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Hello guys, This is a very long post but I felt that the people willing to give me some suggestions will need to know the history of the OTA as well as the setup. I’m swallowing my pride and asking for your help here. I always try to fix things myself before asking for help and I can tell you that the others I have asked for help have not given me the “fix” for my problem here! I’m looking for some help with a problem I have had with a 12” f5 newt. I’m looking for others that have experience with large newts or (very similar optical trains) that have experienced the same or similar problems I’m going to describe.Let me give you some background here because I think you will need to understand just how much I’ve worked on this setup and what all has been done. 1st, I’m a big fan of newts and I use them as my main imaging instrument. My 8” & 10” newts I’ve always had very good luck with round stars etc… Feel free to look at my other images in my profile. They are not what I’d call perfect but they are very acceptable to me and was happy with them. This 12” is NOT acceptable in any way at this point to me! “OTA Background” The OTA is a 12” F5 Steve Swayze primary with an 1/18th wave Antares 3.1” secondary mirror. 2” Moonlite focuser with dual 7”x8” inside/outside reinforcement plates to eliminate tube flexture in that area. See images that I have attached. I have also modified the primary mirror cell with extra heavy duty springs and oversized fine thread collimation screws and added an extra “but lighter” set just for extra support between the collimation screws. This is all sitting in a set of custom Parallax 14” rings with dual 24” Losmandy mounting plates, an Orion CF 80ed riding a top for guiding. This all weighs in at 76lbs and is 47” long from end to end and is housed in my back yard Roll of Roof observatory. “Mount Background” When I 1st had this scope setup for imaging (2yrs ago) I run it on a Celestron CGE Pro that had problems with the drives motors. (CGE Pro owners know what I’m talking about here) but later on with the help of a few people I rebuilt the mount and converted it over to the OnStep drive system. While this made a significant difference it just wasn’t what I had hoped for so I pulled it off and decided if I ever get a better mount I’ll give it another try.Well as it worked out I was lucky enough to acquire an AP1200 mount that has performed incredibly well with tracking numbers that I didn’t even know existed. (I don’t need to give any details on this mount they speak for themselves) So I decided to go ahead and give the 12” OTA another try to see how this mount would handle and track with this scope. Considering this OTA to mount ratio is only 54% (yes its over the 50% rule of thumb for mounts) I’d still have thought I’d get more rounded stars with this setup! “The Problem”It doesn’t seem to matter what length of sub I use, I always seem get these stars that are egg shaped from the middle of the FOV right out to the edge of the field. BTW This is zoomed in 100% to see the egg shaped stars. (See the pic’s of the sample image along with guiding numbers for reference) Collimation is spot on and when I use a single beam laser in the focuser perfectly centered in the primary center ring, it does NOT move regardless of where you point the scope, so I believe that tube flexure and or primary shift isn’t the issue. Am I wrong or missing something here?I have also gone thru and built the jig, and adjusted the tilt of the camera using the method described in this article/tutorial https://www.macobservatory.com/blog/2021/11/28/how-to-remove-sensor-tilt-from-your-astrophotography-camera-while-indoorsWhile this helped it did NOT fix the whole problem, and I am planning on going back and tweaking it in better if possible. “The Imaging Train”I’m using my ZWO 071mc pro with a Starzona filter drawer attached to a Quattro Coma Corrector. The filters I’m using are as follows: Optolong UV/IR CutOptolong L-ProIDAS NBZ Dual Narrowband filter. All filters produce star images that are identical or extremely close to each other. Thanks in Advance to any and all that reply! Dale  |
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| UPDATE:So I decided to go with an OAG to help eliminate what we “all” talk about differential flexture. I have to tell you that I DO NOT think this was/is my problem BUT I’m open to suggestions and explanation’s about this. The second set of images/LINK will show the images taken from this “new” setup and while I think it helped somewhat, its not a perfect fix I still think I have more issues somewhere and I need some help PLEASE!I have taken and image and used APP to process the image with the algorithm for “star shape” here is the link: https://www.astrobin.com/j1z8vv/C/ |
