I have now finally completed building this scope. I had more or less finished a month ago but found a small problem with the main mirror cell;
basically there was some movement / flexure which cause some slight collimation problems when pointing to different parts of the sky, as well as limiting the exposure length to 5 minutes or so when using the separate guidescope / STi. After working the problem I found there were two causes:
- The back plate that the conical mirror sits on was not perfectly flat (I had made it out of CF) which allowed the mirror to slightly rock when pointed to different parts of the sky. I decided to replace this with an aluminium plate that had been precision ground perfectly flat one side (actually a raised annular ring to further ensure good contact around the periphery of the ground flat back of the mirror). I had my machinist friend David Byrnes make this part for me. On a side note, the engineering company that David manages (Anglade - Toolmaking and General Engineering) has won a contract to make parts for the new ASKAP (Australian Square Kilometre Array Pathfinder) project - no doubt on the back of the success of this Bunyip 12.5 project
David has been a big help to me throughout this whole project from teaching me how to use a lathe as well as making various specialised parts for me.
- There was some flex in the CF casting that I made to support the back plate. I remade this part by building up a solid carbon fibre plate (12 layers at a time to a total thickness of 17mm / 85 layers) instead of a casting, and I also increased some of the cross-sectional area.
The end result of these improvements is that the collimation stays much more firmly in place, and I can now use the guidescope to take up to 12-13 minute exposures with no star distortion on the main imaging CCD.
I have attached some images below as follows:
- The first 3 images are various views of the completed telescope.
- The fourth image shows the newly replaced mirror cell in the tube.
The images at the following links demonstrate test results of the overall rigidity of the system by using the external AT102 / STi guider instead of the builtin guiding chip of the STL-11000M CCD. In general I found that at reasonable altitudes (ie. >40 degrees) using the external guider that star images remained fairly tight up to 12 minute exposures; greater than that and the stars would start to become slightly elongated. Considering the size and length of the scope I would consider this quite a reasonable outcome, with 10 minute subs being quite common when imaging. Especially for Ha/R/G/B subs, it will be quite useful to be able to guide unfiltered.
- Here is a link to a 10 minute exposure of M30 using the external 102mm guidescope / STi. Note that all of these images are single sub-exposures that have had a dark subtracted and minimal processing. No flat fields / star minimisation / cropping has been performed.
Full size: http://www.pbase.com/david_fitz_henr...70523/original
Small: http://www.pbase.com/david_fitz_henr...45070523/large
- Here is a link to a 12 minute exposure of M30 using the 102mm guidescope / STi.
Full size: http://www.pbase.com/david_fitz_henr...70533/original
Small: http://www.pbase.com/david_fitz_henr...45070533/large
- Here is a link to a 10 minute guided single exposure using AT102 / STi external guider of the Tarantula nebula. Note that this was taken when the field was only at 28 degrees altitude and with mediocre seeing. This is a more severe test which would exert greater stress / flexure on the whole tube / guiding assembly. The stars are very minimally elongated due to the severity of this test.
Full size: http://www.pbase.com/david_fitz_henr...70545/original
Small: http://www.pbase.com/david_fitz_henr...45070545/large
Anyway, that pretty much brings the construction phase to completion - now I can start to do some actual imaging with it
