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Sorting out the Frankenmount


RLWP

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If you read this: http://stargazerslounge.com/topic/242618-me-and-frankensteins-telescope/ you'll know that in the best Dr Frankenstein tradition, I have assembled a telescope from oddments, including an equatorial mount

The mount has some lovely scales covered with numbers on, most of which I'm not currently using. The one for setting the latitude seems to be moderately important. But is it correct? I decided to find out

I partially stripped the mount and set it up in my lathe:

Checking-1.jpg

That was the first problem - most of the mating diameters are tapered. Must be some perverse astronomical instrument thing... I found one that was a plain cylinder

The scale has a pointer, and from what I remember of school physics (in addition to setting fire to the gas taps or connecting bunsen burners to water taps) it should be near to the scale to avoid errors due to parallax. This one wasn't:

Paralax-1.jpg

So I hit it with a hammer until it was:

Paralax-2.jpg

I then zeroed the scale and pointer and checked how square the base was:

Checking-3.jpg

The dial test indicator is set up on the cross slide of the lathe so I could track across perpendicular to the axis of the mount. at the other end, we have:

Checking-4.jpg

About 0.030" in 2" (ish) which doing some hard sums told me the scale is out by around 50'. I confirmed this by resetting the base until it was parallel to the cross slide axis, then checking the pointer:

Checking-5.jpg

Yep, around a degree out. Easily fixed by moving the pointer:

Checking-6.jpg

And there we are. It's not a highly accurate bit of kit, it's going to be less inaccurate now

Richard

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Next up, the worm and wheel. I set the depth of the worm into the wheel by feel, then tightened the fixings:

depthing-1.jpg

depthing-2.jpg

Next, I checked the worm all the way around the wheel. Rather than count the turns, I put a marker on in pen. As I found tight parts, I marked them on the wheel:

depthing-3.jpg

Taking the wheel off, I found things like this:

depthing-4.jpg

Impact damage. I used a triangular needle file to ease off the ends of the teeth and checked again. Now I have a smooth worm and wheel with no backlash (I got excited and forgot to photograph the filed teeth)

There was a dent in one of the calibration rings:

dent-1.jpg

This is a pretty flimsy aluminium pressing. I didn't want to knock the paint off the outside, so I rubbed the dent out with a plastic screwdriver handle:

dent-2.jpg

So, all back together again. This isn't raising the mount to high accuracy, it does improve an inexpensive mount though for NOTHING!!

Richard

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