Collimation saga. Worth it?

[Welrod50]

So today, having a valuable day off work, I decided to finally get round to collimating my SW 130 f900 Newt. Have not bothered thus far as, looking into the EP holder, the spider veins are all equal and the secondary looks central. My eye peering back at me is in the middle. Star tests have revealed that the circles are concentric and my overall views have been good and sharp. 'Why bother then' I think. But, being a curious sort and having a desire to fiddle with the scope I decided to check the collimation anyway to see if I can tweak it a little and improve matters. The collimator is collimated, scope is nicely cooled (stored in the spare room which is always notably colder than the rest of the house) and with screwdriver and allen key in hand off I go.

Timely here to point out that it is a very straightforward procedure and not to be feared by anyone. Lots of vids on You Tube and it really is easy!! I have also taken the primary out and carefully measured the centre and put a tiny dot on the mirror to aid with centralising the beam. Spot on!!

So I adjust the secondary - bang on first time. I then take out the collimator and put it back in again to make sure all is well. The dot is off (?) So I adjust the secondary again, and do the same again. The dot is off. Now, the collimator is collimated and placed in and locked up in exactly the same way each time so I know this is correct. I even rack the focusser in and out and the dot remains in place. Third time - same again. This time I unlock everything so the screws are not exerting a differing force on the collimator (the OTA is level also and collimator upright facing the ceiling and has been throughout) and repeat. This time the dot remains bang on when I remove and replace the collimator. 'Phew' I think!

Primary mirror adjustment - bang on first time. Tighten the screws up gently and all looks good. Red dot smack bang in the centre of the centre hole of the collimator. Re-check red dot in the primary and guess what - it's off the dot in the middle by half and inch. So I remove the collimator and put it back in again in exactly the same place, whereby the red dot is again in the centre of the primary, but not returning to the centre of the scale in the collimator.

You can see where I'm going with this

After almost two hours of faffing about, the dots now all line up after the collimator is removed and put back in again, but the view into the EP holder shows the spider veins to be slightly decentralised, although I can still see the primary mirror clips, so it is only slightly 'out'. I trust the collimator and also trust my technique and having looked at a far distant lamp post at 8mm on my zoom EP it is nice and sharp - but looks no different than before. In fact the only difference is the view through the EP holder seems to show the spider veins and my own eye peering back at me slightly decentred compared to before.

Thanks for reading my rant. Is it worth it? Does anyone have any similar experiences? I should add that the primary is flat and the scope is not a parabolic mirror. I don't know if this makes any difference? As I say, the view again is nice and sharp. Weird eh?!

Peace and clear skies all.....

[Moonshane]

I never use laser collimators although have done in the past and for 'barlowed laser' techniques (google this) it's great for the final primary collimation.

I tend to use a Cheshire / sight tube eyepiece which works well for me even at f4. I suspect that play in your focuser drawtube might be the issue here.

You are quite right that it's a simple process not to be feared but it is worth it for sure.

[madatter]

Hi, i have the 130p and i found the same as you, i think its play in the focuser, if you take the laser out and than put it back it moves in the holder. i try to limit the movement by only using one ajustment screw to remove the laser

[Moonshane]

the important thing is try to replicate the position of an eyepiece when collimating (especially if there is play in the focuser). keep everything orientated the same way and if you use a thumbscrew with an eyepiece then use one with the collimation tool. in the end you just do your best and after all, you look through an eyepiece not a laser.

[cs1cjc]

Any play in the focuser or adaptors and any lack of collimation of the laser itself will render laser collimation inaccurate or unreliable. Passive collimation, the barlowed laser technique and indeed visual observing are insensitive to these errors and collimation is important for high power viewing...

[Welrod50]

Cheers for the input. I tried using one locking screw, but it was evident in my OTA's case that this had an adverse effect on the angle of the collimator (way out). I also thought about replicating the position of the EP, but my thought process being that the mirrors need to be aligned and the EP's will all be slightly different in positioning when swapped around during a night's viewing, so the mirrors took precedence. In the end I made sure the focus/EP tube was right down as this eliminated any play in the mechanism. I may have another look tomorrow night and try to completely re-centre the secondary/spider veins in the EP tube as they used to be, as although they are pretty damn close now and the view through the EP is still sharp, it is bugging me. A proper star test will be the deciding factor (whenever that ends up being).

Would a parabolic mirror be different? Am i right in thinking that these are a little concave? (or not?)

Cheers all

[Moonshane]

all telescope primaries are concave it's just that parabolic mirrors are polished slightly differently in the final stages. there's no difference as far as collimation goes.

the main thing with collimation is that it's a means to an end - i.e. to provide sharp stars, highest contrast and detail and lowest coma. if your methods produce these effects at the eyepiece then it's not something to overly worry about.

[Welrod50]

all telescope primaries are concave it's just that parabolic mirrors are polished slightly differently in the final stages. there's no difference as far as collimation goes.

the main thing with collimation is that it's a means to an end - i.e. to provide sharp stars, highest contrast and detail and lowest coma. if your methods produce these effects at the eyepiece then it's not something to overly worry about.

