MN190 Collimation Woes (still)

[coenie777]

This question relates to my MN190 but I guess the technical principals at play here may be the same for all Newts.

In trying to tweak my MN190 to show round stars all the way to the edge (I have them round up to around 90% of the edge at this time), I need some advice please.

I build myself a 2" Cheshire sightube and tried my best to get the cross hairs exactly square. They are within 0.5mm I would say.

I started off by trying to get the marking on the secondary mirror centred on the Cheshire. Here I need the first bit of advice;

- For fear of extending the secondary mirror down too much and risking a mirror drop, I also loosened the focuser and moved it up and down the tube.

- I got the crosshair close to the marking, but it was lower than the horizontal crosshair. To explain this, if I moved the focused up and down the tube after loosening the screws, the centre dot would move left and right of the vertical cross hair. The position relative to the horizontal line is not affected by this. Moving the focused only moved the mark left and right (if I do not twist it laterally off course).

- I then figured out that by tilting the focuser on one side, I can actually get that horizontal offset aligned. I added some padding to the one side of the focused and got the dot centred. So I raised the lower fixture point, tilting the focuser to one side.

First question is if tilting the focuser by adding padding is a good idea or not?

Second question is if this scope should actually be able to have the dot centred without having to tilt the focuser.

Once i got the dot centred, it looked like the secondary image circle under the focuser were not 100% following the roundness of the Cheshire. To my eyes it looks like the mirror needed to be dropped further down the tube.

I can see all three primary clips and they actually seem to extinquish  at the same time. It is just that the circle looks to be slightly shifted towards the upper end of the tube (towards the corrector lens).

Before trying to drop the secondary down in order to try and make up for the above, would this be recommended? If the centre dot of the secondary is under the crosshairs now, can it still be a case of the secondary being too high up the tube? Surely if I drop it down now, it would move the dot away from the crosshair centre?

Clouds are going to not see me being able to image anytime this week so I hope someone can share some experience or maybe advice that I can try in the mean time.

I am fairly confident that the secondary twist is correct as I used a laser and got the dot actually on the primary centre dot, taking care while tightening the lock down nut to not twist it again.

[MattJenko]

How sure are you that it is collimation that is the issue with the edges of the field of view?

[coenie777]

Matt, pretty sure at this time as I have photographic proof that my tweaking with the secondary produced either more or less of the effect. I also check the images with CCD Inspector and can see that where the field is not flat is where the stars do not look round. By changing things in the adjustments I have made to date I was able to reduce the coma shapes of the outer stars significantly. So it is definitely something sitting on the optical axis as moving this around I can exagerate or improve the results.

What are you thinking?

[tony210]

Answering question 2 first - yes you should be able to get dot centred collimation without altering the  focuser - the last place to look is the tube and focuser geometry . I would be worried about collimation . There are good commercial collimaters that offer laser enhancement and before altering the focuser I would want to be sure the collimation is accurate . Also the central dot can be off- with my RCT the supplier told me the optical axis can be slightly off the geomteric axis!!- star tests will tell I guess- hope you sort it out soon - Tony.

[MattJenko]

While the Mak-Newt design reduces coma and chromatic aberration down to lower levels than a Newt, they are not eliminated and it still has other aberrations, and wondered if you are now getting to the limits of the design/fabrication. You are using a pretty big chip (assuming the Canon 40d), so presumably you would be looking at a perfect field of view with something like a kodak 8300 chip which wouldn't quite reach the areas you are seeing the edge issues.

[coenie777]

Noted, thanks