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Weird artefact in star images


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Hi All:

I've been taking images over the last few months and I've just noticed some undesirable spike sillhouettes in my images of bright stars, particularly when I do a close crop to expand the image. It was really striking  a couple of nights ago on a shot of M42; I attach a blow up of a region where the brightest star in the field shows 2 "dark beams" shooting out of it. The telescope is a Zenithstar ZS73 with 0.8 FF/FR and the camera  is a Canon 600D.  (The flattener probably needs a bit of adjusting, corner stars on the full-field image are a bit elongated and streaky). This is a stacked image of 24 x 90 sec exposures, ISO 800 unguided, processed throughout using Affinity. I'm not desperate to produce perfect images, and I'm really pleased with what I've taken so far, but I'd like to solve this. I've used a torch to peer up the optical train but can't see anything untoward. There are a couple of machined notches in the rim of the flattener towards the midpoint, but these look part of the item and face forwards so it would seem odd if these are having an effect. Any suggestions would be welcome! Thanks, Phil.

20211130_170743.jpg

Edited by Philip Terry
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On 30/11/2021 at 17:35, Philip Terry said:

Hi All:

I've been taking images over the last few months and I've just noticed some undesirable spike sillhouettes in my images of bright stars, particularly when I do a close crop to expand the image. It was really striking  a couple of nights ago on a shot of M42; I attach a blow up of a region where the brightest star in the field shows 2 "dark beams" shooting out of it. The telescope is a Zenithstar ZS73 with 0.8 FF/FR and the camera  is a Canon 600D.  (The flattener probably needs a bit of adjusting, corner stars on the full-field image are a bit elongated and streaky). This is a stacked image of 24 x 90 sec exposures, ISO 800 unguided, processed throughot using Affinity. I'm not desperate to produce perfect images, and I'm really pleased with what I've taken so far, but I'd like to solve this. I've used a torch to peer up the optical train but can't see anything untoward. There are a couple of machined notches in the rim of the flattener towards the midpoint, but these look part of the item and face forwards so it would seem odd if these are having an effect. Any suggestions would be welcome! Thanks, Phil.

 

If you are seeing "lower order" artefacts in your images, it means that you are doing great! :)

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On 02/12/2021 at 21:09, Tom OD said:

Thats normal for a Takahashi refractor, I m guessing its just the natural baffling on the Zenith star too

Tom

Hi Tom and Philip,

I disagree that this is normal with Takahashis. Yes, the FSQ85 shows similar patterns, but those are caused by internal reflections between lens surfaces, are always in straight lined pairs and rotate around the centre with the pair being perpendicular to the line between the star and the centre of the image: https://www.astrobin.com/full/pmh3a6/0/

What we see here in Philip's image can have two causes: pinched optics or uneven/non-smooth aperture. I have seen the latter in three Esprit ED80s I examined and described it in an article on a Dutch forum. Opening the article in Chrome should properly translate it: https://www.starry-night.nl/het-verwijderen-van-artefacten-bij-een-refractor-met-een-brilletje/

As I describe, the uneven halo can be corrected by stopping down the aperture by 1 or 2 millimetres with a bespoke made field-stop.

Pinched optics can be checked by using the scope at a higher temperature, if the dark lines become weaker it indeed is a matter of pinched optics.

Nicolàs

Edited by inFINNity Deck
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Thanks Nicolàs - very helpful advice and a really interesting article. I wonder if it might be pinching; on your recommendation I've gone back and looked at images from late summer, and the effect is much diminished or absent. On the night of imaging the Orion nebula the temperature was around zero Celsius (I've finally found  a purpose for that thermometer in the ZS focus knob!)  - the coldest I've imaged in so far. Attached is a photo of tne Sadr region nebulosity, from mid-August, and I can't see the dark beams here. They do indeed become more obvious as the year advances. This raises another question - wouldn't it be expected that a dew heater band on the ojective would ameliorate this? Admittedly, I switched mine on after the scope had been sitting outside for a while,  but shouldn't it offset the pinching caused by low temperature, at least to some extent?

