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Correcting cone error- but running out of adjustment!


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I checked my cone error after hearing some people suggest it could be causing issues with my guiding/pointing or polar alignment. I followed the tutorial here: https://www.youtube.com/watch?v=WatdQlPp22Y

So I got my setup roughly pointed at this church a mile or so away. And with the RA axis level (I used a spirit level as instructed) I centered the cross above their bell tower in the field of view using DEC and AZ. I then flipped the scope on RA axis and got it level again, and found the horizon again using the DEC axis. Only to find my FOV had moved some good distance away (probably about 5-6 degrees, maybe more?). I adjusted the cone error seen by raising the rear tube ring away from the dovetail. I eventually ran out of thread for adjustment however before I reached the half way mark I was supposed to target!

The photo on the laptop is after I performed the second iteration of the instructions- I flipped RA back to the starting point and leveled it, centered it back on the cross using DEC and RA, and then flipped RA and centered it again using DEC only. There is still some cone error left, but it's in the same direction as it was before I did any corrections, and I'm out of thread!

I checked my collimation. The spider veins are all within 2mm of the same length as best as I can read, and the secondary mirror looks about right as I can tell, and the primary is properly collimated to the secondary as per my cheshire eyepiece. I just don't know why my scope would be so far away from where it should be in terms of cone adjustment, that I run out of thread!

Does anyone have any ideas as to what I should do to get the rest of the adjustment? Also please forgive the state of my scope, I went a bit bananas on the tape to keep stray light out (especially on the primary mirror cell)

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That looks very far off.

When I tried to align the finder scope of my then new 150pds, I couldn't get it aligned with the main scope, no matter what I tried. For a few weeks I lived with it, and at one point was about to return it to the vendor. But then I checked collimation, and found out that it was miles out. Once I had the scope collimated, aligning the finder was a breeze.

Moral of the story: revisit the collimation. With a Newtonian telescope, the optics don't have to be aligned with the centerline of the tube, and you can still get a good star field. The entire optical path may be tilted, including the focuser. If so, you are trying to compensate for that by moving the tube, while you should be moving the optics. You may need to start by taking the secondary out and make sure that the focuser sits square on the tube.

Edited by wimvb
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One other thought. If you point at that faraway church and then, instead of rotating RA, you rotate the tube in its rings, what cone error do you get? If you get any, shouldn't that be an indication of the optical axis not being aligned with the tube centerline?

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46 minutes ago, wimvb said:

One other thought. If you point at that faraway church and then, instead of rotating RA, you rotate the tube in its rings, what cone error do you get? If you get any, shouldn't that be an indication of the optical axis not being aligned with the tube centerline?

That's also a good point. I'll give that a check tomorrow too.

With collimation checks, If i can ensure that my secondary is straight and my spider vanes are equal lengths, (and ofc, that it doesn't look like the picture attached), does that mean I should basically seek to adjust the spider vanes to shift the secondary further to one side of the tube and then re-collimate there?Astro Babys Guide to Collimation - Newtonian Telescope ...

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One method that I've seen involves removing the secondary and marking a spot exactly opposite the centre of the focuser. Then with either a sight tube or a laser, you line up the centre of the focuser with that spot. This should square the focuser tube.

Next you reinstall the secondary, making sure that the gap between the holder and the secondary is parallell (in your drawing it's not). Measure from the tube to the centerscrew of the secondary along all spider vanes, and make all distances equal. Any offset that is needed is usually built into the secondary already. Ie, it is probably glued in place with an offset.

But you might test my second idea (rotating the tube in its rings) first. It seems a lot easier. 

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