kkara4
11-05-2015, 08:09 PM
OK so I have an interesting problem and that is image shift while guiding. Specifically it is about 2.7 arcseconds every 3 minutes. I am able to repeat this on Eta Carinae, even over time, such that I drift approximately 9 arcseconds every 9-10 minutes. The drift is perfectly opposite to the sky rotation, ie drift opposite to RA. The way I determined this was via stellarium simulation of Eta Carinae with equatorial grid turned on. I took 2 10 minute exposures and the stars had all moved 9 arcseconds (approx). Attached is an image of said star.
This drift is most pronounced near the celestial equator. As expected, the drift is in the opposite direction to RA moving North of the celestial equator as well (but this manifests itself as an image shift the other way on the imaging camera since I don’t change its orientation). This is all while imaging West of the meridian.
Polar alignment was performed in DEC only (and using Az knob accordingly). I got it to 0 drift in 10 minutes. I did not bother with ALT adjustment, since it was fairly close (about 3 arcseconds drift after 5 minutes in RA, according to RA slope in drift alignment tool of PHD2).
Since I was specifically imaging to determine the root of this problem, I also screenshotted my PHD2 graphs (guiding with QHY5L-II + Orion ST80, 1.93 arcsec/pixel). Guidescope FL and pixel size were entered correctly into PHD2, so the graph scale should be meaningful. Here are the two screenshots (second screenshot overlaps because I kept guiding).
The scope was made a touch West heavy (Eta Carinae west of meridian). The mount is a Losmandy G11 with Gemini 2, and PEC done by previous owner. Motors are the new high torque ones, and upgraded worm that people usually do (cant remember if RA or DEC). Guide cam and mount both controlled by ASCOM. PHD2 settings can be seen in above graphs plus settings screenshots attached.
I know the graph is very choppy, but I am really not sure what to change to make it less so. I was afraid to start playing in real-time because I wanted to capture the problem and current state of my setup.
I really don’t think this is flexure (too close to following lines of RA, scalable drift to exposure length, seemingly independent of guidescope/gravity orientation, pretty significant image shift of nearly 1 arcsecond per minute, etc).
It is almost like the scope isn’t tracking RA properly (ie tracking it to fast, so the stars streak in the opposite direction), which shouldn’t happen while guiding???:shrug::question::confuse d2:
I have attached an image showing the imaging camera view with respect to the guidescope too. I then approximately superimposed an equatorial grid (red and orange lines as per stellarium simulation), and blue arrow showing the direction of image shift. Eta Carina star is just off the bottom of the view shown (cant see it in guide scope, but is centered in imaging scope), you can see the other bright stars in the Keyhole region.
Any input is appreciated, except: I am not going to post details of my imaging train/scope. The last thing I want is people telling me my FL is too large, get an OAG, blah blah. I am happy to post up extra information in terms of settings, mount details, etc :).
Cheers!
This drift is most pronounced near the celestial equator. As expected, the drift is in the opposite direction to RA moving North of the celestial equator as well (but this manifests itself as an image shift the other way on the imaging camera since I don’t change its orientation). This is all while imaging West of the meridian.
Polar alignment was performed in DEC only (and using Az knob accordingly). I got it to 0 drift in 10 minutes. I did not bother with ALT adjustment, since it was fairly close (about 3 arcseconds drift after 5 minutes in RA, according to RA slope in drift alignment tool of PHD2).
Since I was specifically imaging to determine the root of this problem, I also screenshotted my PHD2 graphs (guiding with QHY5L-II + Orion ST80, 1.93 arcsec/pixel). Guidescope FL and pixel size were entered correctly into PHD2, so the graph scale should be meaningful. Here are the two screenshots (second screenshot overlaps because I kept guiding).
The scope was made a touch West heavy (Eta Carinae west of meridian). The mount is a Losmandy G11 with Gemini 2, and PEC done by previous owner. Motors are the new high torque ones, and upgraded worm that people usually do (cant remember if RA or DEC). Guide cam and mount both controlled by ASCOM. PHD2 settings can be seen in above graphs plus settings screenshots attached.
I know the graph is very choppy, but I am really not sure what to change to make it less so. I was afraid to start playing in real-time because I wanted to capture the problem and current state of my setup.
I really don’t think this is flexure (too close to following lines of RA, scalable drift to exposure length, seemingly independent of guidescope/gravity orientation, pretty significant image shift of nearly 1 arcsecond per minute, etc).
It is almost like the scope isn’t tracking RA properly (ie tracking it to fast, so the stars streak in the opposite direction), which shouldn’t happen while guiding???:shrug::question::confuse d2:
I have attached an image showing the imaging camera view with respect to the guidescope too. I then approximately superimposed an equatorial grid (red and orange lines as per stellarium simulation), and blue arrow showing the direction of image shift. Eta Carina star is just off the bottom of the view shown (cant see it in guide scope, but is centered in imaging scope), you can see the other bright stars in the Keyhole region.
Any input is appreciated, except: I am not going to post details of my imaging train/scope. The last thing I want is people telling me my FL is too large, get an OAG, blah blah. I am happy to post up extra information in terms of settings, mount details, etc :).
Cheers!