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RASA 8 DISTORTIONS: YOUR THOUGHTS?


ollypenrice

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Hi All, 

First light on our joint RASA 8 project. Lots to do because this will be a robotic setup but we took an initial image image. The cables in front of the corrector are not yet organised so star shapes won't be good until they are. The Centre should come right when this is done.

Centre.

Centre.jpg.93f2a73c767a5b9b68ad4996db1238f6.jpg

However, the corner crops upper left and lower right show stars extended like parts of the circumference of a circle. Here the four corners are presented as they lie in the full image.

 

922694363_1stlightcornersSMALLER.thumb.jpg.0334fed605bea2992a3d34a7421eae67.jpg

My feelings are that it looks like this...

1526915353_RASAdistortions0024.thumb.jpg.93bdc078ab935cc39ebc5ec15e37d657.jpg

This kind of corner distortion is consistent with the chip being too far from the corrector lens...

1923857810_chipdistance.JPG.0de156e82140ef54a456d54e9444c7ab.JPG

(It also arises from poor PA under autoguiding but this was an unguided 60 second sub.)  What's puzzling is why only two diagonally opposed corners are affected.

The chip distance may be too long. We are using the Artsky RASA camera adapter which should give the right backfocus for the ASI Camera but, for now, the Celestron clear glass filter is still in place. Removing it would mean our backfocus would be effectively even longer, in my understanding.

So any ideas, anyone? Why two bad corners? I'm struggling to see how tilt could do this but maybe it could?

Olly

 

 

 

Edited by ollypenrice
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Strange one but guess it could be tilt at 90 degrees to the "OK" axis which would mean one corner is a bit close and the opposite a bit too far, the diagram showing star shapes is alright as a rough guide but would not rely on it too much.

Moving the camera through 90 degrees will show if its scope or camera..

Alan

Edited by Alien 13
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My first thought was that it was a tilt problem - camera tilted about the lower left / upper right axis, which presumably would produce distortions in the two other corners.  But, I would have thought that the stars would be distorted about that diagonal axis, which clearly in your image they are not.  Could it be a combination of tilt and collimation, in some areas working together to make the aberrations worse, and in others working against each other to cancel them out ? Or maybe a combination of spacing and tilt ?  Just a few thoughts to ponder.

For what its worth, my RASA 11 produces nice round stars right across the frame. the clear glass filter has been replaced with Celestron's LPR filter, but I have not yet tried using other filters so cannot comment on what effect these might have on image quality.

It will be interesting to learn what the true cause of the problem is, so once, or if,  you get to the bottom of it, please let us know !

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First take away the optical window on the scope. That was one of the first things I removed and the Artesky adapter seems to work well without it. If you have the window in or a filter then you should use the distance ring that came with the Artesky adapter.

However, I do recognize the distorsion you get from my first attempt with my second RASA8 (although I had it in all corners) and the scope turned out to need some collimation and after that the corner stars looked much better, so another suggestion is that you check the collimation an a bright defocused star. First I thought it looked rather ok but not perfect, but I then had these odd stars in the cornes and thought that collimation may be needed and realized that in the defocused image needs to be perfectly symmetrical.

Tilt is a possibility so try rotating the camera 180 degrees.

Edited by gorann
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8 minutes ago, gorann said:

First take away the optical window on the scope. That was one of the first things I removed and the Artesky adapter seems to work well without it. If you have the window in or a filter then you should use the distance ring that came with the Artesky adapter.

However, I do recognize the distorsion you get from my first attempt with my second RASA8 (although I had it in all corners) and the scope turned out to need some collimation and after that the corner stars looked much better, so another suggestion is that you check the collimation an a bright defocused star. First I thought it looked rather ok but not perfect, but I then had these odd stars in the cornes and thought that collimation may be needed and realized that in the defocused image needs to be perfectly symmetrical.

Tilt is a possibility so try rotating the camera 180 degrees.

Thanks Goran. I don't recall a distance ring coming with the Artsky adapter. Is this just one of those little spacers which fits over M42 threads?

