Accurate collimation for RC8 Primary mirror

[Carl]

6th night now i've been clouded out whilst trying to collimate my RC8; not a happy boy.
So here's the question. I understand how you would use an artificial star to collimate secondary mirror, but how would you do an artificial star test to collimate the primary on a RC8?
Do you place the star at different corners and check for correct concentrict circles when out of focus or can you still use the bahtinov mask technique

I just want to get my collimation done so i can get back to imaging once i get a clear night.

Regards
Carl

[bmitchell82]

... its not a easy task my friend made even harder if you don't have somebody who knows exactly what they are doing standing beside you!

[Visionoz]

See here: http://deepspaceplace.com/gso8rccollimate.php

HTH
Cheers
Bill

[bmitchell82]

thats the kinda thing you have to do but be careful Grahame and I followed that to the T and its backwards. not only in method of primary collimation but other wise.

the focuser is Fixed in position. Collimate the secondary to the focuser then the primary to the secondary that way they are all in alignment or optical alignment the mechanical alignment is upto the original quality and assembly.

Like when i Collimated your Vixen Bill with the auto collimator you then star test it and collimate the primary to the secondary.

[dugnsuz]

Following this thread with interest.

Is a simple Newt-type laser collimator adequate to use with a GSO RC8?

Doug

[mldee]

I'm also following with interest.

FWIW, I have a very carefully centre'd and collimated laser as per your photo, use it with great success (+Barlow) on my 8" f5 Newt. Thereafter, sharpest pics I've ever taken.

I tried using it with one of those Hutech 2" centering adaptors, to centre the secondary mirror return on my RC8, seemed to work a treat but when I looked at a star that night, total disaster.

Spent 30 mins re-collimating the secondary on the star and got it OK, I'd still like to go back and do more, perhaps eventually even the primary, but for the time being, I'll just follow the thread....

[dugnsuz]

Looks like the hard work's going to be done under the stars in the dark!!
Oh well!! Lucky my Bob's Knobs arrived today - hope that will make playing with the secondary a bit easier. Allen key shenanigans in the dark over a mirror are a recipe for disaster for me!

[mldee]

Re Allen Key shenanigans, when I recollimated that evening, I noticed the next day that my pics all had an extra set of diffraction spikes.....

It turned out to be the Allen key still sitting in one of the screws - lucky me!

[TrevorW]

Two heads are better than one when collimating these RC's

[stevous67]

After much experience doing collimations with an RC and CDK, I can confirm a laser is rubbish for setting collimation. The James Mchugh method works, and now a second method definitely works and is easy. For imaging setups, use the camera to set collimation. Place the scope on a open cluster and defocus camera to donuts all over FOV. Defocused stars must populate the FOV, but should not be a crowed FOV. Watching the result on your laptop whilst adjusting you secondary, aim for central donuts to be round, and outer donuts to be distorted evenly and equally around the centre. Outer donuts need to be pointing inward to the FOV centre. Upon achieving a balanced arrangement of Donuts over the FOV, upon returning the camera to focus, you will have achieved the best collimation from just adjusting the secondary. The process works even under unstable conditions. Only where the primary is seriously misaligned, will you have to centralize the primary to point to the middle of the secondary. Some iterations may be necessary to get that part right.

[dugnsuz]

Thanks Steve - becoming apparent that the best collimation will be done under 'real' conditions under the stars.
Cheers

[mldee]

As a totally non-qualified person, I do feel that as long as one respects ALL the laws of optics and physics in general, many of the items mentioned in this thread, and many of the posts, are correct.

Firstly, Doug's question re the laser collimator, I still feel that it has a use, as long as one respects the laws above and makes sure it is accurately centred in its beam, and collimated for zero eccentricity and that it mounts accurately and firmly in the focuser mount.

It should then be capable, once the focuser assembly is also correctly mechanically tested as being aligned, of showing a true picture of the focuser to secondary mirror situation. And only that, as Brendan points out.

The next step, as Brendan says, is to then get the primary mirror aligned correctly with this focuser/secondary mirror unit. This needs an artificial star, again, under correct conditions of distance and size, etc, or alternatively a real star with the mozzies and darkness......I prefer the artificial star gadget, which is basically a laser-drilled very small hole I bought that clamps on to the front of a small one-LED torch, which is then clamped to a sturdy pole some distance away, preferably in a dark location. I use my backyard shed and have the scope about 75m away in the kitchen, looking though a window at the shed door opening.

And yes, there will be some to'ing and fro'ing to get it all right, but one should finish up with a well aligned RC. I realise I've oversimplified things, but the point I am trying to make is that none of it is impossible rocket science, but it does require EACH STEP being correct in all optical detail before proceeding to the next.

I'll be the first to confess that I have yet to do all this with the RC, I did it with the C8s and Newt and it worked out OK. I'm looking for comments before I take the big step with the CF RC, which to date is my favourite scope due to its optical and physical solidity, and performance potential. It's the sweet spot for my EQ6.

[garymck]

Hi,

used to have a VC200L (the disposal of which was the stupidest decision I've ever made!) in which collimation was absolutely critical. Borrowed a Takahashi collimation scope and had perfect collimation in 5 minutes. These are the recommended tools to collimate most of the super expensive CASS variants. Anything else is a waste of valuable imaging time.

cheers
Gary

[mldee]

Obviously a subject I'm interested in...three posts in the one thread! My apologies to the OP if I'm hijacking, but I hope this is all of some use to RC owners.

I just did a Sunday morning Google and yes, the Tak collimation scope (TEC100) seems to be the tool of choice/ $270 at OPT. Best article on collimation I could find was http://tinyurl.com/89eonq8

I also read again on the Hutech SCT collimator ($399 at OPT) but compared to the simplicity of the TCE100, there doesn't seem to be much reason to pay $100 more for a more complex gadget to do the same thing. Although it does have more fancy lights.......Perhaps Marc may like to comment, as I recall he bought one sometime ago for his SCT.

So to cut a long story short, I may take Gary's advice and invest in a TCE100, as I am very happy with the CF RC, and hopefully one day will upgrade to a 10 or 12". so the TCE would still be useful.

One final question: I also read that the GSO RC primary mirror is fixed and that the rear adjustment screws actually move the rear focuser attachment plate for alignment with the secondary and not vice versa. Anyone know the facts?

[davewaldo]

Hi guys,

I'm yet to attempt collimating mine (its quite good now) but I like the sound of this technique using a Bahtinov mask and see no one has mentioned it yet: http://deepspaceplace.com/gso8rccollimate.php

What do others think?

[Visionoz]

Dave - I did - in post#3 above - hehehe!:D

Cheers
Bill

[Marke]

I have a Tak Col scope and its a breeze doing the RC10 with it , I only needed to touch up the primary and all good to go.

[davewaldo]

Nice one Bill, sorry I missed that! :lol: