Flats gone flat, or how my Explorer 250 got straight

[perfrej]

So, I have issues with vignetting. not centered, but more of a gradient top to bottom. It's annoying, so I decided to hack it out. It took the better part of the day...

First, is your secondary where it is supposed to be? Mine wasn't by a long shot. It turns out the mirror is not glued centered on the central fastener. So, I started by measuring and got it straight by adjusting the attachment thingies.

I pointed the scope at an even light source (lit wall is fine) and looked down the focuser. I noticed that while trying to center my eye from about two feet, the secondaty was not quite centered. Moving my head around a little I found a sweet spot for judgibg theat I was actually looking straight down the opening by using the slightly lit walls of the focuser tube as reference. It took a little practice.

So, right to left got centered, but up and down took a different approach to get right. the center screw of the secondary assembly has to be moved up or down and the three adjustments allen-screws have to follow along, bit by bit.

When my eyes told med that the mirror was in the right position I tried collimating and then toom afew flats.

the difference is astronomical (yikes)! I'll check the flats for gradients eventually, but first impression is WOW, what a difference!

pointing the scope at a point light source and putting it out of focus now produces a nicely centered black thing, even though the supports for the secondary show slightly off, the latter being an effect of the mirror not being centered on its support, which doesn't matter.

So, the basic message here is that skywatchers' newtonians are probably misaligned from the factory. Not a big deal since everything is adjustable, but none the less an irritating fact.

If I can muster the energy I'll produce a howto on this.

/p