Quote:
Originally Posted by coldlegs
Mathew
Haven't tried it but it seems to be a great idea although
it relies on a reasonable cross shape for the guide star
and I rarely get even close to this mythical shape they
keep showing. Usually I get some kind of "t" shape. The
biggest pain in the neck with ONAGs is the manual guide
focuser which you have to crudely push/pull until you get
some kind of focus. The focuser is so crude you can tip/tilt
it and get better or worse star shapes. I tried gluing a
3.3mm oval shaped piece of glass in the end of the focuser
tube and although there were indications of better (normal)
star shapes I couldn't get the thickness right so gave up
for the time being. There are rumors that they are making
an optional mini crayford focuser to ease focusing problems
but I suspect that is an optimist daydream as is a correcting
glass insert to give normal star shapes!
Cheers
Stephen
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Stephen!
I also use the ONAG and I can understand your frustration with focusing as I experienced exactly the same. I came up with two solutions. One is crude but does work, the other more elegant but perhaps subject to some flexure. The first you can sort of see here:
http://www.pbase.com/prejto/image/153978120
What you cannot see is blocked in the photo by the ST-i guide camera. It's a long screw that threads through that aluminium plate and presses against the body of the ONAG. I fixed a bit of plastic to the ONAG where the screw hits to avoid scratching the housing. The bubber band provide pressure for the screw to act against. Once focus is achieved you tighten down the ONAG set screw for focus. Admittedly focus works better moving out rather than "in." There just isn't enough spring pressure to pull inwards. This allowed me to get focus because I could just turn the screw a small amount vs trying to slide the focus tube in/out which in my situation was never satisfactory. The advantage of this crude focuser I dreamed up in desperation(!) is that it's firm once set and it doesn't add any back focus to the imaging camera (except for the thickness of the aluminium plate.
However, if you don't have a problem setting up the camera a little further back from the ONAG (and still have enough room to move forward on your main focuser) the Hutech helical focuser does the job. It's not absolutely rock steady but I have not had a problem with flexure.
http://www.pbase.com/prejto/image/152290400
I intend to try Sharplock soon and I will certainly post my results.I'm just waiting for a motorized focuser solution for my TEC180 which I should have in a matter of weeks.
Re star shapes I was able to get the X shape using both focusing methods. I do think the ONAG needs a bit of light to work most effectively. What I'm saying is that there is a discernible difference between my TEC140 and TEC180. The TEC180 gives better shapes, is easier to focus, and guides with shorter exposures. The 40 mm makes a lot of difference though that difference might well be because it is a refractor and these instruments are not so well corrected in IR (compared to reflectors as I understand).
Peter