I recently acquired a 1968 Optical Craftsmen 10" f/6 in its entirety. The cosmetic condition is excellent but the scope sat in one corner of an observatory unused for 40 years.
Anyhow when we threw the foucault tester on it we saw what I would call a "rille" very much akin, in the foucault, to the Valle Marinaris on Mars. It curved round and moved when the mirror was moved.
We star tested it there was an odd effect which I have hand drawn: on the outside of focus (bottom right) you could see the rille in the out of focus star pattern. In-focus you couldn't go very far. It looked like the draw tube was vignetting but actually we determined that the draw tube had nothing to do with it, the effect was at the wrong angle. There was an indentation exactly opposite the rille. Anyhow we determined that this was somehow an asymmetric effect directly opposite the rille.
In focus stars and Jupiter and anything else showed a kind of "teardrop" hanging from one side which we determined corresponded to the position opposite the rille.
The star test was actually excellent in terms of the overall patterns. The view on Jupiter at 200x showed a lot of detail in occasional still moments, so there is potential here.
I'm uploading my hand drawings of the foucault rille and what we saw to give a better idea.
So near as I can tell the next step is to strip the coatings (local club member has done this many times) and see if the rille is still present in the foucault. If the rille disappears I've scored a nice 10" f/6 mirror. If the rille stille shows in the foucault then the mirror will have to be re-ground which I am not keen on doing, I don't want to throw huge bucks at this project and will probably sell the whole scope and mount "as is."
I'm hoping that someone out there has seen a similar effect on a mirror with decades old coatings and that maybe they can say yeah this is probably just the coating. I forgot to label the lower right freehand drawing which is outside focus.
thanks
Greg N
Anyhow when we threw the foucault tester on it we saw what I would call a "rille" very much akin, in the foucault, to the Valle Marinaris on Mars. It curved round and moved when the mirror was moved.
We star tested it there was an odd effect which I have hand drawn: on the outside of focus (bottom right) you could see the rille in the out of focus star pattern. In-focus you couldn't go very far. It looked like the draw tube was vignetting but actually we determined that the draw tube had nothing to do with it, the effect was at the wrong angle. There was an indentation exactly opposite the rille. Anyhow we determined that this was somehow an asymmetric effect directly opposite the rille.
In focus stars and Jupiter and anything else showed a kind of "teardrop" hanging from one side which we determined corresponded to the position opposite the rille.
The star test was actually excellent in terms of the overall patterns. The view on Jupiter at 200x showed a lot of detail in occasional still moments, so there is potential here.
I'm uploading my hand drawings of the foucault rille and what we saw to give a better idea.
So near as I can tell the next step is to strip the coatings (local club member has done this many times) and see if the rille is still present in the foucault. If the rille disappears I've scored a nice 10" f/6 mirror. If the rille stille shows in the foucault then the mirror will have to be re-ground which I am not keen on doing, I don't want to throw huge bucks at this project and will probably sell the whole scope and mount "as is."
I'm hoping that someone out there has seen a similar effect on a mirror with decades old coatings and that maybe they can say yeah this is probably just the coating. I forgot to label the lower right freehand drawing which is outside focus.
thanks
Greg N
Attached Image:
"Scope will get you through times of no money better than money will get you through times of no scope." --Freewheelin' Franklin