I've taken out the Celestron window since trying without it is the obvious first step.

Olly

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Both my Artesky adapters came with a distance ring. It is 0.5 mm thick, goes over the T2 thread and looks like this. You can of course use any 0.5 mm ring. You use them when there is a filter (or glass window) in the image train.

Göran

20210810_152359_resized.jpg

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1 hour ago, gorann said:

Both my Artesky adapters came with a distance ring. It is 0.5 mm thick, goes over the T2 thread and looks like this. You can of course use any 0.5 mm ring. You use them when there is a filter (or glass window) in the image train.

Göran

20210810_152359_resized.jpg

Humph, I didn't get one of those. I'll ask the dealer.

Olly

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Any chance of it being astigmatism induced by something being tightened down too far over a high spot? You might find that just shimming the distance just transfers the fault to the other corners. 

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We collimated the scope last night and the corner elongations are gone. Relief! (And, in my case, surprise, I must say. I'm still not sure why mis-collimation would affect only two diagonally opposed corners but it did. This confirms Goran's experience above.) I'm not going to say that I got the out of focus doughnut perfectly symmetrical but I was going round in circles after a while so called a halt at 'pretty good.' We can always go back to it once we have all the rest of the rig running well.

To collimate the RASA you need access to the collimation screws on the central lens cell. A full size camera covers theses so you can use your guide camera for collimation. (Another good tip from Goran.)

Thanks for the input on this thread, everyone.

Olly

Edited by ollypenrice
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7 hours ago, ollypenrice said:

We collimated the scope last night and the corner elongations are gone. Relief! (And, in my case, surprise, I must say. I'm still not sure why mis-collimation would affect only two diagonally opposed corners but it did. This confirms Goran's experience above.) I'm not going to say that I got the out of focus doughnut perfectly symmetrical but I was going round in circles after a while so called a halt at 'pretty good.' We can always go back to it once we have all the rest of the rig running well.

To collimate the RASA you need access to the collimation screws on the central lens cell. A full size camera covers theses so you can use your guide scope for collimation. (Another good tip from Goran.)

Thanks for the input on this thread, everyone.

Olly

Great news Olly! Eager to see the proper first light!

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Optimism proved premature! This is the current state of our collimation. Star in the centre of the image.

1844892845_brightstarlinear.jpg.ccb706e6996f3a9cc6092ddb2515d47c.jpg

What do you think of it? Close, bad, whatever you think?  It is not giving us acceptable corners in either landscape or portrait, unfortunately. In fact we had better corners with worse collimation!

This is not proving an easy instrument to set up...

(I havevn't routed the cables carefully yet, hence the two dark lines.)

Olly

@gorann 

Edited by ollypenrice
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12 minutes ago, ollypenrice said:

What do you think of it? Close, bad, whatever you think?

Very close. Not sure how much impact will it have on the final image:

1844892845_brightstarlinear.jpg.ccb706e6996f3a9cc6092ddb2515d47c.gif.2d32603dfc076e61d8d2a2050f76dd15.gif

I feel that upper side is maybe 1-2px thinner than bottom side, so just a minor tweak is needed. Left-right seems spot on.

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5 minutes ago, vlaiv said:

Very close. Not sure how much impact will it have on the final image:

1844892845_brightstarlinear.jpg.ccb706e6996f3a9cc6092ddb2515d47c.gif.2d32603dfc076e61d8d2a2050f76dd15.gif

I feel that upper side is maybe 1-2px thinner than bottom side, so just a minor tweak is needed. Left-right seems spot on.

Agreed. Great way to check! Thanks. I did something along those lines but yours is the way to do it. Thanks.

Olly

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On 17/08/2021 at 08:52, ollypenrice said:

We collimated the scope last night and the corner elongations are gone. Relief! (And, in my case, surprise, I must say. I'm still not sure why mis-collimation would affect only two diagonally opposed corners but it did. This confirms Goran's experience above.) I'm not going to say that I got the out of focus doughnut perfectly symmetrical but I was going round in circles after a while so called a halt at 'pretty good.' We can always go back to it once we have all the rest of the rig running well.

