What on earth has happened here?

[BrendanC]

So I'm still getting walking noise with my image acquisition - I'm going to ramp up the dithering, make sure it's dithering in RA and Dec, and try and get my polar alignment better.

However, on inspecting individual frames it seems I have an even worse problem. Take a look (you get a better sense of what's going on if you click the image below then you can see it 100% size...)

 

I have trails that go horizontal to the right of the image (zoomed-in bottom right section below)...

... but then seem to curve up towards the vertical on the left! (zoomed-in top left section below)

 

The brighter stars also seem to have a 'judder' to them.  It's the same for every one of the 155 subs.

How can this happen?

This is from a 130PDS on an NEQ6 mount, camera is a modded EOS1000D with Skywatcher coma corrector. I'm using DARV for polar alignment in APT. The subs were 30 seconds, just tracking, no guiding. Control was via APT, using APT's own dithering module, via EQMOD directly connecting the laptop to the mount. There is no stretching or processing on this image, other than converting from raw to JPG to upload it. It was a really clear, calm night, no wind. The mount is on the grass, not concrete, and I made sure it was level.

I've been able to get 30 sec subs easily before with pin-sharp stars. I'm stumped as to how this could have happened. Tripod not level? Camera not secured? Collimation? PEC? Is it optical, or mechanical? Little goblins coming out at night and shaking the tripod?

Any/all suggestions welcome.

Thanks, Brendan 

Edited July 11, 2020 by BrendanC

[BrendanC]

I've had a suggestion this could be backlash, and that I might need to experiment with being east-heavy. Any comments welcome!

[wxsatuser]

Always useful to set east or west heavy, keeps gears engaged, does'nt need much.

If I remember right counterweight slightly down the bar for east and slightly up to west.
About half an inch from balance should do it.

Edited July 18, 2020 by wxsatuser

[PeterCPC]

Cable snag?

Peter

[pete_l]

sticky bearings? (stiction)

[MarkAR]

Could it be image tilt?

[ollypenrice]

Could you post 5 images cropped from one single sub: centre and four corners. Post them heavily cropped and at full size, but strictly from one single sub.

Olly

[BrendanC]

Thanks for all the suggestions but it turned out I need the mount to be east-heavy. Having done that, the problem is fixed (until it, or another of the myriad problems I've been having, pops up again).

Thanks, Brendan

[BrendanC]

@ollypenrice - help!!!! (and anyone else too!)

So I thought I'd fixed this. I was wrong. Right now, I'm looking at every frame coming through with a ton of problems. I've tried changing dithering times and settle periods, turning dithering off, using anti-vibration pause (this is all in APT), been outside to check for snagged cables, moved weights around - no change.

Comparing this with the very first shots I took in May, when I first got the mount, I seem to have taken several huge steps backwards! None of these artefacts were present then. Since then I've moved the setup to a lawn rather than cement, for better vibration and less heat, but a big change has been to start using EQMOD to bypass the hand controller, which means I'm tracking using Stellarium now. I do wonder whether something is very wrong with this setup, and I might just go back to the hand controller next time to check. I've checked that EQMOD, Stellarium and Stellarium Scope are all using the same locations and epoch settings. Put it this way: if the hand controller works, I'm more than happy to go back to it!

So, as you requested, here are five shots, just crops at original size, from one single sub:

Top left

 

Bottom left

 

Bottom right

 

Top right

 

And finally, middle

 

You can see my kit list in my signature.

I have absolutely no idea what to do about this, so any/all comments welcome. I'm sure my collimation and polar alignment aren't the greatest (I'm still really learning, I've moved to Sharpcap Pro for the polar alignment, only my second time doing it this way), but anything anyone can suggest, please do.

Thanks, Brendan

Edited July 28, 2020 by BrendanC

[wxsatuser]

Not an expert on 130pds but is the focuser solid and not got any movement?

[BrendanC]

I think it's fine from what I can see. 

Personally I think there's some timing issue somewhere. Something isn't set right. When I align stars they're all out more than I would expect. I'm going to go back to using the handset for alignment as a test, unless anyone can pinpoint the problem here. 

[barkingsteve]

I could be wrong but it looks like your focus tube could be impeding inside the ota, it is a known problem with this telescope. Some people cut an inch or so off the bottom of the focus tube to solve this problem, there are threads about it somewhere. I just move the focuser almost all the way out then get close to focus before i lock the camera down.

[BrendanC]

How could that cause these issues though? I never had them in the previous 18 months of using an AZ mount, with the same OTA, focuser and camera. I also didn't have this problem when I first used the new mount with the handset. 

This is a new problem since using EQMOD. But that could be a red herring. 

It's driving me mad frankly. 

@ollypenrice seemed to have an idea about analysing the corners and middle of the image, so I'm hoping he will pop up and help here. 

