secondady mirror sizes

[sunshine185]

Hi, what would the effect be if you replaced the secondary mirror of a 250px with the smaller one from the 200p and vice versa? Would the 250px perform better or worse with the smaller secondary?

[sunshine185]

It was supposed to read secondary mirror sizes, stupid phone.

[Capricorn]

It depends on how much bigger the present secondary is then it needs to be,

If the new secondary is too small then light reflected from the primary does not get reflected into the eyepiece and you lose light. In effect your 250P becomes say a 220P.

Also even if the smaller secondary is big enough to reflect all the light into the eyepiece then the set up will have to be much more accurate, you will have no margin of error. This takes time and any mis-collimation will be more critical.

The final point is if the manufacturer could use a secondary from a 200P on a 250P they would have done so. Their costs will be reduced by only having to stock and have produced one mirror. Less cost to them means greater profit.

[sunshine185]

I see, so its not as simple as it sounds, the size of the secondary on the 8" is 47mm and on the 250px it is 58mm. I thought maybe if the 250px had a smaller secondary it would improve contrast without sacrificing anything, I guess I'm barking up the wrong tree, thnx for clearing it up.

[sunshine185]

Just seen that the oo UK 10" dob, the 10L is 400mm longer than the standard 10" I guess this is so the smaller secondary catches all the light from the primary, like you said,a smaller secondary in a 1200mm tube would be useless.