Images taken last night with the nikon camera at newtonian focus- rather disappointing I must admit but then what can one expect from a spherical surface at f/4. The moon appeared better to the eye when focusing the setup - the camera was not so kind. The blue image shows Arcturus in a 30 second exposure - unguided as I was more interested in seeing how the star trailed images looked like across the field of view - well again not too great :-(( It very much looks like the secondary is not on square to the camera as focus definately worse on the left hand side). The field of view is 1.78 x 1.16 degrees and faintest star is around mag +11. Since it was first quarter moon one could probably add about 2 magnitudes to get a "dark sky" magnitude ( at least for my bright city lit sky), so current limit, as a spherical mirror is about mag +13 . The Nikon is not as sensitive as my FL8300 CCD camera so maybe another magnitude or two possible but then about equivalent to what I can get with my 4 inch f/5 refractor, so the 8 inch is still "under performing". I am pretty certain that once the 8 inch is parabolized it SHOULD out perform the 4 inch. The other image is of Alpha Centaurus - again a 30 second exposure - the star trails are shorter because of the high southern declination, and reaches about a magnitude deeper than the Artcurus image. The star trails on the right of the image are not too bad but bad on the left - some trails actually appear double!. I focussed with one of those aperture masks which has two holes in it - when the scope is focussed the two star images produced when out of focus merge into a single star - well with this scope this didnt quite happen - close but not close enough so is due to the "range of focus" of the f/4 mirror. I think this concludes my tests with this mirror. The next step is to remove the mirror from the tube and mirror cell and take it to my friend Willie Koorts so he can remove the aluminium coating after which I will go on hands and knees and somehow bribe Chris Forder to parabolize it for me :-)) , so until this is all completed this scope will be "abandoned" for the present.
Posted on: Thu, 18 Jul 2013 08:51:54 +0000