Orion Short tube CT80 won't focus.....

[Higgs_Bozon]

I can't get it to focus no matter what, I'm not using a star diagonal right now, is that why? I'm using a 10mm eyepiece btw....

[John]

14 minutes ago, Higgs_Bozon said:

I can't get it to focus no matter what, I'm not using a star diagonal right now, is that why? I'm using a 10mm eyepiece btw....

You do need a star diagonal to get the ST/CT80's to focus visually. 

 

Edited November 1, 2023 by John

[Higgs_Bozon]

I got the optical tube mount and eyepiece together, I'm only missing the star diagonal. I have an extension tube lying around though, I tried using it, the images are forming but the stars are distorted. The best focus has a point in the middle and a spill on the top right, like paint spilling. Do I have to get a star diagonal or is it a scope problem? 

[Cornelius Varley]

33 minutes ago, Higgs_Bozon said:

I got the optical tube mount and eyepiece together, I'm only missing the star diagonal. I have an extension tube lying around though, I tried using it, the images are forming but the stars are distorted. The best focus has a point in the middle and a spill on the top right, like paint spilling. Do I have to get a star diagonal or is it a scope problem? 

A diagonal just bends the light path. Any other distortion is probably due to a problem with the telescope optics.

[John]

1 hour ago, Higgs_Bozon said:

I got the optical tube mount and eyepiece together, I'm only missing the star diagonal. I have an extension tube lying around though, I tried using it, the images are forming but the stars are distorted. The best focus has a point in the middle and a spill on the top right, like paint spilling. Do I have to get a star diagonal or is it a scope problem

What eyepiece are you using ?

 

[Higgs_Bozon]

55 minutes ago, John said:

What eyepiece are you using ?

 

I'm using a 10mm Kellner eyepiece 

[John]

Do your stars look a bit like this and do they look like that across the whole view or just some of it ?:

[Higgs_Bozon]

3 minutes ago, John said:

Do your stars look a bit like this and do they look like that across the whole view or just some of it ?:

Yeah they look like that but even more elongated, and even if they're in the centre focus they look like that. Heck even Jupiter looks like that lol. I have mild astigmatism so I got my friend to verify as well, he saw it like that too.

[John]

It sounds as if your scope is mis-collimated, that is when the objective lens or the focuser tube, or both, are tilted a bit.

Can you post a picture of the scope setup as you were using it ?

 

[tooth_dr]

Is this the same scope with a scratched objective?

[johnturley]

On 01/11/2023 at 00:01, Higgs_Bozon said:

I can't get it to focus no matter what, I'm not using a star diagonal right now, is that why? I'm using a 10mm eyepiece btw....

You might need an extension tube if you are not using a star diagonal, most refractors sold these days require an extension tube to reach focus without a diagonal.

John 

[John]

2 minutes ago, johnturley said:

You might need an extension tube if you are not using a star diagonal, most refractors sold these days require an extension tube to reach focus without a diagonal.

John 

I think he is using an extension tube now, judging by this earlier post:

https://stargazerslounge.com/topic/415113-orion-short-tube-ct80-wont-focus/?do=findComment&comment=4428120

 

[Higgs_Bozon]

8 hours ago, John said:

It sounds as if your scope is mis-collimated, that is when the objective lens or the focuser tube, or both, are tilted a bit.

Can you post a picture of the scope setup as you were using it ?

 

I am using an extension tube although it's not too good, it's just something I found lying around. Should I get a proper star diagonal?

[michael8554]

Depends on how you will be using the scope.

Focal length is 400mm, so focused star images are formed 400mm back from the objective.

Roughly measure this with a tape measure, this will give you an idea of how much spacing you need.

For Visual use a Star Diagonal will work.

For Imaging use standard-size screwed extensions, the focuser will take up the rest.

Michael

 

[John]

8 hours ago, Higgs_Bozon said:

I am using an extension tube although it's not too good, it's just something I found lying around. Should I get a proper star diagonal?

If the extension tube is not a proper 1.25 inch fitting one it could be drooping a little when you use it with an eyepiece or the eyepiece is not being held centrally within it. Either of these things could cause the optical misalignment and the distorted star images that you are seeing.

