Newtonian Collimation

[astrosathya]

Hi All,

I purchased a GSO 6" f/4 Newtonian "Astrograph" late last year and eventually found that stars on one corner were egg shaped while taking images. I narrowed it down to improper centering of secondary mirror from the factory and resulting tilt. 

Long story short, after numerous iterations, I used the Advanced Newtonian collimation technique by Astro Shed guy and ended up with the below pic of the optics. Does it look ok or do I need to do more? I will be checking with a Howie this weekend too.

[Geoff Barnes]

Looks pretty good to my crooked eyes Sathya.

The best and simplest collimation technique that I've used successfully is this one by Gary Seronik, no tools required and it really works a treat.

http://garyseronik.com/no-tools-telescope-collimation/

[astrosathya]

1 hour ago, Geoff Barnes said:

Looks pretty good to my crooked eyes Sathya.

The best and simplest collimation technique that I've used successfully is this one by Gary Seronik, no tools required and it really works a treat.

http://garyseronik.com/no-tools-telescope-collimation/

Thanks Geoff, I will definetly use this method by Gary Seronik. Since its the monsoon season here I'll have to try it with my artificial star. I tried using a Cheshire and for the love of God I cannot comprehend whatever I see through it. The above was done using only a simple collimation cap and my eye. 

[peter shah]

I'm not so sure you have right. At f4 you would expect to see the secondary mirror offset. In your picture everything appears to be concentric but in a fast Newtonian set up there is a need to have the secondary offset to get full illumination. If you have re-centred the secondary on its mount and lined up all of the reflections you will have introduced tilt while through the Cheshire eyepiece everything appears lined up. 

[Tim]

4 hours ago, Geoff Barnes said:

Looks pretty good to my crooked eyes Sathya.

The best and simplest collimation technique that I've used successfully is this one by Gary Seronik, no tools required and it really works a treat.

http://garyseronik.com/no-tools-telescope-collimation/

Ok for basic alignment Geoff, and ok for observing,  but imaging at f4 requires a more rigorous attention to collimation, in my experience.

[astrosathya]

1 hour ago, peter shah said:

I'm not so sure you have right. At f4 you would expect to see the secondary mirror offset. In your picture everything appears to be concentric but in a fast Newtonian set up there is a need to have the secondary offset to get full illumination. If you have re-centred the secondary on its mount and lined up all of the reflections you will have introduced tilt while through the Cheshire eyepiece everything appears lined up. 

How do I offset the mirror correctly? There's a lot of stuff online and a lot of noise too. 

[Doc]

Take a look here

https://www.skyandtelescope.com/astronomy-equipment/offsetting-your-secondary-mirror/

[johninderby]

This thread is worth a read.

 

[astrosathya]

1 hour ago, peter shah said:

I'm not so sure you have right. At f4 you would expect to see the secondary mirror offset. In your picture everything appears to be concentric but in a fast Newtonian set up there is a need to have the secondary offset to get full illumination. If you have re-centred the secondary on its mount and lined up all of the reflections you will have introduced tilt while through the Cheshire eyepiece everything appears lined up. 

Also, given that I have an oversized secondary mirror, would it really make a difference for imaging? In other words, is it really going to be worth it? When the scope was bought new and I tested it on HH, I found that the stars on the top right of the frame were egg shaped. I am going to do an artificial star test tonight to see how different it is now.

[Geoff Barnes]

45 minutes ago, Tim said:

Ok for basic alignment Geoff, and ok for observing,  but imaging at f4 requires a more rigorous attention to collimation, in my experience. 

Aah I didn't see Sathya is collimating for imaging. I am purely visual so his image would be fine for me at f5. ?

[astrosathya]

18 minutes ago, Geoff Barnes said:

Aah I didn't see Sathya is collimating for imaging. I am purely visual so his image would be fine for me at f5. ?

I read through the excellent article by Astrobaby and find that the patterns as seen through the cap, as well as the Cheshire are exactly as they should be for the offset secondary. I am quite certain now that the star test will confirm the same. Boy oh boy, this is so exciting. ?

[astrosathya]

Update:

I did the collimation one time again tonight using the webcam attached to the focuser with the lens on. Below are the results.