Rear View Barlowed Laser Collimation
By Steve Smallcombe
There are many methods and commercial devices available to help collimate a Newtonian telescope, and thus achieve the instrument’s full potential. The various devices and collimation methods however vary considerably in terms of accuracy and convenience.
Collimation will involve adjusting the tilt of the primary and secondary mirrors, typically by adjusting one or more screws or knobs that control the tilt or alignment of the various mirrors. It is also necessary to assure that the focuser and eyepiece are aligned with the center of the secondary mirror.
An accurate collimation will center the “sweet spot” of the image from the primary mirror exactly in the center in the eyepiece. An accurate collimation method is one that achieves this condition reliably and it not subject to error.
A collimation method can be said to be convenient if you can twist one of the adjustments screws and immediately see the effect on the alignment. With instant feedback, collimation is faster and easier, compared to methods where you need to move your head or body in order to observe the effect of any given tweak.
For example, collimation using a Cheshire eyepiece requires observing the alignment by looking through the eyepiece. One can typically reach the adjustment screws that affect the secondary mirror while looking through the Cheshire and thus conveniently align the secondary mirror. This is typically impossible to do however, while adjusting the alignment screws for the primary mirror thus making it impossible to see the effects of the adjustments in real time – inconvenient.
Many collimation devices make use of a laser placed in the focuser with the laser’s beam reflecting off the secondary onto the primary mirror, and back again. The use of a laser seems high-tech, precise and convenient, but may not necessarily produce an accurate collimation, as we will see.
Using a laser for adjusting the tilt of the secondary mirror is convenient, in that one can stand at the end of the telescope, adjust the screws that control the tilt of the secondary mirror, and directly see the effect as movements of laser’s beam on the primary mirror’s surface relative the marked center spot. However, adjusting the tilt of the secondary mirror using a laser is accurate, if and only if, the laser’s position (and direction) in the focuser is well defined and identical to the position of the eyepieces. Unfortunately as many have noticed,it is easy to rock the laser device in the focuser moving the reflected image on the primary mirror by a considerable distance – quite disconcerting when one is interested in an accuracy.
These same laser-based devices are also promoted for alignment of the telescopes primary mirror, as with good collimation, the laser beam returning from the primary should be reflected off the secondary directly back to the source, striking a “target” at the base of the laser or focuser. When the outgoing and returning beams are coincident, one might assume that the telescope has been accurately collimated.
There are really two problems here, one a matter of convenience, and the other a matter of accuracy. The accuracy problem is that laser collimation of the primary is prone to error. A small error when aligning the secondary mirror can translate to an unacceptable error when aligning the primary mirror. Try rocking the laser device in the focuser and notice how much the returned image moves.
This may not be important with slower scopes that have a wide “sweet spot”, but with any scope faster than about F5, including the Orion XT10, errors using this technique for collimation can easily degrade performance.
Fortunately there is a solution – using a combination of a laser collimator
and a Barlow lens, as is well explained in the article on the Barlowed Laser
method by Nils Olof Carlin.
When a telescope is used for astronomical observations, light from distant sources arrives as a parallel rays. Once these rays strike the concave primary mirror they are reflected and converge in the eyepiece at the focal point of the telescope, (after being reflected by the secondary mirror, of course).
Turn this around and you will understand how the Barlowed laser works. In this case a narrow beam of light from the laser (at the focal point of the telescope) is caused to diverge when it passes through the negative lens of the Barlow. When these diverging rays strike the concave primary mirror, they are reflected back towards the open end of the scope, now as parallel rays that will be reflected off the secondary mirror head back towards the focuser.
In the photo on the right, one can see the diverged laser beam on the primary mirror. Notice how the diverged beam is not round, indicating that the laser does not put out a perfect circle of light. This is not a problem as long as the center spot on the primary is illuminated, as it is in this case.
Typically the center mark on the primary mirror is less reflective than the mirror itself, so the returned light from the Barlowed laser contains a shadow of the center spot. If you place a “target”, e.g. a piece of paper with a hole for the diverging laser beam to pass through in front of the focuser, the shadow of the center spot in the pink reflected light can be observed on the target and used to collimate the primary mirror. When the shadow lies directly on the center of the target (and Barlow), the primary mirror is accurately aligned or collimated.
