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Tilt/collimation/other issue with a newtonian sanity check


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Picture 1 is at 90 degree orientation, picture 2 at 0.

Both images are from different sessions, and so have different collimations, although both are collimated via the same method to the best of my ability using a cheap 1.25 inch laser collimator (that i have collimated myself) NOT through the coma corrector, but i am 100% certain it was "right" on both cases. in the first issue the tilt is to the top left of the frame, on the second image that is rotated 90 degrees to the left it is to the bottom left, as i would expect if it was not sensor related. Rotating the camera involves me rotating the entire imaging train, which consists of a filter, the coma corrector, some adapters, and the camera. Am i right in saying that the tilt in my system is located in some place other than the sensor using the above information? I feel like this is a simple question and i should know the answer, but im no expert and am proven wrong left and right.

If the tilt is not in the imaging train, it must be in the scope am i right? That leaves just collimation with my newtonian as the culprit. But here is the issue, i have my imaging train intact when using my scope, so i collimate without the coma corrector. I have a 2'' to 1.25'' adapter where i put my trued cheap laser into and i confirm the colimation each time i use the scope (because all of my imaging is at darksites, so bumpy roads etc will guarantee the scopes needs checking). I think this is due to my coma corrector itself being miscollimated? Sounds reasonable? Just guessing. I cant be certain because it involves removing equipment but im pretty sure i have different collimations with the CC and without.

Easy solution: Collimate through the coma corrector. This step is not what i want to do as it has me unscrewing and screwing out equipment each time i want to use my scope. Rather have a solution without.

??-difficulty solution: Try to un-mess the CC alignment, if it is even possible. Or some other method. Feel free to suggest if you have one in mind.

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I am famously bad at visualizing geometry, but if I understand you correctly, you haven't ruled out tilt in any part of your system. I take it that it's not possible to rotate just the camera, even just a little? That would be my first try.  Next up would be an attempt to simplify the imaging train as much as possible -- eliminate the corrector and filters, and attach the camera as directly as possible to the telescope.

Are all your connections threaded?  What happens when you take the image train "as is" and point to different parts of the sky? Can you induce or change the tilt  with gentle pressure  on the camera? Is it possible to replace the camera alone with your laser? Do you have a "return to sender" collimator, so you can see if the beam bounces off the secondary, primary, secondary again, and back to its source? (And how did you align the cheap laser in the first place? I'd love to do that!)

Edited by rickwayne
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7 minutes ago, rickwayne said:

I am famously bad at visualizing geometry, but if I understand you correctly, you haven't ruled out tilt in any part of your system. I take it that it's not possible to rotate just the camera, even just a little? That would be my first try.  Next up would be an attempt to simplify the imaging train as much as possible -- eliminate the corrector and filters, and attach the camera as directly as possible to the telescope.

Are all your connections threaded?  What happens when you take the image train "as is" and point to different parts of the sky? Can you induce or change the tilt  with gentle pressure  on the camera?

Im gonna be honest and say that i was a few cans too deep into a case of beer at the time of posting and im not sure what my point was or if there even was one, other than tilts exists somewhere in the system 😅. Im sure there was a train of thought going through my mind when posting but i couldn't tell you what that was now.

Connections are threaded as far as they can be, with a 3-point compression ring connection between the CC body and the focuser, which is a Baader diamond steeltrack. I dont see a difference in apparent collimation with removing the collimator and placing it back in, trying to find out if the connection is sloppy.

If i understand my own post right, i made the conclusion that the CC would be somehow to blame, but since it moves with the camera when rotating the imaging train, the tilt should travel with it as well. And seeing as the bad corner has traveled 90 and not stayed in the top left corner of the sensor, the tilt is something before the imaging train (collimation). Of course its possible the CC is miscollimated too and i would have to collimate trough it to get the best result, but i think most of the tilt is in collimation. I did a little test by punching the OTA (lightly) with a laser attached to the CC and it did move a bit every time, erratically. The OTA itself is very thin and soft aluminium in the VX8, so i have reason to believe that might be part of the issue.

