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I hate to throw a spanner in the works but a 4"f10 Newtonian riding on a Newtonian is going to be subject to differential flexure .
As a guide scope you will want to ensure that there is no movement between it and the main scope or tracking errors will result.
This movement can be in the form of slight twisting as the scope moves from one side to the other and slight mirror shift .
Also thermal contractions and expansions of the tube or trusses.
It can be done but will need to be done well to function as guide scope.
If you want a small CO then it will need to be a truss tube as in a solid tube you require about 25mm clearance for thermals to rise up the tube without causing image problems.
This leads to it's own problem in such a small objective of only 100mm as compared to a much larger mirror in a newt.
If you use a 150mm tube it's going mean a CO larger than 20-25% to maintain a reasonable fully illuminated field.
If you use a truss the clearance issue is reduced by design and you will be able to use a smaller secondary.
I put the figures through Newt2.5 and a small CO is hard to get with the smaller the mirror even at f10.
Personally if you want a guide scope come short grab and go with reasonable quality then one of the 80mm ED's or even one of the Synta Orion Saxon Maksutov-Cassegrains ota's.
The later can be subject to mirror flop again for guiding.
I have gone the first route with the 80mmED as it makes a great guide scope , grab and go , wide field imager and 80mm finder scope with 40mmish eyepieces in one package and it's compact.
I have seen 4"f10 mirrors around in the states for under $100us but there getting rare .
Just some things to be aware of , but if you have your hart set on a newt down load a copy of newt2.5 first and play with the figures to make sure it's viable..
Mark
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