William Optics ZS73 field flattener producing the "lighthouse effect" on larger stars

[TerryMcK]

I was out last night taking snaps of M45 and have just done some processing only to find what looks like pinched optics or maybe something else from the scope. The scope was out all day so had plenty of time to acclimatize. I have seen this intermittently before and now is the time to take action. Anybody know how to move the dew shield, not necessarily remove it or dismantle the entire scope, on a ZenithStar to gain access to the cell securing screws?

 

Edited March 15, 2021 by TerryMcK
Title change

[TerryMcK]

It looks similar to this old thread hence my assumption that it may be pinched optics 

 

[KP82]

You really need to zoom in and examine the star shape. If the stars are indeed triangular or in some other irregular shapes, it's pinched optics. The diffraction spikes alone caused by spacers aren't enough evidence to suggest that.

[carastro]

Yes I did Zoom in and look at the stars, they look to be round to me but with diffraction spikes, which is a different issue.  Not sure what that is. 

 

Best way to check it to image a defocussed star.  I posted to examples in my first reply but they seem to have disappeared.  So i will post them again.

Carole 

Edited March 5, 2021 by carastro

[TerryMcK]

2 minutes ago, carastro said:

I posted to examples in my first reply

That was some other place Carole  

[carastro]

No wonder I could not find it Lol.

The stars tend to be triangular from what I understand.  

I am not sure what causes the diffraction, probably need Vlaiv to explain it all.

Found this on line posted by Steve Richards:

https://www.skyatnightmagazine.com/advice/understanding-optical-aberrations/

Carole 

 

Edited March 5, 2021 by carastro

[TerryMcK]

I've been able to remove the dew shield. It was very easy in the end. Hold the aluminium ring where the clamping thumbscrew is and simply unscrew the shield and withdraw it off the main tube. Couldn't have been easier.

Underneath the cell mounting is clearly evident and has a number of screws around the periphery arranged in two lines one per lens. I used a jewellers screwdriver to see if any were tight. Most of them were barely nipped up apart from one which was slightly tighter than the others so I have backed it off a hair. We are talking probably a 1/32 of a turn so nothing much. I have assembled it back together and will wait for a clearance in the cloud test.

If it is something else then happy to listen. 

Here is a zoomed in view with PI STF - an hour at 60 second exposures. I have noticed this before and it seems to occur in the winter. It was about -2C last night but I had the dew heaters on around the cell area. If not the lens cell could it be at the other end where the field flattener is?

 

 

[KP82]

35 minutes ago, TerryMcK said:

I've been able to remove the dew shield. It was very easy in the end. Hold the aluminium ring where the clamping thumbscrew is and simply unscrew the shield and withdraw it off the main tube. Couldn't have been easier.

Underneath the cell mounting is clearly evident and has a number of screws around the periphery arranged in two lines one per lens. I used a jewellers screwdriver to see if any were tight. Most of them were barely nipped up apart from one which was slightly tighter than the others so I have backed it off a hair. We are talking probably a 1/32 of a turn so nothing much. I have assembled it back together and will wait for a clearance in the cloud test.

If it is something else then happy to listen. 

Here is a zoomed in view with PI STF - an hour at 60 second exposures. I have noticed this before and it seems to occur in the winter. It was about -2C last night but I had the dew heaters on around the cell area. If not the lens cell could it be at the other end where the field flattener is?

 

 

The stars do look round to me.

But to be sure try to take a defocused shot of a bright star like Carole has suggested.

[Crackabarrel]

I have noticed a similar effect on my own zs73, so following this with interest, screenshot of a recent image for comparison.

[Adam J]

8 minutes ago, Crackabarrel said:

I have noticed a similar effect on my own zs73, so following this with interest, screenshot of a recent image for comparison.

Actually that looks more like astigmatism. I.e Maltese cross pattern. 

[Crackabarrel]

6 minutes ago, Adam J said:

Actually that looks more like astigmatism. I.e Maltese cross pattern. 

Ah pity, cheers for the clarification 

[Adam J]

On 05/03/2021 at 09:55, TerryMcK said:

I was out last night taking snaps of M45 and have just done some processing only to find what looks like pinched optics or maybe something else from the scope. The scope was out all day so had plenty of time to acclimatize. I have seen this intermittently before and now is the time to take action. Anybody know how to move the dew shield, not necessarily remove it or dismantle the entire scope, on a ZenithStar to gain access to the cell securing screws?

 

You need to do a star test on a defocused star inside and outside of focus to show between three and five diffraction rings. I suggest an artificial source placed about 10-15meters from the scope. FLO sell an artificial star torch type thing. 

Edited March 6, 2021 by Adam J

[Adam J]

6 minutes ago, Crackabarrel said:

Ah pity, cheers for the clarification 

Astigmatism that shows in focus is not going to be diffraction limited so if it's in Warranty you should return it. But I would perform a star test first to confirm. You are looking for a oval star test that rotates orientation by 90degrees just either side of focus.  

Edited March 6, 2021 by Adam J

[Crackabarrel]

6 minutes ago, Adam J said:

Astigmatism that shows in focus is not going to be diffraction limited so if it's in Warranty you should return it. But I would perform a star test first to confirm. You are looking for a oval star test that rotates orientation by 90degrees just either side of focus.  

Cheers Adam, 

I'll have a go at testing next clear night and if necessary look into sending it back, the amount of clear nights we're getting here I don't think I'll be missing much imaging time anyway.

[TerryMcK]

16 minutes ago, Adam J said:

FLO sell an artificial star torch type thing

Just ordered one. Thanks for the heads up Adam. It looks useful.

[Adam J]

7 hours ago, TerryMcK said:

Just ordered one. Thanks for the heads up Adam. It looks useful.

With the focal length of your scope it needs to be at least 10metres away to work correctly. 

[TerryMcK]

OK I have found out what the issue was with my William Optics scope - after quite a bit of testing with the artificial star and different scenarios. It was not pinched optics, nor aperture vignetting in fact it was nothing to do with the front elements at all. It appears that the field flattener had a slight burr that had been raised on the clamping ring probably in the factory. This burr was what could be seen on larger stars. I took a small file to it to remove the burr and all is ok again. No lighthouse effect at all.

The burr can be seen on the tightening slot (slightly out of focus but try getting an iphone to focus this close  ) of the clamping ring. It could be felt quite prominently as a sharp raised point.

 

 

The burr was the slight shark fin shaped lump in the corner of the slot in this close up shot.

The miniature file smoothed it out, I blew the metal away carefully with a blower and then reassembled the flattener. 

[Adam J]

Fantastic news.