I've finished it.
Meridian Telescopes
http://www.meridiantelescopes.com/index.htm were fantastic and sent me a correct 5.2 inch f5 mirror very quickly when I explained what happened so as soon it arrived, I got the 150mm PVC pipe from Bunnings and got to work. The mirror was US$59.95 plus US$21.?? postage which came to about AU$93 at the time. USPS shipping only takes 7 days (both times)
. The other thing is that they were the only company out of 4 that I tried in the States and Canada who responded to my queries. I can recommend them completely.
I worked out the new tube length I needed by simply sitting the mirror in a low table outside and sort of standing to one side, getting some stars in focus looking through a 20mm eyepiece then measuring the overall height and subtracting and adding the appropriate lengths to get the actual tube length. For example, the front of the tube to the focuser centre line is 60mm and needs to be added (I just measured this from the original tube as it is a fairly non-adjustable distance) and the focuser length from the tube centre line to the originally measured point on the eyepiece was 230mm (80mm + 150mm) which needs to be subtracted.
Anyway, my final result was 535mm which seemed a bit short so I cut it at 550mm as I figured I could cut a bit off if it was too long. I should have trusted my calculations. I'll take 15mm off the back end of the tube soon but with the primary mirror as far in as possible (while still allowing collimation) it works although I'd prefer to shorten it so the focuser tube is further out of the main tube and doesn't obstruct the main tube at all at normal focus settings.
Notes:
- I marked, drilled and carefully tapped all the mounting screw holes in the tube so neither nuts nor longer screws are needed.
- The new primary mirror fitted the mounting cell perfectly. The original 127 mirror is a bit bigger than 127mm and the 5.2 inch (132mm) is a bit smaller than 132mm.
- The new total OTA weight is 6.6 Kg against an original 6.2 Kg so it isn't much heavier although it is appreciably longer (see photos).
- I removed both the focuser internal lens and it's holder. The holder was a bit of work. Remove the rack mounting screw (I forgot) and soak the end of the tube in petrol to soften the glue to make it easier.
- My 'new' scope is appreciably better optically than the original (although I have now been spoilt by my GS-680) and I can actually collimate it! Not only that but I don't have to take the lens out to do so.
- Keep in mind that the focal length is now 650mm (or so) instead of 1000mm.
- It now looks like a "real" telescope (but still isn't so long that I can't take it away on holidays with the family).
- I still have no idea what to do with the original spherical mirror.
- I still have to flock the inside of the tube.
- I don't think I can call it a Celestron Powerseeker 127 any more.
If anyone else wants to do this and has any questions I am happy to help. I had some dramas but it should be fairly easy with the info I've given and I believe is definitely worth the effort. I've gone from being disgusted with it to being very happy with the final result.
Steve M