How's my RC8 collimation?

[Adam1234]

I've been revisiting collimation of my Stella Lyra RC8 now that Jupiter is on it's way back, and I think (hope) I have got it more of less there.

I was really struggling at first,  spending days doing lots of research, reading all the websites, watching youtube videos. I refuse to buy a howie glatter as they seem to be nearly as much as the scope, and I'm also convinced that a lot of the instructions out there are either completely or partially wrong (or at least rely on various assumptions being correct for the method to work), or incomplete. 

For example one video on youtube instructed you to align the secondary using cheshire eyepiece - fine.  And then to align the primary using a standard laser collimator, adjusting the primary until the laser hit the centre of the target on the laser. Correct me if I'm wrong but this method makes a massive assumption that the focuser is completely square...which was not mentioned at all in the video. As a result I took my primary WAY of out of whack, to the point where one of the screws was completely loose. 

Anyway, I decided to settle on the good old cheshire eyepiece method, i.e. 1st - align the secondary by getting the dot made by the cheshire eyepiece hole into the donut on the secondary mirror, 2nd -align the primary by adjusting the mirror to get the thin strip of light (optical axis) even around the edge (some also say you should see concentric circles within the shadow of the secondary). 

 

Sounded easy enough, but I couldn't seen the centre dot properly because the crosshair of the cheshire was in the way, and I couldn't see the thin line of white around the edge or concentric circles. 

I ended up removing the crosshair from the cheshire and find it much easier to the centre dot/donut. I also discovered that if I shined a torch directly into the 45 reflective surface of the chesshire I could indeed see the concentric circles., so here is what I did:

1) I lined my camera up a best as possible to the chesshire with 10x zoom, and used the adjustable circles in the collimation aid in Astrophotography Tool while adjusting the secondary to get the centre dot right in the middle of the donut

2) shined the torch into the chesshire to light up the concentric rings, and again used to the collimation aid circles while adjusting the primary to get all rings concentric

3) Repeating 1 & 2 again until I was happy.

 

Here are my final results of steps 1 and 2 after the final check, (with and without the collimation aid active to ease of viewing).

How does my collimation look? Was this a good approach to make and am I interpreted the views correctly?

 

Centre dot/donut 

 

Concentric circles

 

Thanks

Adam

[Knighty2112]

Did you have just the focuser on the RC only, and no extension rings? With any extension rings fitted you would loose the thin outer white ring. For collimation I ended up getting one of these https://www.teleskop-express.de/shop/product_info.php/info/p11188_TS-Optics-2--LED-Collimator-for-RC-Telescopes-and-all-other-Types-of-Telescopes.html from TS Optics. Used it the first time and got it pretty much spot on 1st time, which gave me nearly perfect lunar views.

[Adam1234]

Yeah I removed all the extension rings and only had the focuser attached.

According to FLO website that collimator doesn't work with the stella lyra rc8

 

[davies07]

I think what you've done looks good to me.

Your final tests should should be to test for coma on a centrally placed star. Zoom in on a central star at focus and then slightly defocus it moving the focuser out. Turn off any auto-stretching and defocus only enough to reveal a tiny doughnut. Study the distribution of the light around the central hole. It should be symmetrical but poor seeing  makes it tricky to assess. If the annulus of light shows a soft edge on one side and hard edge on the opposite side, you have some residual coma. Also, look for the Young Spot (a tiny dot of light) in the centre of the dark area. It should be in the centre. Make any corrections to the primary, only.

You could use Metaguide, for example, for checking the collimation on a central star. I've found it found it to work well.

Take a test image on a star field and use Pixinsight to measure  the FWHM and eccentricity of the stars (script/Image analysis/FWHMeccentricity). Click the support button to see the graphs. They should show the smallest and most circular stars symmetrical about the centre of the field. 

[Adam1234]

2 hours ago, davies07 said:

I think what you've done looks good to me.

Your final tests should should be to test for coma on a centrally placed star. Zoom in on a central star at focus and then slightly defocus it moving the focuser out. Turn off any auto-stretching and defocus only enough to reveal a tiny doughnut. Study the distribution of the light around the central hole. It should be symmetrical but poor seeing  makes it tricky to assess. If the annulus of light shows a soft edge on one side and hard edge on the opposite side, you have some residual coma. Also, look for the Young Spot (a tiny dot of light) in the centre of the dark area. It should be in the centre. Make any corrections to the primary, only.

You could use Metaguide, for example, for checking the collimation on a central star. I've found it found it to work well.

Take a test image on a star field and use Pixinsight to measure  the FWHM and eccentricity of the stars (script/Image analysis/FWHMeccentricity). Click the support button to see the graphs. They should show the smallest and most circular stars symmetrical about the centre of the field. 

Thank you! I will try a star test when I get the chance. I tried a star test the other week after using the cheshire, and made a few minor adjustments but then it didn't agree with the cheshire. By the sound of it, I defocused to much!

Out of interest, why make corrections to the primary only when doing the star test? I've seen other people mention this, but others have said seconddary only, and others have said both?

[davies07]

My understanding is that the adjustment of the primary mirror controls the amount of coma in the centre of the field of view - when properly adjusted you should get no coma in the centre.

The secondary mirror controls the distribution of the characteristics in the centre across the rest of the field of view. So a correct secondary mirror will produce symmetrical star characteristics into the corners of the field of view.  If you see elongated stars in just one corner, for example, it indicates that the secondary needs adjusting to make all of the corners look the same.

The primary mirror has a strong effect on the collimation, the secondary mirror less so. So getting the primary correct is pretty crucial. I guess that is why some advise not touching the primary because messing up the primary can do lots of damage to the image.

The advantage of the RC is that, when correctly adjusted, it can eliminate the three key optical aberrations: coma, astigmatism and spherical aberration (SA). The RC doesn't have a strong lens so there is no chromatic aberration (a flattener is a weak lens). Spherical aberration is controlled by the distance apart of the mirrors and you would use a Ronchi test to check it.

Astigmatism looks like oval shaped stars, either radial or tangential. You might see astigmatism if you go too far off the centre of the field of view where the mirrors can no longer operate correctly.  So oval stars at the edge of the field is astigmatism. Using a very large sensor might lead to seeing astigmatism at the edges.

Hope this helps.

[Adam1234]

@davies07that's great, thank you for the explanation. Just need to wait for the next clear night now. Cloudy tonight but I'm sure there will be some more clear nights soon. 

[Clarkey]

From my experience of the RC8 I found collimating the secondary worked well with the TS collimator (even though it is the world's most expensive LED light with a hole in). Following this I use a star test to get the primary aligned. Whether this is the 'right' way is another question but it works for me.

I have tried some other methods including the laser version which completely ruined my relatively good collimation. Once right I think they are excellent imaging scopes.