Please review and advise on C8 collimation

[oymd]

With galaxy season upon us, I've decided to give my C8 a go.

The OTA is 2 years old, very lightly used and I had never collimated it before.

To check that my collimation is ok, I used NINA to slew to a bright star, plate solved and centred, then I moved the electronic focuser out of focus.

I could see the doughnut and it looked centred to me, so I left it alone.

Am I missing something, or does it look ok?

Is that all I need to check regarding collimation?

 

Many thanks

 

[Peter Drew]

It looks "ok" at a glance but it is far too out of focus to give a definitive assessment.  You need very high power and almost focussed to accurately judge the collimation.     🙂

[globular]

Looks like you might not have defocussed quite enough (or far too much).  And you should do it both inside and outside of focus.

Edited March 15, 2023 by globular

[oymd]

17 minutes ago, Peter Drew said:

It looks "ok" at a glance but it is far too out of focus to give a definitive assessment.  You need very high power and almost focussed to accurately judge the collimation.     🙂

Thank you Peter fr your reply

I am using a camera and the PC for the collimation.

Can you please clarify what you meant by "very high power" and "almost focussed"?

If the star is almost focussed, it would be tiny and pinpoint on the screen, and I would not have a doughnut?

Apologies, I am new to this, and I do not follow?

[oymd]

1 minute ago, globular said:

Looks like you might not have defocussed quite enough.  And you should do it both inside and outside of focus.

Oh, I did not know that...so I should de-focus in both directions then?

I did defocus all the way out, but I never say a doughnut similar to the one you kindly provided in your image. It went from a doughnut to completely disappearing. I used my camera and PC.

Is it a pre-requisite to collimate using an EYE PIECE rather than PC and camera?

[globular]

It is probably something like 1 turn of the focuser to get the proper donut (my outer images).  Not enough (or far too much) and you'll get the 'all white disc' donut - which is not 'accurate' enough for collimating.

I always do it visually with a 10mm or so eyepiece - but I suggest you try again with your camera with only a very small defocus before you give up on it - no reason springs to mind why it shouldn't work.

[Cosmic Geoff]

Your collimation looks "good enough for galaxies".  

I have only collimated visually. AFAIR you need to use a high powered eyepiece and put the star (e.g. Polaris) slightly out of focus and see if the diffraction rings look deformed.  A very small tweak (e.g. 1/8 turn) on one of the 3 collimating screws on the secondary holder is enough to make a difference, if the collimation is slightly out. 

If the collimation is very far out, stars can look like a badminton shuttlecock, and a lot of trial and error, and re-aiming the scope may be needed to get it right. 

[Peter Drew]

Defocusing a star to show the doughnut either side of focus is used to examine the correction and/or other defects rather tan collimation.  A nicely concentric large defocused image can often collapse into a less than perfect image near the point of focus.  You need enough magnification to reveal the "Poisson Spot", a small star like appearance in the almost focused image.  The centring of this with minute tweaks of adjustment will give the best available performance.     🙂 

[Carbon Brush]

Good advice from @Peter Drew

Supplemented by my (poor) advice to enjoy the view rather than observe the defects.😁

I am reminded of something said to me me many years ago, about 1970, when a lot of people were trying to perfect the 'perfect' hifi audio system using records. More recently known as 'vinyl'.
......These people are listening to to the scratches, instead of listening to the music....🤣

Seriously. My experience of newts is that they hold collimation quite well and this collimation stuff is often overthought.
Unless of course there are loose screws, wobbly fixings, or the scope gets dropped on the floor.🤔

HTH, Dvid.

 

[RobertI]

This seems to be a pretty good guide to collimating an SCT:  

https://www.highpointscientific.com/astronomy-hub/post/how-tos/collimating-your-sct-for-best-performance

Edited March 15, 2023 by RobertI

[paulastro]

57 minutes ago, Carbon Brush said:

Good advice from @Peter Drew

Supplemented by my (poor) advice to enjoy the view rather than observe the defects.😁

I am reminded of something said to me me many years ago, about 1970, when a lot of people were trying to perfect the 'perfect' hifi audio system using records. More recently known as 'vinyl'.
......These people are listening to to the scratches, instead of listening to the music....🤣

Seriously. My experience of newts is that they hold collimation quite well and this collimation stuff is often overthought.
Unless of course there are loose screws, wobbly fixings, or the scope gets dropped on the floor.🤔

HTH, Dvid.

 

I agree with you.  It's still often stated in books and on chat rooms that Newts will need to be frequently collimated.  In my experience over many years this is not the case.  I've taken  Newts to Kelling and back from West Yorkshire many times for the star party.  Though I check the collimation after every journey, I can't recall ever having to adjust it.

[CraigT82]

12 hours ago, Carbon Brush said:

Good advice from @Peter Drew

Supplemented by my (poor) advice to enjoy the view rather than observe the defects.😁

I am reminded of something said to me me many years ago, about 1970, when a lot of people were trying to perfect the 'perfect' hifi audio system using records. More recently known as 'vinyl'.
......These people are listening to to the scratches, instead of listening to the music....🤣

Seriously. My experience of newts is that they hold collimation quite well and this collimation stuff is often overthought.
Unless of course there are loose screws, wobbly fixings, or the scope gets dropped on the floor.🤔

HTH, Dvid.

 

 

11 hours ago, paulastro said:

I agree with you.  It's still often stated in books and on chat rooms that Newts will need to be frequently collimated.  In my experience over many years this is not the case.  I've taken  Newts to Kelling and back from West Yorkshire many times for the star party.  Though I check the collimation after every journey, I can't recall ever having to adjust it.

Not sure how you both got onto Newtonian collimation as the OP is asking about SCTs. 

My own view of the OP’s situation is that it looks ok in terms of rough collimation (maybe that’s all you need for galaxy imaging?) but for fine collimation I would advise to use an in-focus star image at high power (eyepiece) and observe the brightness distribution of the first diffraction ring around the airy disk.  You may need to tweak the collimation screws to get the brightness of the ring even all around.  I’m my experience it is almost impossible to do this using a camera as you can’t see the diffraction ring. Anyways I think that’s necessary for planetary imaging  where you need the sweet spot to land on the small sensor, but for larger formats you’ll probably get away with a good-enough collimation. 
 

Perhaps you could capture some images and view the distribution of coma in the image, or run the image through CCD inspector or similar, that would give you a clue. Properly collimated the sharpest point of the image would be in the centre and not offset to one side/corner.