How can I collimate a dobsonian telescope with a laser collimator?
I've had a dobsonian telescope for a while now, but I've never been able to collimate it. I thought I had it collimated once, but when I looked through it the star didn't look right, it looked kind of like a round disc with a hole in the middle. I know the basic idea behind doing it with a laser collimator, you need to get the laser to line up with the center of the primary mirror as well as line up in the center of the target on the laser itself. However, I always have a lot of difficult accomplishing this.
For starters, I always have a lot of trouble getting the secondary mirror lined up. I'll get really close, but once I get close I have a lot of trouble doing the fine tuning to get the laser in the middle. I'll get it in, but when I got to tighten the other screws it will no longer be lined up. I'll try to account for the tightening as well, but it's always a guessing game for me when it comes to knowing which screws to loosen and tighten so that the laser winds up where I want.
I don't have that many problems with the primary mirror, and can normally get that lined up fairly easily. However, usually after lining up the primary mirror the secondary mirror no longer seemed to be lined up and I need to do the secondary mirror all over again. I'm assuming this is to be expected, but considering the difficulty I have lining up the secondary mirror it's very frustrating.
I've tried looking around at resources online, but none of them go into enough detail into aligning the secondary mirror. They just say "adjust the screws until it's lined up", which doesn't solve my problem, since I have trouble figuring out exactly which screws to adjust and when. I'm also confused as to what I should see through my telescope when it is properly aligned. Most of the sites just use a diagram, but I'm never really sure what to make of the diagram. The one time I did line it up it looked like a round disc with a hole in the middle, which I didn't think looked right.
So to summarize, the main problems I have are:
- I have trouble fine tuning the secondary mirror, since I have trouble accounting for the movement of the mirror when tightening the two screws I didn't loosen
- I'm not sure what a star should look like through my telescope when it is properly collimated
Can anyone offer some detailed instructions, and possibly a video, of how to collimate a dobsonian telescope with a laser collimator?
On the second part, a properly collimated telescope should show a star as a single point of light. There's not enough magnification available from the Earth's surface to resolve them as anything other than a point light source.
Note to future answerers: We have been working in the Astronomy.SE main chat room with OP to solve this problem and established it's actually a matter of focusing that resulted in problems described (telescope was either too intrafocal or extrafocal), not collimation itself. This is a video on YouTube that OP said was exactly how the problems looked like. Granted, focusing can be technically called collimation too, and indeed that demonstration video calls it as such (albeit misspelled), since it's still about adjusting positions of primary and secondary mirror in dobsonian or newtonian telescopes (dobsonian is a type of newtonian), but the former is a lot simpler to solve by turning focusing knobs and achieve optimal illumination.
Now, let's answer the actual question.
Preparing our laser collimation setup
First thing to know is that any laser collimator will likely have the beam strong enough to cause damage to your eyes if pointed directly in them, or damage strong light sensitive equipment, like for example digital cameras, so be careful with it and take precautions. Carefully read instructions on how to handle and use it, before you do so. This is how a laser collimator might look like:
Next thing to do is make it functional by inserting batteries or plugging its power adapter and testing it actually works by turning it on with the switch, if it has it (probably does). Once you'll establish your laser collimator is turned on and functioning by observing (usually ruby red) light beam spot it makes on any neutral surface, turn it back off and attach any adapters on it you might need to help you attach it to your telescope's focuser. This is how your telescope's OTA (Optical Tube Assembly) might look like:
Now remove the barlov and eyepiece from your telescope's focuser if you're using a direct focuser adapter for your telescope model, tighten the laser collimator's compression ring by rotating its rubber ring and attach the collimator into the focuser. Rotate collimator's compression rings to tighten it in place. If you're not using an adapter, your laser collimator might attach directly to your eyepiece. Consult your collimator's user manual, if unsure.
Rotate your collimator's targeting faceplate towards your telescope's primary mirror (on the telescope's bottom) and turn on the laser.
Angular alignment of the secondary mirror
By looking into the telescope top down from the secondary (top) mirror into the telescope, we will adjust the angular alignment of the secondary mirror by referencing the laser dot projecting on the primary (bottom) mirror. The secondary mirror's support has a center ring, the collimating cap, with three bolts on the top of the secondary mirror holder. Use Allen wrench or Phillip's screwdriver to rotate them individually to bring the laser dot into the center of the doughnut ring of the primary mirror, like so:
Try to bring the dot more directly into the doughnut center than on the photograph above, though. I intentionally didn't select a photograph from when it was in its dead center, because of the light reflection covering the doughnut ring completely, rendering it nearly invisible.
