First time Newt collimation - how does this look?

[JP-S]

I've acquired an 8" F4 Newt for AP as my second scope and I'm still trying to wrap my head around collimation. After watching and reading a bunch of guides and examining various diagrams, it's still not clear to me what things should actually look like when they're good. I think the main thing throwing me off here is the lobey lopsidedness on one side of the reflection of the secondary mirror in the primary as the guides are suggesting that everything should just be concentric circles. Is this the so called secondary offset and is it supposed to look like that?

The first attached image is the view down the empty focus tube, the second image is the best view I could manage to capture through my cheshire combo tube tool thingy. All I've done so far is slightly adjust the primary mirror to align the 'hole' in the middle of the reflective element of my cheshire with the inside of the center spot on my primary. This adjustment was done inbetween the taking of the two photos. I haven't touched the secondary at all as the previous owner of this scope said he'd got it into a good position so I figured I'd check before fiddling with it.

[Piero]

First of all, your technique for aligning 1) the focuser axis against the primary central spot and 2) the primary mirror axis, is okay. You can get a bit more precise, but the technique is sound.

Your secondary mirror is oriented towards the focuser. However, there is a visible tilt error due to the lack of offset, which should be fixed because the primary mirror is currently reflected using the edge of the secondary mirror. To do so, you will need to slightly loosen the secondary holder and tilt the secondary towards the focuser. Doing so, one reflection might appear slightly oval, but that doesn't not matter. What matters is that the whole primary is reflected using the central part of the secondary (not its edge).

For completeness, in the "new model of collimation", offsetting the secondary is achieved by moving the secondary towards the primary and tilting the edge of the secondary towards the focuser. The secondary holder remains centred within the tube. This kind of offset technique is easier and more flexible than the old way of regulating the spider or offsetting the holder mechanically.

I would first try to adjust the secondary tilt using the three collimation screws behind the secondary holder. You might find that you need to adjust the central screw.

Don't be afraid of collimating your telescope. Whatever error you introduce, can be fixed. It's just down to practice.

Besides, good Catseye central spot you have! They are the best and they offer some advantages (e.g. autocollimator), but that's not relevant right now.

Edited December 18, 2021 by Piero

[Piero]

This was taken ages ago, using a Catseye TeleCat collimator and my 12" dob after playing a bit with the secondary holder. The collimation is not perfect in this photo (in particular the primary mirror axial alignment is slightly off), but you can see that the primary reflection is reasonably within the secondary mirror. This last point is fine for visual. For imaging, try to get as precise as possible.

Edited December 18, 2021 by Piero

[JP-S]

Thank you Piero!

Can you help me understand what part of the visual shows the tilt error of the secondary? Is it because the visibility of the tube where it meets the spider vanes is more present on the left of the photo than on the right? Or does it relate it something in the center of the image in the reflection of the secondary in the primary. This is the part of collimation that confuses me as even in your own photo it's not obvious to me how your primary axial alignment is off.

Nonetheless I will attempt to adjust the secondary and maybe things will start to become more clear to me as I see how those adjustments change the visual I'm seeing. To adjust the tilt do I need to loosen the central screw and adjust the secondary by hand or will the loosening of the screw in itself perform a slight adjustment? Or should I instead be using the three thumbscrews also present on the secondary holder?

Thanks again for your help. I will have a play tomorrow and see what results I get. Undoubtedly I will be back for more help 😅

[Piero]

Can you help me understand what part of the visual shows the tilt error of the secondary?

In your first image, the secondary mirror is "more present" towards the left. And you can see that the reflection of the primary mirror on the secondary mirror touches the secondary mirror to the right. It is possible that the previous owner was more concerned about obtaining circular shapes. The issue with that is that the edge of the secondary mirror is the weakest part of the mirror and should be avoided if possible. Your telescope is fast (f/4) and is meant for AP. Therefore, it makes sense to offset it a bit at least and avoid using the edge of the secondary mirror anyway.

 

To adjust the tilt do I need to loosen the central screw and adjust the secondary by hand or will the loosening of the screw in itself perform a slight adjustment? Or should I instead be using the three thumbscrews also present on the secondary holder?

First of all, I would just play with the three thumbscrews behind the secondary holder. If this is not sufficient you might need to play with the height of the secondary by regulating the central screw. 

 

This is a nice collimation guide: https://www.cloudynights.com/articles/cat/articles/collimation-and-the-newtonian-telescope-v4-r2599  .

[DhamR]

I just thought I'd supplement Piero's excellent comment with my PROFESSIONAL* editing of your picture:

(* caveat: I am a massive noob to this, but kind of "get" collimation).  You have everything lined up, but you're not using the best bit of the secondary in the centre of the view.

Edited December 20, 2021 by DhamR

[jetstream]

My 15" f4.8 uses the "new model" ( no offset) of collimation and the 24" f4.1 uses full offset. One of the issues with the new model is the need for larger secondary and another is a slight loss off accuracy with some DSC. The views through either are very good. The 24" has stunning accuracy with the Sky Commander easily re finding objects in my 10mm ortho.

If you want to test you and your scopes ability to collimate, use the Catseye dual hole autocollimator IMHO. I test my reflectors using one but for visual I normally use a combination of Glatters Laser, the Catseye sight tube and separate Cheshire.

[jetstream]

Just saw the new wifi Catseye Cheshire, guess what I'm buying for the 24"... The Catseye cheshire is easier to obtain accuracy with than Glatter TuBlug, but the convienience of the latter is super.

