If do math on square inch minus the diagonal one can see from a ex 3.5” to a 4.5” or even a 5” on ex a 18” mirror the area don’t change much.
Ex a 5” get 17.25” and a 3.5” get 17.5”, so it’s marginal.
As I read most say a mag-drop at 0.2 and even to 0.4 and more really can’t be seen.
This one is told ; One need a 100% field at ex 0.5” and then whatever the mag-drop will be is not important.
Ex my 18” will most see 150-200X and rarley 500X.
As the scope has no tracking 500X start get hard to use.
So mayor work is low or mid powers in this scope for this sky.
So why not use a diagonal that give a 100% field at lowest power ? ( as it not effect the area of primary much )
In this case I need a 116 mm ( from CAD ) diagonal or a 4.5” ( 4.4” C-A ) is almost zero mag-drop.
But I heard to big diagonal is often used for visual so it’s not wrong to undersize it vs gain a better ’contrast’.
My question is ;
If the diagonal has not zero mag-drop it will limit the magnitudes some ( how much I can’t say*) and area of used primary is not much effected by use a bigger one.
-So what is the idea that a bigger one hurt the ’contrast’ ?
How can a area of 0.25 inch less get a better contrast.
* A mag-drop at 0.3 show at 100X ( low power ) from 18” it would be as a 15”.
Edited by hakann, 21 October 2019 - 03:26 AM.