Very strange problem with my refractor

[Acrab67]

Hi,

I have a problem with my TS 125 refractor. I have only used it about 10 or 12 times, and on a couple of occasions the tube has lost collimation, only to recover it later. He has not received any blows or sudden movements. In both, the airy disk appears well defined, but clearly off center to the right. The image in focus was anything but punctual, opening a kind of fan to the left of the star point. It is not an instrument thermal problem or local turbulence.
Last week after having the same problem I put it back in its box and sent it to APM telescopes as it is under warranty. Well, yesterday they told me that they have tested it on an optical bench at 300X, and the optical tube is perfect, no trace of lack of collimation, astigmatism, etc. I am clear that the tube randomly presents some optical-mechanical problem, but how is it possible that without any other manipulation it is kept in its box, travels half of Europe, and in an optical bench test it is found to be correct again?
I am baffled, after 30 years of visual observation I have never seen anything like it.
I have to say that Markus Ludes has shown professionalism and has offered to test it himself under a real sky.
Any ideas?

[michael8554]

You tested without a star diagonal ?

Otherwise has to be a loose element that periodically unjams itself, but not when APM tested it :-<

Michael

[vlaiv]

44 minutes ago, Acrab67 said:

In both, the airy disk appears well defined, but clearly off center to the right. The image in focus was anything but punctual, opening a kind of fan to the left of the star point.

I'm a bit confused by this.

I can't really understand what sort of image your scope is presenting when in focus and did you view star image defocused?

Maybe by mistake you said Airy disk when in fact you meant out of focus disk?

Is this what you are seeing?

either side of focus concentric rings are not quite concentric and in focus image like you say - fans out to one side?

In any case - it could be thermal issue.

By thermal - I don't mean what we usually think of when we say thermal - like scope not cooled down properly and seeing influence of circulating air inside scope - nor local thermal issues like local seeing.

By thermal I mean telescope tube and lens cell expansion / contraction issues. If cell is made out of two different types of metal - they can contract or expand at different rate. This means that they will be of different size while scope reaches ambient temperature and this can introduce tilt of some sorts.

Such effect would be only visible while scope is cooling to ambient temperature - and would present as miscollimation (not wavy like regular tube currents).

[Acrab67]

2 hours ago, vlaiv said:

Estoy un poco confundido por esto.

Realmente no puedo entender qué tipo de imagen presenta su alcance cuando está enfocado y vio la imagen de la estrella desenfocada?

¿Quizás por error dijiste disco Airy cuando en realidad quisiste decir disco fuera de foco?

es esto lo que estas viendo?

cualquiera de los lados de los anillos concéntricos de enfoque no son del todo concéntricos y en la imagen de enfoque, como usted dice, ¿se abre hacia un lado?

En cualquier caso, podría ser un problema térmico.

Por térmica, no me refiero a lo que solemos pensar cuando decimos térmica, como que el alcance no se enfría correctamente y ve la influencia del aire circulante dentro del alcance, ni problemas térmicos locales como la visión local.

Por térmica me refiero a problemas de expansión/contracción del tubo del telescopio y de la celda de la lente. Si la celda está hecha de dos tipos diferentes de metal, pueden contraerse o expandirse a un ritmo diferente. Esto significa que serán de diferente tamaño mientras el alcance alcanza la temperatura ambiente y esto puede introducir algún tipo de inclinación.

Tal efecto solo sería visible mientras el alcance se está enfriando a temperatura ambiente, y se presentaría como una mala colimación (no ondulada como las corrientes de tubo normales).

Correct, but rotating the image to the right.  I agree that it is a cell-lens issue, but when it happened to me the difference in temperature between the inside and the outside was around 4 or 5ºC. It has been working without problems with more pronounced temperature differences

Edited October 6, 2022 by Acrab67

[pipnina]

On 06/10/2022 at 16:53, vlaiv said:

 

Does this happen from focuser misalignment, cell misalignment, or both?

I see this in my refractor and know the focuser is not straight but have been worried that the front cell might be out too.

[RobertI]

Can a fellow SGL’er come up with a methodical troubleshooting process to eliminate each component as the culprit next time it happens? 

[vlaiv]

5 hours ago, pipnina said:

Does this happen from focuser misalignment, cell misalignment, or both?

I see this in my refractor and know the focuser is not straight but have been worried that the front cell might be out too.

It is usually due to lens cell being misaligned.

Focuser being misaligned usually shows as tilt on focal plane - center being in focus and ok - and sides being defocused (one inside other outside).

[Acrab67]

Also, if it were a focuser centering problem only, this defect would always be present.

[Orlando Gonzalez]

I also am baffled. Markus is very reputable. I suspect the problem is in the diagonal. Try a different diagonal if available.

[dweller25]

On 06/10/2022 at 16:50, michael8554 said:

You tested without a star diagonal ?

Otherwise has to be a loose element that periodically unjams itself, but not when APM tested it :-<

Michael

Ditto the above.

I also think you have a loose optical component.

If the fault re-appears you could try observing straight through with no diagonal in place, if the fault is still there it can only be the lens/cell.

Is the lens cell seated correctly and not loose ?

Are the lenses seated correctly and not loose ?

Is the lens retaining ring snugged up correctly ?

Does it have push/pull collimation screws - if so are they all correctly snugged up ?
 

Edited October 10, 2022 by dweller25

[Acrab67]

It's not a diagonal problem, it's the first thing I ruled out. The same lack of collimation occurred with a Baader binoviewer. Right now I'm waiting for Markus to test it under a real sky. Regardless of the outcome, it's clear that the problem must be with the fit of the lenses in his cell. I can't find any other logic.