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Huge tracking errors. #ascom #question #tickmanagement
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Hello.
My fork-mounted homemade telescope seems to be experiencing a lot of tracking error and it tracks a tad too slow. (I'll attach an image of my scope and a ZIP file containing a photo displaying the slow tracking & 30 min with of 5s shots that show the tracking errors). We are trying to resolve this issue with Dan but I was just wondering if anyone else has any information that might help.
Some info that might help identify the problem:
- The telescope is homemade so it may not be mechanically perfect,
- It uses worm drives on each axis(no noticeable backlash on RA and some backlash on DEC),
- It's a very heavy mount/scope(over250kg) and almost all that mass is resting on two small ball bearings holding the scope at a 45deg angle,
- It uses servo motors with encoders from Sidereal Technology,
- I'm perfectly polar aligned when the scope is struggling to track,
- The motors themselves have some play if you try to manually rotate them when they are powered off(they have 10mm of play left to right),
- All the bearings are old and probably dirty,
- I have the scope unbalanced towards East and back to get rid of some backlash,
- Error when slowing on RA is very low however on DEC it's quite bad,
- It's not windy when I'm shooting. Scope: https://imgur.com/a/kkCcYG1
Link to image of slow tracking: https://imgur.com/a/eOdQxcl
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Joshua Hufford
Matija, I'm only able to see the image of the scope and the first one you call slow tracking. What exactly are you trying to show with the image of slow tracking? How long was the exposure? I can't zoom in much but honestly I'm not seeing any obvious tracking problems in that image. It looks a bit out of focus and I'm seeing coma, but the stars in the center look relatively round. Were you guiding in that image? Can you post a FITS image somewhere for download? Josh On Sat, Nov 12, 2022 at 5:12 AM Matija Sircelj <matija.sircelj@...> wrote:
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Hi.
That image of slow tracking is a livestack of 30min at 5s exposure. You can see that the object moved out of view(black borders). The fit files are here(25min - 5s expousures) https://wetransfer.com/downloads/715287fbf95aa0435089be8c9bd611a920221112104409/2a2a278444ea0c7af52e97c0da37e9be20221112104422/04be27?trk=TRN_TDL_01&utm_campaign=TRN_TDL_01&utm_medium=email&utm_source=sendgrid Matija |
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Joshua Hufford
OK I understand what you are saying now. Are you guiding and if so what is your guiding setup? On Sat, Nov 12, 2022 at 6:50 AM Matija Sircelj <matija.sircelj@...> wrote: Hi. |
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No. Unfortunately, I'm not guiding yet.
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SIMON
Matija,
When you say 'I'm perfectly polar aligned when the scope is struggling to track' how are you checking polar alignment? Might be worth using something like SharpCap Pro or NINA to define exactly how far from the pole you are - it will at least reassure you that you can eliminate this. Re Guiding: How are you guiding? OAG or guide scope if the latter, then flex in the guidescope can cause issues, as can slop in the mirror - also - something that has happened to me on more than one occasion is PHD2 starts guiding on noise so it might be worth starting with the polar alignment measurement, and seeing how well it tracks with guiding off, get that as good as possible before moving to using PHD, guiding works better with a well tuned mount. Tracking can be impacted by...(I've probably missed some) - Polar Alignment - Flex in OTA - Flex in Mirror cell - Flex between Guider and Guide Scope (OAG is better approach IMHO) - Servo/gearing incorrect - PHD not calibrated - your mobile set-up looks like you will need to calibrate PHD2 at the start of each observing session. - Poor mount model in SiTech (I say this because I've always found my mount works better without the mount model - but I have a very good mount) - whilst fault finding I'd suggest not having a mount model) - Periodic error in gearing - you can examine this in the PHD log viewer (I think) - or you might need a viewer I vaguely remember finding some software to analyse PHD logs - Poor PHD set-up (defaults seem to work well for me) Hope this helps. Simon. |
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Hi.
