Field rotation although very small PE error?

[uhb1966]

Alright... so i set up yesterday evening, with a small PE error (<10 minutes) and great pictures at low declination.

i went on imaging M31. When it was directly overhead, i noticed again elongated stars while PHD had its guide star *dead* center - practically no correction at all...

The only thing that might be suboptimal was the distance between guide star and M31- maybe 3 degrees.

I checked this website http://celestialwonders.com/tools/rotationMaxErrorCalc.html and i got the following:

16 microns would be like 2.1 pixels in my camera... Hm. I guess i have to minimize the distance between guide star and target?

 

Edited October 3, 2023 by uhb1966

[ONIKKINEN]

It could also be differential flexure between your guider and the telescope, which is why with a reflector you really want to have an OAG instead of a guide scope. The mirror can flop or shift on its own during imaging, mainly as it is effected by gravity in different orientations, but the focuser and the rest of the telescope itself could also be wobbly. All of that will result in the guidescope reporting everything as a-ok while the images show elongation. With an OAG the guider works with the exact same optics as the imaging camera, so there is smaller chance of what you describe happening.

[uhb1966]

Hi Onikinnen, thanks for the quick response! This "hobby" is driving me nuts ...

[tooth_dr]

In my view that doesn’t look like field rotation.  I think @ONIKKINEN is most likely correct in that it looks like differential flexure. Posting a photo of your setup will possibly help identify any weak areas.  
I don’t get too hung up on things like periodic error myself, and let guiding take care of it.  How do you measure your PE?  
 

[ONIKKINEN]

Just seen the image now. Looks like the elongation is suspiciously close to being entirely in Declination, suggesting generic guiding issues (like excessive backlash) rather than simple flexure. Could be flexure too, but this seems rather extreme for just that.

If you have a guide log you can attach here then we could have a look and form a more educated guess.

[michael8554]

M32 is almost due south of M31, so as ONIKKINEN said, it's mostly Dec elongation, PE isn't involved.

Need to see the GuideLog.

Michael

 

[uhb1966]

Hi all, i_ll check if i can find a log in PHD2. But it was really flatlining with a super small error ... but maybe the guide star was far out. 

[uhb1966]

Alright, here comes the guide log.

The interesting part begins with the string "Guiding Begins at 2023-10-03 00:20:28" ... thats the part of the session where these things happened.

 

PHD2_GuideLog_2023-10-02_202444.txt

[uhb1966]

And a BIG "Thank you" to everybody who tried to help me 😙👍

[uhb1966]

On 03/10/2023 at 13:20, ONIKKINEN said:

It could also be differential flexure between your guider and the telescope, which is why with a reflector you really want to have an OAG instead of a guide scope. The mirror can flop or shift on its own during imaging, mainly as it is effected by gravity in different orientations, but the focuser and the rest of the telescope itself could also be wobbly. All of that will result in the guidescope reporting everything as a-ok while the images show elongation. With an OAG the guider works with the exact same optics as the imaging camera, so there is smaller chance of what you describe happening.

I will look into the OAG idea. Thanks!!

[ONIKKINEN]

1 hour ago, uhb1966 said:

Alright, here comes the guide log.

The interesting part begins with the string "Guiding Begins at 2023-10-03 00:20:28" ... thats the part of the session where these things happened.

 

PHD2_GuideLog_2023-10-02_202444.txt 318.17 kB · 2 downloads

Loads of backlash in there, that the mount is trying to fight but just ends up fighting itself by overcorrection:

All those hollow red vertical bars are attempts to guide in declination, which fail until the backlash has been cleared and now the axis is catapulted way beyond where it was supposed to go. Rinse and repeat for most of the night. You also have a substantial error in both axis with 1.48'' in RA and 2.17'' in DEC, and the much larger error in declination here which is why we see elongated stars. Most mounts should be able to do 1'' RMS error on both axis, so something to fix.

Something is mechanically not all right in the setup i would say. How are the basics of the mount, and i should probably ask what mount is it? I see its EQMOD driven but that could be any Skywatcher mount really as they all say that same HEQ5/6 name no matter the actual mount. Things to see to would be for example proper balance of both axis, backlash adjusted to be tolerable and/or compensated for in PHD2 settings (run guide assistant to give pointers on what to adjust).

Edited October 4, 2023 by ONIKKINEN

[uhb1966]

Alright, thanks for the input!

I will get into backlash . It's an EQ-6 from 2014. Balance was way out at first, i corrected but maybe thats still an issue. Thanks!!

[ONIKKINEN]

1 hour ago, uhb1966 said:

Alright, thanks for the input!

I will get into backlash . It's an EQ-6 from 2014. Balance was way out at first, i corrected but maybe thats still an issue. Thanks!!

Do you know if it has been serviced? Might be necessary to check the innards and maybe replace some bearings if it has seen a few too many winters without a cleanup.

I serviced my brand new AZ-EQ6 and still saw an improvement because the factory finish is not all that high tech.

[uhb1966]

@ONIKKINEN no, has not been serviced for 9 years, i'm the first owner 💀

However, last night i got down below one ArcSecond of error with phd2. Backlash is at 2300 seconds, and the power port and the rs232 port seem to have issues... so yes, i need to get it serviced this winter

[michael8554]

"Backlash is at 2300 seconds,"

Hopefully only 2300 msec   😆

Michael

[uhb1966]

@michael8554 Indeed! 😇