Can you help me understand my optics issues?

[old_eyes]

Message in two parts as I can't figure out how to upload all the images into one

I am imaging with a Canon 1000D through an Equinox ED80 mounted on a CG5-GT using PhD for guiding.

Below are a series of images from a session looking at the M81 M82 area.

Ignore the poor processing and focus on what is happening to the stars. The images show:

• The complete image
  
• Where I have taken 5 approx 600 pixel square sub-images from
  
• In the next part of the message, the five sections I have cut out at full resolution.
  

The middle segment shows fairly round stars, but as you go to the corners the stars defocus and elongate in the direction away from the image centre.

Is this the effect of the image field not being flat on the sensor? I figure it can't be tracking or field rotation or the trailing would all be in the same direction.

Is the fact that the lower left corner is showing more defocus due to the sensor not being square to the scope optical axis?

Tryong to knock the problems off one by one . Thanks for your help.

old_eyes

[old_eyes]

Second ppart of message with the sub segments:

• Upper left
  
• Upper right
  
• Lowert left
  
• Lower right
  
• Middle
  

[robbieince]

Yep, That's field curvature. The focal zone is not flat and the sensor is only in focus at the centre. At the edges, the stars are radially distorted as the sensor does not follow the focal area curvature.

Correct with a field flattener with the right spacing.

As for worse in one corner - most likely the sensor centre is not at the focal plane centre ie not collimated.

Regards

Rob

Sent from my iPhone using Tapatalk

[Bizibilder]

Yup -Optical effect. You can buy a "focal reducer / field flattener" that will get rid of this (at a price of course!) Reducers/Flatteners - Skywatcher Focal Reducers

[old_eyes]

Thanks. Glad 'O' level physics is still working for me .

How would I determine the right spacing for the flattener? Will the Skywatcher one for the DS-Pro work?

And how could I collimate the camera to the telescope?

old_eyes

[Bizibilder]

Give FLO a ring - they will tell you how to set the whole thing up.

[old_eyes]

Give FLO a ring - they will tell you how to set the whole thing up.

Will do.

old_eyes

[simon84]

If you use the Skywatcher reducer/flattener with the 56mm adapter on FLO's site and then the correct EOS t ring to fit the reducer the spacing should be as it is.

I used the same setup on my Equinox 120 and it was spot on.

[old_eyes]

If you use the Skywatcher reducer/flattener with the 56mm adapter on FLO's site and then the correct EOS t ring to fit the reducer the spacing should be as it is.

I used the same setup on my Equinox 120 and it was spot on.

Thanks SImon,

I'll talk to FLO

old_eyes

[old_eyes]

I spoke to Martin at FLO and they recommended the Skywatcher Field Flattener. It arrived in time ofr Christmas, but I did not get around to trying it until a couple of nights ago. It fitted easily, did not require any spacers and really improved things!

Thanks guys! Onto the next problem ;-)

old_eyes