200PDS Collimation issues

as said so often now, you have to look at your pinched main-mirror.

I have not the slightest experience with Newtonians but didn't you report that your mirror cell is stuck so firmly in the scope that you can't access it? If this is the case there may just be some strong force acting on the mirror leading to a slight deformation which could result in such an asymmetric contour plot. Just an idea …

Clear skies
Wolfgang

Andy I believe in one of your other posts you mentioned you have a mirror cell that was stuck and your unable to remove your primary mirror to check for pinched optics. Several others including myself had mentioned this. You have actually proved this not to just us but, yourself. Personally this should be telling you what your next move should be. At the very least look at that situation closer and more then likely get it corrected. Many here in this post have pretty much stated the same thing.

Let us know if we can help with your mirror and cell removal.

Dale

I have not the slightest experience with Newtonians but didn't you report that your mirror cell is stuck so firmly in the scope that you can't access it? If this is the case there may just be some strong force acting on the mirror leading to a slight deformation which could result in such an asymmetric contour plot. Just an idea ...

Clear skies
Wolfgang

Wolfgang,

Thanks for the suggestion, however the way Newtonians work is that the mirror is mounted to a very solid cast metal structure before being inserted into the tube.  

The truth of the matter is that I'm learning just how tough it can be to re-align a secondary mirror to the focus tube once it's been messed with.  I have now worked out that for my flats and camera orientation the following applies:

* The centre sweet spot moves upwards as I move my secondary closer to the primary.
* The left/right vignetting moves based on the rotation of the secondary about the OTA axis. This is a really sensitive adjustment and once I get it right I will not touch it again if I can help it.
* I don't think mirror pinching has any discernable impact on field illumination at this level, so it's all down to my lack of secondary alignment/collimation skills.

Here's my latest attempt which I think is actually quite close to where I want it to be.  I will probably move the mirror in to the primary just a tad and then I think I will have nailed it:

I would in no case use a screwdriver to loosen the primary exceptfor removing the screws…. The primary of my Vixen R200SS was also very hard to remove, I kept the mirror fixed to the floor with my shoes exactly as Tobi showed, then kept doing small circles with my hands close to the secondary mirror. It took a little patience but in the end I managed to loosen the mirror assembly.

Michael

andrea tasselli:

I have not the slightest experience with Newtonians but didn't you report that your mirror cell is stuck so firmly in the scope that you can't access it? If this is the case there may just be some strong force acting on the mirror leading to a slight deformation which could result in such an asymmetric contour plot. Just an idea ...

Clear skies
Wolfgang

This is exceedingly unlikely. I think the main contentions brought by other posters is that:

1. You need to fix the pinched mirror issue, possibly related to the cell being stuck into the tube.
2. You need better collimation tools and, possibly, better guidance in using them.

Thanks Andrea!  I have decided to focus on my collimation rather than the pinched mirror theory for the following reasons:

* When I got the scope 4 years ago, it didn't have a pinched mirror, so I'm struggling to think how it has tightened itself up
* I know I messed with the secondary mirror without really knowing what I was doing a couple of years ago
* I need to and want to learn how to do collimation and secondary mirror alignment really well; particularly as I replaced my focusser recently
* I honestly believe that there is no point in fixing a "maybe issue" with pinched optics until I have got my alignment and collimation truly sorted

To get to root cause you need to prove/fix one thing at a time in my book

I hate to ask this, but if you had a moment and could do a FlattContourPlot of one of your Newtonians flats that would be great, just so that I could see what I need to strive for.

andrea tasselli:
Not sure what you're aiming at, absence of symmetry of the flat doesn't automatically mean isn't well collimated, especially if the secondary isn't offset-centred. But here it goes, this from one of my newtonians (not sure here the collimation is 100% spot on, more as a trial test if you like):

Thank you ever so much for doing that Andrea.  I'm just trying to get one thing absolutely nailed before I do the next step.  I'm really happy with my field illumination now as per the below and will now move on to nailing the collimation before stressing about pinched optics.  It was important for me to go through this step as I recently swapped out the focusser and needed to get it aligned.

The below isn't perfect (I probably need to move the secondary about 0.5mm towards the primary), however it is probably good enough, so I will not touch it again before imaging.

For me Bob's Knobs is a game changer in collimation, with barlowed Laser.