BTW, how did the images look? Are you happy with them? Hope so.
Bob
):-{])) <---- madden@netcom.com madden@svpal.org Remember amateur astronomers: "keep looking for the next Universe"
On Mon, 27 Mar 1995, Steve Scampini wrote:
> Hi Guys,
>
> Just experienced first light with my first scope, a ten inch Dob. I had
> to hand hold it (the base is not complete) but it still was a kick. A
> question for the forum:
>
> Everything went well except I came up about 0.3 inch short on my expected
> in-travel. I measured everything at least five times before cutting the
> mounting holes. I measured the focal plane of the eyepiece relative to
> the mounting shoulder (got about 5 mm distal to the eye, manufacturer
> says about 6 mm, OK). I purchased the mirror (didn't have the time to
> push glass). It was marked as 59" focal length and I asked and was told
> it was 59.0" "exactly". I would assume +/- 0.1 inches. I have measured
> everything twice after putting the thing together and still no clue.
> At this point the scope is very roughly colliminated, just by sighting
> by eye but all the mental experiments I do cannot account for 0.3 inches of
> error due to collimation effects.
>
> Bottom line is I can live with the about 0.4 inches of in travel ( I was
> shooting for 0.75 inches to give myself some error margin) but the lack of
> explanation is bugging me.
>
> I keep thinking it may be the mirror but then I think I maybe overlooking
> the obvious. I accounted for:
>
> *Secondary offset
> *Shim plate thickness under the focuser (to give something solid for the
> collimating set screws to push off against)
> * Secondary mount to spider geometry (in fact I made a special fixture to
> measure this external to the scope prior to mounting).
> * Primary to mounting bolts including the radius of curvature of the primary
> * Racked in focuser height
> * Tube wall thickness
> * Tube diameter
>
> It may be that the primary is offset in the tube in a radial fashion but
> can this effect plane of focus?
>
> Thanks in advance,
>
> Steve Scampini
>