MN190 - New owner need some pointers

[coenie777]

I recently bought a second hand MN190 (from first owner).

I get images that appear to show coma in the upper  and lower right hand corners:

Here is a single sub, unprocessed with a Canon 40D (1.18"pixel resolution):

And here is what the extreme lower right corner looks like:

In contrast, the left upper and lower corners look way better:

CCD Inspector looks like this on most of the subs:

My question is:

- Does it look like just bad collimation resulting in these terrible corners?

- When I pick up the scope and turn it on its back (I mount it with the eye piece down so need to turn the tube lenght wise 180 to store it), there is a slight thumb. I am not sure if this could be the primary mirror flopping around. Does this sound like an issue?

- When you focus with the standard focuser and want to lock focus with the set screw below the focuser, I see quite a bit of image shift. I want to know how tight you need to turn this as turning it in fully it actually move focus completely out. Should you just lightly tighten this? Once tightened, does this then disable the fine focuser knob from making any adjustments?

- Lastly, when I collimate the secondary and loosen the fourth screw (lower one), it results in the secondary turning. I tried tightening the round knurled srew a bit but do not want to do something that may damage anything. Are these secondaries prone to turning when being adjusted?

 

 

 

[Peter Drew]

Looks more like a tilt problem to me, take your pick of several potential culprits!. A secondary rotating whilst it is being adjusted and specially when finally tightening it in position is an all too common issue. The usual cure is to interspace a washer of aluminium or plastic between the spider centre hub and the top of the secondary holder for the adjusting screws to bite into. Not straightforward on a M/N unfortunately.