How to Collimate the Primary Mirror on Orion Skyquest XT Telescopes
The Primary Mirror is quite a hefty beast, as can be seen in the photo below (the mirror is shown removed from the OTA tube for illustration purposes only - removing the mirror is not part of collimation). Note that the rubber clamps have been removed from the mirror cell enabling me to lift the mirror slightly out of the cell to demonstrate how thick the primary mirror is.
In the right-hand photo below the mirror has been completely removed from the cell, showing the 3 circular felt support pads on the inner circle, and 4 black rubberised foam pads on the inside circumference of the cell. These provide cushioning to keep the mirror firmly mounted. Note the chromed thumb-wheels on the underside which are used to adjust the tilt of the primary.
[Sidenote: The yellow/green-striped length of insulating tape I use to mark one side of the mirror cell to help me always orient the mirror cell the same way when I put it back in the tube]
Before starting you should be familiar with the standard method of collimating the telescope. This is described in Section 4 - Alignment (Collimation) of the Optical System of the instruction manual provided with the Orion Skyquest XT range.
Better still I highly recommend you view Andy's Shot Glass page on Collimating a Newtonian. This provides an excellent 5 minute multimedia presentation showing exactly how you go about adjusting the telescope.
Standard Collimation Cap
When following the standard method illustrated in the instruction manual, it requires the use of the little Collimation Cap, seen here in this photo.
The "colli-cap" is a simple 1.25 inch black plastic cap with a disc of highly-reflective material on its inside surface, and a small viewing hole drilled through. This ensures that when you look through the hole while collimating the telescope, that your eye is dead central to the axis of the focuser drawtube, and so you will be able to accurately align the mirrors.
While this simple method can be fairly accurate, you can usually get better, easier and quicker results by using a different method provided you have the necessary tools.
The Barlowed Laser Technique
Rather than repeating the information provided in the manual, I will describe the Barlowed Laser Method.
I originally read about this method in the following Adobe PDF document by Nils Olof Carlin: Collimation with a Barlowed Laser.
In summary, the Barlow lens deliberately diffuses/defocuses the laser beam so that it creates a spread out beam of diffuse yet strong light which bounces back from the primary mirror causing a "shadow" from the center-ring (what I will refer to as the "ring-shadow") to be projected onto a collimation cap.
The unique thing about this method is that barlowed laser collimators achieve the accuracy of Cheshire sight tube collimation with the ease and simplicity of a laser collimator! Standard lasers are subject to minor centering errors on the primary mirror that can be difficult to detect. These minor errors are not much of an issue with longer Focal length telescopes (> f5) but in today's fast scopes (such as the XT10 with f4.7 ) this error can make a substantial difference in tack sharp collimation. When using a barlowed laser, it is possible to correct for .5+mm centering errors that make all the difference to achieving superb collimation in a fast telescope.
The method requires;
a) a Barlow lens (mine is the Orion Shorty-Plus 3 Element 2xBarlow),
b) a Laser Collimator (I use the Baader Laser Collimator),
c) a large portable wall mirror (e.g. bathroom mirror)
c) a homemade laser image projection Collimation Cap which can be inserted into the focuser drawtube, as shown in the picture below.
The cap basically has a hole in the centre through which the laser beam can shine, and a white paper projection screen which the "shadow" of the primary mirror centre ring (ring-shadow) gets reflected back up and on to. The dark crosshair lines need to be thick enough to be clearly seen from a distance to aid centering of the ring-shadow.
The internal diameter of the Crayford focuser tube is 52mm, and I had to either make or find something which had that exact dimension. I discovered that the plastic lid of a large salt/pepper dispenser or herb pot from the local supermarket was almost exactly the right diameter (and can be padded out with some electricians tape).
Not only that but the lid normally has a rotating dispenser with different sized holes for pouring the contents (you can just make out the large oval shaped hole in the left-hand picture), and when this part is "popped out", this leaves a central hole with diameter large enough for the wider barlowed laser beam to come out, but small enough to provide accurate sighting of the ring shadow on the crosshairs.
Being made of white plastic I found that the laser beam was bright enough to shine through the back side of the translucent plastic which ruined the ability to see the ring shadow projection on the front-side, and so a thick layer of cardboard was cut to fit into the recess in the top of the lid, before a layer of white paper was stuck over the cardboard. For this I cut a circle from a white adhesive-backed CD disc label - this has a larger hole in the centre.
Next I cut a square from the same adhesive backed label paper, which because of its smaller size was easier to use a paper hole-punch to cut a perfect circle through, and this was carefully centred up over the lid-hole and stuck down. Then I marked a thick cross-hair with a pen.
Finally some blue electricians tape was wrapped around the circumference of the lid to make a good snug fit when inserted into the focuser drawtube, so that it will not drop out and down into the telescope tube.
One of the usual difficulties of collimating are that you need to be in two places at once; a) at the top end of the scope to view the alignment of the ring-shadow, and b) at the bottom end to adjust the primary mirror cell thumbscrews. Normally this entails a lot of to-and-fro'ing between the two locations while making the adjustments. However, this is easily avoided with the aid of a large wall mirror.
The task is better performed in a dim room.
Now that we have the correct tools for the job we can perform the collimation.
Push the Laser Collimator into the Barlow lens, and place these into the Focuser and tighten them up to eliminate any slop and ensure a good tight fit.
