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Tilt on the TAK Epsilon 130

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#1 p088gll

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Posted 26 March 2022 - 03:13 PM

Hello folks,

 

I have been using my TAK Epsilon 130D together with an ASI 6200MM camera for more than a year now. I have learned to collimate the telescope the TAK way. Its easy and straightforward, and the collimation is rock solid, as long as one does not rotate the focuser. This is all well known in the community.

 

I have also found a significant tilt in my optical train.  I bought the big CTU (camara tilting unit) by Astronomik/Gerd Neumann to deal with it and spent many clear nights trying to find the right setting to get round stars - and I think I have some revealing results:

  • The tilt is not due to the camera sensor and not due to the optical train after the field flattener, since I got the same tilt with two totally different optical trains (including the camera)
  • The tilt is huge. I did achieve round stars, but at the maximum possible tilt you can set with the CTU, and that is 1mm on the one side, and 0mm on the other !
  • I found this puzzling, since such a huge tilt should be easily visible when looking at the components of the optical train, but it is not.

 

Finally I am confident that I found the reason for the tilt. The inner drawtube of the TAK focuser is tilted w.r.t. the outer tube. By collimating you can center the optical axis on the center of the sensor. But due to the tilt of the drawtube, the sensor remains tilted w.r.t the optical axis. The tilt of the drawtube can easily by seen when rotating the focuser during collimation: The point where the axis of the drawtube touches the secondary mirror moves by about 2mm during the rotation.

 

This means that the well-known problem that the Epsilons do not stay collimated when the focuser is rotated, always comes with this tilt. It has the same origin - the crappy focuser of the Epsilons.

 

I am writing this so that it may be useful to some Epsilon users. But also because I am looking for a solution.

 

One rather adventurous path might be to disassemble the focuser and somehow try to fix it. I heard that the inner drawtube is running on teflon coated pads of some kind, and the problem is created because the pads dont have the same size. Has someone ever had a look at this ?

 

I know that there are other focusers for the Epsilons on the market. Unfortunatelly, Moonlite has decided to sell their focuser with a build-in stepper motor only. This is incompatilble with my setup including the ASIAIR. Since my observatory is remote controlled and the ASIAIR in my opinion is the best, most user-friendly solution for this, I am not willing to sacrify it for the Moonlite focuser.

 

Other focusers for the Epsilons are rare, so I am curious if someone has an alternative focuser up and running on an Epsilon with  good results ?

 

Goetz


Edited by p088gll, 26 March 2022 - 04:43 PM.

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#2 TxStars

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Posted 26 March 2022 - 04:45 PM

I know the Optec LEO has been used on the E-160's, so making some adapters should allow it to be mounted on the E130 also.

I'm not sure if you would have enough back focus for a rotator, do you need one?

 

Might read through the experienced-deep-sky-imaging section to see if someone has already done this with the E 130


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#3 p088gll

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Posted 27 March 2022 - 05:39 AM


Finally I am confident that I found the reason for the tilt. The inner drawtube of the TAK focuser is tilted w.r.t. the outer tube. By collimating you can center the optical axis on the center of the sensor. But due to the tilt of the drawtube, the sensor remains tilted w.r.t the optical axis. The tilt of the drawtube can easily by seen when rotating the focuser during collimation: The point where the axis of the drawtube touches the secondary mirror moves by about 2mm during the rotation.

 

This means that the well-known problem that the Epsilons do not stay collimated when the focuser is rotated, always comes with this tilt. It has the same origin - the crappy focuser of the Epsilons.

 

After sleeping on it for another night, I have to admit that the above theory may not be true. Collimation should align the optical axis of the telescope with the axis of the drawtube. Therefore a tilt should have its origin in the components on the camera-facing side of the drawtube. So I am, again, totally at loss about the origin of the tilt. Maybe its the field flattener itself ? What I heard from a friend at astrobin is that the center mark on the primary mirror of the Epsilon may not be on the optical center of the mirror. I have no idea how to investigate this, but maybe someone else has done this before ?



#4 rockstarbill

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Posted 27 March 2022 - 12:20 PM

According to Takahashi, the marking represents the optical center.

#5 p088gll

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Posted 27 March 2022 - 02:12 PM

According to Takahashi, the marking represents the optical center.

It sure normally is, and it is unlikely that it is not in my case, but I would like to verify it just to exclude one possible cause for the tilt.



#6 rockstarbill

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Posted 27 March 2022 - 02:20 PM

It sure normally is, and it is unlikely that it is not in my case, but I would like to verify it just to exclude one possible cause for the tilt.


You can ask them. The response I gave you is from my email communication with Tak USA. It makes sense that it represents the mechanical and optical center as their collimation system wouldn't work well otherwise. The whole idea is that you can fully collimate the telescope in the daytime with no need for star testing, since star test collimation won't work correctly on the Epsilon.

#7 rockstarbill

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Posted 27 March 2022 - 02:30 PM

I replaced the stock focuser with the Optec Leo. There are some grub screws on the stock focuser you can adjust to improve the tension of the drawtube and prevent it from flopping. If anything is the cause of your problems it's very likely to be the focuser.

Edited by rockstarbill, 27 March 2022 - 02:33 PM.



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