To collimate the RASA you need access to the collimation screws on the central lens cell. A full size camera covers theses so you can use your guide camera for collimation. (Another good tip from Goran.)

Thanks for the input on this thread, everyone.

Olly

Hi Olly,

you sounded very optimistic here, so what happened? It could now be a chip distance problem, but Vlaiv has a point, maybe it could be even better collimated.

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PS @ollypenrice. Today I got my double RASA rig put together in the livingroom, after spending the day getting the balance point right for each piece of the equipment (you can see my pencil under the dual rig holder). I may be asking for trouble. The total weight is 29 kg so my iOptron CEM70 may struggle (32 kg max payload but as usual it is not stated if that is imaging or visual) and I may have to put it on the Mesu (although I have other plans for that mount). Got an Evostar 72ED as guide scope of one of them and to balance it a vintage Canon FD 200mm f/2.8 with an ASI2600MC as piggy back on the other.

Cheers Göran

20210822_144559_resized.jpg

20210822_112358_resized.jpg

Edited by gorann
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12 hours ago, gorann said:

Hi Olly,

you sounded very optimistic here, so what happened? It could now be a chip distance problem, but Vlaiv has a point, maybe it could be even better collimated.

What happened? Good question, but basically the improved collimation made the corners worse. Why? Not a clue.

Last night we removed the Celestron window and it didn't make a huge difference. However, we then rotated the chip by 180 degrees and there was a vast improvement. My thinking on this is shown in this magnificently high quality sketch :D:

1494081517_doubletilt0025.jpg.be80d6c74e8003507b0acde4b2e171a7.jpg

The beam is directed by tilting the lens cell. On the left the tilt in the cell and the tilt on the chip go the same way, causing maximum deviation from orthogonality. On the right the camera has been rotated by 180 degrees so the tilt on the chip lies opposite to the tilt in the beam, correcting the angle between beam and chip. I suspect that both errors, in themselves, are very small but that they become significant when added together.

In any event we think could live with what we have, especially if we could get a more convincing autofocus software than the one we're using now. That shouldn't be difficult and focus visibly improves the corners when it has no obviously visible effect in the middle.  Comments on my theory are more than welcome!

I think only testing will show what your dual rig can do.  Because our dual Tak 106 just worked like a charm from night one, I hoped the dual TEC140 would do likewise, but it doesn't. The change in resolution brings differential flexure. The RASAs are physically short and, with one possible exception, are also physically very stiff, having no draw tube to sag.  The uncertainty concerns the primary mirror. How much might it move and how much would such movement matter, given that it's spherical?  In the end, there remains the possibility of simply using two mounts for two scopes.

Thanks again for your input, it's very helpful and reassuring.

Olly

 

 

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Interesting Olly with possible minute tilts in both the scope and camera. I have seen a bit of variability in star shapes and now you got me thinking that it has been the result of how the camera happened to be oriented (I often rotate the camera to get the best framing).

Also interesting that it may be a good idea to focus on periferal stars, at least in your case. Will your autofocus allow you to pick any star in the FOV? I have no experience with autofocus.

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5 hours ago, gorann said:

Interesting Olly with possible minute tilts in both the scope and camera. I have seen a bit of variability in star shapes and now you got me thinking that it has been the result of how the camera happened to be oriented (I often rotate the camera to get the best framing).

Also interesting that it may be a good idea to focus on periferal stars, at least in your case. Will your autofocus allow you to pick any star in the FOV? I have no experience with autofocus.

I think Paul's going to try a focus routine which uses multiple stars. The old rule was always to choose a star at one of the four intersections of the 1/3 lines.  This is my first time with autofocus as well.

Olly

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FWIW I always get nice,  smooth symmetrical autofocus curves with the Celestron RASA focuser drive running through NINA. However, that was a while ago, it's not been out of the box since last Autumn.

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