Re: Reflection from primary to secondary mirror

[EMAIL PROTECTED] (Jim English) wrote in 
news:[EMAIL PROTECTED]:

> 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