Collimating a 6" Mak is Darn Touchy

[Renato1 (Renato)]

I bought a 6" Saxon Mak second hand nearly three years ago, but only got around to mounting and using it four weeks ago. The images on stars was great, but it was extremely disappointing on planets.

The out of focus bullseye pattern on a star looked concentric to me, but a small part of the outer ring wasn't as defined as the rest of the ring. Also, when very close to focus, the outer ring went triangular, indicating pinched optics.

Anyhow, it took me six hours over two cold nights of moving that bullseye pattern around to finally get it concentric, and get rid of that tiny smudge in the outer ring. The main problem was that every time I touched the screws, it would then take me 5 to 15 minutes to reacquire the star I'd been using, since the finder was then out of line with the telescope. In the end, I started using the moon to readjust the finder with, and then I would go back to the star.

Thus something that would have taken 3 minutes on a Newtonian, took me around 6 hours.

Anyhow, the pattern also shows less pinching than at the beginning of the exercise, probably because I didn't do the screws up as tightly. And the images of Mars and Saturn were significantly improved.

The exercise of collimating seemed considerably harder with this 1900mm focal length telescope, than with the 1356mm focal length 4" Mak I did a week and a half ago to psych me up for attempting the 6" one. I came to the conclusion that the Moon was pretty much essential for doing the 6" or I'd still be there trying to find the star to check the pattern on.

Has anybody else done this touchy exercise, or does it get easier with practice?
Regards,
Renato

[Steffen]

After centring the star in the FOV defocus inward and check on which side of the defocussed image the diffraction rings bunch up (the head of the "comet"). Then, find one collimation screw on the primary and adjust it so that the star image moves towards the head of the comet. Don't adjust so much that the star leaves the FOV.

If the secondary mirror cannot be collimated re-centre the star image and reiterate. Only adjust one collimation screw before re-centring, and don't let the star leave the FOV.

If the secondary can be collimated then defocus outward and return the star image to the centre by adjusting the appropriate screw on the secondary.

Once the rings appear concentric and aren't obviously bunching up on any side ramp up the magnification and slowly defocus the star. Observe the moment when the first bright diffraction ring splits into two. It needs to do that all around its circumference at once. If the split-up lags behind on one side treat that side as the head of the comet and proceed as above. Note that only minute adjustments are required towards the end.

You need a stable and tracking mount for Mak collimation, if you value your sanity Apart from that, it's straight-forward.

Cheers
Steffen.

[Renato1 (Renato)]

Originally Posted by Steffen

After centring the star in the FOV defocus inward and check on which side of the defocussed image the diffraction rings bunch up (the head of the "comet"). Then, find one collimation screw on the primary and adjust it so that the star image moves towards the head of the comet. Don't adjust so much that the star leaves the FOV.

If the secondary mirror cannot be collimated re-centre the star image and reiterate. Only adjust one collimation screw before re-centring, and don't let the star leave the FOV.

If the secondary can be collimated then defocus outward and return the star image to the centre by adjusting the appropriate screw on the secondary.

Once the rings appear concentric and aren't obviously bunching up on any side ramp up the magnification and slowly defocus the star. Observe the moment when the first bright diffraction ring splits into two. It needs to do that all around its circumference at once. If the split-up lags behind on one side treat that side as the head of the comet and proceed as above. Note that only minute adjustments are required towards the end.

You need a stable and tracking mount for Mak collimation, if you value your sanity Apart from that, it's straight-forward.

Cheers
Steffen.

Thanks Steffan,
I think my error came from following the instructions for the Orion 6" Mak which I downloaded. They say that if you adjust one screw in the way you describe, you have to then also adjust the two other screws in the opposite way - which means also unscrewing each of the other three screws that hold everything down.

When doing the exact same thing with the 4" Mak, the star moved only slightly out of the field of view and I'd find it within half a minute of twiddling the knobs.
Regards,
Renato

[Steffen]

The Orion Argonaut 6" Mak is virtually identical to the Intes MK-67 (except for the focuser), it has a secondary in its own cell that can be collimated. The Saxon has a silvered spot on the meniscus, IIRC.

In either case, you need to sufficiently loosen the primary locking screws before you start, I forgot to mention that step (probably because I'm not using them on my Mak and removed them altogether).

Cheers
Steffen.

[Renato1 (Renato)]

Originally Posted by Steffen

The Orion Argonaut 6" Mak is virtually identical to the Intes MK-67 (except for the focuser), it has a secondary in its own cell that can be collimated. The Saxon has a silvered spot on the meniscus, IIRC.

In either case, you need to sufficiently loosen the primary locking screws before you start, I forgot to mention that step (probably because I'm not using them on my Mak and removed them altogether).

Cheers
Steffen.

I was using the instructions for the discontinued Orion 150mm Maksutov Cassegrain which was sort of similar to mine, as it had the silvered meniscus.

I had thought that only slightly undoing the locking screws would in turn limit the amount of movement of the star outside of the field of view when I then turned the other screws. It didn't work out that way.
Cheers,
Renato

[Steffen]

Sorry Renato, I haven't seen the insides of the silver spot Orions or Saxons, so I can't predict exactly how the screws would respond, but in general you would loosen the locking screws a few turns so they won't interfere with your collimation procedure, but only turn the collimation screws in the order of 1/10th to a 1/4th turn at a time – while looking through the eyepiece of course.

Likewise, not having seen the Saxon primary cell I can't say whether or not it is advisable to lose the locking screws completely. However, if you're game take apart your scope and study its insides. It will make you feel much more confident and in control during collimation.

Cheers
Steffen.

[Renato1 (Renato)]

Originally Posted by Steffen

Sorry Renato, I haven't seen the insides of the silver spot Orions or Saxons, so I can't predict exactly how the screws would respond, but in general you would loosen the locking screws a few turns so they won't interfere with your collimation procedure, but only turn the collimation screws in the order of 1/10th to a 1/4th turn at a time – while looking through the eyepiece of course.

Likewise, not having seen the Saxon primary cell I can't say whether or not it is advisable to lose the locking screws completely. However, if you're game take apart your scope and study its insides. It will make you feel much more confident and in control during collimation.

Cheers
Steffen.

Hi again Steffen,
What? You have the luxury of looking through the eyepiece while adjusting the screw?

I was having a hard time adjusting the screw while looking at the screwdriver just trying to make sure it was straight and actually engaging the screw! This was because the screws were deep inside the telescope, and the notches weren't very deep. There wasn't any latitude - the screwdriver couldn't be the slightest bit off centre.

Also, I had to look at the screwdriver to make sure I was only doing a quarter or eighth of a turn.

I guesss the more expensive Maks are somewhat better built than mine.
I was actually surprised when I looked at the image and saw that it was right.
Cheers,
Renato

[Steffen]

Insert the screwdriver and find the screw, but look through the eyepiece before you start turning. You need to see what you're doing, otherwise it's just guesswork.

Cheers
Steffen.

[Renato1 (Renato)]

Originally Posted by Steffen

Insert the screwdriver and find the screw, but look through the eyepiece before you start turning. You need to see what you're doing, otherwise it's just guesswork.

Cheers
Steffen.

That's very difficult to do on this Mak, and was impossible to do on the 4" Meade Mak, because to access the screws one had to take off the back plate which contained the mounting thread.

But there wasn't any guess work. I use the mnemonic IL-OS (in long, out short) referring to how I turn the screw in relation to the comet head and tail of the out of focus image. That mnemonic has worked on three reflectors, and an SCT and on the Meade Mak.

But it didn't work with this Saxon Mak. Instead I had to use OL-IS, which also had me mystified as to why that should be.
Cheers,
Renato