Mmmm I see what you're saying but I can't help but wonder if I've made a problem where there wasn't one before. I'm going to have another go tomorrow and try to re-centre 100% the secondary through the EP tube, as it is about 10% decentred at the mo. As for the primary, it certainly looked very flat when I had it out, so the curve must be very gradual indeed. To be fair, the primary was really clean and I was impressed as I was at least expecting a bit of atmospheric residue. I will report back tomorrow all being well. A bit of a note to others planning on collimating - please don't be put off by this thread!! I reckon most larger aperture instruments will need doing at some point and it is actually really easy. But I would add that if your star test looks good (as mine did) then perhaps put it off until you know it needs adjusting.

Watch this space ..................

[Moonshane]

logically, it must be concave as it reflects the light gathered by the whole diameter at such an angle as to 'shrink' the light cone to the size of the secondary within less than a metre.

[Astro_Baby]

Well a few points from me.........

First off lasers seem to cause new ies the most aggro. They can work well but generally only if the laser is good qualty and, most crucially, the scope has a solid focuser. Most off the shelf scopes have not terribly great focusers which have slop in them.

The biggest step up in collimation is not a collimator or incredibly clever collimation technique its having a focuser thats rock solid.

On a smallish scope like yours, good as it is, the focuser will never be up to snuff for really suer accurate collimation.

Secondly.....Lasers cant really help with positioning the secondary mirror. My exoerience is always that while your messing with a laser adjusting the secondary tilt screws there is always a danger of the mirror rotating slightly and coming out of line with the focuser. Thats an advantage of a Chesire, you can keep an eye out while your adjusting the tilt that the mirror doesnt rotate around its central screw.

Thirdly, as your scope is a relatively slow focal ratio it will be quite tolerant of collimation errors and in any event the ultimate test of collimation is a good star test. A decent star test will tell you much that collimation with a collimator, of any hue, ever can.

Finally, you asked if its worth it. Well how much have yo learnt ? My bet is quite a bit. How much more will you learn from redoing it ? my bet again would be qute a bit. So your learning which is good and sooner or late you will have to collimate amyway so yo may as well learn it now. Its always tough the first few times but in no time at all you will ge it down pat, you'll start to understand what you can live with, whats worth worrying abot and what isnst. So my take is ...yes its worth it and well done for getting stuck in.

[mikeyrea]

Hi Welrod

As it happens I was peering at Jupiter this evening (after stargazers live, of course) and was rapidly coming to the conclusion that all my EPs were rubbish. I couldn't get the planet really sharp and the moons were looking spikey. If I unfocused, the planed morphed into a nice round doughnut, so I was pretty sure that the collimation was ok. However, before I blew my life savings on a set of Naglers I thought I'd throw in my laser collimator to double check.

Turns out the thing was a bit out as the red spot was right out at the edge of the wee target. Not wanting to totally muck everything up I pulled out the instruction manual for the 200P. This was on page 3 of 5, if you discount the non-English language sections (although the English is not great). As the manual also describes the set-up and maintenance of another three different scopes and also a different mount I was relieved to find there was actually a relevant bit and it was useful, although I skipped the bit about waving your hands in front of the OTA(?).

I unlocked the screws for the primary, got it nice and centred, but noticed that as I tightened them up again the dot began to wander. Was able to keep it in the middle of the target by screwing each one a little at a time, balancing the offset from one screw with the tightening of the next one.

Lo and behold, nice sharp images of planet and moons (when I remembered not to breathe on the ep) so my savings are safe for the time being.

Not sure if this helps but if was definitely worth it for me!

[Moonshane]

I generally remove the locking screws as they cause more issues than they solve.

[Mr Q]

I hope you learned your lesson. NEVER attempt a collimation without first seeing if the scope needs it. A simple star test that takes only a few seconds would have shown you that all was well. Now more time is needed to get the adjustments back where they were.

[Welrod50]

Thank you all for your kind and helpful input

I have this evening, spent almost an hour re-doing it and have to say, I'm rather pleased I did. Before I started I put the collimator in again and it was a little off, so I undid the secondary and started from scratch. The EP tube does indeed have some play in it, but what I noticed quite early on was that when right down or right up, it moves ever so slightly, but mid range (and where the EP's usually focus) it has very little play and the dot was very stable and only slightly off centre. I took great care to lock the collimator in centrally and after about ten removals/replacements established that it does sit ok in the EP tube and locks in bang on. So, readjusted the secondary and also established that you can tell if it's off kilter not only looking through the focuser, but there is some sort of notch on the mirror housing which sits in when the housing is correctly in place and made life so much simpler as its possible to feel when it sits properly. All good. Small adjustment then to the secondary and the laser dot is in the middle of the collimator straight away, so no need to adjust the primary. I have to say, i think i was virtually bang on last night, but having just star tested it again (and had half an hour nosing at Jupiter and M42) I have to say I am of the mind that the views are a little sharper than they were before I ever started, so overall I am most pleased with the result.

Worth it? Oh yes! And a learning curve along the way which although not overly steep, was insightful

One final note to any other SW 130 owners out there, the primary is very robust and well assembled and although small compared to other larger scopes, it is solidly built and I don't now think I need to collimate again for a while.

Cheers all and clear skies