Weird shaped stars...look like UFOs?

[oymd]

Morning everyone.

Have been getting perfectly round stars for sometime, but recently I have noticed that my stars started looking a bit strange?

Any thoughts what might be causing this?

It does not look like pinched optics, and I'm certainly not imaging from a cold site?

Many thanks

Ossi

[paul mc c]

Could be pinched optics...is it a reflector your using.

[oymd]

1 hour ago, paul mc c said:

Could be pinched optics...is it a reflector your using.

Its a WO71 refractor. The five element one which has a built in flattener.

Ambient temps are about 15C where the scope is setup. I doubt its pinched optics? Unless pinched optics can occur even when its relatively warm?

I have not touched the scope in ages, and stars were always perfect, until about 2 weeks ago.

I also checked with CCD Inspector, and there is 1% tilt only, and a pretty flat field.

Edited January 7, 2023 by oymd

[paul mc c]

Could there be a bit of an obstruction some where,even an insect.... seems like you have covered most things.

Edited January 7, 2023 by paul mc c

[Peter Drew]

It looks like a tilt issue to me.  One degree tilt could be enough to spoil a fast refractor image, specially one with a complex lens system.    🙂 

[Adam J]

It looks like pinched optics, its the only way you will get star shapes like that. Its not tilt as that would not notch out the star halo like this. If its not happened before was it very cold when you collected this data?

Adam

[Rodd]

Almost looks like a guiding or periodic error issue.  There is a side to side elongation.  I hope you figure it out.  These kind of issues are really annoying, especially with a quality scope.

[oymd]

2 hours ago, Adam J said:

It looks like pinched optics, its the only way you will get star shapes like that. Its not tilt as that would not notch out the star halo like this. If its not happened before was it very cold when you collected this data?

Adam

Ambient temps are about 15C at night. 
I thought pinched optics happen only in very cold weather?

Could it be too much heat from the dew straps?

[oymd]

1 hour ago, Rodd said:

Almost looks like a guiding or periodic error issue.  There is a side to side elongation.  I hope you figure it out.  These kind of issues are really annoying, especially with a quality scope.

Guiding overall is not great. About 0.8rms on the CEM40. 
 

How can I trouble shoot this?

[oymd]

2 hours ago, Adam J said:

It looks like pinched optics, its the only way you will get star shapes like that. Its not tilt as that would not notch out the star halo like this. If its not happened before was it very cold when you collected this data?

Adam

 

1 hour ago, Rodd said:

Almost looks like a guiding or periodic error issue.  There is a side to side elongation.  I hope you figure it out.  These kind of issues are really annoying, especially with a quality scope.

I’ve just watched a video on YouTube by Lukomatico. 
 

The video is a review of the Askar 130 APO, and I noticed in his video he had a problem with the matching reducer. His target is similar to mine, IC1805. 
 

He also shows similar UFO shaped stars. Specifically at 13:40 in the video. 
 

He describes it as Tangential and Saggital astigmatism. 
 

Could it be astigmatism?

@vlaiv Wish you can give us your opinion on this!!???

[PadrePeace]

Please could you post the whole field of view?

You state the temp was +15C during capture so not obviously the atmospheric conditions. Never seen stars look like this from a guiding issue.

You suggest that the scope is permanently set up on a mount and has just started having issues. If that assumption is true then it’s not likely an impact symptom or a temperature conditioning issue as it will always be ambient.
Please confirm both assumptions. Just covering off some missing context of use.

To my eyes: the larger stars to the left of frame show three extended bright arms in the corona. Hard to tell how smaller stars are impacted. Curiously, this pattern changes to bottom right of the example frame (assumed to be a single sub) similar to a ‘coma’ look (which should not be possible in a refractor) but which suggests focus is changing across the FOV.

SO WHAT?

Check if the optics are clear of obstructions internally and externally ie bugs spider web threads; has a baffle moved?

Have you got an additional dew shield fitted that might now be impacting the light path?

Were you imaging through branches of a tree or similar distant obstruction which may give you weird stars?

Did this issue happen on all subs, ie over a prolonged session? 

What about with other filters in the optical path?

I’m not an expert but offer some less technical questions to address before you try dismantling the scope!!! Things seldom just break if not abused so look for the less obvious possibilities. 

Hope this helps.

 

 

[vlaiv]

Yes, still linear sub would be great to examine.

What sort of filter was this shot with? Was any filter used at all?

