Collimation and mirror clips

[MarkF]

Hi All,

I have a Newtonian f8 114mm reflector which came with a collimation cap, small hole in centre surrounded by a wide ring stuck to the inside of the cap.

I understand the principle of making sure the circle is centred when looking through the pin hole which it all is. However should I be able to see the three primary mirror clips throughout the range of the focuser? ATM I have to wind it almost all the way in before I can see all three clips evenly spaced.

I am guessing I need to tweak my secondary ever so slightly to see all three clips regardless of where the focuser is as I suspect the collimation is only tight at the point where the focuser is wound almost right in.

[kniclander]

it doesn't matter how far in or out the focuser is - usually, you would wind it in/out just so you can just see each mirror clip - if it's too far out and you can't see them at all, you can't do it; if it's too far in and you can see too much of them, it's hard to tell if they are all the same size.

[NGC 1502]

Hi Mark, It sounds like your scope is fine. It's quite normal to only see the mirror clips when the focuser is racked well in. If as you say the mirror clips look evenly spaced then " if it ain't broke, don't fix it " Cheers, Ed.

[MarkF]

cheers for that, my only concern was that i can see 2 clips wherever the focuser is only all three after winding it in.

[kniclander]

if there's a point where you can see two clips and not the third that suggests that the secondary is not quite in the right place (or your focuser is not square but that's a whole other story...)

[MarkF]

if there's a point where you can see two clips and not the third that suggests that the secondary is not quite in the right place (or your focuser is not square but that's a whole other story...)

That is indeed the case and why I thought my secondary was out a bit, been like that since I bought it though probably why I can't see too much ;-)

So I guess i need to adjust the secondary to get all three clips in view at all times?

[kniclander]

yes - if as you rack in and out, you lose one and still can see the other two, that means that your secondary is not in the right position.

[MarkF]

yes - if as you rack in and out, you lose one and still can see the other two, that means that your secondary is not in the right position.

Thanks for that, time for some swearing and sweating then

[NGC 1502]

Hi Mark, I did read on your original post that " I have to wind it almost all the way in before I can see all the mirror clips evenly spaced " and thought all was OK. But if as you rack the focuser out one clip disapears before the others, then adjustment of the secondary mirror will be needed. They should all "disapear" together. Keep at it, take it slowly, have a break for a cuppa, and you will get there !! Ed.

[MarkF]

Cheers Ed, its very difficult to explain on a forum sometimes, will try it over the next few days.

[LukeSkywatcher]

If one of the clips is vanishing before the other two then the secondary is slightly out of alignment. The fact you can see all 3 clips anyway is a good indicator that your collimation is ok............just have to centre the donut then.

[MarkF]

I'm going in wish me luck!!!!

After seeing out of focus planets and double vision last night I need to really sort this out. So far realigned the secondary so all three clips visible throughout travel of focuser, now that was the easy bit, getting the secondary centred on the primary and the doughnuts lined up is proving a task. just a case of keep twiddling til its done? Any technique?

[kniclander]

if you can see all three mirrorr clips and as you rack in/out the disappear all together etc then your secondary is centred on the primary. for the primary, do you have a cheshire? if not, i don't now how the do the primary but I am sure someone else will.....

[MarkF]

if you can see all three mirrorr clips and as you rack in/out the disappear all together etc then your secondary is centred on the primary. for the primary, do you have a cheshire? if not, i don't now how the do the primary but I am sure someone else will.....

I can confirm my secondary is centred on the primary, it is a fixed arm so no adjusting its position in the tube. Whilst adjusting the primary the image of the secondary moves off centre, I can put it on centre easily enough but I am using a collimation cap through which you see concentric rings in the secondary and the secondary should form a concentric ring on the primary, they all need to be even ie. like a ringed doughnut. So if get the doughnut image the secondary is not central in the primary, if I centre the secondary in the primary the doughnut is oval. Does that make sense???

I am guessing I need to adjust the primary off centre with adjustment then bring it back to centre with another adjuster to achieve concentric rings.

[NGC 1502]

Hi, Having sorted the secondary, as in post #13, then using your collimation cap, get the reflection of the secondary centred as best you can (using the primary adjustments only) then finish off with a defocused bright star, again, primary adjustment only. The defocused bright star needs to be nice & concentric within the visible rings when it is in the centre of the field of view. Start this process at medium power then at high power, 150x -200x should do it. Hope this helps, you will get there, Ed.

[MarkF]

Thanks Ed, that is how I have left it ATM, the secondary is centre of the primary but a slightly offset doughnut, reading around this is probably acceptable, but then again I am sure it is a flat primary as opposed parabolic so I think it is not that critical.