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Help - How to correctly collimate a Newtonian... with a twist

Started by RobertM, Nov 08, 2011, 10:44:03

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RobertM

Well, it's about time we had a practical guide on this and my MN190 is well up the creek.  I've never managed to get the collimation spot on, though it has been close.  When the scope was delivered (but I never noticed till too late) it had the following problems:

1) Secondary was nowhere near under the centre of the focuser.
2) Secondary was rotated.

This resulted in a horribly out of centre secondary when collimated (which I could never get spot on).

I've been homing in on correct collimation but it's not helped by the secondary assembly so I've decided to start off from first principles and try to address all the issues over the next fwe nights.

The twist is that as it's a Maksutov Newtonian the tube is enclosed.

This is my checklist:
1) Remove corrector plate.
2) Align the focuser faceplate so a collimation laser bisects the tube.
3) Set the secondary so that it's at 45 degrees to the corrector (initial position).
4) Blank off the primary (so I can unambiguously see the secondary spot).
5) Put the corrector back and check that the secondary spot is centered under the focuser and if not adjust till it is).
6) Remove corrector, and primary blanking and refit corrector.
7) Adjust the secondary rotation ?
8) Collimate primary/secondary normally (laser/cheshire)

Anyone have any comments or ideas, I'm especially concerned about adjusting the secondary rotation as that is just about the most critical.

Any help or advice greatly appreciated.
Thanks in advance
Robert

MarkS


Sounds like a nightmare but your approach looks OK to me.

But how do you rotate the secondary?  On a normal Newtonian you just grab hold of the secondary assembly and twist it.  But you have the whole meniscus corrector in the way.  It doesn't seem right to rotate the meniscus assembly because that ought to have been accurately aligned with the primary during manufacture.

mickw

How far out is the rotation ?

Rather than touch the secondary, perhaps remove the focuser, enlarge or relocate the fixing holes to align the focuser with the secondary.
Growing Old is mandatory - Growing Up is optional

RobertM

The corrector was originally about 15 degrees out and the nut that holds it in place on the miniscus was incredibly tight.  I did manage to loosen it using pump pliers and lots of padding so now rotation is not a problem.  So far I reckon rotation is within about 3-5 degrees of being correct.

I'd prefer not to change the tube assembly and since I can rotate the secondary easily (not that it's loose) from outside the tube it shouldn't be necessary.  The main problem is aligning the secondary rotation in an accurate manner rather than by mk 1 eyeball as there are so many interdependencies.

Robert

RobertM

Things are looking better 8)

I didn't need to move the secondary in or out (I'd done that already on a previous attempt) as it seemed accurate enough.

The rotation did need tweeking and looks to be good.  Now the view through the cheshire looks like I think it should do and agrees with the laser but the proof of the pudding will be in the next image (when ever that is !).

I noticed that rotating the secondary moves the laser image horizontally across the primary so what I ended up doing was repeating this routine:

1) put laser in and adjust primary only.
2) center the secondary spot in the cheshire
3) replace cheshire with laser.  At this point the laser will be either pointing left or right of the primary spot.
4) if the pointer is to the left (looking down the tube) rotate the secondary clockwise to reduce the distance to 1/3rd of the way to the center spot.  If it's right then move it anti clockwise.
5) repeat from 2 till no further adjustment is necessary.

I just hope it performs better than it did.

Robert

The Thing

Hi Robert,

You left out:
0) Collimate the laser

When I had a Meade laser collimator it was way off and had to be adjusted out of the box, not that it was any use for an SCT.
Cloudy? ¯\_(ツ)_/¯ (Gallic shrug :))

RobertM

Yes thought of that... I gave up with cheap lasers after taking one apart and putting it back together - I was shocked at how naff they are.  I now have a second hand hotech crosshair jobbie and that's spot on 8)

Robert

RobertM

Well, unfortunately that didn't work, the images just weren't good enough

Have taken a different approach.  Using some of the collimating instructions in 'How to build a Dobsonian' I realigned the secondary laterally along the tube length.  I then used a camera to accurately set secondary rotation so the centre of the flat was even radially.  The secondary was then accurately aligned using a laser and the whole process repeated till perfect both visually and using camera/collimator.  Rounded off with a Cheshire as a double check.

It looks good but it's looked good before.  Will see what the results look like tonight on Jupiter and with an image if the cloud gives me a chance.

PS.  The laser is a Hotech and looks to be perfectly adjusted, the eyeball however is a bit the worse for wear so needed a bit of adaptive optics help ;)

JohnP

Blimey Robert - all sounds like hard work.. I know I used to bang my head against the wall with MN56. Just think though if you get it sussed it'll be a great astro scope. Good luck for tonight - John

RobertM

Thanks John.

Well, Jupiter looked really good though it was bouncing around a bit which made it difficult to focus.  Even with the 4.7mm UWA there was plenty of detail visible so that's a promissing start.  Unfortunately the imaging run ran into problems straight away.  I could not get the M25 to reach focus, there was just not quite enough reach from the moonlight and with the SW extension there was just too much - I only needed another 1-2mm :(  Thinking about it now I could have tried the AP reducer which will have brought the focus a lot closer !  I'll also remove one of the spacers under the focuser to give a bit more inward travel so I can use the extension.  The imaging I did was with the camera set rather precariously and with so much slop in the gears the results were not good.

Until the next installment I guess...

Robert

MarkS


It all sounds like a real pain! 
I'm sure you'll get it sorted though.

Mark

The Thing

Sounds like when you first got your Tak Mark. PITA but worth it.
Cloudy? ¯\_(ツ)_/¯ (Gallic shrug :))

RobertM

Just stacked the results and all the stars are elongated in exactly the same way evenly across the frame :)  Phew, that's a relief !!! Next thing to do is remove that focuser spacer, adjust the EQ6 Gear train then retry with guiding against a star field to see what the detail looks like.

RobertM

Update: the AP 0.66 reducer reaches focus so the scope could theoretically do 660mm f/l @f/3.5 with the correct spacing.  Vignetting might be a bit hairy though...

MarkS


Assuming you can achieve good stars over your field of view, 660mm f/3.5 is a very worthwhile setup!

Assuming it's clear tonight I intend to tackle my C11 reducer problem again - this time with a tilting adapter.

Mark