I've seen strange stars like this when no filter is placed on refracting optics. It is usually due to out of focus IR/UV light.

[Peter Drew]

Change of focus or shapes across the field of an image is usually the result of tilt either in the optics of the telescope or any supplementary optical components including the camera.  My money is still on tilt somewhere in the system.    🙂  

[vlaiv]

How do people figure it is a tilt?

1. Pinched optics will make all the stars equally affected. Some are and some are not affected.

2. To be able to diagnose tilt - we need whole image. This is zoom x2 of the center of the field - showing very small section of the center.

Note position of the sliders in the vertical and horizontal direction on that screen shot - they are somewhere around the middle. Title is pointing that there is zoom involved - 2:1, so we are basically looking at very small central part of the frame. No way to tell the tilt from that.

3. Effect seems to be present only on some stars. Stars have different brightness in IR / UV part of spectrum depending on their temperature, some will have larger "purple" halo and some won't.  Next to Filter / No filter part of the title and knowing what stars look like on fast achromat (which is very similar to what stars look like on a refractor that is not using UV/IR cut filter) - my money is on that until we get further info (if for example UV/IR cut filter was indeed used).

[oymd]

Thank you all for your replies.

I've attached a THREE subs, one at the start of the session, one in the middle and the third close to the end.

Also attached the Master LIGHT.

The gear used was:

WO71 Apo, 350fl

ASI294MC Pro

CEM40

Optolong L-eXtreme filter

Ambient temps at night are 15C.

Optics on telescope, filter and sensor window are pristine.

No obstruction what so ever.

Total 69 subs x 300s.

Calibrated with Master flat and master dark

masterLight_BIN-1_4144x2822_EXPOSURE-300.00s_FILTER-NoFilter_RGB_autocrop.xisf 2023-01-05_20-01-44__-4.90_300.00s_0001.fits 2023-01-05_21-48-51__-5.30_300.00s_0020.fits 2023-01-05_23-19-44__-4.90_300.00s_0003.fits

[oymd]

2 hours ago, PadrePeace said:

Please could you post the whole field of view?

You state the temp was +15C during capture so not obviously the atmospheric conditions. Never seen stars look like this from a guiding issue.

You suggest that the scope is permanently set up on a mount and has just started having issues. If that assumption is true then it’s not likely an impact symptom or a temperature conditioning issue as it will always be ambient.
Please confirm both assumptions. Just covering off some missing context of use.

To my eyes: the larger stars to the left of frame show three extended bright arms in the corona. Hard to tell how smaller stars are impacted. Curiously, this pattern changes to bottom right of the example frame (assumed to be a single sub) similar to a ‘coma’ look (which should not be possible in a refractor) but which suggests focus is changing across the FOV.

SO WHAT?

Check if the optics are clear of obstructions internally and externally ie bugs spider web threads; has a baffle moved?

Have you got an additional dew shield fitted that might now be impacting the light path?

Were you imaging through branches of a tree or similar distant obstruction which may give you weird stars?

Did this issue happen on all subs, ie over a prolonged session? 

What about with other filters in the optical path?

I’m not an expert but offer some less technical questions to address before you try dismantling the scope!!! Things seldom just break if not abused so look for the less obvious possibilities. 

Hope this helps.

 

 

 

2 hours ago, vlaiv said:

Yes, still linear sub would be great to examine.

What sort of filter was this shot with? Was any filter used at all?

I've seen strange stars like this when no filter is placed on refracting optics. It is usually due to out of focus IR/UV light.

 

1 hour ago, Peter Drew said:

Change of focus or shapes across the field of an image is usually the result of tilt either in the optics of the telescope or any supplementary optical components including the camera.  My money is still on tilt somewhere in the system.    🙂  

All conditions and gear used provided, and uploaded subs and master light.

Many thanks for your help.

[vlaiv]

@oymd

Could you restack your data but using simple average method without fancy things like rejection and such? I think that strange effect might be caused by guiding issues and strange stacking settings.

Also, if you can - post fits of stack, xisf is PixInsight only format

[oymd]

8 minutes ago, vlaiv said:

@oymd

Could you restack your data but using simple average method without fancy things like rejection and such? I think that strange effect might be caused by guiding issues and strange stacking settings.

Also, if you can - post fits of stack, xisf is PixInsight only format

HI Vlaiv

I do not touch anything in the stacking process.

I use WBPP. Load masters dark and flat, and load all the lights.

I then stack using the default WBPP settings, and do not touch anything.

Can I save the master light as a FIT and post it here?

[vlaiv]

10 minutes ago, oymd said:

Can I save the master light as a FIT and post it here?

Yep, that would be great.

13 minutes ago, oymd said:

I do not touch anything in the stacking process.

I use WBPP. Load masters dark and flat, and load all the lights.

I then stack using the default WBPP settings, and do not touch anything.

Do you know how to do it manually? Just calibrate, align and use simplest integration method - average without any weighing of subs or pixel rejection

[oymd]

11 minutes ago, vlaiv said:

Yep, that would be great.

Do you know how to do it manually? Just calibrate, align and use simplest integration method - average without any weighing of subs or pixel rejection

I'll try and do that.

As I am writing this, I am actually uploading the Master Light in FITS format for your consideration

masterLight_BIN-1_4144x2822_EXPOSURE-300.00s_FILTER-NoFilter_RGB_autocrop.fit

[oymd]

29 minutes ago, vlaiv said:

@oymd

Could you restack your data but using simple average method without fancy things like rejection and such? I think that strange effect might be caused by guiding issues and strange stacking settings.

Also, if you can - post fits of stack, xisf is PixInsight only format

Hi Vlaiv

Master saved as FITS and uploaded!

Thanks

[vlaiv]

I don't think it is tilt, but I do believe it is some sort of collimation issue - maybe decentered lens or something like that.

Here we have two stars in R, G and B channel - and they have different size and shape.

I can't be sure how much of it is down to stacking (we need simple average stack to check that) and how much is down to optics.

This pattern repeats over the whole image and is not prominent in one corner so I don't think it is tilt issue. Fact that colors are affected differently suggests that something is going on with lens. There might be some spherochromatism or something like that. This flaring that is most obvious in red is also very interesting. It looks like some sort of obstruction is present, but I have no idea what it might be.

First thing that I would recommend would be loosing the filter and trying to image simple star field to see if this would repeat.

Well, actually - first thing to do is to stack using regular average stacking to see if there will be any difference to this stack, and second - loosing filter.

Third thing to do would be to image star in/out of focus to check collimation and perform sort of star test in general.

[oymd]

8 hours ago, vlaiv said:

I don't think it is tilt, but I do believe it is some sort of collimation issue - maybe decentered lens or something like that.

Here we have two stars in R, G and B channel - and they have different size and shape.

I can't be sure how much of it is down to stacking (we need simple average stack to check that) and how much is down to optics.

This pattern repeats over the whole image and is not prominent in one corner so I don't think it is tilt issue. Fact that colors are affected differently suggests that something is going on with lens. There might be some spherochromatism or something like that. This flaring that is most obvious in red is also very interesting. It looks like some sort of obstruction is present, but I have no idea what it might be.

First thing that I would recommend would be loosing the filter and trying to image simple star field to see if this would repeat.

Well, actually - first thing to do is to stack using regular average stacking to see if there will be any difference to this stack, and second - loosing filter.

Third thing to do would be to image star in/out of focus to check collimation and perform sort of star test in general.

I recall from our previous conversations that you use DSS. 

I will restack the subs in DSS. I haven’t used DSS for a while. 
 

What sort of parameters do you advise to use in DSS to avoid any rejection and keep it simple as you suggest?

[oymd]

8 hours ago, vlaiv said:

I don't think it is tilt, but I do believe it is some sort of collimation issue - maybe decentered lens or something like that.

Here we have two stars in R, G and B channel - and they have different size and shape.

I can't be sure how much of it is down to stacking (we need simple average stack to check that) and how much is down to optics.

This pattern repeats over the whole image and is not prominent in one corner so I don't think it is tilt issue. Fact that colors are affected differently suggests that something is going on with lens. There might be some spherochromatism or something like that. This flaring that is most obvious in red is also very interesting. It looks like some sort of obstruction is present, but I have no idea what it might be.

First thing that I would recommend would be loosing the filter and trying to image simple star field to see if this would repeat.

Well, actually - first thing to do is to stack using regular average stacking to see if there will be any difference to this stack, and second - loosing filter.

Third thing to do would be to image star in/out of focus to check collimation and perform sort of star test in general.

Here is the stacked TIFF file from DSS.

Under the LIGHTS tab, I left it at default, which was AVERAGE.

Everything else was also left at default.

WOW, DSS completed the whole process in under 5 minutes!!

WBPP took about 20 minutes, if not more!!

Autosave.tif

[AstroMuni]

Doesnt seem to be tilt