I'm not guiding and I'm checking my PA in SiTech directly under features. I was off by: (Move Up 00:11:11 and Mone Right 00:01:51). My mount model was decent I think. 30 points and the PointXP values were (RMS=00:03:06 and Peak=00:07:27). My tube may be flexing. not sure about servo/gearing. Thanks! Matija |
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Joshua Hufford
Honestly, while certainly not ideal, that isn't a huge amount of drift in 30 minutes and could be easily guided out. If your goal is astrophotography (assuming here) the short term tracking accuracy is what is more important, at least that has been my experience. Slow gradual drift can be guided out, short erratic drift is what can ruin an image because guiding can't easily correct for that. If that were my setup, I would first rotate the tube so the focuser is mounted inward, I found it is easiest to balance a Newtonian style scope with a camera on it this way. You can see how I have mine oriented here, Then I would try to start guiding, even if you just put your guide camera in the focuser and nothing else, get your mount guiding well first. If your mount points to targets reasonably well then your ticks must be at least pretty close. Once you get it guiding well then you can move on to taking pictures. Josh On Sat, Nov 12, 2022 at 7:33 AM Matija Sircelj <matija.sircelj@...> wrote: No. Unfortunately, I'm not guiding yet. |
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I'm planning on starting to guide soon.
About rotating the tube. The scope is sometimes used as an "observatory" so that people can see some DSOs, so I can't rotate it like this. If you looked at the Wetransfer file you'll see that there is a lot of short term error. every third image is ruined. Thanks for your help. Matija |
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Joshua Hufford
I would concentrate on what is causing every third image to be ruined before I try to figure out the slow drift. Josh On Sat, Nov 12, 2022 at 7:47 AM Matija Sircelj <matija.sircelj@...> wrote: I'm planning on starting to guide soon. |
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I completely agree but after over a month I still have no idea what could be the problem. A few options(tell me if you agree):
1.) the 200kg mount + 25kg telescope is resting on two small old ball bearings, which could be a problem when tracking, 2.) the motors(SiTech servos) have a lot of play when I try to move them with my hand, 3.) a lot of backlash od DEC, 4.) the telescope tube is very long and very thin(1mm) so it could be bending and the mirror could be moving, 5.) controller problem? Another thing I noticed is that even after a good 30-point model, the scope still misses a target in a FOV of 0.3deg x 0.3deg. The scope is homemade but I don't know how well since I didn't make it. Hope I can resolve this issue because it's sad to see a mount like this struggle with 5-second exposures. Matija |
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SIMON
So I'm guessing... 1) My old EQ6 got a lot better when I cleaned and re-greased the bearings - so old poorly maintained bearings and grease can be a problem 2&3) I *think* play/backlash is more likely to be a concern for pointing/goto than tracking - the calibration for guiding in PHD should calculate the backlash for guiding purposes. 4) Difficult to assess, if it was my problem I'd look into mirror cell before the tube - just because it is probably easier to fix 5) More for Don/Dan
I personally wouldn't use the pointing model until you have fixed the guiding - I still think you need to measure the actual polar alignment error Good luck - you will solve it eventually
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Joshua Hufford
My suggestion would be to use your camera for guiding, don't worry about taking any pictures. Orient the camera so RA is exactly left to right and DEC is exactly UP and down in your image. Sit next to your mount and watch the guiding graph. Watch for patterns, listen for unusual noises when you see large guiding errors. Even with the drift you show over 30 minutes, you should be able to take images of 30 seconds with round stars. Figure out what is causing the short term tracking errors, once you have that corrected you can work on figuring out the long term drift. And who knows, the short term errors could be adding up to the long term drift. Also make sure everything on the mount is rock solid tight, any flex or movement even that can't be seen to the eye can result in large tracking errors. Also I would suggest not using a point model for now, if you have bad data going in that can make your tracking worse. If you need to use it to get your polar alignment close fine, but then remove the model, and then start guiding. Josh On Sat, Nov 12, 2022 at 9:29 AM Matija Sircelj <matija.sircelj@...> wrote: I completely agree but after over a month I still have no idea what could be the problem. A few options(tell me if you agree): |
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Don W
Hi Matija,
One thing you can do to help pointing accuracy is to set SiTech to always approach DEC from the same direction. That control is in Change Config / Mount Parms. This minimizes the effect of backlash in DEC. Don W |
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