Rotation after meridian flip... why?

[Uranium235]

Just to further add to the mystery, I had the 200pds out last night and it had exactly the same issue (key stars didnt match up with artemis markers post flip).

[vlaiv]

Just a guess here, what would happen if mount is not perfectly level? if it's tilted to one side? could this be a cause for slight rotation of field pre/post meridian? If mount is a bit tilted to west or east then meridian line on sky and meridian line of mount will be at a slight angle.

Now when I come to think of it, yes, this might be a reason - sky / stars flip around sky meridian and sensor flips around mount meridian, on one side - east of meridian - consider them aligned, but on west side frame is rotated with angle 180 degs + difference between sky meridian and mount meridian. Well frame rotates 180 degrees but stars inside frame rotate a bit more. I made a mess out of explaining, let me try to make a diagram.

Edited November 17, 2016 by vlaiv

[vlaiv]

No, I was wrong, it can't be due to mount being tilted. Only way to get angle between frames is due to polar alignment, or lack of it. One way to explain it would be - misalignment is such that on east side circles are aligned almost ok, while on the other side they start do diverge.

Edited November 17, 2016 by vlaiv

[Uranium235]

I've already considered the tripod being titled, but ruled it out. And Im also thinking bad PA cant be a cause either, otherwise it would show rather quickly while imaging @ 1000mm (even when guiding). Not unless youre talking about just a small error being a potential cause.

Next time im out (whenever that may be!) I'll run another test to see if I can replicate the error.

[steppenwolf]

I haven't got any further with this topic either because the skies have been cloudy but mount tilt isn't going to be the answer I fear.

[Davey-T]

No help to you Rob but if using Maxim and plate solving you can check if the sensor is square to the "horizon" and see if the error is different after flipping, never bothered myself as I just align it to the top of the scope rings but will have a look just out of interest if it ever stops raining.

Dave

[vlaiv]

Yes, I'm thinking about small error, also depending on polar alignment error - it could be present on one side and almost none on the other side - look at two circles - place where they intersect - tangents are at the angle, further away from that point tangents are more and more parallel - 90 degs from intersection - tangents are fully parallel.

Contributing to this would be guide scope alignment. Are you guiding with guide scope or OAG? Also what is length of your exposure? If long, check for field rotation inside frames. Make sure that guide scope is aligned well with main ota. You can easily do this by slewing to a bright star and checking fovs of both cameras - imaging and guiding - star should be centered in both.

[martin_h]

46 minutes ago, steppenwolf said:

I haven't got any further with this topic either because the skies have been cloudy but mount tilt isn't going to be the answer I fear.

I was out last night, did the flip and post flip images are rotated as before.. am out again tonight, flip coming up soon, I'll measure the rotation.

[martin_h]

OK just did the flip, the plate solve and sync pre flip gave an angle of 87.44 post flip plate solve 268.64 to give a move of 181.2 deg

Edited November 17, 2016 by martin_h

[vlaiv]

So it has nothing to do with actual exposures, its just scope orientation pre/post flip?

[carastro]

Very interesting topic, am following this with interest.  

I am not that technical so can't offer any thoughts except for tilt - if the camera is being locked into the draw tube by only two screws.  Then if there is slight tilt on one side of the meridian, then the tilt would be the opposite way after a flip, doubling the error.

I am probably completely wrong and probably doesn't apply to scopes which don't use the 2 screw method at the draw tube, but just thought I'd throw that into the ring.

Carole

 

 

[ollypenrice]

Given that it took over 200 years of telescopic astronomy to find the stellar parallax on a baseline of two astronomical units or nearly 200 million miles I think we can safely dispense with the idea that it arises from the distance between one side of Rob's pier and the other.  We all know that he's a stickler for fine resolution but... 

If there were neither cone error nor non-orthogonality then surely the images would have to align perfectly.

So what can it be? Non orthogonality of the camera? When the mount flips, any difference between camera angle and lines of RA and Dec will turn into twice that angle of difference between the image before and the image after.

Also, when we align our cameras along RA and Dec, by slewing them while exposing, we are doing so with any cone error already in place, so that cone error is factored into our alignment. Now surely this error we have factored in is 'sided.' That is, it matters which side of the mount we are on when we carry out the procedure. Again, any error will be doubled when measured as a comparison between 'before' and 'after' images. And to take this a step further, won't the cone error we have factored into our camera angle be local to the area of the sky in which we carried it out?

In a nutshell there is a complex and interactive relationship between cone error and camera angle.

Olly

 

 

[martin_h]

Yep that sums it up, I only align my camera by slewing in RA during an exposure, and have never adjusted for cone error(life is to short). Close enough for jazz is my philosophy!

[ollypenrice]

30 minutes ago, martin_h said:

Yep that sums it up, I only align my camera by slewing in RA during an exposure, and have never adjusted for cone error(life is to short). Close enough for jazz is my philosophy!

Mine too, though I don't get much rotation so far as I'm aware.

Olly

[martin_h]

I suppose I could buy a £1000 rotator to correct the 1.5 deg, on the other hand...............  Four fingers and a thumb!

[vlaiv]

Not sure that I'm convinced with this argument.

Cone error - in essence same thing as pointing to a different part of the sky. Only important in the terms - goto / mount / computer thinks that scope is pointing to a certain place, and the scope knows it's pointing somewhere else. After meridian flip, frame will be rotated 180 degrees and point where scope is pointing will be "rotated" 180 degrees - meaning it will flip cone error to other side - it will not contribute to FOV rotation - only offset - this is corrected with RA/DEC offset (which does not rotate FOV).

