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Desperate again with my E130-D

Focuser Tilt Troubleshooting
Takahashi Epsilons •5 replies•245 views
Götz Golla avatar
Götz Golla avatar
After many months of wonderful sharp images, a few days ago hell broke loose again. It was a very warm night. On that night I found that stars were almost straight lines in one half of the image and perfectly round in the other half, indicating a significant tilt. Moreover, during a two hour session with 5min exposures, the images were slowly rotating by about 3 degrees in total. All this even though nobody moved or even touched the telescope on its mount since the last sucessful observation in May.

First I adjusted the two small screws to hold the drawtube of the focuser a bit tighter, because the rotation comes from the drawtube rotating in the outer focuser tube when weight of the camera and filter wheel shifts.
Then I tried to readjust the tilt starting from scratch. I havent succeed after three full nights of work. Whenever I thought I finally got it and move to an object to observe, I see the same tilt again. I just cant find a reason and a handle to deal with this situation. During all the adjustments I checked the collimation several times. It always was 100% OK. Maybe its a temperature effect related to the teflon bearings of the focuser ? Has anyone successfully disassembled the focuser ?

I like the Epsilons very much when they work. I have heard from some people that they also have massive problems with tilt and collimation. Others dont seem to have any problems at all. I also heard that the E160D is more stable with less problems. What do you think ? If it could be verified somehow that the E160Ds have less problems I would be willing to give my E130D back to Takahashi and go for an E160D instead (- or go for a Stellarview SVX-140T, but thats a different story).
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Thorsten Glebe avatar
Thorsten Glebe avatar
Hi Götz,

since some months I am an owner of the E160ED, which is my first Takahashi scope, so I cannot comment on the difference to the E130. But from your description I don't think it is much different with the E160.
The scope is for sure not "plug and play" and I am still learning how to adjust it perfectly. I also started to play with the 1.5x extender, trying to find a setup where I do not need a recollimation in case I change between 530mm and 800mm focal length...

My experiences so far:
1. Collimation: with the Tak collimation tool simple and easy to learn, collimation can be performed during daytime with good precision, only a little fine adjustment with stars required - see e.g. the collimation primer)
2. Backfocus: I played with backfocus a bit, but with inconclusive results. So I just take the backfocus from the specs, adjusted for filter and took this value.
3. Focuser: the E160 focuser can be rotated, but this will impact collimation - so I keep it in a fixed position. It might happen that the focuser is rotated by accident when changing the optical train - this is a source of errors, which is bad.
4. The focuser screws you mentioned do not help to align the focuser tube with the optical axis, this is why the problem in 3. cannot be easily solved - avoid to rotate the focuser, maybe even mark it's position.
4a. The focuser is not light tight - this is a different story we discussed in another thread (not relevant here).
5. Tilt: I also experienced that tilt adjustment during night time is cumbersome with suboptimal results. It is hard to determine when the optimum is reached and one can easily overdo it. I therefore eliminate the major part of the tilt during daytime using the following tutorial. As a rotatable holder for the optical train I use a self constructed 3D printed holder which I can mount on a table  (see below). After some practice, tilt adjustment is easy and quickly performed during daytime. The residual tilt needs to be corrected with the help of some stars, but you are much closer to a good result in the first place.



I have mobile setup and I have to put up and take down the instrument on every night. So far I did not experience that the instrument looses collimation.
On the other hand, up to now I was never satisfied with the star shapes, maybe I am still not good enough or there is another degree of freedom which I am not yet aware of.

Hope this helps a bit.

Kind regards,
Thorsten
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Götz Golla avatar
Götz Golla avatar
Yesterday I experimented with the focuser and collimation during daytime. I checked that the following actions do not change the collimation at all:
  • operating the focus motor
  • move the telescope in any direction
  • pull, push and press with full force on the focuser in various directions
  • push and try to move the primary mirror from the back side

To me this looks exactly like what people always say: the collimation of the Epsilon is rock solid. It may also indicate that my earlier statements about the bad focuser of the Epsilons is not true.

The problem may rather be in the rest of the optical train towards and including the camera, the part which is not connected when you use the TAK collimation eyepiece.

I found a very interesting thread on Cloudy Nights: https://www.cloudynights.com/topic/821151-direct-measurements-of-image-surface-tilt-and-curvature/

It basically outlines a way to adjust the backfocus. The method described is implemented in NINA, but before I buy a Windows PC and use NINA instead of ASIAIR I will try to utilize the method once manually. It should work by doing a focussing sequence and determine the focus distance for stars in the center and in the four corners seperately. Dependend on whether the focus distance of the stars in the corners is smaller or larger than in the center the backfocus is to small or too large. So this should be a good way to adjust backfocus.

When backfocus is adjusted, the assessment and adjustment of tilt will come next.
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Shinpah avatar
Shinpah avatar
Götz Golla:
move the telescope in any direction


What's the method you're using to verify this?


I've had little luck using the Hocus Focus plugin in NINA for backfocus specifically; it pretty much always recommends adding more spacing even after the corners are very much so bad. Incremental .1mm adjustments and evaluating the corners manually seems to be the way to go.
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Götz Golla avatar
Götz Golla avatar
Götz Golla:
move the telescope in any direction


What's the method you're using to verify this?


I've had little luck using the Hocus Focus plugin in NINA for backfocus specifically; it pretty much always recommends adding more spacing even after the corners are very much so bad. Incremental .1mm adjustments and evaluating the corners manually seems to be the way to go.

I did this in daylight and moved the telescope manually and checked collimation in every position. Of course all this is not very useful since it is done without the image train (camera, filter wheel, CTU), but it verifies that collimation is not  the problem.

I try to make 1/4 to 1/8 revolutions of the CTU screws.  Last night I tried the aberration/tilt view of ASTAP in monitoring mode.  Unfortunatelly I had the position of the three CTU screws wrong, but in the next clear night I might have better luck.

BTW, one idea about the origin of the tilt is the use of plastic rings for backfocus adjustment. I ordered metal rings now. Another chance for an improvement.
Götz Golla avatar
Götz Golla avatar
OK, I managed to fix the adjustment of the E130 again. As last time it took me 4 full nights. Here is what I have found and what I have done:
  • using plastic spacer rings for a telescope like the E130 is not a good idea. They are not very accurate and they get "softer" in higher temperatures. When we are talking of tilt adjustments of the order of 0.1mm this is relevant.
  • In a thread in Cloudy Nights the software ASTAP was praised for making tilt adjustments easy. In another thread NINA was praised similarly. Although you can indeed adjust your CTU so that your software says the tilt is almost gone, there may still be a very noticable tilt in the edges of your image. The only way to correct this is to carefully observe the edges of your image yourself. There is  no magic software  that makes tilt adjustment easy.
  • This is even more so for a reflector like the Epsilon, because whatever software, it does not take into account the information the different star shapes give you about which part of the image is too far away and which is too close to the sensor.
  • It is essential to know which of the taper screws of your CTU is where in your image. If you utilize several programms to analyse your images, these images may be rotated or flipped differently.  I finally made the following sketch for my setup. 
  • I lost almost a whole night because one of the nylon threads of the collimation tube was shifted. Doing a "perfect" collimation while the crosshairs are not centered does make very bad stars. This was a nasty suprise and it took me a long time to discover what was wrong.

But now - finally - the E130 is adjusted again and I am happily working on the next image - problem solved
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