What is wrong with this telescope?

[JamesF]

And more to the point, can it be fixed and how?

It looks a lot like coma to me, but in a refractor? (600mm fl f/7.5 achro).

James

[Alien 13]

7 minutes ago, JamesF said:

And more to the point, can it be fixed and how?

It looks a lot like coma to me, but in a refractor? (600mm fl f/7.5 achro).

James

Looks like someones had the lenses out and put them back in the wrong order, curvy less curvy inverse curvy flatish is the normal configuration I think.

Alan

[JamesF]

Thanks, Alan.  I can certainly test the hypothesis.  I don't reckon it could get a lot worse

James

[michael8554]

24 minutes ago, Alien 13 said:

curvy less curvy inverse curvy flatish is the normal configuration I think.

I can't wait for the opportunity to slip that into the conversation at the Royal Institute........

Might first be worth holding the scope vertically by the focuser end, and giving the front a gentle bump on a carpeted floor - might just be the elements need joggling into the correct positions, since the coma is in one direction.

Michael

[Alien 13]

4 minutes ago, michael8554 said:

I can't wait for the opportunity to slip that into the conversation at the Royal Institute........

Might first be worth holding the scope vertically by the focuser end, and giving the front a gentle bump on a carpeted floor - might just be the elements need joggling into the correct positions, since the coma is in one direction.

Michael

That might work, Achros do usually have a bit of free play in the lens cell to avoid pinching effects.

Alan

[JamesF]

Well, the current arrangement appears to be less curvy, curvy, inverse curvy, flat-ish.  Judging by the dirt on the inside lens faces someone has definitely had it apart in the past (I bought it used).

James

[Alien 13]

16 minutes ago, JamesF said:

Well, the current arrangement appears to be less curvy, curvy, inverse curvy, flat-ish.  Judging by the dirt on the inside lens faces someone has definitely had it apart in the past (I bought it used).

James

Usually the inner two surfaces of the lens elements match each other in curvature, if they are dirty might as well clean them and try reversing the front element but make sure this doesn't cause any glass to glass contact. The flattish surface should point towards the eyepiece end of the scope like this.

Alan

Edited March 25, 2020 by Alien 13

[JamesF]

If I gently put the most convex and concave faces together then there's a definite vacuum when I try to separate them, so I reckon they must go together.  There's also a thin plastic spacer that is bent on one side as if it were caught around the edge of the inner lens, so perhaps that's the cause of the problem.

The cell is plastic and possibly not entirely round, so I'll see how reassembly goes...

James

[Alien 13]

2 minutes ago, JamesF said:

If I gently put the most convex and concave faces together then there's a definite vacuum when I try to separate them, so I reckon they must go together.  There's also a thin plastic spacer that is bent on one side as if it were caught around the edge of the inner lens, so perhaps that's the cause of the problem.

The cell is plastic and possibly not entirely round, so I'll see how reassembly goes...

James

The spacer is there to prevent any direct glass to glass contact, have made suitable spacers from various things like coke cans/milk cartons in the past but that damage you mention is probably the main culprit.

Alan

[geordie85]

Could it just be severe tilt? The stars in my st80 look a bit like this through my guide camera. Which has a fair bit of movement in the focuser. 

[JamesF]

2 minutes ago, geordie85 said:

Could it just be severe tilt? The stars in my st80 look a bit like this through my guide camera. Which has a fair bit of movement in the focuser. 

It would have to be pretty extreme I think, but perhaps I can test it.  I don't really seem to have improved things so far.  My first test image doesn't look a whole lot better than the one I originally posted.

James

[Thalestris24]

Looks a bit like thermals to me - was the scope temperature stabilised?

Louise

[JamesF]

5 minutes ago, Thalestris24 said:

Looks a bit like thermals to me - was the scope temperature stabilised?

Louise

It was.  It lives in the observatory all the time.

James

[Thalestris24]

11 minutes ago, JamesF said:

It was.  It lives in the observatory all the time.

James

But maybe the ambient temperature dropped rapidly? If it's a biggish triplet it can still take a while to stabilise.

Louise

ps this is just an idea - I don't have any great expertise 🙂 

Edited March 26, 2020 by Thalestris24

[JamesF]

It's only a simple 80mm achro.  I can't see thermal problems being the cause.

James

[Thalestris24]

1 minute ago, JamesF said:

It's only a simple 80mm achro.  I can't see thermal problems being the cause.

James

Oh ok - sorry, I should have read it more carefully...