Orthogonality of camera being X/Y orientation to RA/DEC? - no impact on meridian flip as it rotates FOV by 180 no matter how it's oriented - if it's 30 degs to "horizontal" (being RA) - after 180 deg rotation it will continue to be 30 degs to horizontal.

[vlaiv]

I gave it a bit more thought and here is my argument:

Properly polar aligned setup:

Any movement in either RA or DEC or combination of those will not introduce FOV rotation. Cone error (up down) is movement in RA. Sideways movement of OTA is DEC. So anything that can happen to scope is combination of RA and DEC if properly aligned. Except tube / focuser / camera rotation.

So possible causes of field rotation would be: improper clamping of tube - so it tilts left right (looking down the tube with mount being at the bottom) under gravity when lying on its side (just before and after meridian) - but I guess this is not case - easily spotted, one would feel ota being loose on a mount. Any kind of focuser rotation, camera rotation due to gravity and uneven distribution of weight in configurations west / east of pier.

And of course - polar alignment. We know that field rotation is problem for long exposure if polar alignment is not good. So to me this is obvious reason for field rotation. Mind you, although points before and after meridian flip are close on sky, they are 180 degs apart in both RA and DEC when meridian flip is performed.

[Stub Mandrel]

How much does the scope tube flex between being loaded on one side and then on the other?

(I was going to suggest parallax - those stars are closer than you think!)

[vlaiv]

Also - rotation of FOV for 180 degrees after flip is just apparent rotation - it's not real rotation so it can rotate a bit more or a bit less - its two times mirror image - once in vertical, once in horizontal - two flips combine to give same result as 180 deg rotation. This is due to 180 degs RA and DEC movement when performing meridian flip.

Edited November 18, 2016 by vlaiv
correction to make it more clear what I was trying to say ... :D

[martin_h]

I get the same result regardless of scope mounted, 350 mm focal length or 1.6 mtrs so I don't think it's the tube shifting.

[ChrisLX200]

2 hours ago, vlaiv said:

Not sure that I'm convinced with this argument.

Cone error - in essence same thing as pointing to a different part of the sky. Only important in the terms - goto / mount / computer thinks that scope is pointing to a certain place, and the scope knows it's pointing somewhere else. After meridian flip, frame will be rotated 180 degrees and point where scope is pointing will be "rotated" 180 degrees - meaning it will flip cone error to other side - it will not contribute to FOV rotation - only offset - this is corrected with RA/DEC offset (which does not rotate FOV).

Orthogonality of camera being X/Y orientation to RA/DEC? - no impact on meridian flip as it rotates FOV by 180 no matter how it's oriented - if it's 30 degs to "horizontal" (being RA) - after 180 deg rotation it will continue to be 30 degs to horizontal.

There's more than one way of getting Cone Error - one way is if the scope does not sit orthogonal to the DEC axis (the saddle may not be orthogonal to the DEC axis), another is if the DEC axis itself is not orthogonal (perfectly @ 90deg) to the RA axis. Consider the latter - the pointing error will be doubled after a meridian flip (if the pointing offset was 1deg East before the flip it will be pointing 1deg West afterwards thus requiring a 2deg movement in RA to get back on target). So you have 2 different RA positions needed to point at the same place in the sky leading to a rotational offset.

ChrisH

[vlaiv]

39 minutes ago, ChrisLX200 said:

another is if the DEC axis itself is not orthogonal (perfectly @ 90deg) to the RA axis.

Yes, I see what you mean, still not sure if that would result in frame rotation. It might, but if we reason that 180deg rotation in each axis is flipping FOV (in either vertical or horizontal direction), not sure if it would lead to FOV rotation more or less than 180 degs. Pointing error will surely be present - GOTO would suffer no doubt. Center of FOV might end up in wrong place after flip - offset by some RA and DEC amount but not sure about rotation.

Lets think of it this way. Instead of having tube pointing only in one direction let's consider tube that is hollow and points forward and backward - empty OTA for sake of argument, we are looking at central line - line of sight of OTA both forward and backward. 180 deg rotation around any axes that crosses OTA central line will result in flip. Yes, indeed - if we flip on two axes that are not orthogonal we will end up with rotation that is not perfect 180. This is possible solution to problem.

So if RA is not orthogonal to DEC - frame rotation will happen!

This case is not even hard to imagine. On my HEQ5 when I did adjustment of backlash and general maintenance - at certain point in procedure DEC axis is adjusted only on one side of RA (next to the worm, the other side, down where weights bar is is held by bearing in place), by moving couple of millimeters back and forth - enough to deviate from 90 degrees.

Edited November 18, 2016 by vlaiv

[alexbb]

7 minutes ago, vlaiv said:

Yes, I see what you mean, still not sure if that would result in frame rotation. It might, but if we reason that 180deg rotation in each axis is flipping FOV (in either vertical or horizontal direction), not sure if it would lead to FOV rotation more or less than 180 degs. Pointing error will surely be present - GOTO would suffer no doubt. Center of FOV might end up in wrong place after flip - offset by some RA and DEC amount but not sure about rotation.

If you have the offset and then you try to center to the same FOV, you need to rotate the axes. Hence the rotation.

You should be able to experiment this by pointing to horizon during the day and center on the same point, on both sides of the mount.

[vlaiv]

1 minute ago, moise212 said:

If you have the offset and then you try to center to the same FOV, you need to rotate the axes. Hence the rotation.

You should be able to experiment this by pointing to horizon during the day and center on the same point, on both sides of the mount.

Well if you have proper polar alignment and RA and DEC are orthogonal (I edited my post above, I think it is plausible explanation for field rotation - RA and DEC not being perfect 90 degs), any offset will be corrected with motion in DEC and RA - these do not create FOV rotation.

[martin_h]

This is turning into a very interesting/ complex post, lots of input. Well done everybody