Field rotation after flip - is this cone error?

[BrendanC]

Hi all,

Continuing to familiarise myself with my ASI1600MM-Cool. I think I'm starting to understand that I have light pollution that my trusty old EOS1000D didn't pick up as readily, which I can see when I compare before/after flip images because the sky gradient shifts around too.

HOWEVER, on going through this, I've noticed something I knew was happening, and decided to take the opportunity to get some insight into it here.

Watch this:

(Ignore the very faint sat trail in the after shot, that's not the issue).

See how there's a field rotation before and after?

This is a 130PDS on an NEQ6 mount, so the mount shouldn't have a problem with field rotation. 

Is this cone error? I've never seen an example of it before so this is the closest guess I can make.

It's not a serious problem in that I can still get images out of this, but it does mean I have to crop more than I'd like to, for example with nebulae.

Any takers?

Thanks, Brendan

[Adreneline]

I used to get something very similar to this with my Samyang135+ASI1600 mounted on a CEM25-EC - I know this is a different scale in terms of size of scope/mount but the effect was just the same. It turned out that when the flip occured the OTA was moving a very, very small amount because the mounting arrangement I was using at the time was not as tight/firm as I thought it was. I improved the mount of the lens and camera and the problem went away. I was essentially only using a 'central' mount on the Vixen bar which was holding the lens with camera attached. I revised it so I was mounting both the lens and the camera separately on the bar and it stopped the small amount of twist that was happening when the mount flipped.

Hope this isn't a red herring!

 

[BrendanC]

Hmmm, interesting. I just have the standard Sky-Watcher mounting, nothing fancy, but it seems to grip the OTA fine. Definitely one to think about though, thanks. I can totally see how even a slight twist pre- and post-flip could cause this.

I still don't really actually know what cone error is though, and whether this is it. 

[carastro]

Looks like what l get after a flip and l put mine down to cone error.  I just live with it and crop the edges.  
 

Carole

[BrendanC]

That's what I've been doing for the past 2 years too! It's just that while going through my subs I spotted this and thought it was an ideal example to show what's going on.

If this is cone error then I might give Cone Sharp a go - or, I might just continue to live with it.

https://www.sharpcap.co.uk/conesharp

Edited April 27, 2022 by BrendanC

[ollypenrice]

The cross below roughly marks the centre of the frame, so that's more or less where the telescope is pointing (there being the possibility of slight misalignment of the optical axis within the OTA, droop in the focuser etc.)

The centre of rotation, however, is very close to the marked star.

Now, what does that tell us?

Ah, now I'm not very good at these spatial awareness conundrums. Thinking aloud, the mount thinks it has pointed the OTA to what lies at the centre of the frame but actually it has offset it slightly to the small star at the centre of rotation. A difference between the direction of the saddle plate and the direction of the telescope (ie cone error) seems like a good fit.

However, if the marked star were the guide star and there were polar misalignment, wouldn't we also see rotation around that point? I think we would. How can we distinguish between the two? By knowing where the guide star was. If you were using an OAG or had aligned your guide scope fairly well with your main scope (which serves no real purpose) then the star at the centre of rotation might be the guide star. I think it more likely that it's not, making cone error more likely. Another way to check would be to look for field rotation between the first and last sub on the same side of the meridian. If it's the polar alignment solution you should see some rotation around the star in question even without a flip.

 

None of this comes with a guarantee!

Olly

 

Edited April 27, 2022 by ollypenrice
Typo

[symmetal]

It suggests to me that the dovetail isn't sitting orthogonally to the RA axis in the 'left to right' direction. That is in the home position with the RA axis pointing down, one side of the dovetail is slightly higher than the other so the scope mounting is rotated slightly cw or ccw when standing behind the scope looking North.

Cone error usually implies that the orthogonal error is in the front to rear direction, that is the front of the dovetail is sitting 'higher' than the rear of the dovetail or vice versa. The screws at the ends of the dovetail can allow this front to back error to be corrected.

Whether Conesharp will show this rotational error I'm not sure. If the dovetail adjustment screws are sitting in the puck you could try adjusting them on just one side to allow the dovetail to be rotated slightly, or it may be easier to put a thin strip of metal under the dovetail down one side to effectively rotate it when it's clamped.

A bit of trial and error should show where it's needed.

Alan

[tomato]

Here is another example, NINA executed the meridian flip and platesolved and adjusted the scope pointing until it was within NINA's tolerance (whatever that is). I'm running a very heavy dual rig so I never get my cameras  perfectly aligned so I always have to crop anyway.

Apologies for being so cavalier with my sensor real estate, but  my excuse is I'm imaging galaxies.

[BrendanC]

Thanks everyone.

Given that the NEQ6 is rated for 25KG, and my kit probably comes in at less than 6KG, I think I'm tending towards the cone error suggestions rather than OTA or saddle sag.

Right, I'm going to stop typing now before that animated GIF causes me to have a fit...

[tomato]

Sorry, my face starts to turn the same shade as the gif if I look at it for too long.🤢