Stellalyra 8 inch dob collimation question

[greenery]

Hi guys

I won't get a chance to use my new scope for at least a few days. In meantime, I've checked the collimation per astrobaby's guide here https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/.

The only issue seems to be that the crosshairs of my Cheshire collimator, the centre of the primary and the centre of the secondary are all in slightly different places. Before I do anything, this can be solved by only adjusting the primary, right? And I'm aiming for all 3 to align?

TIA

[DAT]

Good luck and well done for asking, I went straight in and managed to de-collimate even further. Lots of nice people will soon give you good advice on here

[Spile]

It’s important that you do everything in the right order. Start by getting the outside edge of secondary concentric beginning with the offset (a=b). See https://astro.catshill.com/collimation-guide/  Then get c=d and e=f by making small tilt/rotation adjustments. 

[greenery]

Thanks Spile.

As far as I can tell, the earlier steps seem fine already and the mirrors appear circular and concentric.

Beyond that, I'm now confused. Your guide says the cross hairs and primary centre should be aligned by adjusting secondary tilt, with primary adjustment only used to align primary centre and reflected Cheshire centre. The astrobaby guide (which I gather is well regarded) says only use Cheshire for primary adjustment, which will align all 3: "It may be that that the pattern is offset and will show the cross hairs as being adrift from the central circle created by the Primary Mirrors centre spot and that the centre spots are adrift. If this is so then you will need to adjust the primary mirror". Idid think that aligning all 3 with one adjustment sounded challenging.

Which guide is right? Or am I misunderstanding one of them?

 

Edited December 22, 2023 by greenery
Clarity

[greenery]

Sorry, don't know why that's highlighted in white or how to undo.

[Mr Spock]

I've fixed your formatting 

[Ricochet]

 

2 hours ago, greenery said:

Which guide is right

Astro baby is wrong. Cheshire for all secondary steps, collimation cap for the primary. 

1. Use the edge of the Cheshire to get the secondary circular and concentric under the focuser. Insert the Cheshire the appropriate amount so that the circular edge and your secondary appear to be the same size to make this step easier. 
2. Use the Cheshire crosshairs to adjust the secondary to align with the primary doughnut
3. Use a collimation cap to adjust the primary so that the peep hole is centred in the primary doughnut. You can still use the Cheshire here but the crosshairs obscure the dot and doughnut. 

[Mr Spock]

1 hour ago, Ricochet said:

Astro baby is wrong. Cheshire for all secondary steps, collimation cap for the primary. 

That's a wide sweeping incorrect statement. There are many preferred ways of doing it.

 

[DAT]

I have been utterly confused by it all but I must say I had some very sound advice on a thread I started. There are many guides and lots of good advice and they may contradict each other, I struggled to find the right format for the way my brain goes.

I have read the guide you mentioned and got totally confused but I did find the another one suggested to me much easier to follow and I think I have gotten somewhere even though I read it like 6 times and I am going to read again tonight:

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

Thank you @wookie1965 for that link and everyone else who chipped in, I think I got somewhere last night.

Its good to know different ways of doing the same thing anyway

 

Edited December 22, 2023 by DAT

[Mr Spock]

You can mostly ignore the position of the secondary. It isn't likely to be far out. It is flat so as long as the optical train is in alignment it will be ok.
If you insist on doing it correctly then a concentre will allow you to position the secondary in the right place.

The first time I looked down a Cheshire my reaction was what is all that? Cross hairs here, there, mirror clips this and that, circles, shadows and who knows what else. No wonder people get confused.

That's why I use a laser. I can collimate any scope in under 30 seconds. As soon as it stops raining and I can get my scope inside I'll produce a guide. For now the two steps are:

1. adjust the secondary so the laser points in the centre of the primary's doughnut.
2. adjust the primary so the laser in in the centre of the collimator's scale.

I check my 12" before each session - with a laser you can do it in the dark and you don't struggle to reach the primary adjusters - and it's good for x461 on the moon.

The only caveat with a laser is you have to make sure your laser is collimated. Most will be ok but they are adjustable if not - and that's easy to do too!