Primary mirror collimation
Unlock the primary mirror plate locking screws, position yourself towards the panel side of the primary mirror tube so you can observe the laser's center spot on the targeting faceplate on the collimator and start adjusting collimation screws by turning them gently until the laser dot is in the center of the targeting faceplate:
Another way to check your collimation progress when adjusting collimation screws is by checking the projection laser dots on the secondary (top) mirror and making sure both laser dots are overlapping into one single spot, but that might make your adjustments of the primary mirror somewhat harder, so using the targeting faceplate of the laser collimator would be my suggestion and perhaps check alignment of the secondary mirrors laser dots projection for the last finishing touches to collimate your mirrors as precise as possible.
Now you're done collimating your dobsonian or newtonian telescope, so it's time to remove the laser collimator by first switching it off, then untightening its compression ring and taking it out of the focuser. Once you've removed the laser collimator, insert in the focuser barlov and the eyepiece, and tighten them back in place.
Additionally, and as requested, here is the Youtube video of the Newtonian Collimation using SCA Laser Collimator that are also the source of the last two example photographs and inspiration to write the collimation procedure, courtesy of HoTechUSA YouTube channel. Make sure to browse through other similar videos on telescope collimation in this same channel.
Collimation of a dobsonian without a laser collimator
Standard collimation technique for the exact dobsonian telescope make OP mentioned has can be read in this Instruction Manual for Sky-Watcher dobsonians (PDF):
Collimation is the process of aligning the mirrors of your telescope
so that they work in concert with each other to deliver properly
focused light to your eyepiece. By observing out-of-focus star
images, you can test whether your telescope's optics are aligned.
Place a star in the centre of the field of view and move the focuser
so that the image is slightly out of focus. If the seeing conditions
are good, you will see a central circle of light (the Airy disc)
surrounded by a number of diffraction rings. If the rings are
symmetrical about the Airy disc, the telescope's optics are correctly
If you do not have a collimating tool, we suggest that you make a
"collimating cap" out of a plastic 35mm film canister (black with
gray lid). Drill or punch a small pinhole in the exact center of the
lid and cut off the bottom of the canister. This device will keep
your eye centered of the focuser tube. Insert the collimating cap
into the focuser in place of a regular eyepiece.
Collimation is a painless process and works like this:
Pull off the lens cap which covers the front of the telescope and
look down the optical tube. At the bottom you will see the primary
mirror held in place by three clips 120º apart, and at the top the
small oval secondary mirror held in a support and tilted 45º toward
the focuser outside the tube wall (Fig.h). The secondary mirror is
aligned by adjusting the central bolt behind it, (which moves the
mirror up and down the tube), and the three smaller screws
surrounding the bolt, (which adjust the angle of the mirror). The
primary mirror is adjusted by the three adjusting screws at the back
of your scope. The three locking screws beside them serve to hold the
mirror in place after collimation. (Fig.i)
Aligning the Secondary Mirror
Point the telescope at a lit wall and insert the collimating cap into
the focuser in place of a regular eyepiece. Look into the focuser
through your collimating cap. You may have to twist the focus knob a
few turns until the reflected image of the focuser is out of your
view. Note: keep your eye against the back of the focus tube if
collimating without a collimating cap. Ignore the reflected image of
the collimating cap or your eye for now, instead look for the three
clips holding the primary mirror in place. If you can't see them
(Fig.j), it means that you will have to adjust the three bolts on the
top of the secondary mirror holder, with possibly an Allen wrench or
Phillip's screwdriver. You will have to alternately loosen one and
then compensate for the slack by tightening the other two. Stop when
you see all three mirror clips (Fig.k). Make sure that all three
small alignment screws are tightened to secure the secondary mirror
Aligning the Primary Mirror
There are 3 hex bolts and 3 Phillip's head screws at the back of your
telescope, the hex bolts are the locking screws and the
Phillip's-head screws are the adjusting screws (Fig.l). Use an Allen
wrench to loosen the hex bolts by a few turns. Now run your hand
around the front of your telescope keeping your eye to the focuser,
you will see the reflected image of your hand. The idea here being
to see which way the primary mirror is defected, you do this by
stopping at the point where the reflected image of the secondary
mirror is closest to the primary mirrors' edge (Fig.m). When you get
to that point, stop and keep your hand there while looking at the
back end of your telescope, is there a adjusting screw there? If
there is you will want to loosen it (turn the screw to the left) to
bring the mirror away from that point. If there isn't a adjusting
screw there, then go across to the other side and tighten the
adjusting screw on the other side. This will gradually bring the
mirror into line until it looks like Fig.n. (It helps to have a
friend to help for primary mirror collimation. Have your partner
adjust the adjusting screws according to your directions while you
look in the focuser.) After dark go out and point your telescope at
Polaris, the North Star. With an eyepiece in the focuser, take the
image out of focus. You will see the same image only now, it will be
illuminated by starlight. If necessary, repeat the collimating
process only keep the star centered while tweaking the mirror.