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Tilt corrector location on imaging train

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#1 alarmclock

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Posted 26 November 2020 - 03:35 AM

Hi,

 

Having some tilt issues with a Borg refractor, I decided on purchasing a ZWO T2 tilt adapter.

 

My setup is as follow : Focuser > Borg rotator > OAG > 11mm ring > Filter wheel > camera

 

Considering my backfocus requirements, and keeping permanent access to the setting screws of the tilter, there is only one option that fits : straight after the rotator, just before the OAG.

The diagram would then be : Focuser > Borg rotator > Tilter > OAG > 11mm ring > Filter wheel > camera

 

I know most of the times imagers put the tilter just before the camera, but this isn't possible with the filter wheel blocking the setting screws. I don't want to disassemble the whole thing to work on my tilt.

 

My question is : with the tilter placed "first"  and once the tilt adjusted, will reframing by rotation of the whole imaging train will have an action on the tilt ?

 

Cheers
'clock



#2 michael8554

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Posted 26 November 2020 - 05:32 AM

Where is the tilt occurring ?

 

If the tilt is always in the same position on the image wherever you rotate to, and when checking in the east / west, / high Dec / low Dec, then you can place the adjuster anywhere.

 

But if is varying with rotation or sky position eg due to focuser sag, I'm not sure there is any position where it will work.



#3 alarmclock

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Posted 26 November 2020 - 05:35 AM

I am pretty sure the focuser does not sag. It is a 3" Feather Touch, solid as a rock.

 

Star elongation is upside down after a meridian flip. There seem to be tilt at the camera sensor, and I also suspect the OAG connection to be a bit hasardous.

 

Note this is a f3.9 refractor, very sensitive to... everything!

 

So you reckon Tilter first is fine ? 


Edited by alarmclock, 26 November 2020 - 05:54 AM.


#4 Monkeybird747

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Posted 26 November 2020 - 11:37 PM

Tilter on OAG should be fine. A very common position in the image train for it. Like the previous poster mentioned, take image, rotate 90 degrees, take image. Does tilt change direction relative to sensor? If so tilt is in focuser or rotator or some other connection point. If it remains the same then possible it is sensor tilt (my last ZWO camera had tons of it). It's important to know if the tilt is fixed or variable. If fixed then you can tilt it out and probably be fine when you rotate. If variable (due to focus sage, etc.) then it will drive you crazy trying to chase it down.

 

ZWO camera packages are so light that I would be surprised if the FT was sagging. But, they are not completely immune.



#5 alarmclock

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Posted 27 November 2020 - 05:55 AM

I investigated the focuser sag hypothesis and could actually find a fair amount of play in the draw tube. A quick sketch gave me an approx error of 0.4° tilt at the camera sensor.

I tightened the top screw of the focuser, enough to not have any wiggle, but not enough to make the motor focus stall.

I have an artificial star (actual stars are pretty rare nowadays where I live) and will try a 4 corners + center focused bahtinov image (+ rotation test as advised) to see if that solved the problem.

Will post the results here shortly.


Edited by alarmclock, 27 November 2020 - 05:56 AM.


#6 sn2006gy

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Posted 27 November 2020 - 10:45 AM

If the tilt isn't focuser sag, then put the tilt adjuster anywhere where you can actually use it more easily.  I find the zwo plate in front of OAG and behind a chonky reducer to be a disaster... the CTU design that sticks out much further than the zwo is something i'm looking to get once the new SW .77 reducer is out and i have the backspace for it.

 

Having to teardown image train to adjust zwo tilt plate is such a hassle



#7 FlankerOneTwo

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Posted 28 November 2020 - 09:41 AM

I'll second the recommendation for a Gerd-Neumann CTU (Camera Tilt Unit). The adjustment screws are on the edge so you don't have to unscrew everything to get to the screws. It is thicker and heavier, but infinitely more useful.




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