Turn the laser on. CAUTION - IF THE MIRRORS ARE BADLY OUT OF ALIGNMENT THE LASER BEAM MIGHT BE MISSING THE SECONDARY ALTOGETHER AND SHINING OUT OF THE FRONT OF THE SCOPE - THE BEAM COULD GO INTO YOUR EYES. CHECK FOR THIS WITH A PIECE OF PAPER HELD IN FRONT OF THE TUBE BEFORE MOVING YOUR HEAD INTO THE LIGHT PATH OF THE TELESCOPE.
Note that due to the Barlowed Laser method, it does not matter if the laser beam is not aligned dead on the centre of the primary mirror! All you need to do is ensure that the diffuse "blob" of light going down the tube is reasonably centred on the primary centre mark ring (left), so that a bright image with a dark ring shadow is reflected back up onto the Collimation Cap (right).
[Click photos for larger images]
Now place the wall mirror angled against a chair or wall in front of the telescope, then point the telescope at the mirror, at an angle suitable for you to be able to look down inside the tube from the rear end of the telescope, then tighten the Altitude CorrecTension wheel to hold this position firmly. You can see my reflection taking the photo, but more importantly notice how you can now see the collimation cap in the focuser drawtube, with the laser beam reflected onto it.
Unfortunately it was a little difficult to get satisfactory pictures of the collimation cap in place with the red laser beam and ring-shadow showing, suffice to say that in practice its more easy to see things clearly.
Easy Accurate Adjustment
You should now appreciate that with the big mirror, it is now possible to view the "ring shadow", while positioned comfortably at the primary mirror cell to adjust the thumb-wheels.
I love this part! It feels a bit like "driving" your telescope, because you can easily and immediately see the effect turning a particular thumbwheel has on the position of the ring-shadow. Once you get the hang of it, you can adjust two thumbwheels simultaneously.
Remember that when adjusting a thumbwheel, the other two thumbwheels pivot points are acting like a hinge for the plane of movement of the thumbwheel being adjusted.
Tip: If this is the first time ever you have collimated the Primary I recommend that you undo ALL 3 of the thumbwheel Lock-Screws completely, and then rotate ALL 3 of the Thumbwheels clockwise to close the mirror adjustment fully, until their springs are fully compressed, and the thumbwheels can be tightened no further. Using a permanent marker pen, mark a single black dot on each thumbwheel to denote their 12 o'clock position when fully tightened. Now undo each thumbwheel an exact number of turns, say 5 or 8, so as to relax the springs again and put the travel about half-way, while using the reference "mark" to place them each at exactly 12 o'clock again. Try to be very accurate with the position of the turns because even just 1/32nd or 1/64th of a turn makes a visible difference to the position of the ring-shadow.
This means you are starting out your adjustments knowing that the mirror will be evenly spaced all round, before you make final collimation adjustments.
Tip 2: Final Adjustments: One thumbwheel at a time, make large adjustments (half a turn) clockwise/anti-clockwise initially because this makes it obvious which axis of movement a particular thumbscrew is affecting, then home into the precise position with smaller and smaller movements around the crosshair centre-point. You will soon learn how a small rotation makes a big difference.
Tip 3: Note how the top-left thumbwheel (in pic below) is positioned in the same direction or plane of movement as the focuser tube. This means that if you were to adjust this one while the telescope is near horizontal and looking through the focuser from the side, the movement would appear to be a left-right motion. Or viewed another way, if you looked at the actual collimation cap from the front of the tube (not a reflection), while adjusting the same thumbwheel, the ring-shadow would appear to be moving vertically up/down the cross-hairs (ie. the length of the tube). So its easier to start by adjusting this one first, because its affect is more easily understood.
The shot below shows a view looking sideways into the telescope tube front, at the Collimation Cap focuser where the "ring-shadow" can be clearly seen. To prove to yourself why the Barlow-Laser method is so accurate, deliberately grab and wobble your laser collimator+barlow combo in the focuser tube. Notice how the diffuse red light patch moves about, yet the ring-shadow remains almost perfectly still.
So now its simply a case of making the fine adjustments to the 3 thumbwheels to move the ring-shadow exactly over the centre of the cross-hairs, finishing off by tightening the Lock Screws, and removing the Collimation Cap from the drawtube, and turning off the laser.
Click the photo below for a larger view, which shows the ring-shadow on the collimation cap, and we're trying to move it from poorly-collimated position A to properly collimated position B, central over the cross-hairs.
- Baader Laser Collimator - I bought mine from David Hinds UK (link goes to catalog for the laser collimator)
- Andy's Shot Glass - Collimation Videos - Excellent, well presented website by Andy Raiford providing information for hobbyists on a budget. This link goes to his Collimation videos for Newtonian Reflectors.
- Observations on Newtonian telescope collimation - Newtonian collimation with lasers and using barlows in the Barlowed Laser Technique.
- Kendrick Astro Instruments - Laser Collimators - While this is a sales page for Kendrick Astro, it displays several types of Laser Collimators, and the Q&A section at the bottom of the page provides some very useful information.
- Skywatcher Telescope Star Testing Techniques - This page provides excellent graphics showing what a star test should look like, including errors such as poor collimation, abberation, coma, atmospheric turbulence, tube currents, pinched optics, astigmatism, zonal errors and surface roughness!