Frustrating Newtonian Collimation Problen

[jeff46]

I've had this 'scope for years (20cm f/4 Newtonian - probably an early GSO) and been able to keep it pretty well collimated. However, in recent times it has developed a so-far insoluble problem: cannot get it to focus. The 'near focus' (still terribly out of focus) occurs well within the physical limits of the focuser.

Using a collimating cap, I have followed the prescribed step to align the secondary mirror (including checking its centrality - the 3 spider-legs are all the same length), aligned the primary mirror, sat and pondered my predicament, rinsed and repeated (at least three times!!).

I can get 'collimation' i.e. the mirror-clips showing equally, the 'black dot' centre of the collimating cap in the centre of the mirror but no focus. The mystery for me is what seems to be a 'misalignment of the focuser and the secondary. I'd attach a diagram to help to describe the problem but can't see how to. (Help needed there :confused2: )


I'm stumped and hope some one can help get be back on track before to many clear skies, Saturn and Jupiter have passed me by. (Judging from the recent weather patterns, cloudy skies here are outnumbering clear night by about 5 to 1!)

[astro744]

Over the years you have probably moved the primary down the tube by tightening all three screws during the collimation process thereby moving the focal point. I suggest loosening all three collimation screws as much as possible and then re-collimate. This will gain you the distance you have lost to regain focus by pushing the primary back up the tube.

[bojan]

:thumbsup:

[jeff46]

Thanks, I'll try that next. I did take the mirror cell out yesterday but didn't consider your action. (Can't imagine why I didn't think of that!! :-)

[jeff46]

Thanks again, astro744, for your suggestion which got me pretty well focused this afternoon on a target some 400 metres away. :) Now I just have to wait for a clear night sky to see if it works on a target a billion+ km away! :lol:

Oh, and make a note of it on my collimation howto, and then remember where I filed it...

[astro744]

:thumbsup:

[jeff46]

In my original post I mentioned what I described as a misalignment of focuser and secondary - but hadn't been able to work out how to attach the relevant diagram. (Not quite sure why, but there was an 'upload' feature evident tonight which I don't recall being available the other night.) If you can see the diagram it means I've worked out how to attach such things. :)

I'm looking for ideas as to why this apparent misalignment should be and how it can be overcome.

[bojan]

Have a look at this page:
https://www.bbastrodesigns.com/kolli/kolli.html

[croweater]

Hi Jeff. If you look up "astrobaby newtonian collimation" she shows by diagram (bottom of page) that what you are seeing is offset which happens in a fast newt. It's normal and I see this in my f5 newt.
Cheers Richard

[jeff46]

Thanks for pointing this out, Richard. I hadn't previously read all of Astrobaby's page (typical!!) and had missed this reassuring information :rofl:

[croweater]

No worries Jeff. That threw me too many years ago.
Cheers, Richard

[jeff46]

Thanks, everyone for your help. I'm about 90% satisfied with where I've got to. I've attached a sample result of my labours.

(Yes, I know, the image got a bit more processing than it needed. :lol: Still learning:nerd:)