9 replies to this topic

[kb38758]

I recently was forced to remove my secondary mirror on my Orion Xt10g Dob telescope to clean the mirror, due to a large spot spot on it.

After replacing it and collimating as I usually do I now have a odd extra coma on the right side of all images.

I've checked and recheck collimation and can't spot the error.

I suspected that the secondary is a tiny bit closer or farther from the primary mirror than before but I can't see any issue.

I use Cheshire, Laser and pinhole cap for collimating.

I did not remove the primary mirror.

Any help would be appreciated.

Attached are the full image, the left side magnified that seems normal for my scope and the right side  magnified which shows the image issue.

 

 

Left Side of Image

 

Issue Image, Right Side of Image

Edited by kb38758, 30 March 2023 - 11:22 AM.

[David Castillo]

Perhaps the secondary's long axis is rotated counter-clockwise 45 degrees (or more) causing vignetting of the light cone. It would be helpful if you could tape a sheet of white paper to wall of the OTA -opposite the focuser, and post a picture of the view down the focuser tube. Something in the optical pathway is amiss. 

Edited by David Castillo, 31 March 2023 - 05:57 PM.

[smiller]

I get something similar to you on the bottom image when I have tilt because the camera isn’t flush when I insert it in the focuser.  Not exact but similar to pic #3

Edited by smiller, 01 April 2023 - 09:13 AM.

[MellonLake]

In step #1 of collimation you need to make sure the secondary mirror is centred and rounded in the focuser.  If you don't do this perfectly the mirror clips will not look equal after collimation.  I suspect you are trying to force the mirror clips to look equal and imparting a tilt error on the secondary that is leading to the focal plane not being square to the focuser.  

 

Collimation is 3 Steps;

 

1) Centre and round the secondary. Do this with your Cheshire by making sure the outer diameter of the secondary mirror is concentric to the inner diameter of the Cheshire (with the focuser and Cheshire all the way in (I am presuming it is a 1.25" collimating eyepiece (combo tool) and not a Calibrated Farpoint or Catseye Cheshire)  

2) Align the secondary.  Do this with the laser.

3) Align the primary.  Do this with the collimation cap.

 

YOU HAVE TO DO THIS IN ORDER ENDING ON STEP #3.  If the mirror clips don't look equal after this step then step #1 has not been done perfectly (go back to step #1 and do it all again).  The mirror clips really only need to be perfect for photography.  If you are just doing visual, the clips can be a bit off and it would be detectable at the eyepiece (this just leads to a slightly non-uniform field illumination which our eyes are not sensitive to)

 

Rob

[kb38758]

In step #1 of collimation you need to make sure the secondary mirror is centred and rounded in the focuser.  If you don't do this perfectly the mirror clips will not look equal after collimation.  I suspect you are trying to force the mirror clips to look equal and imparting a tilt error on the secondary that is leading to the focal plane not being square to the focuser.  

 

Collimation is 3 Steps;

 

1) Centre and round the secondary. Do this with your Cheshire by making sure the outer diameter of the secondary mirror is concentric to the inner diameter of the Cheshire (with the focuser and Cheshire all the way in (I am presuming it is a 1.25" collimating eyepiece (combo tool) and not a Calibrated Farpoint or Catseye Cheshire)  

2) Align the secondary.  Do this with the laser.

3) Align the primary.  Do this with the collimation cap.

 

YOU HAVE TO DO THIS IN ORDER ENDING ON STEP #3.  If the mirror clips don't look equal after this step then step #1 has not been done perfectly (go back to step #1 and do it all again).  The mirror clips really only need to be perfect for photography.  If you are just doing visual, the clips can be a bit off and it would be detectable at the eyepiece (this just leads to a slightly non-uniform field illumination which our eyes are not sensitive to)

 

Rob

Thank you for the response!

This is very helpful!

My issue began when I was forced to remove the secondary to clean it.

The last time I did this it took several tries to get the secondary back into the right spot.

I recently tried moving it a tiny bit closer to the primary and this gave the the issue ALL around the image.

Next time I'll try moving it further away from the primary instead.

It has all the looks of a backfocus issue to me.

