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Any Newtonian produces four diffraction spikes around bright stars as a result of the spider holding the secondary mirror. So far so good. My Skywatcher BKP150 produces more than four spikes and as a novice I am wondering where I should look to find the cause. Collimation looks good, no ambient stray light. The outer surface of the focuser's inner tube is silver while the the edges of the secondary mirror are not blackened. Before I paint all black, I thought it might be a better idea to post this question in this fabulous forum  Cheers, Robert  |
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| Robert, can you post this image full size, maybe in your normal gallery? |
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Hi Michael, Certainly, here is a full size image: https://www.astrobin.com/421949/ Cheers Robert |
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First thing to notice is the massive coma, especially in the upper borders of your image. This tells us that you need a coma corrector. And the coma is not centered in your image, that means that your optical center is not the middle of your CCD-chip. This can happen because of several reasons and can be - in combination with your coma - the cause of your distracted spikes. I suggest that you take a flat and use CCDInspector to center your optical path on your CCD. This would also be visible in your images, your coma will become like a symmetrical tunnel (or warp)-effect. When your image is centered, you can try to beat the coma with a coma corrector. Keep in mind that coma-correctors are very sensitive to the distance between corrector and chip - this should be calibrated on a mm-scale with distance rings. Hope that helps and my english doesn't take away the meaning of my explanation-try :-/ |
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Hi, I will try it in english  Regardless of the COMA problem already discussed by Michael, the spikes in a scope come from diffraction from any straight shape in the optical path. That is the reason why the common shaped spider in a nexton brings the main spikes. On your picture, there is some other secondary spikes, looking like a diffusion (-they are not dramatic). Most of the time this is due to the focuser tube or the 3 supports (holding mirror from top and side) on the miror cell. Sometimes, the coating (primary and/or secondary) might also have some defaults on the border, making straight shapes that can lead to spikes or diffusion. CS JF |
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had same problem with my old scope, this is result of several errors : Tilted optics Mirrors need to be collimated reflections in the tube dew on the mirrors once you solve these problems everything will be fine, first thing - clean mirrors and colimate them, next thing - avoid any artificial light to enter the tube, do it using wrapped arround black mated yoga matt it will help against the dew and reflections in the tube + you can remowe mirros and paint inside of the tubus with matt dark paint. to fix tilt you need to make sure you have stop ring for your camera so you can put it in the focuser parallel to it its because focuser has a hole thats about 0.2mm bigger than your camera adapter is, use some office tape to fix it, but be carefull in warm weather it will cause problems, or buy adapter that allows you to fix tilting, costs about 50 usd max. |
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Hi Michael, Both your English and explanation look wonderful to me. I confess, I am not using a coma corrector >_< I will definitely check the optical center of my photo train. Thank you so much! Robert |
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Hi JF, That sounds like spot-on. I will check the seating of both mirrors. Fortunately (or unfortunately), everything in a Newtonian is adjustable. Thanks a ton! Cheers Robert |
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Hi peleks, Indeed, dew could be playing a role as well. I am living in a sub-tropical region on this planet. Just recently I read about tilt-adapters for fast optics. Will do my homework. Thanks a lot! Cheers Robert |
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Experimentation is the best way to understand and resolve issues. I am not an expert at any of this, however, my background is science. You could acquire another set of data using the same exposure and same setup, to remove possibly some unknown, like dew. If the results are the same, start modifying the optical train, starting with some of the suggestions. Just like in business, risk or defects must be eliminated or reduced using steps that can be replicated. If you modify too many things all at once, well no knowledge will come out of it. What will come of slow modifications, is a better understanding of your optical setup. Good luck, and keep looking up. R Dew is a real possibility for your issue. If you look at this image, the issue is frost. The image below was taken with a RASA in cold weather -32 C. https://www.astrobin.com/422048/?nc=user Now, compare the spikes with this other image. https://www.astrobin.com/422045/?nc=user Different nights, different conditions, different spikes... |
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Hi Rich, Thanks so much for your kind advice! Robert |
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Hello ! I had exactly the same problem last year, I posted the same question on forums and I had the same responds (they are relevant), but the problem comes not from the collimation, the coma or the focus. It comes from the imperfections of the mirror edges. This problem is current, and it's not very difficult to solve it. You just have to cover the edges, with a cercle made of plastic for instance. I have a Skywatcher Quattro 200P, the primary mirror is 200mm large, so I cut a cercle in a plastic sheet, like a notebook cover, of 195mm of internal diameter. I put it in the tube, in front of the mirror, and there it goes ! No more issue. Ask me if you want more details or photos. Clear skies, Max |
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| I can echo Max's comments. I too had a turned-down edge on my primary mirror that caused this kind of asymmetrical spikes like this. If after first adjusting the collimation as others have mentioned the spikes are similarly bad an edge-mask is a very easy option. I made mine out of mattboard, trace the width of the mirror and size of the edge I wanted to mask off with a compass, and used a razor to make the cuts. |
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Hello Max and John, Thanks a ton! This is a new aspect to me. I will try this out first, weather permitting. The stars in your images on your great images are looking exactly as I would expect my Newtonian to produce them. Cheers Robert |
