Square stars!

[Priesters]

Hi,

Just wondering if anyone could have a look at these images and work out why the stars have a square-ish appearance.  One image is the stacked and stretched version from SharCap (no further processing done) and the other is a single sub frame.  I might be being paranoid but I wondered if I had a problem somewhere in the imaging train.

Any help appreciated.

Andrewframe_00010.fits

[pipnina]

I can't see the problem, but it could be the web browser interpolating the image.

When you zoom in on an image displayed with nearest neighbour interpolation, stars may look a bit square if they are relatively sharp.

I can't see anything much wrong though. I think your image looks quite nice.

[The Lazy Astronomer]

I don't see square stars, but what I do see is possible evidence of pinched optics. Can happen when the lens cell gets cold and contracts, and as it's been very cold recently, l wouldn't be that surprised if that was the case.

[ONIKKINEN]

Not seeing square stars, but i guess your star shapes can look a bit square-ish with the aberrations around them. Took your .PNG of the stack and turned sharpening to 11 to show starshapes better:

Lots of things could be to blame, like collimation if its way off, obstructions to the light path, something in the way causing diffraction (tree branches, powerlines), pinched optics and so on.

[Priesters]

7 hours ago, The Lazy Astronomer said:

evidence of pinched optics.

 

6 hours ago, ONIKKINEN said:

pinched optics and so on.

I suspect it is pinched optics as this is the first time I’ve used this scope (WO GT81) in such cold weather.   Only other change has been a new camera (Altair HyperCam 183C).  Is there anything I can do about pinched optics?

[The Lazy Astronomer]

13 hours ago, Priesters said:

 

I suspect it is pinched optics as this is the first time I’ve used this scope (WO GT81) in such cold weather.   Only other change has been a new camera (Altair HyperCam 183C).  Is there anything I can do about pinched optics?

A dew heater tight to the lens cell may help. There's probably also some adjustment that could be done, but it's not something I've ever done (or indeed would ever want to do, for fear of messing something up). 

Wait to see what the stars are like on a less cold night - it may be that under more normal winter temperatures all is well again. If it's still bothering you after that, I'd look to have a professional sort it out, personally. 

[Priesters]

1 hour ago, The Lazy Astronomer said:

I'd look to have a professional sort it out, personally. 

Thanks for the advice and yes I’ve learnt the hard way in the past about fiddling with stuff I don’t understand.  Looking back at previous images I can see the effect, but to a lesser degree, when the weather was warmer.  I bought it from Flo in Feb so I’ve emailed for their advice.   However, I am a little worried as after Googling ‘WO GT81 pinched optics’ there seems to be an unnerving number of threads!

[scrufy]

That’s why we don’t use square telescopes for imaging!

Sorry, all joking aside it does appear to be pinched optics.

Is it an inherent problem with that scope design or just some particular samples?

Maybe FLO can exchange it?

[Priesters]

3 hours ago, scrufy said:

Is it an inherent problem with that scope design or just some particular samples?

Hopefully not 🤞.  Some of the threads I’ve read ended with a satisfactory resolution, usually by slackening the collimation screws, other threads with less satisfactory conclusions.  That said,   I don’t think this is unique to WO optics.  

[powerlord]

I get a similar thing with my Redcat. Like yours - only really visible with bright stars. I just sort it in post processing. Let's face it - it's still doing a very fine job.

[AstroMuni]

I dont see square stars. Just a few dust spots and a bit of what maybe vignetting? Both would be removed by flats. If this were my scope I wouldnt worry about it at all as the image looks great!

EDIT: And some stats on the tilt : Stars: 965, Truncated mean[FWHM]: 2.83, Sensor tilt[FWHM]: 0.08 (3%), Off-axis aberration[FWHM]: 0.13

 

Edited December 21, 2022 by AstroMuni

[vlaiv]

On 20/12/2022 at 08:16, Priesters said:

 

I suspect it is pinched optics as this is the first time I’ve used this scope (WO GT81) in such cold weather.   Only other change has been a new camera (Altair HyperCam 183C).  Is there anything I can do about pinched optics?

Yes, this sort of looks like some pinching, but I'm not 100% certain it is.

This star maybe shows the effect the best. It looks like flaring in 4 directions.

It reminds me of issues with 130/150PDS line of reflectors that have issues when focuser is protruding into optical path - but these tend to be to one side only:

Maybe some screws on 4 sides do the same with your setup?

[Andyy]

This is a very common problem with WO and other telescopes made by the same manufacturer under different names. It’s well documented online in several different forums. If I remember correctly it’s something to do with how the lens cells are retained inside the OTA.  The problem gets worse in cold weather.

[Priesters]

4 hours ago, AstroMuni said:

EDIT: And some stats on the tilt : Stars: 965, Truncated mean[FWHM]: 2.83, Sensor tilt[FWHM]: 0.08 (3%), Off-axis aberration[FWHM]: 0.13

I’d suspected tilt before so it’s useful to have that confirmed.  What software did you use to ascertain that?  You may have gathered I’m still on the lower slope of the steep learning curve 🙂 I was also wondering what my next Astro purchase might be so that’s the tilt adaptor added to the list. 🙄

[AstroMuni]

46 minutes ago, Priesters said:

I’d suspected tilt before so it’s useful to have that confirmed.  What software did you use to ascertain that?  You may have gathered I’m still on the lower slope of the steep learning curve 🙂 I was also wondering what my next Astro purchase might be so that’s the tilt adaptor added to the list. 🙄

I used Siril (its free 🙂 ) Your tilt is minimal if any. I dont think you can easily achieve perfectly zero tilt as any movement in scope could cause tilt due to gravity.

Here is your image showing the tilt.

 

 

[ONIKKINEN]

Agree, tilt is excellent. Trying to fix that tilt will make it worse for sure.

[Priesters]

Great news - what should I spend the money on instead then 😂

[Priesters]

Response below from Flo in case anyone else experiences the same issue.  Great service, as always 🙂.

It is pinching of the lens caused by the lens cell radial adjustment screws when the cell contracts in cold temperatures.

We can slacken them off slightly, but only by so much otherwise when the temperature increases the lens cell will be too loose and the scope runs the risk of falling out of collimation.

We recommend using a dew heater band, placed as close to the lens cell as possible which often cures this, as a first step and if that does not eliminate the pinching we can have the scope back and adjust the lens cell for you

[scotty38]

I have a GT81 and a dew heater and haven't knowingly seen this issue. Maybe I have it but the heater hides it 🙂

Worth a try for sure.....

[ollypenrice]

Dew heater is the obvious first step...

Olly

[Priesters]

Not tested yet but I think I may have discovered a contributing factor.  Forgot about this as I’d hidden the gap underneath the OTA 🙄