Mak Cass Collimation

[beamish]

I hope you don't mind but I thought one or two may find my documented process of bench collimation useful or at the very least insightful.

In the following process no diagonal was used as this would have had an effect on the collimation and since I don't plan to use a diagonal except on a few occasions- on the whole I'll be using an SCT Crayford focuser, so this was in place during the collimation process with a collimation cap in place in the focuser

With both the primary and secondary and meniscus having been removed from the scope, collimation was going to be necessary and here it got a little worrisome. However the beauty of the internet again comes through to illuminate the slow witted

I joined the Mak scopes Yahoo group and found help and encouragement there and also dug up a copy of a "How to collimate a MK67 " which is similar in it's construction, in addition the archives of our very own SGL proved fruitful. An article collimating a Kletsov 200 by Kaptain Kletsov proved very helpful.

Collimation Process :-

Obviously I needed to start from a known starting position, so regarding the Primary I unscrewed all the M4 "pull" screws ( 3x pairs, 6x in total) to the point where they offered no purchase. Then the M5 "push" screws 3x were tightened up but only to the point where they began to tighten up ( bare in mind these are what hold the whole of the back end and primary to the OTA !!). Then using KK's method, I set the scope up on a work bench arranged to make a large "V" block and given the scope's fl is 2700mm I then took up a position close to 3m in front of the scope and aligned my eye with the centre of the 2ndry holder such as to just allow the baffle to be hidden behind the holder.Holding this position I am then able to judge the concentricity of the rings/reflections etc. In a collimated scope all rings would be concentric !

Now I was unaware as to which collimation screws were responsible for which set of rings/reflections so through a bit of trial and error I discovered that:-

A)

The secondary collimation screws are responsible for the concentricity of the larger "2ndry baffle reflection" which is the first coming in from the edge of the mirror. This can be identified by the appearance of the ribbed edge of the retaining collar for the secondary holder. So adjust these to get this result !

At this point ignore the reflections/rings* within this larger, magnified ring/baffle image *These are actually the reflected image of the 2ndry mirror image reflected in the primary, don't be too scared if they look completely skew-wiff , even if the smaller 2ndry reflection/baffle and window can be seen !!

So! At this point the ribbed edge of the the locking/retaining collar of the 2ndry should be visible running around the inside edge of the mirror, if this is concentric to the mirror edge then you're done with the 2ndry collimation screws !!

Now to deal with the rings/reflections within the larger , magnified image of the 2ndry baffle. The primary collimation screws are responsible for this.

Again, if you are incredibly lucky, all rings /reflections will appear concentric when viewed from the position taken up earlier (in my case 3m, but depends on the fl of the Mak !)

However I wasn't lucky and clearly they were amiss. At this point you will have make an adjustment to JUST one set of screws. Try to use the set that are clearly the nearest regarding the position of the eccentricity, make the adjustment and judge the effect upon the eccentricity. If the eccentricity has improved then great, if not then reverse the adjustment to return to a similar position prior to the adjustment. ie if pulling the mirror out with that particular set made the appearance worse then clearly the mirror may need to be pushed instead. However remember that the "push" screws can only go so far so at this point it may be worth considering "pulling" the mirror from the other two sets.

I'm afraid it is a case of trial and error- but here's the nice bit, once you start to see improvement in the concentricity it all gets quite easy to follow and will seem easy.

Eventually you will notice that the ring/reflections when viewed ALONG THE CENTRE AXIS from a distance roughly equal to the fl of your Mak will be concentric , or least very close as can be judged. Just be sure that the baffle behind the 2ndry holder appears to be hidden revealing only an annular eclipse of itself, if that makes sense, because the 2ndry holder is incapable of hiding the baffle completely !

Go back to "A" and check the 2ndry collimation, the ribbed collar for the 2ndry holder should still appear concentric to the inside edge of the mirror, if not then make the necessary adjustments and then check the overall concentricity of all the rings & reflections.

Your scope is near as damn it collimated, ENSURE that the "pull" screws (M4 x 6) on the back plate have been screwed in to the point where they begin to bite and offer resistance DO NOT TIGHTEN BEYOND THIS POINT !!

CONGRATULATIONS !

Your scope is now ready for StarTesting and final tweaking !!and just to prove it look through a colly cap and it should all look tickety boo !

I will not go into Star Testing since this will be covered by an additional note regarding the MK67 ( credit provided), but you ought to find that collimation will be very close and only a slight tweak will nail it ! Just make sure the scope is cooled and the subject star is viewed under a high power and in the centre of the FOV

NB REMEMBER !

Intra-focal image (rings but no central dot) = Primary collimation screws

Extra-focal image (rings with a central dot) = Secondary collimation screws

Thank you & Good night xx

I found this document many many moons ago -never thought it would one day save my bacon !

collimation_of_intes_mk67_and_mk66.doc

courtesy of Thierry Gauthier

All I need now is sort out the motor focuser

[beamish]

I hope all this makes sense to people ??

[samtheeagle]

Thanks Karlo Some good info in there... My only commment would be that a picture is worth a thousand words

You mention a colli cap... Now I know laser collimators don't work with Maks, so the old school tube with a hole style does? What would I expect to see through one of these with a Mak?

[beamish]

not even that Sam just a cap with a small hole- I use an Astro Engineering 1.25" dust-cap, Astro Engineering Astro Engineering Posh 1.25" dust plugs (AC756)

remove the logo and hey presto a perfectly centred lickle hole to look thru. PS they take a really good polish too for added reflectivity.

Should see the reflection of the primary in the 2ndry and your eye down the centre all concentric.

I'll try and take pics:)

what's the fl of the mak? put it on a Black & Decker style workbench "V" formation and go and stand as far away from the front of the scope as the fl of the scope. Then take a looksie down into the scope from a position that is in line with the centre axis and have a look at the reflection rings !

[samtheeagle]

Magic, one silver dust plug ordered I think that the FL is 1900mm, not 100% on that, but it's certainly around the 2 meter mark.

[womblemaster]

@An article collimating a Kletsov 200 by Kaptain Kletsov proved very helpful.@

go go astro superhero!!