand behind the secondary mirror about 3 or so inches
and noticed light apparently passing by the secondary and reflecting
onto my hand. I am a novice and assumed that the all the light from
the primary should be reflected back to the secondary in a cone shape.
Is this normal.

Thanks

a couple of things could be happening here.

first only on axis light should be reflected to the secondary and on to the
eyepiece. if there was considerable off axis light (the moon was out last
night) then your hand may have been illuminated by that.

second even with on axis light some of it always misses the secondary unless
the secondary is unusually large.

have fun

jtm


"Jim English" wrote in message
om...
I have a dobsonian telescope. Last night while I was observing the
moon,I put my hand behind the secondary mirror about 3 or so inches
and noticed light apparently passing by the secondary and reflecting
onto my hand. I am a novice and assumed that the all the light from
the primary should be reflected back to the secondary in a cone shape.
Is this normal.

Thanks

If on-axis light misses the secondary then the secondary is much too small!!
All on-axis light should be reflected by the secondary mirror.

Del Johnson


"Jim Miller" wrote in message
...

second even with on axis light some of it always misses the secondary
unless
the secondary is unusually large.

have fun

jtm

Hi Jim,

The secondary only needs to be big enough to catch the light that would end
up in the eyepiece. If the secondary was greatly enlarged so it reflected
the light that was hitting your hand, that light would be seen as an
illuminated circle around/outside of your eyepiece. For an extreme example,
imagine removing all tube elements completely. We'll set the secondary to
hover without support in the right spot (someone else on the list can
explain how we do this). Now imagine the moon off to the side so its light
hits the mirror at a 45 degree angle. Obviously the moon is not going to be
in the view at the eyepiece. But the light will reflect and leave the mirror
at a (roughly) 45 degree angle going out the other side. To catch that
light, your secondary would have to be a lot bigger than your primary, which
means it would completely block the primary for any object the telescope is
actually pointed at. So the secondary is made just big enough to catch all
the light for an object straight in front of the telescope (on axis) and
depending on the intended use, most or all of the light that comes in at a
slight angle. That angle is the angle of how far off from center an object
can be and still be seen in your widest field eyepiece. Or to be technical,
to illuminate the largest field stop on an eyepiece that you are going to
use. If you use 2" eyepieces, it will be a little wider than if you are only
going to use 1.25" eyepieces.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/



"Jim English" wrote in message
om...
I have a dobsonian telescope. Last night while I was observing the
moon,I put my hand behind the secondary mirror about 3 or so inches
and noticed light apparently passing by the secondary and reflecting
onto my hand. I am a novice and assumed that the all the light from
the primary should be reflected back to the secondary in a cone shape.
Is this normal.

Thanks

(Jim English) wrote in
om:

I have a dobsonian telescope. Last night while I was observing the
moon,I put my hand behind the secondary mirror about 3 or so inches
and noticed light apparently passing by the secondary and reflecting
onto my hand. I am a novice and assumed that the all the light from
the primary should be reflected back to the secondary in a cone shape.
Is this normal.

Thanks

It is normal. The size of the secondary mirror is a compromise. A larger
secondary will increase the extent of the illuminated field at the expense
of contrast. Since a dobsonian is normally optimsed for visual use, the
secondary doesn't have to produce a large illuminated field and
consequently will have better contrast than a newtonian optimised for wide
field photography.

Regards Llanzlan

The reflected image produced by your primary mirror is can be as large as
you wish, several feet across in fact. The size of the secondary is such
that it reflects what is required for your eyepiece, but it could be much
larger.

Del Johnson



"Jim English" wrote in message
om...
I have a dobsonian telescope. Last night while I was observing the
moon,I put my hand behind the secondary mirror about 3 or so inches
and noticed light apparently passing by the secondary and reflecting
onto my hand. I am a novice and assumed that the all the light from
the primary should be reflected back to the secondary in a cone shape.
Is this normal.

Thanks

(Jim English) wrote in message . com...
I have a dobsonian telescope. Last night while I was observing the
moon,I put my hand behind the secondary mirror about 3 or so inches
and noticed light apparently passing by the secondary and reflecting
onto my hand. I am a novice and assumed that the all the light from
the primary should be reflected back to the secondary in a cone shape.
Is this normal.

Thanks

I want to thank all those who replied to my question. I believe the
telescope needs to be collimated. I did use a laser to check the
alignment and the return beam from the primary missed the secondary
altogether. I think I have alot of work to do.

Jim English