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If the star elongation is uniform across the frame, and it's consistent regardless of exposure length (>1s), i wonder if this is a high frequncy oscillation. Something like this typically won't show up in guiding error. To observe high frequency error (<1Hz), try recording high speed video (>90fps) of a bright star at high resolution (<0.15arcsec/px). Record in SER file format to retain time data for each frame. Good luck with troubleshooting. Clear skies, Nikko |
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Hi Dale, Can you provide Some more images of the problem, e.g. a full resolution single sub? I have been through quite a few problems with star shapes but it's hard to rule out a few things without a full high res image. Regards Robert |
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Hi Dale, I'm just soooo envious of your mount performance. So very envious.... Back to the problem you're experiencing. First, I'm having it from time to time and it is related to a couple of things: 1. Collimation is a little askew. You should most defnitely use an autocollimator and a good one too. Not cheap ones! This is the real game changer in terms of collimation. Do not trust you laser! it is ok for centering the secondary but that is about it. Also, I'd change in due time the center donut with one of the center marker from Cat's Eye tools, you know the triangular reflective ones. As an aside note, I'd say I keep checking collimation in my sub-optimal mirror cell. They do change with time. 2. Tilt in the filter drawer adaptor. Remove it from your image train, attach the filter to the front of the CC and see if things improve. If it is the tilt in the image train you should defintely invest in this: https://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/ctu-camera-tilting-unit.html As an aside, do NOT overdo things. Putting extra strength springs risks bending the cell and then the mirror. Might be not your case but is worth mentioning. |
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If the star elongation is uniform across the frame, and it's consistent regardless of exposure length (>1s), i wonder if this is a high frequncy oscillation. Something like this typically won't show up in guiding error. Thank you I’ll look into this Nikko! Dale |
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Robert Shepherd: Hello Robert, I attempted to upload fits files and was unable to do so but, I took the individual subs and saved them to JPEG format, so I could upload them here. There has been absolutely NO processing, only converted RAW subs to JPEG format to be able to upload the files. BTW, all subs had very similar guiding to them, regardless of the guiding method. OAG/Piggyback. This 1st image is a single 2-min sub at what I'm calling 0 degrees.  This 2nd image is a single 3-min sub. I rotated the camera assembly 170 degrees. Because of a thumbscrew collision, I was unable to rotate 180 degrees fully. But you can see there is a slight improvement here. Especially towards the center of the image.  I attached this image I took of M101 about 3-4 weeks ago, WO the OAG. This was with my CF 80ed guide scope riding piggyback on the main scope. Same scenario 1- 3 min sub.  Dale |
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andrea tasselli: Hello Andrea, Thanks, I've been really happy with my (new to me) mount. With the guiding that I get from it, you can see why I'm so FRUSTRATED why I can't get better star roundness then I'm getting. Collimation: Yes, I used both the single beam laser and AutoCollimator from AstroSystems. Sorry, I should have put that in the original post. This Primary has been just recoated and the center spot I am just using a round center spot, so the Cats eye center spot I'd have to get and pull the mirror cell and replace it. BTW I do throw the defraction pattern and final tweak it with the camera assembly in the scope. Filter Drawer: Good thought! I could do that and see if that makes a difference. This crossed my mind but seeing as though I'm using it I figured I should keep it in the imaging train, but again I'll look at doing that. Tilt Adapter: I guess I forgot to mention that my camera is the ZWO ASI071MC Pro which does come with the tilt adapter. I will say that the filter drawer does cover up the tilt screws and every time I need to make an adjustment I have to unscrew everything from the camera body. I'm sure you can see the issue here in that trying to make everything perfectly consistent every time is next to impossible! Springs: While I don't think this is the issue, it is a good thought, I'll keep this in mind. When checking collimation by throwing the defraction pattern inside and outside of focus (with camera in the draw tube) collimation is spot on along with it being perfectly round. I'm guessing if the mirror cell was causing an issue, I'd see it in the defraction pattern. Your thoughts on that? Dale |
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To give you more background on the camera imaging train I have taken some pix for you to see and maybe you can see something I’m not. 1st one is just the complete system off the scope. The other 2 are installed in the focuser. Also on the 2nd image you can see the extra reinforcement plate on the outside of the scope. There is also another one directly below it inside the OTA for added strength. Dale    |
