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Refraction artefacts on bright stars in a f/5.5 100mm refractor?

Sharpstar Z4 Refractors Artifacts
Andrew RambautOscarandrea tasselliZoltan KIRALY
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Andrew Rambaut avatar
Andrew Rambaut avatar
Hi,
I am getting some radial gaps in the star glow/halo for bright stars. This is with a SharpStar Z4 f/5.5 refractor, a ZWO ASI2600MC and no filter. There is nothing but 55mm of M48 tube between the telescope glass and the camera. And nothing at the front. It is consistent between different targets and different nights. Anyone know what this is?

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Oscar avatar
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Oscar avatar
Oscar avatar
Gosh, I've seen that pattern before and I don't know where. I looked all over google for it and I cannot find it.

Based on my memory and reasoning, it's either:

1. a natural and unavoidable star pattern for your telescope

2. pinched optics
Andrew Rambaut avatar
Andrew Rambaut avatar
Thanks - that gives me some hints of what to look for. I had the scope for a while now and not noticed it until the last few sessions. But they were cold nights so pinched optics could be a possibility (although there is a dew heater on the front element).
Stjepan Prugovečki avatar
Stjepan Prugovečki avatar
Most likely something is protruding into your beam path (collimation screws, lens spacers , who knows..) .  It is easy to test this, just make a mask that has the aperture 1 or 2mm less than your scope diameter is and try. (you can just use black cardboard and precisely cut the hole in it) . If it helps, you can make a good one (3d printed for example) and use it all the time and forget that you ever had a problem. Or you can do scope "forensics" to find out where exactly the protrusion is. It could be very small (a fraction of mm) and difficult to spot it visually.  It could even be due to thermal contraction due to low temperatures.
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Oscar avatar
Oscar avatar
Stjepan Prugovečki:
It is easy to test this, just make a mask that has the aperture 1 or 2mm less than your scope diameter is and try.


That's genius!

This also means, and this is irrelevant, and I might be daydreaming, instead of using an aperture/mirror mask on my reflector to hide the clips, I could just directly block the light from directly hitting the clips by putting a ring of cardboard on the front of the reflector. If this works for me, you just saved me a lot of time, money and work.
Andrew Rambaut avatar
Andrew Rambaut avatar
I am not seeing the pattern in an image I took in warmer weather which maybe suggests the cold temperature makes the obstruction protrude more (which would suggest it is deeper in the scope than the front lens which has the dew heater on it). Thanks, everyone, for the tips.
Andrew Rambaut avatar
Andrew Rambaut avatar
And 3D printing a mask is a great idea - if mounted on the dew shield it would need to extend in a bit further to mask the objective but I could over-compensate  and then try printing ones with larger 'holes' until the artefact reappears.
Rob Kiefer avatar
Rob Kiefer avatar
Hi Andrew, I had a similar pattern on my Esprit 100 and on my WO61. Esprit - it was dirt and WO I had to loosen the set screws. Check for any dirt, obstruction, screw etc. that might get in the light path. I was able to fix both. Good luck!
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Andrew Rambaut avatar
Andrew Rambaut avatar
That is interesting - i would assume the 180 angle between the two shadows suggest there is a symmetrical obstruction on opposite sides. The only thing I can see in the front objective lens are a pair of notches for unscrewing the ring that hold the cell in place. Obviously these don't protrude (in fact they do the opposite) but perhaps they have an effect somehow that isn't obvious to me. I could probably work out from the image what angle the obstructions must be at in the scope and see if that fits the notches.
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andrea tasselli avatar
andrea tasselli avatar
If it is dependant on the distance from the centre then it's cat's eye, otherwise something like clips are protuding in the light path.
Zoltan KIRALY avatar
Zoltan KIRALY avatar
Hi Andrew,

Thank you for bringing up the topic.

I am experiencing the same phenomenon. Just yesterday evening at the request of the dealer I conducted tests and after a detailed analysis.
I observed the conditions shown in the attached images.

On the first lens retaining ring one of the notches is more recessed and the paint has chipped off, revealing a shiny aluminum color.