If you want to see what the scope optics can really do I would get a star diagonal for the scope. I don't know where you are in the world but you can get diagonals for as little as £10 on the used market here. Even new a basic but suitable one is not too expensive:

https://www.firstlightoptics.com/diagonals/astro-essentials-90-degree-mirror-diagonal.html

[Higgs_Bozon]

1 hour ago, John said:

If the extension tube is not a proper 1.25 inch fitting one it could be drooping a little when you use it with an eyepiece or the eyepiece is not being held centrally within it. Either of these things could cause the optical misalignment and the distorted star images that you are seeing.

If you want to see what the scope optics can really do I would get a star diagonal for the scope. I don't know where you are in the world but you can get diagonals for as little as £10 on the used market here. Even new a basic but suitable one is not too expensive:

https://www.firstlightoptics.com/diagonals/astro-essentials-90-degree-mirror-diagonal.html

I've found a Mirror Star Diagonal which I can get. It says OD in its name though, what does that mean? The supplier is a trusted source in my country, so what should I ask them to gauge the quality of the Diagonal?

[michael8554]

Probably OD = Outside Diameter

eg 1.25 inch OD

You can add your country to your Profile that is displayed to the left of your posts.

Michael

[Louis D]

23 hours ago, Higgs_Bozon said:

The best focus has a point in the middle and a spill on the top right, like paint spilling.

Is the light spill mostly violet?  If so, that is completely normal for a fast achromat like yours.  Try a #8 Yellow filter to tame it down a bit.

Try focusing with a #56 Green filter to see if the star images are sharper.  Correction at the red end of the spectrum might be as bad as at the blue end with your scope.  The green filter will cut out both and will probably show a sharper image.  I'm not saying it will be a pleasing image, but we'll start to narrow down the problem source.

[Higgs_Bozon]

3 hours ago, Louis D said:

Is the light spill mostly violet?  If so, that is completely normal for a fast achromat like yours.  Try a #8 Yellow filter to tame it down a bit.

Try focusing with a #56 Green filter to see if the star images are sharper.  Correction at the red end of the spectrum might be as bad as at the blue end with your scope.  The green filter will cut out both and will probably show a sharper image.  I'm not saying it will be a pleasing image, but we'll start to narrow down the problem source.

No the image spill will depend on the color of the object. I've done it with Jupiter, the spill is the same yellowish and with Sirius, the spill was mostly the same colour as the centre.

[John]

Until you get a proper diagonal on the scope and have a look at some stars with that in place we can only make guesses, which is not going to help you much.

 

[Higgs_Bozon]

11 hours ago, John said:

Until you get a proper diagonal on the scope and have a look at some stars with that in place we can only make guesses, which is not going to help you much.

 

Yeah, I'm ordering one, dunno when it'll get here though 

[Higgs_Bozon]

On 04/11/2023 at 23:18, John said:

Until you get a proper diagonal on the scope and have a look at some stars with that in place we can only make guesses, which is not going to help you much.

 

The issue still persists. Exactly the same event with the mirror diagonal.

[John]

23 minutes ago, Higgs_Bozon said:

The issue still persists. Exactly the same event with the mirror diagonal.

But no extension tube ?

It is beginning to seem as if there is something amiss with the objective lens of the telescope. Maybe it has been taken apart for cleaning and one of the lens elements put back in the wrong way around ?

 

Edited November 10, 2023 by John

[Higgs_Bozon]

37 minutes ago, John said:

But no extension tube ?

It is beginning to seem as if there is something amiss with the objective lens of the telescope. Maybe it has been taken apart for cleaning and one of the lens elements put back in the wrong way around ?

 

No extension tube, just the normal arrangement with mirror diagonal and eyepiece. I'll take out the objective and try putting it the other way

[tooth_dr]

1 hour ago, John said:

But no extension tube ?

It is beginning to seem as if there is something amiss with the objective lens of the telescope. Maybe it has been taken apart for cleaning and one of the lens elements put back in the wrong way around ?

 

I posted earlier on in this thread. This scratch might give a clue to the issue.  Dropped or taken apart