As I progress through building my telescope, it is important to know and understand the focal length, or distance where all of the light from the star forms into an actual image at one point. There is a simple yet effective way to do this. You don’t need much, but a checklist is helpful.
When I perform this test, I use:
a bucket halfway full with clean water
a tape measure that extends to about 100 inches
the mirror (duh)
a piece of chalk or card to mark the distance
and a white poster board (optional but it helps)
Here is the setup:
So here’s the steps from Building a Reflecting Telescope:
As you can see, the process is so simple and straight to the point that I will agree not to explain what is being depicted. My results came out well as expected. I was aiming for an f/12 telescope, which would yield a 48 inch focal length, and it came in at 47 inches. No harm done, as an inch will not effect the performance in the least bit.
As I was grinding on the weekend, I am on the last 2 abrasives; aluminum oxide 9 and 5 micron. It is said that when grinding has been successfully completed, the mirror will feel like the finest satin to the touch, and newsprint can be read through it. It’s already unbelievably smooth, and not quite able to read through, but nevermind.
I caught a snag while finishing my abrasive: I scratched the mirror’s surface near the edge. I’m not quite sure how it happened, but my best guess is that somehow, dirt of some sort got between the glasses, and scratched. If this furrow was near the center, I wouldn’t be stressing over it as much. It takes about 3 times as long to remove such a thing at the edge than one near the center, as the glass naturally grinds out fastest at the center. Oh well. I knew that I would get a scratch at some point, but I’m happy it wasn’t during polishing, or I would have been in some serious trouble.
Hope to be finishing this project soon, because I’m itchy to look through this telescope!
The Young Astronomer 