Big Bertha Improvements

I mentioned a few days ago that Big Bertha's optical quality problems was at least partly because of non-repeatability--that moving it across the sky would cause the primary mirror to move because of the rather approximate way in which the mirror was held in place.

Here's a picture of the gate hasp that the original builder used to hold the "tailgate" in place--and you can see one of the studs that now helps hold it in position.


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This helped a lot; image quality improved, and more importantly, it was repeatable. Since I already had the mirror out of the tube, I decided to remove the diagonal mirror as well, vacuumed out all the spider webs, bugs, and dirt, and applied a fresh coat of flat black spray paint to the interior of the tube, to suppress internal reflections.

While I had the diagonal mirror out, I found myself looking at the gadget that was holding the diagonal mirror in place. It was clever, but not exactly right. Traditionally, good diagonal mirror holders use three spring-loaded screws so that you can adjust the diagonal mirror's angle relative to the primary mirror and eyepiece focuser. (Really good diagonal holders have an additional screw that lets you move the entire mirror up and down in the tube.) Here's a picture; it isn't really covered in dust. That's an artifact of trying to adjust the contrast and brightness of the image to bring out detail.


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This adjustment process of part of what is called collimation--and for Newtonian reflectors, it is absolutely critical. I can't count the number of Newtonians that I have looked through that had a serious collimation problem severely impairing the telescope's performance.

Anyway, I decided to disassemble the diagonal holder's three screw adjustment scheme to find out why it was so clumsy. It turned out that there were no springs in between the screws and the piece of metal to which the diagonal is glued. This has probably been a contributor to the poor performance of Big Bertha, because without springs between the mirror holder, and the part that screws into the spider, gravity can cause minor movements of the mirror holder against those screws--destroying the collimation.

I dug around and found appropriate sized springs--and it certainly makes a difference. Last night was pretty clear, although there was a little turbulence, but I was able to crank Big Bertha up to 400x on the Moon, with the image just starting to get soft. I couldn't decide if this was a limitation of the optics, or because of turbulence. I could certainly see the image quivering a bit. Jupiter was limited to about 222x, but this may have been because a quarter Moon puts out a lot of light. I'll try this again when I don't have tht pesky natural streetlight in the sky.

Partly because of all the rearrangement I made to the primary mirror's mounting method when I first bought this telescope, it has always been a bit tail heavy. One solution has to been to hang the bungee cords that hold the dust cover in place off the front of the tube. I decided to relocate the finderscope bracket a little closer to the front end of the tube, partly to rebalance the tube assembly, and partly because where it was before made it clumsy to sight along the bracket to get the finder aimed properly. (Because it is an 8x finderscope, you often have to do a very coarse aiming of the entire telescope to get an object in the finderscope.)

You can see the old bracket holes in the picture:


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