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Dale, Thanks for providing the raw images. So, as the effect on the stars is the same direction over the whole image and does have the same orientation relative to the object/ sky, no matter how you turn the camera, I would conclude the following: - the effect isn't that pronounced, so we're looking for a rather minor cause. - probably no problem with tilt, as some corners would be better than others and the effect would vary more strongly over the image. - as the effect stays oriented in the same direction relative to the sky/scope it is very likely a collimation issue- there is one more effect that could show the same "behaviour" and that is a turbulence layer along the primary or in the tube. But there would be more "flare" on one side of the star corresponding to "up" in the tube. For collimation on a bright star I recommend the free program metaguide. You can get collimation absolutely perfect with that program after getting it close with collimators. Unfortunately it takes some effort to get behind the UI and use. (But that is just a one time thing and will for ever be the most precise Methode) The most important aspect of all collimation methods is: collimating with the camera on the scope so you're collimating the entire system. Good luck Robert |
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Robert Shepherd: Hello Robert, I’ll look at the program your referring to for sure! I do collimate with the whole system in the focuser and like I mentioned in my reply to @andrea tasselli I’ve felt that the collimation is spot on using the defration pattern method. If it is collimation it must be very slight as I cannot distinguish any difference zoomed in at 100% with the defraction patten on either side of focus. I’m glad that you feel tilt isn’t the big culprit here, I have spent a lot of time and energy getting the tilt on this system as close as I can possibly get it. Although I would say I was considering making another tweak in 1 direction to see if it would help any. I seen in a post about this “Reflectix” (I think you commented in the post) about slowing down the “chimney” effect so I wondering if that would make a difference in my situation? @andrea tasselli also replyed in that thread about that stuff. Maybe this is why others with these bigger newts use that stuff??? BTW, I have a cooling fan that runs on the primary mirror 24/7 but I do shut it off when I start my imaging sessions. Dale |
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Dale Penkala: Hi Dale, When you're using the autocollimator how many reflections of the center donut you can see overlapping, or partially overlapping? At any rate if the Fresnel pattern looks identical in and out of focus then you should be pretty close. There is still the issue on non-perfect orthogonality between the focuser and the primary axis but this can be checked by moving in and out the focuser with a laser on and see if the return beam shifts. As a method to nail down the issue(s) I'd venture to suggest that you should remove everything attached to the focuser and camera and leave the imagining train as simple as possible, that is CC and camera with right spacing. Point at the NP with the mount not tracking and take images from there , say 5-10s long, and look on how round the stars are. They should look all pretty round. If not than you have an issue with either tilt or spacing (or both) and can start to work on that. Once this is sorted you can start rebuilding the image train one item at the time. The nice thing about the Neumann tilting adaptor is that it is easy to do the adjustment without disturbing anything or without anything being in the way. BTW, I don't think the issue got anything to do with the guiding method. |
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andrea tasselli:Dale Penkala: Hello Andrea, Autocollimator: when I used this I had a total of 3 circles that were superimposed on each other. From there I did the defraction pattern method with the imaging train in the focuser and tweaked it in closer from there. After reading @Robert Shepherd post I went and read the collimation part of that MetaGuide users file and I’m honestly surprised that it states that in some cases a star can come to focus better with the central obstruction showing slightly of center. I do plan to give this program a try and see what kind of difference it makes to the stars after using its collimation method. I’m quite interested in this method and how this program works. BTW what do you think about the Refletix wrap for the OTA? Do you think this could help with what Robert was talking about? Tube current chimney effect? Nailing down: I like your thoughts on this Andrea, I was thinking right along that line start with a simple (but necessary setup) and work from there. However I wanted to see what you guys had to say about my problem and may see something I’m just flat out missing. BTW: Thank you SO MUCH for this! I don’t think so either but so many people have told me my problem is with differential flexure so I had to just give it a try for myself to rule it out and I think that I have done that now. Dale |
