C6: How Bad Is My Collimation?

[Ags]

This is how my collimation of my C6 is right now - I haven't touched the three front screws since it came out of the box. Does it need tweaking?

[dweller25]

Yes it needs tweaking for best results

Edited April 16, 2020 by dweller25

[vlaiv]

Provided that star was right in the center of the field - I would say your collimation is off. Scope needs collimation.

Here is my scope (larger defocus) for reference:

[Ags]

Yes the star was dead center. I couldn't see the issue clearly at the scope, but now I processed the SER file it seems pretty obvious. 

For my reference, how bad is my collimation currently? @vlaiv's collimation seems perfect!

[Captain Scarlet]

It’s not far off, and not too difficult.

With the star centered, find out which is the most relevant of the three adjusting screws by putting your hand over the edge of the front tube, seeing where it appears on the pattern, and moving it until it corresponds to the “most squashed” bit of your pattern. If you’re lucky, the “gap” will appear either next to or opposite one of the three screws.

Adjust that screw, in small increment, and see which way the pattern goes, and thereby discover which direction of turning makes the pattern better.

if your hand gap is not perfectly aligned with an adjustment screw, pick the one nearest (ie nearest to “next to or opposite”) it, get it as good as you can, then shift to another one which has now likely become closer. You should home in on perfection in this way.

hope that makes sense ... always adjust in 1/16 of a turn at most at a time.

Good luck 

M

Edited April 16, 2020 by Captain Magenta

[johninderby]

This is good guide to collimation.

https://skywatch.brainiac.com/collimation.pdf

[newbie alert]

10 hours ago, vlaiv said:

Provided that star was right in the center of the field - I would say your collimation is off. Scope needs collimation.

Here is my scope (larger defocus) for reference:

Is that a RC Vlaiv?

[vlaiv]

31 minutes ago, newbie alert said:

Is that a RC Vlaiv?

Yes it is. Large secondary, isn't it?

[newbie alert]

Looks very different to a sct.. 

[Ags]

I dared to take a screwdriver to my SCT tonight. This is the result of my efforts...

I think I could still go a bit further, but I think this is a definite improvement.

Edited April 17, 2020 by Ags

[vlaiv]

2 minutes ago, Ags said:

I dared to take a screwdriver to my SCT tonight. This is the result of my efforts...

I would say much better, but I still don't like the shape of that central bit.

Could be that tube currents are present. Was the scope properly cooled? Could you do smaller defocus than this and make image of that?

[Ags]

Yes the central bit is not point-like. I had the scope cooling for three hours outside so no tube currents I expect. I am reprocessing the SER now - I just spotted there was no alignment point on the middle bit.

[Ags]

Red face. The central bit was misshapen due to processing error in Autostakkert (missing alignment point). Here is a correct stack.

[vlaiv]

1 minute ago, Ags said:

Red face. The central bit was misshapen due to processing error in Autostakkert (missing alignment point). Here is a correct stack.

I would say much better. I have a feeling that it is still a bit "squashed" to the right? Just a tiny bit.

Here is good way to judge it. I took your image and made a mirror copy - flipped around vertical axis. Then aligned centers:

Still some work left to do?

[Captain Scarlet]

Well done. Homing in...

[Ags]

@vlaiv thanks for that - that was quick work! It makes my current collimation state look "not good".  I think however I will leave the scope in this state for a couple of observing sessions to see if I have actually improved the views, and whether I need to go further. I knew my collimation was out because I could see it in the eyepiece - diffraction rings were arcs and seeing blur seemed to spray out to only one side of the star. So I would like to see if that has improved now. Also I would like to try some lunar imaging like the rilles around Gassendi - possibly I will be able to draw out more detail now.

I think it might be easy to get caught up in the pursuit of perfect images of Fresnel rings without paying due attention to what the scope is doing in focus 😀... 

Also I need to wait for a really stable night - tonight's work was done in gusts of wind and drifting cloud, so it was hard to judge what effect I was having. For the finer adjustments I need to wait for much better conditions.

[Captain Scarlet]

43 minutes ago, Ags said:

@vlaiv thanks for that - that was quick work! It makes my current collimation state look "not good".  ...

A better description would be “not bad, nearly there.”

”not good” is when the central spot is on or even outside the rings, making a tear-drop, and the at-focus view of a star is not even a splodge, it’s an actual line. I re-assembled a Mak recently and have been through all the stages of terrible collimation, and made sure to see what they all looked like.

You are nearly there...

[Ags]

I tested out the adjusted scope tonight. The night was very promising by my Bortle 8 standards - I could see all the stars in the question mark in Leo and I could see two stars in Canes Venatici.

I started with Arcturus - I am stunned by how bright first magnitude stars are in the C6. I was pleased to see the star surrounded by several concentric and complete diffraction rings - to my eye collimation was perfect. I zipped over to Izar, and it was much better resolved at 150 magnification than before my collimation tweak. The stars were also nice and round, so overall a much more pleasing image. I also noticed I could use higher magnifications post-collimation without the target stars going dim and soft.

I ran through a few globulars - M3, M53 and M13. I did not detect an improvement in M3 compared to my observations pre-collimation. I might have been able to pick out some more of the cluster's stars, but if so that was probably due to the particularly dark skies tonight. But it was my first look at M13 with the C6 and it was phenomenal!  So many stars... 😀

I realized earlier tonight that M94 is really close to Cor Caroli so I decided to try and bag M94 (no, not a test of collimation...) It was very awkwardly positioned at the zenith, but I found it eventually (the ST80 and Berlebach Castor make a great combination for scanning a region of sky). I couldn't make out any structure in the C6, but could at least see the bright fuzzy core. 

I then moved down to the Double Double but my C6 still struggles with this one. In moments of good seeing the pairs are split but it is a struggle - more so than with my 4 inch Maksutov. Maybe I need to wait for this one to get higher and away from the rooftops.

A pleasing evening all in all! Hopefully in a few days I can try image Gassendi and see if I can pick out the rilles more successfully with my adjusted telescope. 

Edited April 20, 2020 by Ags