Sensor tilt star shapes help please

[RayD]

I managed to get first light with my Star 71 and SX Trius 814 last night and have some concerns over the stars across the field.  If I have sensor tilt will all the stars across the whole sensor show an aberration on one side as that is what I seem to have?

Below is a lightly cropped and stretched .jpg from a Ha stack of 28 x 900s subs and to me when looking at the larger stars they all seem to blow out on the left but more of a flaring or aberration than just an elongation (sorry I'm struggling to explain how it doesn't just look like elongation).  

It was take using the AVX and no auto-focus, but using a bahtinov mask.  It is much more evident on the .fits image.

Any ideas welcome as it was a checked OTA from Flo so I wouldn't think it is anything sinister just I'm not sure what would cause this oddity.

[Starlight 1]

I  have seen this and the ink wells are fulling more on the one side. ask steve as I see it in a post , and terry  will tell you how move it over. 

[RayD]

1 minute ago, Starlight 1 said:

I  have seen this and the ink wells are fulling more on the one side. ask steve as I see it in a post , and terry  will tell you how move it over. 

Ah OK so you think it is camera related rather than OTA?

[Uranium235]

Have you used the rotator ring on the focuser at all?

If so, you need to make sure all three screws are undone by the same amount (maybe one turn), then done up by exactly the same amount after rotational adjustment. Then tweak each one up by a small (equal) amount until theyre tight.

Edit: On closer inspection, there seems to be distortion across the whole image - was your guiding as good as it could have been?

[Starlight 1]

Yes Steve how do the book on here.

[RayD]

2 minutes ago, Uranium235 said:

Have you used the rotator ring on the focuser at all?

If so, you need to make sure all three screws are undone by the same amount (maybe one turn), then done up by exactly the same amount after rotational adjustment. Then tweak each one up by a small (equal) amount until theyre tight.

I have but only undoing and doing up the thumb screw. I didn't give that a thought so will check that. 

Thanks. 

[Uranium235]

Just now, RayD said:

I have but only undoing and doing up the thumb screw. I didn't give that a thought so will check that. 

Thanks. 

Its what I need to do with mine to ensure a good field, so I only rotate when I really need to. Also, when I got mine I had to pinch up the two brass screws on top of the focuser - but mines a Mk1 that was pre-testing (fortunately it turned out to be a good one!).

[Starlight 1]

Look at this post ray  

star shape issue

By Erquy, August 17 in Imaging - Discussio

[RayD]

2 minutes ago, Uranium235 said:

Its what I need to do with mine to ensure a good field, so I only rotate when I really need to. Also, when I got mine I had to pinch up the two brass screws on top of the focuser - but mines a Mk1 that was pre-testing (fortunately it turned out to be a good one!).

Yes makes sense. I'll have a check of all of them. 

Once set it shouldn't really need moving any more I hope. 

[RayD]

12 minutes ago, Starlight 1 said:

Look at this post ray  

star shape issue

By Erquy, August 17 in Imaging - Discussio

Thanks I'll have a read. All ideas been really helpful. 

[RayD]

Thanks @Starlight 1 I've emailed Terry as reading through that post it does seem like I have the same issue, but as I was imaging NB it seems it may be fairly prominent as the OP in that post noted he didn't see in NB images.

We'll see what Terry says.

[Starlight 1]

Was a good post and one of them post not to be over looked as olly  said. 

[RayD]

48 minutes ago, Starlight 1 said:

Was a good post and one of them post not to be over looked as olly  said. 

Terry has got back to me already and noted it could be an issue with the anti-blooming adjustment.  I've sent him a .fits file to have a look at so fingers crossed.

Thanks again 

[RayD]

OK as as update Terry has looked at the .fits file I sent him and has pretty much eliminated the camera (on a Sunday, what a good bloke).  He has noted that the issue seems to be more diagonal than lateral, so is highly unlikely to be something sensor related as any anti blooming problem would show to one side only, and usually the right.

@Uranium235 he tends to agree with your diagnosis of it being an optical issue probably to do with positioning and/or tilt, so I will look at that rotator thingy today.  Thanks for your help.

[Starlight 1]

Well that that one off the list .

[RayD]

16 hours ago, Uranium235 said:

Have you used the rotator ring on the focuser at all?

If so, you need to make sure all three screws are undone by the same amount (maybe one turn), then done up by exactly the same amount after rotational adjustment. Then tweak each one up by a small (equal) amount until theyre tight.

Edit: On closer inspection, there seems to be distortion across the whole image - was your guiding as good as it could have been?

Sorry just seen your edit.

Yes I believe the guiding was pretty good but I will definitely check on the graphs to make sure, but it was OAG and didn't raise concerns at the time.

