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Rotation between imaging sessions, no change in camera orientation?


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I captured some data about a month ago, marked down both the camera orientation relative to the focuser and the Newtonian OTA relative to the tube rings and still had an 8 degree rotation between sets yesterday.

 

In both cases the polar alignment has been pointlessly good, at below 10 arcseconds as it is pretty easy to do with Sharpcap pro and the small adjustment screws of the EQM-35PRO. Both sessions have also been pointed entirely by plate solving, to 30 arcsec accuracy. I am not guiding in DEC so it drifts a bit before i re-center, but never more than lets say 45 arcsec from the image center. In both sessions the platesolver has reported an image rotation of around 77 degrees, confirming that everything was where it was supposed to be.

 

What can cause this? First session i got 2h, yesterday i got 3h. I am planning on capturing a lot more and cant really take a big hit on rotation for cropping.

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Polar misalignment causes field rotation. Are you sure that your polar alignment was within 10 arcseconds? I can't manage such accuracy with my AZ-EQ6 on a pier. What polar misalignment does PHD report? The best wsy to avoid rotation, in my experience, is to align the sensor with RA. Take a 30 s exposure, and some 5 seconds in, start slewing RA+ at 1 x sidereal.  Preview the exposure and turn your camera into the star trails. Repeat until the trails run along the frame's edge.

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I am certain that it was under 10 arcsec when i did it, and when i started imaging. Getting accurate PA is no problem, since the azimuth screws have quite tight threads (M5 or M6) on the EQM35. The altitude bolt however is quite sticky and its difficult to tell whether its just stuck or well secured.

However now i have my doubts and checked the guide logs, and it varied from 1 to 12 arc MINUTES between guide sessions... I did not start guiding straight after PA, as i did some tests with snapshots on various targets due to weather conditions. I started the actual sequence and imaging after about an hour since polar aligning. The tripod must have sunk or the altitude bolt was not properly secured. It was also windy so i had to babysit the telescope every now and then, which is also why i didn't pay any attention to drift. With 30s subs most shots were between 0.5 and 0.8 arcsec RMS anyway.

I was also shooting towards 74 degrees or so in DEC, does this have a greater or smaller effect on the rotation with polar alignment issues? I might have to re-check the polar alignment from now on after a while once the mechanical parts settle after a while.

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I think (but could be wrong) that field rotation is more of a problem at high declination, and it's the only reason I can think of for rotation between subs. Also, if polar misalignment is 1 arc minute for each session, there can still be 2 arc minutes between two sessions (from -1 to +1).

When I do polar alignment (using a routine that is similar to Sharpcap), and measure misalignment after I think I got it nailed, I can still get a value that is a few arcminutes off. Generally, I'm pleased when I get it lower than 5 arc minutes. But I always try to have my camera sensor aligned with RA. This makes framing between sessions easier.

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59 minutes ago, wimvb said:

Generally, I'm pleased when I get it lower than 5 arc minutes. But I always try to have my camera sensor aligned with RA. This makes framing between sessions easier.

5 arcmin is probably always fine, but as i don't guide in DEC i want it to be as good as possible so i don't have to babysit the guidegraph. Aligning sensor with either 0 or 90 degrees to RA sounds really foolproof, think ill try that next time. How do you align it, is there an overlay in sharpcap or some other software? I dont think there is one in NINA.

 

Below is what i usually get when things go right, maybe polar alignment is one of the strong points of the otherwise weak mount? Who knows.

931039242_Successfulexample.PNG.bbf666b2115e206c7e392c09bee374dd.PNG

 

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I'm willing to believe software said you were within 10 arcsecs but much less willing to believe the software is right! It is incredibly difficult to get as close as that in reality.

I think there are lots of possible sources of rotation if you are dismantling the gear between sessions.  Wim's suggestion is foolproof: use star trails to orientate the camera along RA and you are sure to have the same orientation.

Olly

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These is no such thing as perfect polar alignment. The position of the apparant mean refracted pole depends on the declination of the target. On some pro wide field scopes they change the alt of the mount when imaging different parts of the sky.

Regards Andrew 

Edited by andrew s
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9 hours ago, ONIKKINEN said:

How do you align it

Take a 30 s exposure with trails. View on screen and rotate the camera (eyeballing it). Take a new exposure with trails. View on screen and rotate camera. Repeat until satisfied. It takes about 3 such iterations, normally. All inall less than 5 minutes to ensure camera alignment. Well worth it, imo. If you like spending money, you can of course buy a camera rotator, but really not needed.

Btw, I'm curious why you don't guide in DEC.

Edited by wimvb
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2 hours ago, ollypenrice said:

I'm willing to believe software said you were within 10 arcsecs but much less willing to believe the software is right! It is incredibly difficult to get as close as that in reality.

Very true, while sharpcap PA is excellent i doubt it is really consistently all that accurate below 1 arcmin, the reported value fluctuates a bit based on which knee is on the ground and what type of ground the tripod is on. I am polar aligning with the main telescope at 0.9 arcsec per pixel so it should be reasonably accurate, at least i hope so.