Third go.jpg

Edited by Philip Terry
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Hi Philip,

that Sadr-region image looks much better indeed, I cannot find those same dark beams in it. A dew-heater could possibly help when wrapped directly around the lens-cell. Instead we commonly use them wrapped around the dew-cap, just to warm the air directly in front of the objective lens, reducing the chance that the dew-heater has a significance effect on the cell.

With one of the ED80s I tested I first cooled it to around 4°C by having it in the fridge for a whole night. Then next morning I let it warm-up to room temperature in front of a collimator with the following result:

afbeelding.thumb.png.947874b63da8f9f2f0dbc80d632021c8.png

The left image shows three dark spikes at about 4, 8 and 11 o'clock. In the right image they are absent. This is of course the difference of 16°C, a dew-heater is unlikely to produce such a difference.

The method, however allows us to test. So what you could try is to have the scope indoors fully prepared to image, then (once at room temperature) take it outside and immediately start taking images and continue to do so until the scope is fully cooled-down (that ED80 I tested took just over two hours to reach equilibrium).

Nicolàs

 

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The "spikes" you are seeing are artifacts from collimation screws of the lens cell. It varys with temperature and apparently cold makes it worse/more visible. Very common on William Optics and similar telescopes from the same manufacturer. Not sure if it's classified as a defect. Had the same problem with mine If you can live with it don't bother.

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52 minutes ago, Andyy said:

The "spikes" you are seeing are artifacts from collimation screws of the lens cell. It varys with temperature and apparently cold makes it worse/more visible.

Just for the record and to avoid confusion: this is indeed what I meant with pinched optics. So the lenses get pinched between the screws.

Loosening the screws should be done with utmost care to avoid that the individual lenses lose their mutual positions. You also want to avoid that they get too loose.

Nicolàs

Edited by inFINNity Deck
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10 hours ago, inFINNity Deck said:

Just for the record and to avoid confusion: this is indeed what I meant with pinched optics. So the lenses get pinched between the screws.

Loosening the screws should be done with utmost care to avoid that the individual lenses lose their mutual positions. You also want to avoid that they get too loose.

Nicolàs

Pinched optics usually results in trefoil-shaped stars. This is not the case here. In this case I think the collimation screws slightly enters into the light path while holding the lens cell.

I do not recommend trying to fix the problem yourself.

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Hi Andy,

perhaps the following article is good to read: http://interferometrie.blogspot.com/2014/08/esprit-tuning-how-we-finetune-esprit80.html

As can be seen there are two types of collimation screws:

afbeelding.png.a63d8294d166fd41f24ef3d82eab4e64.png

So the lens-cell collimation screws are the ones that hold the lens-cell and are there to be able to set the whole lens-cell perpendicular to the optical axis. The Lens-alignment collimation screws (here they are in sets of 3 as this is a triplet) are there to hold the lenses in places and to arrange their mutual alignment within the lens-cell. In neither case  is it possible that those screws enter the light path. The lens-cell collimation screws are not even within the tube and are parallel to the optical axis. The lens-alignment collimation screws do move inwards, but cannot enter the light path as if that were the case, light could pass between the lens-cell and the lens (or the lens is missing).

What some manufacturers do, is using small spacers between the lenses (taken from https://www.cloudynights.com/topic/647909-refractor-doublet-lens-spacer-material/):

Polaris Objective_1958_Newton Rings_Orig Spacers_.jpg

These are able to protrude within the light path if not well cut and/or placed. They can also explain why the dark spikes are only 2 obvious ones and that they are not separated by 120 degrees.

