New RC8 StellaLyra - Collimation needed?

[Doversole]

Hi Everyone,

Just got my RC8 delivered and after a few days of rain (again), I managed to get a very first image yesterday on M27 (14x300s, before the clouds rolled in...). I look at a defocused star and didn't find the shape too ugly, then took this image. It's clear that the stars, in particular and the bottom left corner are not round.

What is the cause of this? Bad collimation? For info I can't tilt the focuser. Is a tilt plate needed? Thanks for your views and clear skies!

[vlaiv]

Yes, that is poor collimation.

Although central part is not problematic - edges are and that is due to primary mirror tilt.

When collimating RC - you go thru set of adjustments - you adjust primary to fix peripheral issues and then align secondary to get round defocused pattern in the center and then you repeat until no further alignment is necessary.

I have found this guide to be particularly useful: https://deepspaceplace.com/gso8rccollimate.php

[Doversole]

I just tried CCD Inspector on some of the images. Surprise, it says the collimaiton is pretty good? Well obviously the image above shows the collimation is crap. So what do I do wrong? 🤪

[CraigT82]

Advanced warning: I know next to nothing about RCs! 

But to me the upper right corner looks pretty good, the bright yellow star looks nice and sharp/round. The stars lower right and on the left all have aberration that are pointing away from this upper right zone.

I'd say the zone of good collimation is there in the upper right and the collimation needs adjusting to be brought back to the centre of the field? 

[vlaiv]

26 minutes ago, Doversole said:

I just tried CCD Inspector on some of the images. Surprise, it says the collimaiton is pretty good? Well obviously the image above shows the collimation is crap. So what do I do wrong?

First thing to try out would be to rotate the camera by 180° and see what happens.

If there is tilt in camera / camera adapter - it can be revealed that way as aberrations will rotate with camera. If they stay the same - then it is collimation issue.

8" version has primary mirror fixed to back tube and those are aligned at factory and should not get out of collimation. Larger models have decoupled primary and back port and have tilt adjuster at the back.

I doubt that back port is out of collimation with primary - as they are "one unit" and move together - therefore you should try primary collimation using single star positioned in different corners and looking at defocus - you want level of defocus in each corner to be the same / minimal - compared to center of the field.

[Doversole]

3 hours ago, vlaiv said:

First thing to try out would be to rotate the camera by 180° and see what happens.

If there is tilt in camera / camera adapter - it can be revealed that way as aberrations will rotate with camera. If they stay the same - then it is collimation issue.

 

Thanks, I didn't think about that. I will try on the next occasion.

[Doversole]

I took this image, slightly defocused and centered on M57 to test the collimation.

I still need to rotate the image train to make sure it is not the image train that is misaligned... Objective for the next clear sky opportunity!

From the image, would you believe this is an off axis coma issue linked with the secondary mirror? I agree @vlaivthe primary is unlikely to have moved during transportation. Just wanted to share here before I try to follow the DSI methodology !

[vlaiv]

Just fix the secondary and repeat the test.

Tweaking secondary is rather easy - find bright star - place it smack in the center of the frame, defocus - and tweak secondary until you get concentric rings.

 

[Doversole]

Thanks @vlaiv, I tweaked the secondary with a bright star in the centre of the frame, then took another picture of M27. Much better indeed and not that difficult. Thanks again for your help!

[jjosefsen]

On 12/08/2021 at 10:30, vlaiv said:

Just fix the secondary and repeat the test.

Tweaking secondary is rather easy - find bright star - place it smack in the center of the frame, defocus - and tweak secondary until you get concentric rings.

 

Sorry to but in here, but I am working on collimating my RC6 as well, and have a question. 

 

I have been using this Deep Space Place to collimate, but haven't gotten it really great yet.

But last night I was looking at the DSI method, and noted something.

 

DSI: On axis collimation done with primary mirror, off axis collimation done with secondary.

DSP:  On axis collimation with secondary mirror, off axis with primary mirror.

 

Does it not matter in the end? I have an aweful lot of weirdly shaped stars around the edges.

[jjosefsen]

On 13/08/2021 at 08:56, Doversole said:

Thanks @vlaiv, I tweaked the secondary with a bright star in the centre of the frame, then took another picture of M27. Much better indeed and not that difficult. Thanks again for your help!

This is looking great! I wish my stars looked like that.

[vlaiv]

16 minutes ago, jjosefsen said:

DSI: On axis collimation done with primary mirror, off axis collimation done with secondary.

DSP:  On axis collimation with secondary mirror, off axis with primary mirror.

 

Does it not matter in the end? I have an aweful lot of weirdly shaped stars around the edges.

Maybe you should think of it this way:

If you have your secondary out of collimation - no stars will look good, but if your primary is out of collimation - your secondary can compensate for central part of field. This means that star in center will look ok and when defocused - you'll get concentric rings, but edge stars will be a mess.

That is why procedure is as follows:

1. adjust primary so that stars have roughly the same FWHM in each corner (same level of defocus due to field curvature)

2. then adjust secondary so that you get concentric rings in the center of the field

then go back to point number 1 (because messing with secondary will change edge stars again but less this time)

Hope this helps.

[Clarkey]

I struggled with my RC8 trying to collimated the secondary using the DSI method. Eventually I got good collimation by using the TS RC collimator (Reego) to collimated the secondary and then the DSI star test (or car headlights 5 miles away) for the primary. Unfortunately the TS RC collimator is no good for adjusting the primary mirror on the GSO 8" RC. By doing it this way, at least there is a fixed point of reference (focuser centre).

It might not be 'textbook' but it worked for me. I am planning to get a Ronchi grating to confirm the FL of the mirrors but it plate solves to 1626mm.

[jjosefsen]

13 hours ago, vlaiv said:

Maybe you should think of it this way:

If you have your secondary out of collimation - no stars will look good, but if your primary is out of collimation - your secondary can compensate for central part of field. This means that star in center will look ok and when defocused - you'll get concentric rings, but edge stars will be a mess.

That is why procedure is as follows:

1. adjust primary so that stars have roughly the same FWHM in each corner (same level of defocus due to field curvature)

2. then adjust secondary so that you get concentric rings in the center of the field

then go back to point number 1 (because messing with secondary will change edge stars again but less this time)

Hope this helps.

It’s not exactly what I’ve been doing, but it makes sense. I will try it. 
 

What confused me was the fact that they seemed to collimate different on/off axis differently. But I suppose it doesn’t matter as long as the optical axis just ends up aligned.