Is this field rotation?

[Euan]

Since getting my 383L I've been scratching my head at the CCD Inspector results I'm getting from it

I have a feeling that's it's just that my polar alignment is out for some reason, as after I did a pier flip the other night, the tilt has completely reversed

Has anyone seen this before?

[steppenwolf]

This is not field rotation - it would appear that your CCD is most likely tilted with regard to the light cone.

[Euan]

This is not field rotation - it would appear that your CCD is most likely tilted with regard to the light cone.

Would it not be the same orientation tilt after a pier flip though as nothing has changed with the light cone? Surely it should only do this if I rotated the CCD 180 degrees in the focuser?

Both the images used above are the same orientation, and when I look at the corners the bent stars do actually move from one corner to the other

[steppenwolf]

If the connection to the telescope is slack or the focus tube is loose then the pier flip change the position of the camera resulting in the light cone to CCD plane flipping too.

[Euan]

If the connection to the telescope is slack or the focus tube is loose then the pier flip change the position of the camera resulting in the light cone to CCD plane flipping too.

Ah yes that is a good point

I will see if there is any perceivable movement

[Ben Ritchie]

Agree with Steve, looks like tilt - my first guess would be the focuser sagging under the weight of the CCD.

Before worrying too much about CCDInspector, can you see this effect in your images? I spent a while chasing down some CCDI results at one point before realizing that they were all but invisible in the final image...

[Euan]

I've checked the whole imaging train and everything is rock solid

The Moonlite can take some amount of weight, and with the 383 on it's own, that only amounts to 500 grams

[Ben Ritchie]

You don't need much sagging for CCDI to see it - i'd suspect the spacers between the TRF-2008 and the CCD, I had a very similar setup once with a TRF-2008, assorted spacers, and relatively light SXV-H9, and that had very similar issues.

Nice neat cabling, by the way - my observatory looks like an explosion in a spaghetti factory!

[Euan]

Before worrying too much about CCDInspector, can you see this effect in your images?

Very much so, it's quite noticeable. Here is a corner crop:

[Euan]

You don't need much sagging for CCDI to see it - i'd suspect the spacers between the TRF-2008 and the CCD, I had a very similar setup once with a TRF-2008, assorted spacers, and relatively light SXV-H9, and that had very similar issues.

I've always had issues with spacing and reducers, it's made me seriously think about having custom length ones made each time. At least that would mean only one piece holding it on

[Ben Ritchie]

Ok, I had this setup, which appears to have similar issues with stars in the corner of the field - it was much worse on one side of the CCD than the other

[steppenwolf]

Having seen the image train on your 'scope, I am even more convinced that this is a tilt problem as there is just so much potential for sagging - it is amazing how little 'sag' is required to cause a problem.

[Euan]

Thanks for the help guys, at least I now know what I'm looking for

I had it nailed with the QHY8 as well, the plot from that is below

It's never easy is it?

[Euan]

Nice neat cabling, by the way - my observatory looks like an explosion in a spaghetti factory!

The trick was to send all the DC supplies up a bunch of 1.5mm speaker cable, along with a USB extension cable for all the data going to a USB hub mounted on one of the scopes.

It means its all in one nice long flexible loom

[Ben Ritchie]

It's never easy is it?

Nope!

Trying to keep things precisely orthogonal as you slew all over the sky is a real pain in the neck - I was struck by how similar your TRF-2008/spacer setup was to mine, looking at the damn things it looks impossible that they can be anything other than rigid ... but ...

[roundycat]

I would suspect the push fit connections. I use the TT 'T' thread spacers without any problems as they are rock solid. What's more I hang an ST10 off the end. With threaded connections it always seems to be solid, put in one push fit connector and it's solid no more.

Dennis

[Ben Ritchie]

I guess another possibility is that the focuser itself is not orthogonal?

[Euan]

I would suspect the push fit connections.

What do you mean? Do you mean the focuser connection?

[roundycat]

anthing that is a push fit and locked in place with one, two or three grub screws is suspect. Always do the screws up fingertight and then a further one eighth of a turn with pliers.

Dennis

[ollypenrice]

Agree with Dennis. I take care which nosepiece I use with which female adapter. Some are not happy together, in one case when both are from the same manufacturer. The non standard and non-necessary compression ring do-dahs have a lot to answer for. Pushing the nosepiece firmly inboard as you tighten is essential.

However, your cabling is indeed neat but is it flexible? Could it possibly be putting pressure on the camera and inducing the non-orthogonality in some positions? This might also account for the assymetry across the sky.

Olly

[Euan]

Ah I see, I will check the focuser out. I don't see the point of those type of connections you would think there would be a better way?

The cabling comes from the mounting bar for the two scopes, so there's no pressure on the imaging train

[ollypenrice]

On both the Tak FSQ and the TEC 140 there are collet EP holders which tighten by rotating and should give greater orthogonolity. I like this system and it doesn't mark whatever is going in the scope.

Olly

[Euan]

Solved!

One of the t-thread adapters had managed to thread itself and twist itself out of shape by about 1mm

The before after thing was just me being a div... I made the pictures the same orientation of course this flips the results in CCD Inspector