Bad focus issue resolved

[Pinwheel (Doug)]

Hi all, for a while now I've had an issue of bad focus & distortion on my 200mm (8") reflector. I was getting feed up with the problem as it was not fixable no matter how much collimation adjustment I did. Well two days ago I found the problem & thought I'd share my embarrassment but also this may affect other newbies. I discovered that my primary mirror was set back 6mm further back than it should have been. This caused the focal length of the Primary & the secondary mirrors to not focus. Even though my collimation laser showed everything was in line, so moving the mirror forward to 12mm all became sharp & clear. I now know with great hindsight that when I dismantled the primary mirror for cleaning a year ago I failed to note the exact position of the mirror mounted on the primary mirror assembly. I hope this information helps anyone with similar issues. I am now happy to report pin point sharpness of stars & I have been out viewing two nights in a row, & am very happy to be back watching the sky.

[brian nordstrom (As avatar)]

Cool hobby or what ? Doug .
Well done .
Brian.

[erick (Eric)]

That Pink Floyd will sound even better now with crisp sharp views!

[Pinwheel (Doug)]

Originally Posted by brian nordstrom

Cool hobby or what ? Doug .
Well done .
Brian.

It took me a full year off & on trying to nut this issue out. I almost gave up & flogged the telescope on eBay as faulty. It wasn't until I started to think of the problem being like a camera lens aperture & how the depth of field from f16 to f2.8 changes the focal length that made the penny drop and gave me a solution. I was of the belief that the primary & secondary mirror distant didn't matter because of the focus knob position, how wrong I was...Anyway I have a greater understanding now of how a Reflector works.

[brian nordstrom (As avatar)]

Onya Doug, you will be glad you stuck with it mate , now you can get out and observe ...and enjoy.
Brian.

[Poita (Peter)]

I think many of us have had the exact same facepalm moment. Glad to hear you sorted it out!

[barx1963 (Malcolm)]

Well done Doug!! Good to know you will be out observing. I have been through similar issues and learnt a lot by solving them!
I think the lesson to be learnt is that collimation is just one of the things that can go wrong and that it is not a guarantor of good images.
The classic example is trying to fix coma, which is an inherent property of all Newtonians and the best collimated scopes, especially fast ones will still display it.
The clue that it is a focal position error is that if focus is really bad in one position but improves as the focuser is wound, but the focuser runs out of travel before focus is reached, then the distance between objective and EP needs adjusting.
If you move from poor focus to better focus then back to poor focus, the focal length is all good and it is another cause (collimation, astigmatism, poor optics, bad seeing etc) that is causing the poor image.

Malcolm