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Hi Dale, Just as a quick test, try keeping the fan running when imaging. I turn my fan on about 30 min before imaging and then have it on while imaging all night. As soon as I turn it of I will develope "flares" on my stars. Its worth a try even if it doesn't really look like the phenomenon on your subs. It could look differently on individual systems though. Cheers Robert |
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Robert Shepherd: I will give this a try Robert! Thanks and I’ll keep you and @andrea tasselli posted on my findings. I’m downloading the program you told me about and plan to go get the mount and cameras connected to it so that I’m ready for the next clear night of imaging. Dale |
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Dale Penkala: Hi Dale, With the right autocollimator you should be able to see 6 reflections, one fainter than the next. When they all overlap then you pretty much nailed down collimation at that point. As for the Refletix wrap it won't make any difference in your case. From what I have seen it is more a mix of collimation and tilt then other (transient) issues. Do keep us abreast on your progress, as it teaches us all something about the behaviour of these newtonians "beasts", as it were... |
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The number one thing to work out first is optical performance on-axis. You said that the stars on-axis are elongated no matter what exposure length you use. If you can see elongated stars even with 5 second exposures (with the mount guiding), that rules out a guiding or mount problem. You've also said that the secondary is reasonably well aligned so that the optical axis of the primary falls on (or near) the middle of the sensor. That means that optical alignment is not a problem. (It may not be perfect, but when you are seeing seriously astigmatic stars on axis, it doesn't have to be. It just has to be close.) That leaves just one possibility--anamorphic wavefront error. There are just two things that can cause on-axis astigmatic star images with a parabolic primary (assuming no other optics in the system): 1) Incorrectly made optics or 2) Improperly mounted components. In my opinion, a mounting problem is the most likely source of the problem. Here are two things to think about: 1) How thick is the primary mirror and how is it mounted? If it's a thin mirror (aspect ratio<~1:8), it is pretty easy to warp the mirror. Does the mount include a stress distribution system like whiffle tree floating mount? If not, how "hard-mounted" is the mirror and is it possible that it is mechanically being deformed? 2) How is the secondary mirror mounted? At F/5, it only takes a very small amount of mechanical distortion on the secondary mirror to introduce significant astigmatism. This is the very first thing to check. The best way to mount a secondary is with a flexible cured RTV potting compound over three kinematic hard points on the rear of the mirror. Did you build this scope or is it a commercial product? Most commercial scope manufacturers have solved this problem, but you never know. One last question: If you have a coma corrector in the system, how is it aligned and how do you know that it is good? Here's the next thing to do: You can easily check for astigmatic error with a B-mask. Position the B-mask over the end of the scope and VERY carefully focus on a well centered bright star. Then rotate the B-Mask by 90 degrees to confirm that the image stays precisely in focus. If your scope has zero astigmatic error, the star image will not show any focus change as you rotate the mask. On the other hand, if focus changes with mask angle, you've got a problem with your optics. John |
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Hi Dale, May not be your problem but worth looking at maybe. I had similar issues with elongated stars mostly in the top RH corner of the frame, collimation was checked many times, the solution was the tilt of the Moonlight focuser to the tube. The Moonlite has four small adjuster screws on the base , I adjusted based on the vignetted image when taking flats (The coma corrector I use is a TSGPU which gives quite a bit of vignetting) only required about 1mm adjustment but it sorted my problem. Rob |
