Distance from sensor

[erg121]

Hi,

I'm now taking photographs with my ED80 and Zwo 1600mm cooled after setting the correct distance between the sensor and the field flattener.

My problem is that I want to start guiding for longer exposures but I am unable to get focus when my ZWO 120mc is attached to my Skywatcher Evoguide 50ED guidescope?

Does anyone know what the distance should be between the sensor and the guidescope and if further spacers may be required?

I have tried with and without the extension on the back of the guide scope?

As usual your help is much appreciated.

[almcl]

Not familiar with that exact scope, but from the spec (242 mm focal length) I think it will be 242 mm back from the objective, which *looks* from pics on the web to be some distance behind the adapter.  

Here's an image that uses several assumptions which may or may not be correct

[michael8554]

Quote:

 Generous backfocus of 20mm and, after removing the 40mm extension, even 60mm. By doing this, even DSLR cameras can be used.

The ASI 120MC sensor is 12.5mm in from its front face.

I don't know where those measurements are taken from, so spacing will depend on whether you attach with T rings or 1.25" nosepiece.

Start with the front of the ASI 242 - 12.5mm from the objective as almcl suggests.

Michael

 

 

 

 

[vlaiv]

you need something like 15mm extension (depends on focuser on Evo 50ED).

Scope has T2 thread and camera has T2 thread - but if you use it directly you will not reach focus as focal plane is 20 mm after T2 thread - and as stated above back focus of ASI120 is 12.5. This means that you are at least 7.5mm short when using T2 directly, and you need T2 extension at least that long (but I would use longer one so you can comfortably find focus).

 

[erg121]

1 hour ago, vlaiv said:

you need something like 15mm extension (depends on focuser on Evo 50ED).

Scope has T2 thread and camera has T2 thread - but if you use it directly you will not reach focus as focal plane is 20 mm after T2 thread - and as stated above back focus of ASI120 is 12.5. This means that you are at least 7.5mm short when using T2 directly, and you need T2 extension at least that long (but I would use longer one so you can comfortably find focus).

 

Thanks for the information vlaiv, almcl & michael8554 .

I believe I may have the information I needed and hopefully something amongst my spare bits and pieces I can use.

[erg121]

Added a T2 extension to my setup and can now get focus with my guide scope so thank you to everyone for their invaluable help and advice.

Onwards and upwards ???