The effect is most apparent in stretched and rainbow rendered flats below:

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In this view reds represent the brightest pixels and purples the coldest. The bright spot doesn't stay in the same spot over the sensor when the imaging train is rotated so id say its just my collimation is out of whack.

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Flex in the tube at different altitudes? My SW 200p was terrible in that respect and I don’t expect my own VX8 to be any different. If I put my glatter laser in, the spot moves as I change altitude. That’s why I got a carbon tube. Tomorrow I’m returning to the UK for a fortnight where my VX8 currently resides so I’ll give it another test.

Edited by Captain Scarlet
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9 hours ago, Captain Scarlet said:

SW 200p was terrible

I think most out of the box Newtonians are, but fortunately easy to bring up to imaging standard.

The only way we've found is to seal the mirror to the cell to prevent lateral movement, fit 6 stronger springs to the primary cell, set the tube rings further apart on a Losmandy plate and tie the rings using rigid aluminium profile. 

I think any assessment of tilt needs to assume the integrity of the mechanical and optical components. Only then can you begin to fine tune focuser, secondary and camera mechanics.

Cheers and HTH

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3 hours ago, alacant said:

I think any assessment of tilt needs to assume the integrity of the mechanical and optical components. Only then can you begin to fine tune focuser, secondary and camera mechanics.

Agreed, too many variables in play to point towards a clear problem. Ill see what i can find from the hardware store to beef up my current kit somehow. Rather not spend a fortune on this right now.

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On 15/03/2022 at 13:17, ONIKKINEN said:

Rather not spend a fortune

The most expensive item by far is the dovetail plate which retails at €silly for what is simply a length of rectangular aluminium. A 500x100x15 lump and an angle grinder is all you need. Your local window frame supplier will sort you. Claim a 500mm length of profile for the top of the rings from his scrap bin. 1.6mm springs on AliExpress are around €3 per box of 10. Silicone sealant, even cheaper.

Cheers

Edited by alacant
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On 16/03/2022 at 17:27, alacant said:

The most expensive item by far is the dovetail plate which retails at €silly for what is simply a length of rectangular aluminium. A 500x100x15 lump and an angle grinder is all you need. Your local window frame supplier will sort you. Claim a 500mm length of profile for the top of the rings from his scrap bin. 1.6mm springs on AliExpress are around €3 per box of 10. Silicone sealant, even cheaper.

Cheers

I have attached some hose clamps i think made for AC tubing to tension the tube and hopefully improve rigidity, like i am pretty sure @Captain Scarlet did to his 8'' (cant remember whether it was the VX). I also drilled some holes through the existing tube clamps and fit a Skywatcher vixen dovetail i had lying around on it to act as a top rail to tie the tube rings together. I lack the critical astronomy resource known as a local window frame supplier 🤣 so this will have to do for now, but looks like these fixes while ugly have improved the mechanics of the tube quite a bit. Trying to squeeze or lightly punch the tube with a laser in the focuser reveals that it doesn't move nearly as much as it used to be so you are correct this operation did not in fact cost a fortune.

The springs and silicone thing im not worried about (yet). It looks like the VX mirror cell already uses this method, seeing as there are 9 (or is it 6? Not sure) small blobs of silicone between the mirror and the cell itself. I dont have the cell out right now but last i checked there was enough clearance between the clips and the mirror to slide something like a credit card in without much resistance. Not sure why the clips are still in place though if its already been glued. Precaution by design perhaps?

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Poking this silicone reveals that it is glued to both the mirror and the cell and not just something thats in between the 2. The springs themselves are also very strong and it takes considerable force to move the collimation nuts, so much that its actually a bit annoying. Im sure it still can be rattled around in transportation but during session, i dont think so.

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