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Hi guys! I got myself a Sky-Watcher Newtonian 150PDS recently and i've been struggling with collimation for at least two months now. I am inexperienced woth newtonians so i've been reading articles and watching videos for collimation, so i've tried every method of collimation, including star testing in the field, and when i take an image i still have distorted stars on one side of the image. I thought it might be my cameras sensor so i've had it checked and repaired at a professional and i also bought a new camera and tested it and the distortion is still there at the same place. So i am guessing at this point that the problem is the focuser. it seems to be sturdy and doesn't shift around but i don't know anymore. Is there a test i can do to check if this is really the problem? and how do i fix it? thank you guys |
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| You can check it with a laser collimator. Test it by moving the focuser from the minimum distance to the maximum distance. |
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kim younghoon: i have actually done that and the laser dot moves while extending focus. So this means i have focuser tilt? |
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If it deviates when moving and stopping, the optical axis may also be a problem. The solution right away is to mark your focal length and precisely adjust the optical axis there. |
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kim younghoon: The focuser and axis are not exactly aligned. The problem for all Newtonian users is the focuser. |
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I am also dealing with this with an Orion 6” f/4 Newt. I will be squaring the focuser with shims, guided by my Howie Glatter laser. There are various methods suggested on Cloudy Nights & YouTube. I’ll also be checking out my old ATM for building a Newt & installing a focuser. Any favourites methods here? |
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The only concern with any focuser is variable tilting along its own axis otherwise collimation would take care of any relative dislocation between primary and focuser axis, which is exactly what "collimation" aims to achieve. So, if you can collimate a newt than you are guaranteed that focuser tilt isn't an issue. What coma corrector are you sing, btw, and what method of securing it to the focuser/camera are you employing. For all that needs to be known on collimation Vic Menard's "New Perspectives on Newtonian Collimation" is a must. |
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andrea tasselli: i am using the baader mpcc iii which i think works very well for now. It is directly screwed on the T ring of my DSLR with some spacer rings to give me the right distance. But i think that because i collimate with out of focus stars then when i focus again then it would be out of collimation because the focuser is tilted. Right? |
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i am using the baader mpcc iii which i think works very well for now. It is directly screwed on the T ring of my DSLR with some spacer rings to give me the right distance. But i think that because i collimate with out of focus stars then when i focus again then it would be out of collimation because the focuser is tilted. Right? In principle, no it isn't right but then it also depends on how far out of focus you are (hint: just a touch to see 1-2 rings) and then anyway critical collimation is always achieved in-focus. As far as collimation is concerned then you're done and everything else is tilt induced by the connections or the sensor itself (which isn't too surprising). The MPCIII isn't a great CC but at f/5 it shouldn't struggle. |
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andrea tasselli: Which is the best CC for you? |
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Paolo Banci: GPU (and clones) or ParaCorr II |
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| Telescope service ? |
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Paolo Banci: Or SkyWatcher or SharpStar. They are basically all the same. |
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I have the SW 130 PDS the SW 14 inch GOTO Dobson. They both have the same clunky focusser- I assume the 150 PDS is the same. There are three pairs of adjusting screws and lug nuts at the base of the focusser that allow you to adjust the tilt of the focusser. During the day I generally adjust these such that using a Chesire the secondary mirror stays perfectly centered as I move the focus tube up and down. I find when the tube is well aligned using this method , followed by collimation with cheshire, and precise spacing of the coma corrector to the camera sensor, it translates well into having round stars (i.e. no or very little coma) in all 4 corners of the FOV (as opposed to having strong coma in once corner, or one side, and no coma on the other side). This old link from Stargazer lounge - is very helpful. You will see that you are not alone in this struggle! https://stargazerslounge.com/topic/380815-how-to-adjust-focuser-tilt-for-sky-watcher-130pds/ By the way, I've been using the SW F5 Coma Corrector for years. It works fine. http://skywatcher.com/product/f5-coma-corrector-2/ Hope this helps! Mike |
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It was trying to collimate with the Ocal that persuaded me the focuser was off - no matter how much I adjusted the offset, I could not get the green circle to align with the focuser opening. So, I guess I’ll continue with the Ocal - remove the nuts from the bottom of the focuser screws & see if that helps with the off-set, insert the appropriate shims, & carry on! |
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| One issue I had with my SW Quattro 250p was the eyepiece clamp was atrocious quality. I posted about it on CN looking to find a replacement here: https://www.cloudynights.com/topic/889824-skywatcher-telescope/#entry12920046. When I measured it, it was 1mm difference in height from the high point to the lowest point which of course created a large amount of tilt in my images. I reached out to SW about it and to see if they would replace it but they stated it was within tolerances. Might be worth checking. |
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| I recently received a T-Ring from Amazon with built in tilt. It was off by about 0.15 mm. This is enough to cause issues. Definitely worth a check. |
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I recently received a T-Ring from Amazon with built in tilt. It was off by about 0.15 mm. This is enough to cause issues. Definitely worth a check. Interesting! How did you measure the tilt? |
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I recently received a T-Ring from Amazon with built in tilt. It was off by about 0.15 mm. This is enough to cause issues. Definitely worth a check. *** With a simple caliper. I have measured all the way around to figure out the thickest and the thinnest part. After that I measured it with a precise height gauge in my companys measuring lab. But the result was basically the same. I got a refund, but it was wasted imaging time still. *** |
