A friend of mine gave me the Idea to use my spare 8" f6 Newtonian as an inexpensive double star/ planetary scope. Was thinking of changing the secondary to something smaller and putting the curve vane spider to diffuse the diffraction spikes to find close doubles.
your friend has a good ideas
First of all I like to say that perfect collimation is vital.
Although not very fast an 8" f/6 must be near-to-perfection collimated to give best views.
The next point is thermal behavior of the telescope.
Depending on the glass-type of the mirror the mirror has to cool down
longer or shorter to re-gain it perfect parabolic curve.
But even if the mirror is made from ultra-low-expansion material
the mirror needs cool-down.
Otherways the mirror warms up the air surrounding the mirror,
the air gets lighter than the rest of the air in the tube and so it rises up
and causes density differences of the air, disturbing the incoming wavefront.
Therefore you need to control the thermal behavour of the scope.
I recommend to add a fan behind the mirror wich sucs the air out of the tube.
The fan should not cause vibrations!
The tube should not consist of thin metal. This has disadvantages.
During a clear night a tube wich is directed to an object wich is 45° above the horizon
cools down on the upper side due to infrared radiation.
The metal cools down the air at the inner tube wall and this begins to fall down.
Again you will get density differences inside the tube an this disturbes the wavefront,
lowering the contrast and if it is bad, even the resolution.
Non-metal tube material plus flocking inside can reduce these problems a lot.
Here in Europe the Gerd Neuman HP Tubes are well known.
In the USA there are these from fpi-protostar:
The tube should be longer than the original one. I would make it 20cm to 30cm longer.
Or add an additional 20 to 30cm long removeable dew-cap.
All this preventy the observer's breath from crosing the light path and prevent straylight.
Having done all this it is time to know about the optical quality of the primary mirror.
If it is good enough (I would consider a 0.95 Strehl as "good enough", but better is welcome)
this is fine, but if it is only 0.85 or worse one should get a better mirror, or refigure it.
The secondary should be very good. It will be a replacement,
because in many cases the original secondary is not good enough and it is unnecessarily big.
Take a low focusser. A helical or one with 1:10 reduction for precise focussing.
You can tak a curved spider like this one
to avoid spikes. Spikes can be bothersome if you want
to split a double in wich the second star is much less bright.
The spike emerging from the brighter star can hide the second star.
Ed Turco has decided to make an optical window, another solotion,
wich you can read in his paper there:
... my window gets rid of the problem of spider diffraction ...
While close doubles and planets do only need a small field,
the moon is so big that you might find the edge correction of a newtonian a bit too bad.
You can largely increase the diameter of the diffraction limited field of a newt
by using a coma-compensating device.
This can be a coma-corrector (there are some of good quality and some less-good),
a coma-compensating eyepiece (in former times there was a "Pretoria Eyepiece"
designed by H.Klee and distributed by by University Optics,
and a Brandon Comacompensating Eyepiece, unfortunately all were discontinued)
or a coma-compensating barlow (2,8x or 2,2x University Optic Klee Barlow, made in Japan,
discontinued, and today there is a APM comacompensating barlow, made in China).
And you should use eyepieces that do not show light scatter.
The cheapest solution is a good upper-mid-class Orthoscopic
like the U.O. HB Abbe Orthos:
Behind a good barlow they perform very well in my own 8"f/6 ATM newtonian.
I hope I could give you some useful hints.