A single .fits image shows the issue clearer and it shows as a kind of flaring to the left rather than just an elongation which I would expect to see with bad guiding (I don't know this for sure of course as self admittedly I am not very good at anything).

[Uranium235]

4 minutes ago, RayD said:

Sorry just seen your edit.

Yes I believe the guiding was pretty good but I will definitely check on the graphs to make sure, but it was OAG and didn't raise concerns at the time.

A single .fits image shows the issue clearer and it shows as a kind of flaring to the left rather than just an elongation which I would expect to see with bad guiding (I don't know this for sure of course as self admittedly I am not very good at anything).

I'll still go with either guiding, or some tilt/loose connection. I say that because the issue is present in the very centre of field - where you would expect the best stars to be. Just be patient, tighten everything up (but not too tight!) and start off with short exposures (60-120s) with a luminance filter on a good starfield. Then inspect it closely - centre, and all four corners. If all is good, push it a bit more progressively until youre at the exposure length you want to be at.

Its always hard to diagnose these things at first, so its just a process of elimination.

[geordie85]

As this topic is about star shape and possible tilt, I hope you don't mind me adding to this thread.

I'm having an issue with mainly the top left corner of my images too. Guiding was fine and there was very little wind to speak of. 

Taken with an atik 383l on a Mk1 WO star71 that was re-collimated by Es Reid before I bought it. 

[RayD]

2 hours ago, Uranium235 said:

I'll still go with either guiding, or some tilt/loose connection. I say that because the issue is present in the very centre of field - where you would expect the best stars to be. Just be patient, tighten everything up (but not too tight!) and start off with short exposures (60-120s) with a luminance filter on a good starfield. Then inspect it closely - centre, and all four corners. If all is good, push it a bit more progressively until youre at the exposure length you want to be at.

Its always hard to diagnose these things at first, so its just a process of elimination.

You were right with the rotator.  I checked having removed the thumbscrew and it flopped around like a pea in a bucket!  I'm a little surprised (and worried if I'm honest) as this was a checked on from FLO that I only got last week so haven't used it.  I've snugged them up (not tight) so will see if there are changes tonight.

Thanks again.

[Starlight 1]

5 minutes ago, RayD said:

rotator

 or is going round like this

[Uranium235]

14 minutes ago, RayD said:

You were right with the rotator.  I checked having removed the thumbscrew and it flopped around like a pea in a bucket!  I'm a little surprised (and worried if I'm honest) as this was a checked on from FLO that I only got last week so haven't used it.  I've snugged them up (not tight) so will see if there are changes tonight.

Thanks again.

Looks like we have found the culprit then! Once the tree brass screws on the rotator are done up, the thumbscrew kind of becomes redundant, I wouldnt rely on just that thumsbrew to hold things squarely in place.

[Uranium235]

1 hour ago, geordie85 said:

As this topic is about star shape and possible tilt, I hope you don't mind me adding to this thread.

I'm having an issue with mainly the top left corner of my images too. Guiding was fine and there was very little wind to speak of. 

Taken with an atik 383l on a Mk1 WO star71 that was re-collimated by Es Reid before I bought it. 

 

Point the telescope lens end downwards.. (eg: hold it between your legs sitting down)

Remove one of the three rotator screws then pop it back in just enough for the thread to hold it in place (and so the slot is in an easy to remember position eg: 12 and 6 o'clock). Next, do the same for the remaining two screws. Once all three are off, do not tilt or nudge the telescope becuase the rotator ring may come off (been there.. done that!)... also dont forget to cap it first with the M48 threaded cover so bits dont fall onto the rear corrector.

Now, tighten up each screw using the same amount of twists of the screwdriver. When the screws are close to binding with the rotator, use smaller increments (ie: 1/4 turn) until it is tight - do not use the silver thumbscrew (tighten that one up last when everything else is tightened up).

Its pretty much the same principle as collimating a newtonian secondary where you use equal, measured adjustments to ensure things arent pushed too far in one direction.

The only other thing I can add is use the "right" screwdriver - one that matches the slot size because those brass screws to burr up quite easily. And if for any reason you need to rotate the focuser again in future - undo the screws by equal amounts, before tightening up by equal amounts.

[RayD]

2 hours ago, geordie85 said:

 

As this topic is about star shape and possible tilt, I hope you don't mind me adding to this thread.

I'm having an issue with mainly the top left corner of my images too. Guiding was fine and there was very little wind to speak of. 

Taken with an atik 383l on a Mk1 WO star71 that was re-collimated by Es Reid before I bought it. 

 

No I don't mind at all.  I know how disappointing it is to think you've had some good skies to get not so good images.

[RayD]

1 hour ago, Starlight 1 said:

 or is going round like this

More the latter by the looks of it! 

[RayD]

Am I right in thinking that if I rotate the camera and the aberration moves around then it is within the optics and not the camera?