In this latest image I now see the "triple stars" on BOTH sides since I tried CLOSER to the primary.

 

[smiller]

In step #1 of collimation you need to make sure the secondary mirror is centred and rounded in the focuser.  If you don't do this perfectly the mirror clips will not look equal after collimation.  I suspect you are trying to force the mirror clips to look equal and imparting a tilt error on the secondary that is leading to the focal plane not being square to the focuser.  

 

Collimation is 3 Steps;

 

1) Centre and round the secondary. Do this with your Cheshire by making sure the outer diameter of the secondary mirror is concentric to the inner diameter of the Cheshire (with the focuser and Cheshire all the way in (I am presuming it is a 1.25" collimating eyepiece (combo tool) and not a Calibrated Farpoint or Catseye Cheshire)  

2) Align the secondary.  Do this with the laser.

3) Align the primary.  Do this with the collimation cap.

 

YOU HAVE TO DO THIS IN ORDER ENDING ON STEP #3.  If the mirror clips don't look equal after this step then step #1 has not been done perfectly (go back to step #1 and do it all again).  The mirror clips really only need to be perfect for photography.  If you are just doing visual, the clips can be a bit off and it would be detectable at the eyepiece (this just leads to a slightly non-uniform field illumination which our eyes are not sensitive to)

 

Rob

I expect my secondary can still be improved as I still get a tiny bit of these types of artifacts with by Orion XT12G Dob with the Nexus 0.75x corrector/reducer.  I mainly get it if my camera isn’t snug against the stop so I blamed it on that but it’s never quite perfect.

[kb38758]

I expect my secondary can still be improved as I still get a tiny bit of these types of artifacts with by Orion XT12G Dob with the Nexus 0.75x corrector/reducer.  I mainly get it if my camera isn’t snug against the stop so I blamed it on that but it’s never quite perfect.

I find that the secondary is the most difficult to get right.

It took me many tries last summer to get it right.

Most tips I find on the internet are vague and subjective.

I know my camera and primary are the same as before but these artifacts are new.

As I said above, my next try will be with the secondary further AWAY from the primary.

I think the secondary is now too close to the primary, causing me to tilt the secondary to get the primary clips in sight.

With 6 clear nights a month it takes a long time to see if the results are good.

So far this is the clearest tip I've found and will add it to my collimation info folder.

 

https://www.stargeez...irror-alignment

[smiller]

In step #1 of collimation you need to make sure the secondary mirror is centred and rounded in the focuser.  If you don't do this perfectly the mirror clips will not look equal after collimation.  I suspect you are trying to force the mirror clips to look equal and imparting a tilt error on the secondary that is leading to the focal plane not being square to the focuser.  

 

Collimation is 3 Steps;

 

1) Centre and round the secondary. Do this with your Cheshire by making sure the outer diameter of the secondary mirror is concentric to the inner diameter of the Cheshire (with the focuser and Cheshire all the way in (I am presuming it is a 1.25" collimating eyepiece (combo tool) and not a Calibrated Farpoint or Catseye Cheshire)  

2) Align the secondary.  Do this with the laser.

3) Align the primary.  Do this with the collimation cap.

 

YOU HAVE TO DO THIS IN ORDER ENDING ON STEP #3.  If the mirror clips don't look equal after this step then step #1 has not been done perfectly (go back to step #1 and do it all again).  The mirror clips really only need to be perfect for photography.  If you are just doing visual, the clips can be a bit off and it would be detectable at the eyepiece (this just leads to a slightly non-uniform field illumination which our eyes are not sensitive to)

 

Rob

By 1.25” Cheshire you mean this?

 

https://www.amazon.c...908517342&psc=1

[kb38758]

By 1.25” Cheshire you mean this?

 

https://www.amazon.c...908517342&psc=1

That is the one I use.

Be forewarned...it's not the easiest device to use with our big scopes.

It took me months to train my eye to use this bad boy.

It's still the LAST item I use when collimating and ONLY if I have an issue that my usual collimation doesn't cure.

Like now.

[MellonLake]

By 1.25” Cheshire you mean this?

https://www.amazon.c...908517342&psc=1

Yep. That is the one. It will only work f/4.7 or higher.