 

2 hours ago, wimvb said:

Take a 30 s exposure with trails. View on screen and rotate the camera (eyeballing it). Take a new exposure with trails. View on screen and rotate camera. Repeat until satisfied. It takes about 3 such iterations, normally. All inall less than 5 minutes to ensure camera alignment. Well worth it, imo. If you like spending money, you can of course buy a camera rotator, but really not needed.

Btw, I'm curious why you don't guide in DEC.

Sounds easy enough, shouldn't waste imaging time since the scope will take a while to cool anyway so think i will do this from now on. Rotation is not an issue since the imaging train is not threaded to the focuser, it is clamped with a compression ring.

 

As for the DEC guiding: I am imaging with an 8inch newton on the EQM35 at around 9kg or 90% of the claimed payload capability. The mount is very much overloaded for photography. I have dismantled, relubed and checked everything but its hardly improvable. The RA is workable, but the DEC axis has horrible backlash and/or stiction problems leading to failed calibrations that sap out imaging time each session. The backlash is also not consistent, there are wildly different backlashes based on which side of the gear is touching. Poor quality manufacturing on some of the parts. I could guide in one direction but honestly not worth the trouble as it requires very good balance, something that is also very difficult with the sticky mount. I dont want the DEC axis doing anything at all during operation to reduce the chance of failed subs with the once a month type of weather im getting.

 

Not guiding in DEC is a minor issue with good polar alignment and short exposures. I am shooting mostly broadband targets from light pollution so 30s exposures with the 8inch aperture are more than enough, and short enough to make DEC drift a non issue. I would take shorter subs if i had more storage space, but processing a thousand subs is a bit of a pain already so probably wont. Now dithering is something i cannot do in DEC, but i don't really see walking noise with the low noise IMX571 so its all good.

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I like to leave my calculator in its case when doing astrophotography, but sometimes it has its merits.

What accuracy is needed in the altitude and azimuth bolts to get 10 arc seconds polar alignment?

Suppose that the bolts press at a distance of 7 cm (70mm) from the central altitude or azimuth axis. (For an eq35, this may be an overestimate.) 10 arcsecond rotation at 70 mm, gives 0.0034 mm sideways movement; that's just shy of 3.5 microns, or 1/20th of the thickness of a human hair. Next, suppose the alt and az bolts have a 1 mm pitch (probably coarser). One would need to turn a bolt only 1.2 degrees, 1/300 of a turn, to move it 3.5 microns. With all the friction and stiction present in a coated, cast aluminium mount and base plate, I'd say that's just not possible to accomplish repeatedly. That's why I'm happy enough when I get the polar alignment error down to (less than) 5 arc minutes.

Btw, if your mount's backlash doesn't allow guiding in DEC, you should really consider saving up for a bigger mount. The eq35 is based on the eq3, which lacks ball bearings in the ra and dec axes. In stead it has teflon washers to reduce friction, but those aren't as good as proper bearings. Nevertheless, spending a few evenings/nights getting dec backlash sorted is worthwhile. I got backlash down from 4000+ ms to below 700 ms (at a guiding rate of 0.5) in one night, and it really improved my guiding. But you have to do it in very small steps, and avoid binding at all cost.

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3 hours ago, wimvb said:

I like to leave my calculator in its case when doing astrophotography, but sometimes it has its merits.

What accuracy is needed in the altitude and azimuth bolts to get 10 arc seconds polar alignment?

Suppose that the bolts press at a distance of 7 cm (70mm) from the central altitude or azimuth axis. (For an eq35, this may be an overestimate.) 10 arcsecond rotation at 70 mm, gives 0.0034 mm sideways movement; that's just shy of 3.5 microns, or 1/20th of the thickness of a human hair. Next, suppose the alt and az bolts have a 1 mm pitch (probably coarser). One would need to turn a bolt only 1.2 degrees, 1/300 of a turn, to move it 3.5 microns. With all the friction and stiction present in a coated, cast aluminium mount and base plate, I'd say that's just not possible to accomplish repeatedly. That's why I'm happy enough when I get the polar alignment error down to (less than) 5 arc minutes.

Btw, if your mount's backlash doesn't allow guiding in DEC, you should really consider saving up for a bigger mount. The eq35 is based on the eq3, which lacks ball bearings in the ra and dec axes. In stead it has teflon washers to reduce friction, but those aren't as good as proper bearings. Nevertheless, spending a few evenings/nights getting dec backlash sorted is worthwhile. I got backlash down from 4000+ ms to below 700 ms (at a guiding rate of 0.5) in one night, and it really improved my guiding. But you have to do it in very small steps, and avoid binding at all cost.

I can move the azimuth bolts in increments of somewhere around 20 arcsec, but not much less and not consistently. Sounds about right for your calculations, a 2 or so degree turn doesnt sound impossible. But the take away from this is that any slight bump will move the alignment more than that so not worth the effort trying to go under few arcmin. And of course sharpcap reported alignment is not the whole story.

 

A better mount is in the shopping list, but seeing as i have some success with the current contraption im in no hurry, not that i could afford one now anyway. Currently with short exposures the effects of seeing and guiding go pretty much hand in hand for high DEC targets so a better mount would be just a convenience purchase, not a necessity.

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