Collimation

[IanT]

Hello everyone.
Great site for a beginner like me. Can someone please explain how to collimate the secondary mirror on a GSO (Bintel) Newt mounted on a Dob base? I'm not sure what the center screw in the 4 vane spider assembly is used for. Does it move the whole mirror up and down the OTA or allow sideways twisting? I am a bit scared to undo it! Do the three other screws simply allow a little adjustment and should I loosen them right off before attacking the big center screw? Also, I have a Saxon laser collimating thingo but it is clearly not collimated itself. It rolls around on a homemade jig and moves everytime I turn it in the focuser or even tighten it up. It doesn't have any ajusting screws or mechanisms that I can see. I tried to make a target on a Barlow in order to use the Barlowed laser technique. Unfortunately, the end of the Barlow (with the afixed target) doesn't protrude far enough to get a good view of any return beam and center spot shadow. I tried a hand mirror but it still hard to see. Any hints?

Cheers
Ian

[Dennis]

I strongly suspect the centre screw physically holds the secondary assembly onto the spider vane assembly, so I wouldn't be undoing it for fear of the secondary assembly dropping onto the primary mirror.

The 3 smaller screws at 120 degrees are the collimation screws that allow the secondary to be collimated. I've never done this on a Newtonian so I'll let someone else chime in with the how to. Also, try doing a Google search on "Newtonian collimation" you should turn up so excellent material.

If you live near a local club, try to get along to a meeting – the fear and mystery of collimation evaporate when you can see it being done.

Cheers

Dennis

[DJVege]

Hi Ian,

Check out this site. http://www.andysshotglass.com/Collimating.html

It should answer your questions. And it has images during the video as well, so you should be right.

Good luck.

[rmcpb (Rob)]

Ian,

The centre screw allows the adjustment of the mirror up and down in relation to the focuser. The three smaller ones allow for the mirror to be tilted to align it with the primary.

Cheers
Rob.

[norm]

Hi Rob,

The centre screw allows the adjustment of the mirror up and down in relation to the focuser. The three smaller ones allow for the mirror to be tilted to align it with the primary.

Out of curiosity, which (if any) of these screws would make the 2ndry mirror fallout. Or is there something else holding it in place?

Has anyone experienced the 2ndry falling out ?

[GTB_an_Owl (Geoff)]

there should be a little retaining clip on the bottom of the assembly to hold the mirror in

geoff

[Paddy (Patrick)]

Hi Ian,

Best tip from the excellent andy's hot glass site on collimation (link already posted) is to have the scope horizontal when working on secondary mirror to prevent it falling onto primary. Having said that, I have found that I can loosen the three adjusting screws on the secondary of my GSO 12' dob enough to adjust the mirror without it falling off. I always suspected that the central screw for adjusting the depth of the mirror is the one that ultimately holds it on - reassuring to know that there is a clip that does this job. Anyway, don't panic, but keep it flat to be sure!

[erick (Eric)]

I'm yet to pull mine to pieces, but I'm sure the centre screw holds the secondary mirror assembly in place, ie. unscrew it all the way and the secondary mirror plus its holder would fall away. The three "collimating screws" just butt against the secondary mirror holder and so can tilt it slightly in different directions. Obviously, to push one hard in, the other two have to relax out a little, so you tweak the three of them a little bit in turn, until you get to the correct secondary mirror angle with all three screws fairly firm against the holder.

My experience is that, when the three screws are loose, the secondary holder will rotate with the main screw (so the mirror begins to face away from the focusser. If you reach in and hold the mirror holder (watch you don't touch the mirror surface, screwing the screw moves the mirrorholder closer to or away from the primary. Now, for a Bintel GSO newtonian, unless something has happened to it since it left Bintel, I would expect this up/down position to be fairly accurate. Unless someone has screwed the centre screw, it's hard to see how that will have altered. Maybe if the focusser has been replaced??

Looking through a sight tube (Cheshire, collimating cap, 35mm film cannister with hole in bottom etc), it's fairly easy to locate the secondary mirror - particularly the rotation around the centre screw. Just hold the mirror holder and turn it while watching, until the secondary mirror looks like a circle. Now, there is a lot of stuff about secondary mirror offsets - search these threads and you'll find some discussion. Any detailed article on collimating a newtonian scope will go over this matter for you. My understanding is, if through the sight tube you see a round secondary mirror well centred in the view, you are close enough.

Now the laser collimator. You do have to get it collimated in itself. That means, as you rotate it on a suitable homemade jig, the dot stays within a few mm radius circle on a wall 2-3 m away. They usually have the back of the little laser assembly held firm, maybe in a rubber O-ring, but the front of the laser assembly normally has three grub screws holding it in place. Are they under some sort of cover or trim? Once you can get at those, you can make slight adjustments, loosen one a 1/4 turn, tighten another a 1/4 turn and check to see if improved or not.