[greenery]

I appreciate all the input guys. I stayed clear of laser as folk suggested more to potentially adjust. It would be good if somebody could give me definitive advice on how to align crosshairs, primary doughnut and reflection of dark centre dot of Cheshire. When everything else seems fine ie circular and concentric. Possibly I just need to get in there and see what works but there's so many variables it might get worse before better.

Edited December 22, 2023 by greenery
Mispost

[Chandra]

In my case I think that the Astro Baby guide is very correct, in fact it is the one I used in the first collimation of my Newton F/5.9.

In stage 2 (alignment of the secondary mirror with the focuser tube) the important thing is that the figure of the secondary mirror with respect to the circle of the focusing tube, and that of the reflection of the primary mirror in the secondary mirror with respect to its edge, are concentric circles, as indicated by @Spile in the guide he shares. In this phase 2 is when you work with the displacement and inclination in its different axes of the secondary (the primary is not touched in this phase, and more than its center what matters is its edge and the clips that hold it). Once a reasonably correct figure is achieved and the screws are adjusted, under normal conditions we can forget about this collimation phase for a long time.
 

Phase 3 is when the primary mirror must be aligned to the secondary mirror using Chesire, although the use of the perforated cover is also sufficient. In fact, in most Chesire it is possible to unscrew the eyepiece to screw it into a 1.25" pitch barrel, obtaining a perfect collimation cap.

 

On the other hand, a new telescope should not arrive in a poor state of collimation, so it would almost be better not to touch the secondary (phase 2) at the outset, and to verify if, in fact, the figures are circular and concentric. We would then go directly to phase 3. If we do have to proceed to check the alignment from the beginning, we must be patient, the first collimation is always complicated and we have to dedicate a lot of time to it, it is very important to go step by step and verify what is being done and if it is being done well.

 

But for visual observation, "perfect" collimation is not necessary, so it is not advisable to fall into the obsession of achieving it either. You have to consider that there are many parameters that can make collimation NOT "perfect", and in entry level or budget telescopes the margin of error for these parameters is greater, and sometimes we simply cannot control them. A star test that offers a reasonable result is a good indication that your optical tube is working fine.

 

All pictures have been taken during collimation processes with my Newton.

 

Edited December 22, 2023 by Chandra

[bosun21]

2 hours ago, DAT said:

I have been utterly confused by it all but I must say I had some very sound advice on a thread I started. There are many guides and lots of good advice and they may contradict each other, I struggled to find the right format for the way my brain goes.

I have read the guide you mentioned and got totally confused but I did find the another one suggested to me much easier to follow and I think I have gotten somewhere even though I read it like 6 times and I am going to read again tonight:

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

Thank you @wookie1965 for that link and everyone else who chipped in, I think I got somewhere last night.

Its good to know different ways of doing the same thing anyway

 

My offer to collimate it for you the conventional way and talk you through each step still stands. I will also show you how to collimate it with a laser. I am roughly 30 minutes away from you. The decision is yours. I myself use a concentre , cheshire and cap to set everything up correctly and then a Hotech laser everytime thereafter. The laser is fantastic for checking before each session.

[DAT]

49 minutes ago, bosun21 said:

My offer to collimate it for you the conventional way and talk you through each step still stands. I will also show you how to collimate it with a laser. I am roughly 30 minutes away from you. The decision is yours. I myself use a concentre , cheshire and cap to set everything up correctly and then a Hotech laser everytime thereafter. The laser is fantastic for checking before each session.

I think I did a lot better last night, I am going to fine tune tonight and post pictures later so I can get some feedback. I will take you up on your offer but I am busy at the moment and away for a couple of weeks after Christmas so maybe in the new year if that is ok?

[Spile]

I’m not going to criticise another guide but one of the reasons I decided to write my own was because I either found the others confusing, unclear or contradictory. I wanted to ensure that what I had written was accurate which was why I spend a lot of time with Vic Menard (and also @Jason D to ensure this was the case. I am of same opinion as @Ricochet , the Cheshire eyepiece and sight tube combination tool to place the secondary and the cap to correctly tilt the primary. 
I also do not recommend covering the primary mirror when aligning the secondary because as my guide shows the reflection of the primary is a very useful point of reference so why would you want to hide that?