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John Hayes: Hello John, 2) How is the secondary mirror mounted? At F/5, it only takes a very small amount of mechanical distortion on the secondary mirror to introduce significant astigmatism. This is the very first thing to check. The best way to mount a secondary is with a flexible cured RTV potting compound over three kinematic hard points on the rear of the mirror. THIS IS THE EXACT METHOD OF MOUNTING MY SECONDARY MIRROR. I DON'T KNOW IF YOU KNOW ABOUT THE DBA CERTIFIED LINE OF SCOPES I USED TO HAVE BUT THATS HOW I MOUNTED MY 1/18 WAVE ANTARES 3.1" SECONDARY. I MACHINED MY OWN SECONDARY MIRROR HOLDERS AS WELL. 1) How thick is the primary mirror and how is it mounted? If it's a thin mirror (aspect ratio<~1:8), it is pretty easy to warp the mirror. Does the mount include a stress distribution system like whiffle tree floating mount? If not, how "hard-mounted" is the mirror and is it possible that it is mechanically being deformed? I DO NOT RECALL THE EXACT THICKNESS OF THIS PRIMARY BUT ITS NOT "THIN" THATS FOR SURE. WITHOUT MEASURING THE PRIMARY I'D HAVE TO SAY ITS AT LEAST 1-1/2" THICK. BTW THIS IS A REFIGURED PRIMARY BY THE LATE STEVE SWAYZE. OPTICALLY I DO NOT HAVE ANY QUESTION ABOUT THE QUALITY OF THE WAVEFRONT. THE MIRROR IS SUPPORTED BY 3 TRIANGLES THAT ARE FREE FLOATING. THERE IS A TOTAL OF 9 CONTACT POINTS IN THE SUPPORT SYSTEM. THERE ARE 3 MIRROR CELL CLIPS THAT HOLD THE PRIMARY FROM COMING OUT BESIDES THE ROUND STEEL RING THAT THE MIRROR SETS IN. THE MIRROR CLIPS ARE NOT TOUCHING THE PRIMARY MIRROR FOR THE EXACT REASON YOU BRING UP/DISTORTION/PINCHING OF THE OPTICS. THESE ARE ALL VERY GOOD THINGS YOUR BRINGING UP BUT I'VE BEEN VERY CAREFUL AS TO COVER THESE BASIS'. THATS NOT TO SAY I MISSED SOMETHING. I'LL CONTINUE TO KEEP THIS IN PERSPECTIVE AND THANKS FOR YOUR INPUT! DALE |
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andrea tasselli:Dale Penkala: Hummm I only was able to see 3 and they were defiantly fainter consecutively! Ok on the Refletix wrap! Thanks! I defiantly will! I think this is a good thread for people with larger newts. The shear size of these monsters can cause a lot of problems as we can see here with my setup. I've done everything I can think of with caution to everything that @John Hayes brings up in his post but all that being reiderated keeps me grounded to basics. Dale |
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Dale Penkala: That all sounds good. I'd sill use the B-mask trick to confirm that you don't have an astigmatic error. If you don't, then there's another problem. One other thing that I've seen that can cause elongated images is fan vibration, which shakes things at a pretty high frequency. On many ZWO cameras you can turn off the fan by disconnecting the power wire and still be able to take uncooled images. You might try disconnecting the camera fan (and any other fans that might be on the OTA) to see if that makes a difference. Image a bright star with the shortest possible exposure, just below saturation, with the mount running to see if the elongated stars disappear. If you get round stars, something is vibrating. John |
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Dale, One thing I forgot to mention is a fireproof way of checking whether you have tilt or not. If you have a bahtinov mask (or can get one), put it on and center of a fairly bright star. Carefully focus manually so that you have a perfect symmetrical pattern. Thne, without touching the focuser, move the star to the 4 corners and check if the pattern is still symmetric. If not then you have a problem with tilt. If you have APT you can actually measure the amount of tilt with the electronic focuser so you can nail down the amount you need to correct. To check for mirror flop move to different parts of the sky and see whether anything changes. |
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As a 12" newt owner myself there are 2 things to consider that largely affect your images. Main mirror shift and coma corrector tilt. To determine main mirror shift, put a laser in your focuser, flip east or west pointed at 45 degrees center the laser on the primary dot and then flip the scope 180. I am going to bet that laser will not be centered anymore, it will be a little or it will be a lot. Some is tolerable. This is due to the common 3 clip system, a heavy mirror will shift left or right depending where in the sky it is. For the coma corrector, if there is any amount of looseness between the CC and the drawtube you WILL have tilt. 3 thumbscrews might hold it in but it will teeter on that point. A paper shim wrapped around the length of the coma corrector should work in most cases if it is too loose. |
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John Hayes:Dale Penkala: Hello John, I will try the B-Mask, I’ve not heard of doing that so I’ll give it a try! I’ve heard of the cooling fan on the back of a primary mirror cell causing vibrations which is the reason I shut mine off, but really haven’t heard of the cooling fans in ZWO cameras having that effect. I will DEFIANTLY try this! Thanks for that suggestion as well! Dale |
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Rob Johnson: Hello Rob, I’ve squared my focuser to the ota and centered the secondary when setting up the optics getting things ready for collimation, so I’m pretty confident that this is set right but I’ll defiantly keep this in mind! Dale |