Nicolàs

 

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20 minutes ago, inFINNity Deck said:

Hi Andy,

perhaps the following article is good to read: http://interferometrie.blogspot.com/2014/08/esprit-tuning-how-we-finetune-esprit80.html

As can be seen there are two types of collimation screws:

afbeelding.png.a63d8294d166fd41f24ef3d82eab4e64.png

So the lens-cell collimation screws are the ones that hold the lens-cell and are there to be able to set the whole lens-cell perpendicular to the optical axis. The Lens-alignment collimation screws (here they are in sets of 3 as this is a triplet) are there to hold the lenses in places and to arrange their mutual alignment within the lens-cell. In neither case  is it possible that those screws enter the light path. The lens-cell collimation screws are not even within the tube and are parallel to the optical axis. The lens-alignment collimation screws do move inwards, but cannot enter the light path as if that were the case, light could pass between the lens-cell and the lens (or the lens is missing).

What some manufacturers do, is using small spacers between the lenses (taken from https://www.cloudynights.com/topic/647909-refractor-doublet-lens-spacer-material/):

Polaris Objective_1958_Newton Rings_Orig Spacers_.jpg

These are able to protrude within the light path if not well cut and/or placed. They can also explain why the dark spikes are only 2 obvious ones and that they are not separated by 120 degrees.

Nicolàs

 

The Esprit does not have this problem not that I know of anyway.  We agree something is entering the light path causing the artifact maybe spacers. It’s not serviceable by the end user.

I made a aperture mask on my telescope to fix the problem. It made the telescope slightly slower but stars were nice and round.

Edited by Andyy
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Thanks Andyy and Nicolàs. It certainly seems to be temperature-dependent; I tried Nicolàs's experiment of imaging straight after taking the scope outdoors, and there was no sign of the dark beams. They crept back subtly as the night progressed. No trefoil-ish shapes either. Fits with the late Summer images being free from the effect. I won't mess with the cell screws, I'm not looking for perfection!

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16 minutes ago, Philip Terry said:

Thanks Andyy and Nicolàs. It certainly seems to be temperature-dependent; I tried Nicolàs's experiment of imaging straight after taking the scope outdoors, and there was no sign of the dark beams. They crept back subtly as the night progressed. No trefoil-ish shapes either. Fits with the late Summer images being free from the effect. I won't mess with the cell screws, I'm not looking for perfection!

Use aperture mask. I used a 3D-printer to make one, but for testing just make one out of cardboard. This will help on star shape.

Edited by Andyy
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I have seen multiple Esprit 80EDs with pinched optics and the company in the link I posted earlier today has seen several as well, they even have designed a new lens-cell for SkyWatcher to mitigate the issue.

An aperture mask may indeed remove certain artefacts as I described above (last Friday at 21:11). But the aperture should be extremely smooth to avoid new artefacts arising as I shown in that article. I made mine of aluminium and polished it.

Nicolàs

 

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3 hours ago, inFINNity Deck said:

I have seen multiple Esprit 80EDs with pinched optics and the company in the link I posted earlier today has seen several as well, they even have designed a new lens-cell for SkyWatcher to mitigate the issue.

An aperture mask may indeed remove certain artefacts as I described above (last Friday at 21:11). But the aperture should be extremely smooth to avoid new artefacts arising as I shown in that article. I made mine of aluminium and polished it.

Nicolàs

 

To be clear my understanding of pinched optics is that something mechanical puts stress on glass and cause artifacts usually triangular star shapes.

Pinched optics I have seen on Esprit but not mechanical parts protuding into the light path. 

It doesen’t suprise me though these cheap scopes all have their quirks. Some more expensive ones have them too… 

We are at pixel peeping level here… I want to say great image @Philip Terry keep up the good work!

Edited by Andyy
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Thanks Andy, the CN thread looks very relevant. But I'll hold off from adjusting any lens cell screws for the time being, I might go for a "cold weather mask" in the short term and see how that works out. Interesting to see how common this issue is! Thanks again for all of the advice; Phil.

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