7 replies to this topic

[Dirk84]

Hi,

 

I recently bought a used Canon 60D and did a full spectrum mod myself. (Removing the IR filter)

 

This week I did my first test on the Heart & Soul nebula.

I noticed some issues and I was wondering what the problem could be.
 

 

 

Single sub of 100sec at F4 ISO100.

 

There is a clear difference in star 'roundness' between the center of the frame and its edges.

I believe this is called lens aberration?

 

- Is this caused because I did not level the sensor perfectly after the modification? (I'm a ware of the 3 spring loaded screws that hold the sensor in place)

OR

- Is this normal abberation for the 70-200 f2.8 Canon lens (even at F4)

 

 

I really hate that the weather is so bad lately. Otherwise I could spent more time troubleshooting. I wonder if the effect is less at 150mm for example... And worse on f2.8

 

Thanks for the help.

 

Cheers

Dirk

 

 

 

[michael8554]

Hi Dirk

 

Not the best image to be making star-shape judgements on, as a 100secs exposure will include tracking errors.

 

Take a 10 sec exposure at high ISO and look at that.

 

The other corners have coma (teardrop shaped stars) worse on the left side and bottom corners of the image

 

The centre also shows elongation.

 

So a combination of tilt, coma, and tracking errors.

[asanmax]

michael8554 is right, you need to take some shorter exposures to at least eliminate guiding errors.

This could well be a lens issue and/or the sensor tilt. I've used and tested lots of lenses including Canon L series and I can definitely tell that there are lemons among them.

On the bright side, I can definitely see the Heart and Soul nebulae in your image 

[kg74]

You really need to pull the lens test sheets. For that lens there will be plenty. 
 

It is unclear where in the 70-200 range you were shooting. (150mm?). It may be soft in the corners at said range at f4. 

 

I had that lens, now own the IS version.  It is a good lens. Check your results with the data sheets to see if you have a bad copy or if you need to close it down some at your elected focal length. 

 

I also have a 60D and believe it is an APS-C sensor - so your results are troubling. Is it only one side showing softness or is it consistent outside center? 
 
Consistent and sharp center hints at a good lens and conversion and the wrong f stop at your elected focal distance. 
 

[Dirk84]

Hi Dirk

 

Not the best image to be making star-shape judgements on, as a 100secs exposure will include tracking errors.

 

Take a 10 sec exposure at high ISO and look at that.

 

The other corners have coma (teardrop shaped stars) worse on the left side and bottom corners of the image

 

The centre also shows elongation.

 

So a combination of tilt, coma, and tracking errors.

 

 

michael8554 is right, you need to take some shorter exposures to at least eliminate guiding errors.

This could well be a lens issue and/or the sensor tilt. I've used and tested lots of lenses including Canon L series and I can definitely tell that there are lemons among them.

On the bright side, I can definitely see the Heart and Soul nebulae in your image 

 

 

You really need to pull the lens test sheets. For that lens there will be plenty. 
 

It is unclear where in the 70-200 range you were shooting. (150mm?). It may be soft in the corners at said range at f4. 

 

I had that lens, now own the IS version.  It is a good lens. Check your results with the data sheets to see if you have a bad copy or if you need to close it down some at your elected focal length. 

 

I also have a 60D and believe it is an APS-C sensor - so your results are troubling. Is it only one side showing softness or is it consistent outside center? 
 
Consistent and sharp center hints at a good lens and conversion and the wrong f stop at your elected focal distance. 
 

 

Thanks for the reply guys.

 

Basically it comes down that I need to take a better test image to judge or determine the cause of the issue.

 

What would be a good test image. Short exposure high ISO with no tracker involved? And what a about the f stop? 

Thanks again.

Dirk


 

[asanmax]

Thanks for the reply guys.

 

Basically it comes down that I need to take a better test image to judge or determine the cause of the issue.

 

What would be a good test image. Short exposure high ISO with no tracker involved? And what a about the f stop? 

Thanks again.

Dirk


 

I would take a series of images at ISO 800 with the use of the tracker. 1, 5, 10 sec exposures should be enough. Try to use 70mm and 200mm at F/2.8. Stopping down will make the lens imperfections less noticeable.

[17.5Dob]

At that scale it's tough to see, but it looks like it's simply coma, as the stars look to be equally elongated in all 4 corners and all point back to the center of the frame.

[kg74]

Pull a test sheet for your lens. Find and print a test page which are easy to find on the net.  Put it on a tripod and shoot your own test. Keep everything controlled and repeatable.
 

Compare your tests w tests for your lens. Look to see if you have issues. 
 

Forget about AP - focus (pun) on f stop and lighting (use iso 400 or less).  You need to eliminate camera issues, lens issues and determine ( assuming camera and lens pass) optimal f-stop for your desired focal length. 
 

Coma doesn’t care about ISO or exposure duration.