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I have a question about what I'm seeing in the attached image ... are these star artifacts normal for a refractor? This was taken with an WO GT102, Optolong filter (Blue) and ASI2600mm. I have my educated guesses, but just want some additional advice. By the way, the objective was recently cleaned with Photonic Polymer Cleaning (just an FYI) |
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Michael, Anything that impinges on the light path can create these spikes. The smoothness of the lens cell edge or internal baffle edges really matters wherever they impinge on the light path. Usually baffles shouldn't impinge on the light path if properly designed. You mentioned you cleaned your optics. Did you have these spikes before cleaning? If not, check that the glass-cell margin that there is no gunk there IR cotton filaments left behind. I have no idea what photonic polymer cleaning is. Clean glass means nothing left behind on the glass surface when cleaning is complete. Hard to imagine anything better than distilled water and pure alcohol. Polymer sounds suspiciously like a film is left behind. If so, would probably defeat the antireflective coating action and possibly cause spikes if streaky. I'd a stronger cleaner is need to remove tough dirt, there should always be a follow up with distilled water and alcohol to remove that cleaner and leave a pristine surface. Having said that, this looks a lot like I got when baby spiders got into my optical tube. The spiders are so small they can get into just about anything. Also the web is super fine and invisible in daylight. The only way I could see them was at night with an LED headlight shining down the telescope front. They may seem nearly invisible, but they are extremely efficient at creating random diffraction spikes. But also check for roughness within and around the front edge of your focus draw tube. Dust there can be an issue |
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| Thanks for the insight Alan. Clouds have moved in so I'm set to check the optics. I was thinking that maybe the cleaning polymer left a rough edge around the cell mount. I'll check that out and see. |
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Michael J. Mangieri: Michael, check my revised response for more info on cleaning. |
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| Photonic Cleaning : https://www.photoniccleaning.com |
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Michael J. Mangieri: Oh yes! I now remember seeing this in a video a while back. It certainly seems like a good product. I do not remember everything about what I saw, but now I understand why you would want to check the margins of the glass and lens cell. I recall that being discussed, if my memory serves. The product seemed to be ideal for professional cleaning. I say that because I know that observatories will go a very long time before they will do a cleaning on many of their larger instruments. They typically will take a significant hit on light throughput before they are motivated to clean. And in doing so, the surfaces will get very dirty. Post if you figure out the cause of the spikes and your resolution. Can be helpful for the community to know your experience. CS Alan |
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Maybe pinched optics, images from my refractor looked like this before I fixed it:  |
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| Am not an expert. But it appears like Iron cross. I have seen these usually happening during winter. The temperature drop causing pinched optics. Most refractors I own had this issue in winter. |
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| Something is protruding into the light path . As it is symmetrical, most likely the (collimating) screws holding the lens together . It could be that it is just due to low temperature ,so it gets "pinched". There is one easy medicine to try : make a mask with diameter one or two mm les than your lens diameter is and it should be gone . 3D printed one should be OK, or even carefully cut from black paper. |
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Deepan Vishal: Yes, thanks. I’ll try that. |
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Johannes Möslein: Thx. I’ll check this out. My example was also from the Pleiades. |
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Anything that is being caused by diffraction will scale as the inverse of the wavelength. You posted an image taken with a blue flitter. Does the pattern look the same or significantly different with a red filter? Off the top of my head, things that can cause this sort of thing are: 1) Irregular or rough surface finish on an optical component. 2) Scatter from the internal edge of a refractive component. This is why edges should be painted black. 3) A large halo could be caused by high order spherical aberration. 4) Trefoil (and highest order) figure errors caused by mechanical stress. 5) Diffraction from small structures like spider webs in the optical path (as others have mentioned). 6) Inhomogeneity of the optical glass. Striations, pull lines, and stress induced index variations can cause effects that look like this. 7) Fungus growing between air-spaced components. Look at the image with another filter to see if it looks the same. Then make an aperture mask to see if stopping the aperture down a little fixes the problem. If it does, see how large you can make the aperture before the problem reappears. If you just have to trim a little bit of the aperture to fix it, I’d leave it that way. Otherwise, this experiment might provide some clues about the source of the problem. John |
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John Hayes: *One thing I noticed upon closer inspection of the diffraction spikes was that they had some odd features. For example, some were not perfectly straight when radiating from center, being slightly curved. And some seemed to cross others. I wonder if those oddities might point to a favored hypothesis, such as pinched optics. John may have the understanding to address this. Fungus scares me! If I was forced to stop down my optic just to cure such an issue, that would be the trigger issue to call it and send it back for professional service (limited to any quality refractor, of course). Though if it was during my imaging high season here in the Pacific NW, I would do that and wait for my Fall-Winter weather dearth to send it out. |
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Thanks all who gave me suggestions as to the cause of the odd diffraction spikes on my GT102 images. I finally got around to checking out the objective lens of my GT102 before starting my imaging run on Sh2-261. I noticed (as I expected) that some of the polymer was still adhering to the lens along the edge of the cell mount. The rubber oring was supposed to prevent the polymer from seeping into the OTA but it apparently didn't seal well enough and so let some of the polymer remain behind when the film was removed. I decided to place the oring on the objective lens to act as a aperture mask as John suggested. I then took two photos of one of the Pleiades stars, one with the oring in place, one without. As you can see, the mask solved the issue and verifies that the rough edge caused by the polymer film was indeed the culprit. Since the oring is fairly thin I may decide to simply leave it in place but I will also contact First Contact to determine the best way to remove that film remaining on the lens. Without the oring mask:  With the oring mask:  The remaining spikes (to left and right) may be due to the fact that the oring was custom fit and has a glue joint. Hope this helps others who might have this issue - and thanks again to John, Alan, Johannes and others for their comments and suggestions. |
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Michael, Glad you found the problem. You should be able to remove the residual polymer film with a piece of Scotch tape. In my experience, polymer cleaning film will bind more strongly to the surface over time so removing it sooner rather than later is a good idea. Properly applied optical coatings won't be harmed at all by the tape--just don't use crazy sticky tape like Duct tape, or other high strength tapes. Regular old Scotch tap should easily do the trick. Just be sure to fold one end over so that you don't accidentally wind up with a piece stuck to the surface with no way to easily lift it off. That would be a way to possibly damage the surface. John |
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Michael J. Mangieri: Great news Michael! |
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| Final update. I was not able to completely remove the residue and I didn't want to force the cleaning too much and risk any damage to the optics. Placing the oring against the cell solves the issue and so I am going to acquire a new oring and place it there as an aperture mask. Need to decide how to mount it, but I'll figure something out. The First Contact Polymer would act as a gentle adhesive for the ring. |