[Pixies]

7 hours ago, jetstream said:

Just saw the new wifi Catseye Cheshire, guess what I'm buying for the 24"... The Catseye cheshire is easier to obtain accuracy with than Glatter TuBlug, but the convienience of the latter is super.

Why does the Catseye website look like it's  1995 again! Even the pictures download sooooo slowly, like I'm using a modem! 

The wifi Cheshire looks nice, though!

[JP-S]

Well I let loose and starting twiddling all the knobs to see what does what. I made a big mess and after tinkering for a few days this is the best I've been able to get it back to:

I'm sure this is much worse than when I started but it's still not clear to me exactly what needs to go where. Can someone look at the image below and explain it to me? I've added coloured circles for reference. Do I want to center the red circle inside the yellow circle? Or the blue circle inside the green? Or are these not the important circumferences?

The other thing I'm struggling with is trying to gauge things by eye. My cheshire/sight tube doesn't quite let me see the whole mirror (one of the above pics is through this tube) and when just looking straight down the empty focuser I get a lot of variation with my perspective of the green/blue circles as I can never really be sure my eye position is central. I've drilled out the center of a dust cap so maybe this will help going forward. Thanks.

[Spile]

I can see tilt/rotation error. https://astro.catshill.com/collimation-guide/. I’ll put some overlays on the image when I am back at the PC.

[Ricochet]

21 hours ago, JP-S said:

I've added coloured circles for reference. Do I want to center the red circle inside the yellow circle? Or the blue circle inside the green

Everything you have marked should be concentric. The only thing that is not concentric is the shadow of the secondary mirror, which is the dark shadow outside of the red circle, which will be offset towards the primary mirror. 

21 hours ago, JP-S said:

My cheshire/sight tube doesn't quite let me see the whole mirror

Try extending the focuser and/or pulling the cheshire out of the focuser until you can see the whole secondary, if it is one like the red FLO premium cheshire that can be clamped at any point along its length. 

[Mr Spock]

When I did my secondary I used a (home made) collimation cap. I loosened all the fixings to the spider - a heart in the mouth moment for sure - and proceeded to align the now loose secondary. Firstly to ensure it was directly under the focuser and correctly rotated (my Skywatcher 250 secondary was off axis when it arrived). Once that is done I then got the primary located exactly in the secondary. Once I had that done I put my laser in, and it shone on the inner edge of the primary doughnut, showing how good the collimation was just from sight. A few tweaks and it was good to go.

The primary then is a doddle. First adjusted with the laser, then with a star test.

The difference in image quality between a nearly collimated system and one that is spot on is dramatic. It is worth the effort!

As an aside, I'd fix your twisted spider vane while you are at it. That can't be a good sign. Also check the length of the vanes to ensure the secondary mount is central.
 

[wookie1965]

https://web.archive.org/web/20200223041214/http://www.schlatter.org:80/Dad/Astronomy/collimate.htm

Use this as a step by step as you will stop all reflections meaning you can go circle by circle to get everything lined up.

As said make sure your Vane`s are all the same length and tight before starting. 

[Spile]

This is my interpretation of the error. Usual caveats about parallax errors and interpreting images from a camera. 

Ref https://astro.catshill.com/collimation-guide/

Edited December 25, 2021 by Spile

[JP-S]

Ok, I think the penny's finally dropped for me after your comments. I want to have the secondary mirror round and concentric with the edge of the focuser (this part I was missing) and have the reflection of the primary concentric with the edge of the secondary mirror. If these 2 conditions are met then that's it for secondary mirror positioning, correct? The results of my last session are below.

Collimation cap view:

I added the concentric circles after the fact using Al's collimation aid. Does that look like a lot of error or is this good enough? It's honestly difficult for me to tell that there's error without the circles and I certainly can't tell by eye when using my collimation cap/dust cover. I'm sure I'm also getting a certain amount of variation from the camera given I'm just holding my phone up to the cap as best I can. I'll give it another crack though if need be.

Cheshire/sight tube view:

Is there any thing wrong with this view? All the mirror clips are visible and seem to disappear at the same time if I rack the focuser in/out.

Mr Spock mentioned I may have bent/twisted vanes. I am a bit concerned as I did go a bit overboard with the tilt adjustments when I first started fiddling with the secondary. Tracing lines suggests the vertical vanes are off:

Is this something to worry about? How should I go about remedying it? I haven't actually attempt to square the focuser or adjust the vanes. The manual for this scope (Skywatcher Quattro 8") suggests the secondary is positioned 'full offset' so I wasn't sure if the vanes all need to be the exact same length from the tube to the center screw like the collimation guides suggest. I did a quick measure and it looks like they're 1 or 2mm out from one another.

[Ricochet]

27 minutes ago, JP-S said:

|If these 2 conditions are met then that's it for secondary mirror positioning, correct?

No. After you have the secondary centred, you must adjust the tilt so that the cross hairs at the bottom of your cheshire intersect with the doughnut in the centre of the primary mirror.

Once that is done, you can collimate the primary using a collimation cap. You adjust the primary so that the dark hole in the centre of the cap appears to be in the centre of the doughnut.

32 minutes ago, JP-S said:

Tracing lines suggests the vertical vanes are off:

Is this something to worry about?

No. It will make almost no difference aside from slightly modifying the diffraction spikes on bright stars. It is far more important to have the mirrors in the right place. The vane in question is the right hand horizontal one that appears thicker. If this is because the vane itself is twisted you can attempt to straighten it. If this is because you have a wire fixed to that vane for a secondary heater the only improvement would be a thinner wire.

[JP-S]

Oh right, yes that's thicker due to the wire from the secondary heater.