Once that is done, try to get the laser to be a snug fit in the focusser adapter or barlow or barlow adapter. I read a recommendation of wrapping one layer of "contact" (yes, for the kid's schoolbooks) and I found a nice flowerly bit and put it on. Perfect, very little slop now. I put my laser in and tighten up the set screw (brass compression ring design) and it seems very snug and well set in position.

Re the barlowed laser - get a piece of white card and punch a small hole in it - up to hole punch size. After having collimated secondary and primary mirrors as best as you can with the direct laser, now introduce the barlow and hold the card down the inside of the scope between the focusser and the secondary mirrror such that the laser beam passes through the hole. Presuming it is illuminating the ring binder centre spot well, you'll see the shadow of the ring binder on the card. If I have collimated with my laser, that shadow is exactly centred, on the white card, around the laser beam every time. I think I'm lucky - my setup and laser have been well behaved.

Let us know how it goes.

[GTB_an_Owl (Geoff)]

NO Paddy

the clip only holds the mirror in place

NOT the holder

the center screw does that

geoff

[Paddy (Patrick)]

Thanks!

[Tannehill]

Yep, the entire secondary mirror assemby - the cylindrical base in which the secondary mirror is clipped - attaches to the spider via that center bolt, which is tightened or loosened from above the spider - that's the bolt. This bolt has a spring around it, which serves a purpose in assisting with collimation (below). But the bolt also holds that secondary mirror assembly on the spider, so don't be loosening that unless you are prepared to catch the assembly if you loosen too much. Best to do this with tube horizontal!

As you turn that bolt, the spider will move up or down. This movement gives you freedom to position the secondary mirror under the focuser drawtube correctly. Bintel does a good job in dialing that in for you, but it's not uncommon for that to be way off for other "out of box" purchases. This centering the secondary under the focuser is part of collimating a dob. If it's off you will be losing some of your mirror surface area: vignetting.

That center screw atop the spider, when turned, will either move the spider "up" and thereby compressing the spring, or "down" and thereby loosening the spring. If you removed that spring, you'd notice that the secondary mirror assembly actually pivots or wobbles a bit on the end of the bolt; it isn't fixed solid with the bolt. That wobble is intentional. When compressed the right amount, that spring provides dynamic tension in the assembly, against which the OTHER three smaller bolts - the three collimation bolts - push in the opposite direction. They push on the top of the secondary mirror assembly to fine tune the 'tilt' of the spider. There should be a metal washer (a fender washer) against which those three bolt ends push.

My spring was too lax, actually. I had to add a washer to take up space, make it more compressed, to achieve the fine tuning of the secondary mirror.

Here is a cut-and-paste of my description on the skyquest_telescopes Yahoo group regarding a related question.

___________________________________ _______________________
I have a GSO dob, and the secondary looks identical. This GSO came
with a fender washer.

I switched the stock secondary collimation bolts with Bob's Knobs,
which, I think, are similar to the Meade Lightbridge specs. Much easier
than dangling a screwdriver over that end...

I also experienced that erratic secondary collimation. I finally
figured it out in my particular situation. At least I think so, and
I'll try to explain it. I obviously can't confirm that is what you are
experiencing.

Smooth collimation of the the 2nd mirror depends on the right tension
balance between the spring-loaded center bolt and the three collimation
screws. The center bolt at its top isn't fixed to the vane, but rather
wiggles and pivots a bit, thereby allowing the secondary to "wiggle"
when all three collimation bolts are loose. This wiggly center bolt is
holding the secondary to the vane, but the spring around it is pushing
away. This degree of wiggle is determined by the tension of the
spring of the center bolt. The tighter the secondary bolt, the more
tightly compressed is this spring. If the spring is too loose then the
three collimation bolts, as they move down to push against the fender
washer, don't have much to move before they "push" the secondary beyond
the tension of the spring. The result is clunking of the secondary when
you try to adjust the collilmation bolts. You need a longer spring,
or...

My solution was to increase the tension of the spring by inserting
several washers over the "upper" end of the center bolt, thereby
increasing the tension of the spring when the collimation bolts would
make contact with the fender washer. I also took the liberty of
putting small thin rubber caps on the business ends of the collimation
bolts so the bare metal end of the bolts doesn't dig grooves into the
fender washer, thereby causing erratic jumps as the bolt rides over,
then off, one of these grooves in the washer. I rotated the fender
washer after apply these caps so the capped bolt ends are making
contact with smooth unmarred metal.

Now it collimates with wonderful predictability, not the erratic
jumping of earlier.

After you make any changes, re-confirm that your secondary mirror is
correctly centered under the focuser tube. Lots of back and forth
adjustments of the center bolt might leave you unexpectedly off by a
significant amount.

Hope this makes sense and helps.

Scott
_____________________________

[Tannehill]

Assuming you've collimated the laser itself (sounds like it's not, tho) you can use the un-Barlowed return beam as a "poor man" collimation for the primary mirror. You move the primary mirror bolts until you see the return beam merge into the center hole of the back of the laser collimator.