 

[DAT]

Hello everyone, I think I am getting somewhere. After reviewing all the information passed on and reading 3 guides quite a few times I have tried to do the following:

1) Align the secondary under the focuser until it is centred , 2) adjust the tilt until I see a nice circle and 3), get the dot from the collimation tools centred on the doughnut of the primary 

I am confused as one guide said that turning the central screw of the secondary holder clockwise made the secondary mirror move down the ota. I found the opposite. 
I think what I see through the chesire and the cap looks good, I can see the three mirror clips and the dot is in the centre. What worries me is that I felt I had to adjust the secondary holder lots so that the outside screws have ended quite far out . I moved it a fair bit out of the tube until I felt is was directly under the focuser.
Please someone have a look at the pictures and tell me what you think. I would perform a start test but can only see Jupiter and the moon right now. Scope is outside acclimatising just in case. My wobbly hands do not take good pictures and the dot is more centred than appears, I promise 

[bosun21]

14 minutes ago, DAT said:

I am confused as one guide said that turning the central screw of the secondary holder clockwise made the secondary mirror move down the ota. I found the opposite. 

Turning the centre screw clockwise moves the secondary mirror holder up towards the open end of the telescope and anti clockwise moves it down towards the primary mirror. I owned the same scope as you and the three secondary screws were not as protruding as yours. You have tightened the central screw too much as your secondary is not centred in the focuser draw tube. It's too far up towards the open end of the scope.

 

Edited December 22, 2023 by bosun21

[DAT]

1 hour ago, bosun21 said:

Turning the centre screw clockwise moves the secondary mirror holder up towards the open end of the telescope and anti clockwise moves it down towards the primary mirror. I owned the same scope as you and the three secondary screws were not as protruding as yours. You have tightened the central screw too much as your secondary is not centred in the focuser draw tube. It's too far up towards the open end of the scope.

 

Glad you said that about turning the screw clockwise, did not make sense….

I thought something was up because I assumed the screws should not protrude that much, there is clearly something I am missing because through the focuser the secondary does look aligned to me, I am not seeing this. I am coming your way when I can!

[DAT]

I must be close, I have measured as precisely as possible with a ruler (looking through a cap) and positioned the secondary further down the tube. It must have all been looking aligned to me because I think I confused the reflection of the primary with the outline of the secondary and tilt was way off too so it all “looked “ ok to me. I think this is better:

[bosun21]

It looks better than before. I can't see the focuser draw tube outline to check the position of the secondary though.

[Steve Ward]

 

[Spile]

Place a sheet of paper behind the focuser and take the picture with the outside of the sight tube visible because that’s the key point of reference.

The centre screw and three outer bolts work in push pull mode. When you are getting a=b the outer bolts will be loose and you’ll be either moving the secondary towards to primary by turning the central screw anti-clockwise OR moving it away from the primary by tightening the screw. Either way, the next step will be to tighten the outer bolts so the secondary is not “left hanging”.

The edge of the sight tube of the long Cheshire will confirm that a=b but a laser will not.

[Spile]

Looks good...

[DAT]

8 hours ago, Spile said:

Place a sheet of paper behind the focuser and take the picture with the outside of the sight tube visible because that’s the key point of reference.

The centre screw and three outer bolts work in push pull mode. When you are getting a=b the outer bolts will be loose and you’ll be either moving the secondary towards to primary by turning the central screw anti-clockwise OR moving it away from the primary by tightening the screw. Either way, the next step will be to tighten the outer bolts so the secondary is not “left hanging”.

The edge of the sight tube of the long Cheshire will confirm that a=b but a laser will not.

Excuse my ignorance but what do you mean by a=b?

I understand how the secondary is moving and how the outside screws need to be loosened or tightened as the secondary holder gets moved up or down the tube by turning the central screw screw cw or ccw. I think I have achieved this (it is better than it looks in the photo :)):

[PeterStudz]

He mentions “a=b” earlier in the thread with a link to his guide which explains it…
 

 

Edited December 23, 2023 by PeterStudz