Star testing a refractor for beginners

[adarshajoisa]

I'm not sure if this is the right place to post this, but it's what I thought is the most appropriate. I have some experience in star testing newtonian reflectors, and have tested a few scopes. One of my friends wants to buy a 102mm meade refractor from a seller who's located in a different city, and he's planning on going there to test the scope. He asked for my suggestions on how to guage the quality. I suggested he do look at Jupiter or Saturn and judge the quality, and also try to see if the stars are pin-point, etc, but I'd ideally like him to star test it. Is the star test for a refractor any different from star testing a reflector? Of course, things like the secondary shadow breakout, etc won't apply, but how else does it differ from a star test on a newt? Is there a better way to test a refractor? Any pointers would be really appreciated.

[Peter Drew]

Not a great deal of difference really. Although there is no secondary shadow, the out of focus star will still show a small but bright centre spot for collimation judgement. The main issues to look for are coma with a star placed at the centre of the field which would indicate considerable miscollimation, an oval shaped ring of which the axis changes inside and outside focus, this is astigmatism usually caused by the objective lens retaining ring being too tight and finally, the edge of the out of focus ring should be clean and not exhibit spikeyness. If it satisfies these criteria it should be worth buying.

[Merlin]

Spherical abberation shows itself as overcorrection when there are well defined deffraction rings in the star image outside of focus, but mushy on the inside. Undercorrection is the opposite and shows sharp rings inside focus but mushy rings outside focus.