Other option is to buy a Chesire....Which doesn't un-collimate when dropped, and never runs out of batteries!

(An uncollimated laser collimator is just an expensive lecture-style laser pointer...good for little else..).

If you have a laptop-style webcam, you can easily set it up to watch the back of the laser collimator by clipping it to the spider vane, and do "remote" collimation this way, watching the image on your laptop screen. But that's really only going to work if you have a barlowed laser technique; the bright dot of the un-barlowed laser beam doesn't image well on a webcam.

Scott

Originally Posted by IanT

Hello everyone.
Great site for a beginner like me. Can someone please explain how to collimate the secondary mirror on a GSO (Bintel) Newt mounted on a Dob base? I'm not sure what the center screw in the 4 vane spider assembly is used for. Does it move the whole mirror up and down the OTA or allow sideways twisting? I am a bit scared to undo it! Do the three other screws simply allow a little adjustment and should I loosen them right off before attacking the big center screw? Also, I have a Saxon laser collimating thingo but it is clearly not collimated itself. It rolls around on a homemade jig and moves everytime I turn it in the focuser or even tighten it up. It doesn't have any ajusting screws or mechanisms that I can see. I tried to make a target on a Barlow in order to use the Barlowed laser technique. Unfortunately, the end of the Barlow (with the afixed target) doesn't protrude far enough to get a good view of any return beam and center spot shadow. I tried a hand mirror but it still hard to see. Any hints?

Cheers
Ian

[IanT]

Hello again
Jeez you people know a lot. I'll give it a go tomorrow when there is a bit of light about. I feel much happier now about the secondary and it's ajustments. I am still a little perplexed about not being able to see the center spot shadow on my target on the end of the barlow. Perhaps the collimation is so out of whack that it will be invisible until I do the secondary etc as suggested by all of you good folk. Thanks heaps for your advice.
Cheers
Ian

[Glenhuon (Bill)]

I'm pretty new to colimating newtonians as well, but here's my experience.
If the secondary is not too far up or down the tube (can you see the either end of the mirror on the long axis ?) if not, leave the centre screw alone.
To check the colimation on the laser, cut a Vee in a block of wood or use an engineering Vee block, mark a point on a wall 2 or 3 metres away and point the laser at it. slowly turn the laser in the block and see if the dot moves off the marked spot. If it does there are screws that can be adjusted to bring it back to centre. They are usually small hex socket screws down near the business end of the laser. Adjust by slackening and tightening (gently) 1/2 to one flat at a time.
Once the laser is lined up, slip it into the focuser and back the draw tube out a few millimetres. I've found that the ends of the drawtube bodies aren't always square and can "kick" the laser out of line if the draw tube is right in.
Look down the scope tube and see if the laser dot is in the middle of the primary mirror, most mirrors have the centre marked in some way. Adjust the 3 secondary colimation screws by slackening and tightening until it is (you can see the dot move so it doesn't take long to get the feel of which way to go).
Once this is done you can go on to the next stage of adjusting the primary until the laser dot returns back up the OTA tube and disappears into the hole in laser colimator from whence it came.
Hope this is clear. If not I have a doc here that helps, kindly provided by another member, I'll scan and post it if needed.

Bill

[GeoffW1 (Geoff)]

Originally Posted by IanT

Unfortunately, the end of the Barlow (with the afixed target) doesn't protrude far enough to get a good view of any return beam and center spot shadow. I tried a hand mirror but it still hard to see. Any hints?

Cheers
Ian

Hi Ian,

It certainly is difficult with a hand mirror because if you stick it in there too far it blocks the reflected image from the primary mirror anyway. You can get it using just the very corner of a small rectangular mirror, but a dental style mirror (as sold by some car accessory shops) is better I have found.

Cheers

[Tannehill]

If the Saxon laser you have is out of collimation and has no feature for adjusting, return it!

You wouldn't see that return beam/image on the Barlow target if the primary is way off, so that's maybe the cause.

If you run up against some obstacle, bring the scope (or just the OTA) to a local club meeting (ASV, MPAS, etc) where I'm sure you'll find someone to help.

s

Originally Posted by IanT

Hello again
Jeez you people know a lot. I'll give it a go tomorrow when there is a bit of light about. I feel much happier now about the secondary and it's ajustments. I am still a little perplexed about not being able to see the center spot shadow on my target on the end of the barlow. Perhaps the collimation is so out of whack that it will be invisible until I do the secondary etc as suggested by all of you good folk. Thanks heaps for your advice.
Cheers
Ian

[astro_nutt]

Hi Ian..just a thought regarding any tools used when adjusting the secondary or anything inside the tube..please secure a cord or shoelace into the handle or around the tool itself and wrap it around your wrist..even though the tube is horizontal when you're working on it..it only takes some applied force or someone knocking the tube to send something horrible towards the primary..

[IanT]

Thanks again for all of your help.
Cheers
Ian