Andrew Rambaut avatar
Andrew Rambaut avatar
Thanks for that - I was coming to the conclusion that it was the notches causing some light scattering. I was going to look at the angle of the spikes and whether that matched the location of the notches but I will need to go and look at what angle I had the camera first. If there is a good match, then I think that would nail it. I should say it is only happening on the very brightest stars but it does suggest that some of the 'halo' on these stars is the result of scattering off these rings and the notches are actually 'fixing' the scattering but that then looks bad.  So I think the solution I want to try is to make a very clean 'mask' or perhaps some flocking over these rings to see if that fixes the halo and the notches in the halo. Will report back here but I may have a while to wait for a clear night.
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Zoltan KIRALY avatar
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Update: I was able to test tonight and based on my experience, the recess on the first lens retaining ring is causing the issue. I placed a diffraction mask precisely where the recess is.


Oscar avatar
Oscar avatar
It's amazing how such a small recess caused such an issue
Andrew Rambaut avatar
Andrew Rambaut avatar
Interesting that only one of your notches causes the effect - I get the effect on both sides. I have ordered some flocking material which I intend to try sticking over the entire inside of the ring. Will post results here.
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Zoltan KIRALY avatar
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It's amazing how such a small recess caused such an issue

I don't know, but one recess is deeper than the other. It also doesn't perfectly align with the ring above it, which has inscriptions.
Zoltan KIRALY avatar
Zoltan KIRALY avatar
Andrew Rambaut:
Interesting that only one of your notches causes the effect - I get the effect on both sides. I have ordered some flocking material which I intend to try sticking over the entire inside of the ring. Will post results here.

I'm curious to hear about your experiences.
Andrew Rambaut avatar
Andrew Rambaut avatar
Finally got a chance to try an experiment. I put a strip of self-adhesive flocking material around the entire inside of the rings holding the objective in place (including covering both notches). I then took the same exposure of M42 that I did before (60 x 60s). Here is the before and after - the big star is Hatysa. I tried to match the framing. Obviously there will be differences in seeing, skyglow etc. I am not sure what to make of it but it would seem to rule out the 'notches' as the direct cause of the gaps in the diffusion pattern. It is possible that the flocking has improved things - the stars seem tighter but that could be the seeing. The spikes are more evenly distributed round the star (looks a bit better IMO).

Before flocking - 
After flocking:
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andrea tasselli avatar
andrea tasselli avatar
Realize that those diffraction effects have nothing to do with reflections. So covering them in anti reflective material would do nothing to improve the situation.
Andrew Rambaut avatar
Andrew Rambaut avatar
My aim was to cover the 'notches' in the retaining rings as a post above suggested these as a possible cause. I guess we are back to pinched optics - the difference between the two images could be temperature? It seems the solution for pinched optics is to loosen the retaining ring a bit.
Stjepan Prugovečki avatar
Stjepan Prugovečki avatar
Andrew Rambaut:
My aim was to cover the 'notches' in the retaining rings as a post above suggested these as a possible cause. I guess we are back to pinched optics - the difference between the two images could be temperature? It seems the solution for pinched optics is to loosen the retaining ring a bit.

As I mentioned earlier, a simple mask with opening that is let's say 2 or 3mm smaller than the diameter of your lens will give you an answer whether there is a protrusion of something into your beam path .
andrea tasselli avatar
andrea tasselli avatar
It is not pinched optics. It is either cat's eye effect or protrusions in the light path. If the effect isn't present in the center of the field then it's the former otherwise it is the latter. Many of us get this effect. Even in catadioptrics.
Observatório Astrográfico do Boqueirão avatar
Observatório Astrográfico do Boqueirão avatar
Andrew Rambaut avatar
Andrew Rambaut avatar
Thanks everyone - I will try fashioning a mask perhaps with a few more layers of the flocking material or some thicker material (high density foam or neoprene). I think putting layers on the inside of the retention rings will produce a more precise mask than cutting a circle in material. The pattern seems to have gone from 2-fold symmetry to 6-fold - note the first image was taken on a much colder night (sub-zero). 

@andrea tasselli - I have not found a good source about the cat's eye effect or how it manifests itself (other than in gemstones?) - I will dig into this further. 

@Observatório Astrográfico do Boqueirão - thanks. Certainly interesting, but perhaps a bit more hands on than I am prepared to go!

I will try picking a bright star and taking images with the star centred and then in each quadrant around the edges - get some more info. Then add the mask and try again. Clear nights are precious in Scotland but these experiments are useful way of using a few hours of cloudlessness.
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