Collimation: 12" Newtonian secondary mirror position question

[vanwonky (Dave)]

Okay, so I finally received my Hotech collimater and went to work. Got the laser pointing on the center mark on the primary by tweaking the secondary (thanks Bobs knobs) and then lined up the primary so the laser was returning correctly as indicated on the collimater faceplate. Job done - easy! Too cloudy to do a star test, time for a beer! Hmmm, lets have a bit more of a read about collimation just to check.

Damn, OK now I am confused. I think I am reading that I need to make sure the view from the focus tube (using the 35mm canister trick at least) is exactly centred in the secondary mirror. I know I can use the centre screw on the secondary to position it a bit higher or lower to try to do this as it does look like the reflection is a bit higher than centre.

So my question is: despite the laser collimater saying all is good do I need to do the above adjustments and if not am I decreasing the efficiency of the scope with reflections off the primary missing the secondary? BTW I can see all the primary clips evenly if that means anything with this issue. I swear I have only had one beer

Cheers
Dave

[pgc hunter]

For best image quality, you'll want your axial collimation (getting the primary and secondary aligned with the focuser axis) to be dead on, by adjusting the secondary so that the laser hits the primary center dot, then adjusting the primary, which is what you've done.

A secondary that's slightly off-center won't affect image quality to a noticable degree, as long as you can still see the entire primary reflected. Your last paragraph has me confused - you're implying that part of your primary reflection is missing the secondary, then immediately you go on to say that you can see all the mirror clips?

If you cannot see the entire primary reflection, you are effectively reducing the aperture of the scope. If you can see the entire primary reflection, even with the secondary slightly off-center as seen through the focuser, you won't lose image quality but the field illumination might be uneven, but visually you won't notice it.

[vanwonky (Dave)]

Yes I can see the whole primary evenly through the focuser if the clips are to be used as a guide. So from your reply I gather that I have things going OK but would be better to get the secondary dead on centre to have the best setup possible?

[mental4astro (Alexander)]

There may be also a misalignment of the focuser too. You can "achieve" collimation just by tweeking the secondary and primary, but the focuser is often overlooked, resulting in still unsatisfactory collimation. Frustrating if you've dumped a whole pile of cash on a new tool.

I've had a little issue with collimation too. No mater how well "collimated" my 17.5" was, I always had a tiny tail come off the brighter stars. I've also tried different types of colliamtion tools, yet they all gave the same result. If there was a problem with the primary, this would have been noticed in variations to the image across the FOV in and out of focus, but the orientation of these tails was unilateral.

I then decided to check my focuser. Using nothing more than an old Kodak film canister, I made a little hole in the centre of its lid & bottom. I also marked the centre of the secondary, with its off-set that I've built into the scope. I stuck this canister in the 1.25" size eyepiece hole, and looked through the holes to see where the secondary was. Guess what? The focuser was off alignment by a tiny amount. I'd say two degrees. That was all that it was. The focuser's alignment was adjusted, and those little tails have finally disappeared.

The focuser is often never suspected as a collimation issue. Sab has suggested this too.

[vanwonky (Dave)]

Thanks Alexander. I did make myself the canister tool as well and I think that is where it all went pear shaped! I may have been totally happy in my ignorance thinking I had all spot on.

I will keep the focuser in mind but I guess that the star test will be the decider. Although knowing myself I will still have that little niggle in the back of my mind knowing that everything may not be "perfect" even if the star test is fine.

[astro_nutt]

Hi Dave.
While your laser is shining a dot on the primary via the secondary, move the focuser fully in and out and see if the dot moves on the primary. If it does then the focuser needs collimation.
Cheers!

[vanwonky (Dave)]

Originally Posted by astro_nutt

While your laser is shining a dot on the primary via the secondary, move the focuser fully in and out and see if the dot moves on the primary. If it does then the focuser needs collimation.

Thanks for that advice. I think I have tried this during my many goes at collimation and if I remember it was OK but I will give it another go. I did however notice that the returning laser on the collimator faceplate would go out of alignment when the focuser was moved fully in or out assuming I started collimating with the focuser at a halfway point and that showed to be correct there. Would this also mean the focuser may be out?

Unfortunately for me (but not our dams) we have had a long run of rainy days and cloudy nights so a proper star test has been out of the question.

[ngcles]

Hi All,

Yep plenty of good advice here so far but I'll just add this snippet: The aim of adjusting the focuser in this way is to ensure the draw-tube on the focuser is parallel to the light-beam produced by the 'scope and that consequently the eyepiece elements are square to that incoming beam.

It's easy to see that if it is skew, so too will be the view!


Best,

Les D

[vanwonky (Dave)]

Thanks for the advice so far everyone. I managed to get the scope out last night when we had 10 mins of clear sky of sorts. The scope was collimated using the 35mm film canister at this point.

Without waiting for the scope to cool the star test showed an even circle with the black (secondary) centered correctly at both ends of focus. I could not however see any 'Airy Disc' that I have read about, mainly turbulence. Saturn looked pin sharp! So should I be happy?

These images describe more accurately what I meant in my initial post about position of the secondary. This from the website - "You should note that fast scopes of f5 and above are less tolerant of collimation errors and they also show slightly offset collimation patterns."

http://www.astro-baby.com/collimatio...ollimation.jpg

http://www.astro-baby.com/collimatio...mation%202.jpg

That is what I am seeing and concerned me initially i.e that I could not get everything centred. This seems to explain why. Any thoughts.

BTW I got the laser collimater out after the star test and it (as always so far) didn't agree that the collimation was spot on - the secondary being out by a fair bit!

Cheers
Dave

[jjjnettie (Jeanette)]

Ugh, I'm having issues collimating my F4 newt atm.
I NEVER had this much trouble with my 10"Dob, I only ever used a film cannister to collimate it.
F4 is a PITA.
The laser says yes, it's all good, but the images so far show bad coma on one side and minor on the other.
Next I'm going to try it with the laser at the focus point for the dslr and see if that makes a difference.

[vanwonky (Dave)]

Argh!! I feel your pain. I know things need to be more accurately collimated for astrophotography so that must be extra painful. I have ordered a chesire eyepiece as well just to have a third opinion. Hopefully it will match one of the others measurements just for some piece of mind.

[astro_nutt]

Hi jjjnettie.
Out of curiosity, have you checked that the mirror isn't pinched in the mirror cell? Also, I have seen a scope with one of the spider vanes loose and it moved a tad once you started looking at things. Hope this helps.
Cheers!