Help fault finding with 8" f/4 Newtonian optics.

[astroboof (Steve McN)]

^ that's why I had to shim under the focuser of the R130sf, but it had a R&P focuser that needed 'the treatment'.

Well, it's beyond me other than that, sorry.

At these times it's a good thing to have a nice refractor to take solace in, or any old faithful scope as you suggest.

CS, and congrats on the Obs completion there.

[Vermin (Tom)]

Have you checked that your secondary holder is not pinching the secondary mirror?

[Astroman (Andrew Wall)]

Originally Posted by Vermin

Have you checked that your secondary holder is not pinching the secondary mirror?

Yep, I did this, one of the many mods I have attempted to do. Although it is still in the plastic secondary holder, I have run a knife between the plastic and mirror. this is probably the best I can do without completely removing the mirror.

I will re-look at it at a later stage.

[Shiraz (Ray)]

Originally Posted by Astroman

Not sure if this helps or not, but I did a quick "star" test using an artificial star and a philips 900NC in the scope. Left is inside focus, right is outside. Bit of turbulence due to ground heating. Took a 500 frame avi and processed in registax. Still something not looking right there.....

maybe that doesn't look too bad - suspect that most of the distortion could be due to tube currents - did you allow much time for thermal stabilisation? With a fan running, mine takes around an hour to settle down with typical temperatures. Tube currents could possibly also explain the pacman effect in your star images. apart from the distortion, the primary figure looks like it might be pretty good.

[Satchmo]

Hard to tell much from the defocussed star test except there is some strong aberration there amounting to waves. There is about 6 waves defocus and the out of roundness would be over a wave . A sequence of photos would show if those out of roundness was a transitory seeing thing or not . The collimation is also way off as well.

I would suggest collimate better and take more pictures which will help with a diagosis.

I can't help thinking provided the mirrors are ok that there is some mirror pinching going on there.

[jjz (Joe)]

Are you using any filters? I recently tried using a cheaper UHC filter on my setup and ended up with triangular stars. Attached is a pic I took lastnight with and without the filter....

[Astroman (Andrew Wall)]

Thanks for your input guys, no filters were used during this test... I did some minor reworking of the mirror cell over the last week or so. I siliconed the mirror to the cell and also added some stronger springs which seem quite adequate now. Time will tell when I get it back up on the mount. Looks to be good so far. I collimated using a cheshire, then flipped the scope over and rechecked, the collimation didn't move. I then flipped it again and it was still as I had left it.. I will wait till I can do a star test on it before saying it's fixed or now, as we all know the proof is in the image from it. Fingers crossed...