[ollypenrice]

Well, what the five crops from one sub tell us is that the tracking is not the primary problem. If you had a guiding error in one axis, all the trailing would be along that axis. We don't see that. The two left hand crops show distortion more or less in this axis:  \ .  The bottom right shows distortions comparable to these but more vertical. However, the top right distortions are angled just into this direction: / .  It would be absolutely impossible for a tracking error to produce this pattern. All elongations would run along the same axis.  Very severe polar misalignment will produce rotation (around the guide star) resulting in a pattern like this: top left and bottom right / . Top right and bottom left the opposite: \ . Connect all these elongations up and you'd get circles, just like those of a star trail image and the pole star.

Also, the distortions, though elongations, are not neat lines as they would be with a tracking error in a well-collimated scope. In the middle of the image they are clearly 'shuttlecocks' of a kind often seen in mis-collimated optics. I once had a TeleVue Genesis which had been bumped and gave these shuttlecock stars at the eyepiece. It was remarkably easy to re-collimate the front lens cell and turn them into nice concentric rings in and out of focus and a good pinpoint at focus.

So we can be pretty sure your problem is optical. I'm not an expert on collimating or troubleshooting Newts but there are plenty of people on here who are. It might be worth reposting with 'Optical problems with 130PDS' in the title.

The distortions are bad enough to mask very small tracking errors which is why I say tracking isn't your 'primary problem' but your tracking might indeed be excellent. It absolutely is not the main problem.  If, you've used the instruments you have, and followed the collimation routines you know, you are left with a choice. Do you believe the tools and the routines which tell you it's right or do you believe the image which tells you it's not? I would believe the image...

Olly

 

[BrendanC]

Brilliant. I am so very grateful for your help with this. I will take another look at the optical setup and might repost as you suggest with a new title.

If I still have problems I'm going to plan B (actually, plan Z given all the issues I've had): use the handset and see if that improves things at all.

Thanks again.

[barkingsteve]

3 hours ago, BrendanC said:

How could that cause these issues though? I never had them in the previous 18 months of using an AZ mount, with the same OTA, focuser and camera.

Here is an image of an out of focus 130 pds with the focuser tube intruding, which can cause mis-shapen and elongated stars. You can see why i thought this might be the cause, the tube travels in further than this and with coma, tilt you could get very streaky stars, but if you haven.t change the optical setup in any way then it is obviously not the cause.

 

[BrendanC]

Interesting, I'll take a look at that too.

I just collimated the hell out of my OTA, and reconnected the handset.

If that doesn't work then I'll be out of ideas.

I've had so many issues over the past few months I'm seriously considering throwing in the towel.

[ollypenrice]

1 hour ago, BrendanC said:

Interesting, I'll take a look at that too.

I just collimated the hell out of my OTA, and reconnected the handset.

If that doesn't work then I'll be out of ideas.

I've had so many issues over the past few months I'm seriously considering throwing in the towel.

Personally I like handsets and dislike computers, so I run handset mounts - albeit good ones. (Mesus with Argonavis handsets.) But this is not your problem, the optics are the problem. The intruding focuser should not be dismissed.

Don't give up! It's so sweet when you get it all working.

Olly

[bottletopburly]

Be careful cutting tube back as too much will cause tube to roll off runners before hitting tube stop , I got a 130pds where this had been done now fixed though only need about 10-15 mm off .

[BrendanC]

The intruding focuser is a candidate, I agree, but I just don't understand why it should suddenly have become a problem.

Also, wouldn't the images all be in the same direction? The examples I posted show that the corners all have different directions, which @ollypenrice pointed out.

[ollypenrice]

57 minutes ago, BrendanC said:

The intruding focuser is a candidate, I agree, but I just don't understand why it should suddenly have become a problem.

Also, wouldn't the images all be in the same direction? The examples I posted show that the corners all have different directions, which @ollypenrice pointed out.

I don't know about diffraction artifacts and how they behave. We need Newt experts on the case!

Olly

[barkingsteve]

I would suggest asking here:

You may have more luck, plenty of experts with the 130 pds there.

[barkingsteve]

I have blown up a random star from each section and to me it still looks like focus tube intrusion with coma or tilt, you can see the hard edges cut off on the right side of the stars. with coma and /or tilt the intrusion will move up or down but still be mostly on the right. I am no expert btw but this is what it looks like to me. There could be other things going on here as well though.

 

I still suggest you ask in the previous thread i linked, they are imagers, while i am more of an EAA guy  

Edited July 30, 2020 by barkingsteve

[MarkAR]

Certainly does look like the focus tube is protruding. Seen very similar on another thread a few months ago.

[BrendanC]

Well, blow me darn wiv a fevver.

I never considered actually zooming into a star like that to see what was going on! Doh...

OK, well, this is all very interesting. I don't quite understand why this should have started becoming a problem - although it's entirely possible that it's only revealing itself to be an issue now that I'm using a decent mount, compared to the old AZ mount.

I'm EXTRAORDINARILY reluctant to start sawing bits off the tube however. Are there any other ways around this?

Oh, and one more very quick question: would this also account for the way the blurs are in different orientations? Or would that be something else entirely?

Thanks for all the help so far.