Repairing my F5 refractor
Posted: Tue Oct 17, 2023 3:30 pm
With a big bang, my F5 Startravel refractor came to rest on the floor from a side table. Swept off its spot by a diligent cleaner's shirt stuck behind a protrusion. Immediately I stood startled and praying in the opposite direction. Refractors are collimation proof, but they can't take shocks of that much G.
Apparently there was no damage. But a little test on a fake star (bright flashlight behind a black cardboard with tiny holes in it) showed that one of the two lenses had shifted. Not much, but enough to annoy me. So that had to be set right again.
Because in this cheap scope there are no adjustment bolts for that, the whole lens cell has to be taken off, three rings removed, lens has to be lifted out of the lens cell, slid back together properly, lens lowered back into the lens cell, replace rings, mount lens cell back to the tube and finally test.
That test in particular is crucial, of course. Because ten to one that it is still not positioned correctly, corrected too much or an additional error has been introduced. Several times I was close, but not good enough. After a good dozen or so attempts it was back as I remembered it from the correction I had made once after purchase. Besides common sense, good luck comes around the corner, but trial and error is also just around the corner without adjustment bolts.
Lens cell and instrument to loosen the lock ring.
A metal ring, a flat plastic ring and an O-ring. Lens is already lifted here.
Fortunately, I had set markers last time, so the orientation of the lenses remained the same. And now just "sliding" those lenses around. Of course, we are talking about tenths of millimeters. Or even hundredths. Human fingers are amazingly accurate at judging whether a side is slightly protruding or smooth.
Raising the lens cell again must be done without touching the lens, an extremely precise job.
By way of illustration a simulated image from Aberrator. All values are estimated and thus not measured. What matters is the illustration. The top row is before correction, the bottom after.
From left to right : Focus, Intrafocus and Extrafocus. Especially the extrafocus image gives good info on the degree of skew.
On the fake star I used a barlow and a Pentax XO2.5 eyepiece, so a magnification of 400X. More than enough, because in practice this F5 scope does not reach that.
There you go, it's good to go again!
Apparently there was no damage. But a little test on a fake star (bright flashlight behind a black cardboard with tiny holes in it) showed that one of the two lenses had shifted. Not much, but enough to annoy me. So that had to be set right again.
Because in this cheap scope there are no adjustment bolts for that, the whole lens cell has to be taken off, three rings removed, lens has to be lifted out of the lens cell, slid back together properly, lens lowered back into the lens cell, replace rings, mount lens cell back to the tube and finally test.
That test in particular is crucial, of course. Because ten to one that it is still not positioned correctly, corrected too much or an additional error has been introduced. Several times I was close, but not good enough. After a good dozen or so attempts it was back as I remembered it from the correction I had made once after purchase. Besides common sense, good luck comes around the corner, but trial and error is also just around the corner without adjustment bolts.
Lens cell and instrument to loosen the lock ring.
A metal ring, a flat plastic ring and an O-ring. Lens is already lifted here.
Fortunately, I had set markers last time, so the orientation of the lenses remained the same. And now just "sliding" those lenses around. Of course, we are talking about tenths of millimeters. Or even hundredths. Human fingers are amazingly accurate at judging whether a side is slightly protruding or smooth.
Raising the lens cell again must be done without touching the lens, an extremely precise job.
By way of illustration a simulated image from Aberrator. All values are estimated and thus not measured. What matters is the illustration. The top row is before correction, the bottom after.
From left to right : Focus, Intrafocus and Extrafocus. Especially the extrafocus image gives good info on the degree of skew.
On the fake star I used a barlow and a Pentax XO2.5 eyepiece, so a magnification of 400X. More than enough, because in practice this F5 scope does not reach that.
There you go, it's good to go again!
