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Hello, I see this issue from time to time and I've never been able to fully put my finger on what it is. Maybe someone here could help me shed light on the cause for these odd looking stars.   Some notes: It's consistent throughout the frame. They all "point" the same way. Guiding is under 0.8" RMS. No obstructions in the light path. My collimation is spot on:  Tilt seems to be manageable:  Thoughts are welcome! |
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| My main theory is that since I live in the city and the scope is on a second story balcony and that the "tail" is completely vertical compared to the ground, it must have something top do with hot air from perhaps the apartment under mine (which would explain why it only every now and again) |
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Hi, This looks like a 'tilt' problem - something in the image train is not orthogonally perfect. I've had this issue with some manufacturers' "2-inch" accessories that aren't perfectly milled. If this is the case with anything in the train, a winding or two of masking tape can tighten things up. - - Steve |
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Hi, I included a detailed run from Hocus Focus and the tilt according to it's analysis is very minimal (I've certainly had worse without my stars looking like this). The focuser is also a brand new TS RAP that is rated well above what I am loading it with. |
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Over the span of 30 minutes it corrects itself to some degree with further leads me to believe that it isn't something mechanical.  |
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| Try to solve the image to find out the angle of the tails. |
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Guiding performance? Bad polar alignment? Mentioned in the original post, guiding is under 0.8"RMS and my PA is within 0.9 arc min according to PHD2 |
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Mentioned in the original post, guiding is under 0.8"RMS and my PA is within 0.9 arc min according to PHD2 Sorry, just read it. |
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Try to solve the image to find out the angle of the tails. As mentioned in my other reply: My main theory is that since I live in the city and the scope is on a second story balcony and that the "tail" is completely vertical compared to the ground, it must have something top do with hot air from perhaps the apartment under mine (which would explain why it only every now and again) |
8.51
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| Thermal plume is the most plausible explanation although it needs to be assessed whether is internal or external. |
24.64
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What kind of telescope is this? It is very difficult to diagnose anything without detailed information about how the data was taken. John |
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| I am nearly certain those are thermal effects, I have seen identical features on stars with my mak-newt when it is taken straight out of a warm room and not allowed to cool to ambient. If you are on a balcony you probably have rising heat as you suggest. Try looking at one of the problematic stars out of focus in live view - the thermal plume should be quite obvious. |
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| Hmm, I see you are on a second story balcony. Maybe it could be vibrations? Not sure how your building or balcony is built. Even a solid concrete structure will transmit some vibrations which could affect your image? |
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| This is most likly tubus seeing. My setup is always outside in my observatory and even I have it here in munich. It takes 1-3 hours to finaly be gone, depending on your amperture. Its extremly annoying, but ive installed a fan on my mirror and now the air flow is great, and its gone in under an hour. Question: is this error the whole night? I even had nights were this was the whole night and the next day it was gone. I think the target location (low on the horizon) is also a reason. The hot air wants to escape. Fastes way is to look directly in the horizon 90 degrees. Tubus seeing was extremly during summer here. The black tubus got hot during the day 20-25 C and then the night with 10. it took ages... |
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The scope is an 8" ONTC from Teleskop-Service with their TSRAP focuser, TSGPU and 294MM. I use vibration pads on the balcony (which is an old concrete building). Looks like my theory wasn't far off, thanks for everyone who pitched in! I am currently building my own observatory so I assume this may not be as big of an issue in the future, will still look into getting a fan for the primary. |
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| Thanks for posting this. I seem to have the same issue on my own 8" ONTC and suspected thermal effects as I have done a fair amount of work both on the collimation and the mechanics. The tails seem to get worse at low altitudes and I am relatively certain my primary mirror does not move much with my new TS mirror cell (which reduced the pinching issue). Maybe I need to use less heat on my primary heater, because I am not at a balcony. |
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That aberration field is not due to image tilt. Image tilt will introduce out of focus star images on either side of a line through the image. The shape of the stars is also not consistent with coma. This looks like it may be due to a thermal effect but I'm not convinced that it isn't due to a mechanically distorted component that may change with temperature. Have you defocused a bright star in the center of the field to see what it looks like? (Your image above is too far out of focus and way too over-exposed to see what's going on). If this is due to a thermal plume, you'll see it right a way as a moving plum of air in front of the defocused star image. You'll also be able to tell if the mirror is being distorted in some way. That will look more stable--only changing with the overall seeing. I recommend star-testing this scope to better understand what's happening. Your "collimation image" showing the shadow-donut does not display a properly aligned Newtonian, but that depends a little on how the secondary is sized. A properly aligned Newtonian should have a secondary that is slightly de-centered in the entrance pupil. Yours looks well centered and that can indicate that you may have some coma; but again, I don't think that's what is causing the comet shaped stars. One way to ensure proper alignment is to use SkyWave. It can tell you pretty accurately what the wavefront looks like and it can provide information on secondary de-centering. It can also help to dial out any alignment induced wavefront errors. John |
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| Was this ever solved? I have the same issue with my 10inch Newt. All tails are point directly to centre. I have collomination done with Ocal and pretty sure im centered. |
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Same problem with a Sky-Watcher Esprit 120. Tonight I've been trying to debug things and have concluded there isn't any issue with tilt or collimation. Aldebaran out of focus, 500x500 px crop in center of frame. So it's a little off the center but best I can tell it looks good  Abreaction inspector on the same field, 2 seconds with L filter. Star tails are wider than original post.  Tilt, as far as I can gauge, is manageable. Last tilt corner measurements tonight after dialing it in (which was done before the above images were taken):  Scope had been outside for many hours by the time I took these. I don't see similar results with longer focal length (reduced C8) and it doesn't matter when in the sky I point. So different temps from various surrounding materials don't appear to be a problem. Other equipment to note: EQ6-R Pro and ASI2600MM. |
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| For a refractor this kind of aberration can be due to a tilted optical element. Triplet refractors and spaced doublet are aplanatic systems and shouldn't show any coma whatsoever. |
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andrea tasselli: Does that mean adjustment of the lens tilt? Or is it something more subtle and less a DIY fix, such as lens elements not aligned to each other? Would love to get this working back to 100%, thanks for any tips  |
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Does that mean adjustment of the lens tilt? Or is it something more subtle and less a DIY fix, such as lens elements not aligned to each other? Would love to get this working back to 100%, thanks for any tips In general, and I'm talking by experience, it's either one of the elements which is either shifted or tilted, tilting more likely. And is not a DIY fix unless you are uncommonly bold and take the lens cell down and open it up. And you need precision instruments to correct the issue. Me, I'll return it if I could. |
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| @jewzaam Maybe focuser sag. I've seen it with Esprit scopes before. |
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You can test to see if it is an internal thermal causing the diffraction effect by taking a single image while blowing into the telescope OTA with a fan. The star shape will change if the internal thermals are the issue. Then you will know for sure without modifying or waiting for anything. I have this issue with my Dall-Kirkham. This does not look like a sensor tilt issue at all. If it were a tilt issue the stars at the center of the field would most often still be normal looking. Optical tilt could be a concern but is less common. This looks like thermals or a mount related /vibration related concern. But, due to the tapering of the effect it is likely thermals. The simple test that I mentioned above will give you a positive answer in 30 seconds time. Even refractors can show thermal issues like this, though it might be less common. |