So why is the Barlowed laser method better? Try rocking the laser device in the focuser. In this case you will see that the reflected center shadow does not move – much more reassuring than the dancing dot achieved with the non-Barlowed laser. As long as the center spot is illuminated, and the center spot does not move, its shadow will not move either. The real advantage of this technique then is that the primary mirror can be collimated in a way that is not affected by errors in aligning the secondary mirror or focuser slop.
We now have an accurate method of collimating the primary mirror, what about the convenience issue? The article by Nils Olof Carlin describes building a “target” on the end of the Barlow, perhaps using an old filter, so that the returned image can be observed relative to the center of the Barlow. Alternately, a target can be built on the end of the focuser, and that is how I started. Unfortunately, with a target on the focuser or Barlow, one needs to tweak a knob at the primary end of the scope, and then move to the open end of the scope to observe the new position of the center spot shadow on the target – accurate, but not necessarily convenient especially with a solid tube scope like the XT10.
Fortunately, the Orion LaserMate Delux (and some other laser collimators) has a 45° reflecting surface, a “rear window”, outside the focuser, so that the returned laser beam’s position can be observed directly from the far end of the scope. The documentation provided with LaserMate Delux describes using this window to observe the returned laser beam without use of a Barlow, and that is how this device was intended to be used. However, as described above, this method is likely subject to unacceptable error when used with fast scopes.
It occurred to me however, that I could potentially combine the LaserMate Delux’s rear window with the Barlowed laser method and get the best of both worlds – convenience and accuracy.
Since the center spot on my XT10i is an annular ring, (like a donut), one might hope to see a shadow of that center spot in the LaserMate Delux’s viewing window when the scope was close to calibration and that is exactly what happens!
In the photo on the right one can see the shadow of the center spot when centered in the viewing window, as viewed from the bottom, or primary end of the telescope. As can be seen, the diameter of the shadow is similar to the diameter of the viewing window.
When one of the primary’s alignment screws is turned slightly, the shadow shifts off center, to the left is this case, as can be seen above. The photo below is a close up of the shadow with one of the other alignment knobs turned a bit.
Watching a moving shadow that correlates with movements of the various knobs and alignment screws is actually far more obvious and effective than perhaps these photos indicate. The moving annular ring was pretty obvious, even in a room with some ambient light. The Rear View Barlowed laser method works well in the dark as well, another aspect of convenience.
One potential drawback of this method is that the returning parallel rays and the shadow of the center spot pass back through the negative Barlow lens on their way to the external target window, thus causing those rays to diverge and thus somewhat expand (and blur) the shadow. In practice this does not seem to be a real problem.
As can be seen in these photos, this Rear View Barlowed Laser method allows convenient observation of the moving primary center mark shadow on the viewing screen while the user is positioned at the far end of the scope tweaking the primary mirror. It is fast and it is accurate. Perhaps, best of all, I didn’t actually have to buy or build anything to get this to work – I just had to use the bits already owned.
Make sure you have a fresh set of batteries in the LaserMate Delux. As the returned light is spread out over several inches, you need a fairly bright laser beam to start with.
If you don’t see the shadow, chances are your scope in considerably out of collimation. Start without the Barlow and get it close, then add the Barlow for the final tweaks, and periodic tune-ups.
Comparing the RVBL and the tuBlug
The main difference with using a rear view laser in an ordinary Barlow, the Rear View Barlowed Laser (RVBL) described above vs. the tuBlug made by Howie Glatter, is that in RVBL case the return beam passes back through the negative lens on the way to the target thus causing the rays to diverge and the shadow of the center spot to enlarge. The enlarged shadow has somewhat less contrast and brightness than if it hit a target before the lens as is the more preferable case with the tuBlug. The enlarged shadow can also fill or overfill the target window making it hard to identify, although with experience as one moves the primary collimation screws, identifying the moving shadow becomes fairly easy, especially if there is minimal ambient light. Perhaps the biggest problem with the RVBL technique is that because the shadow fills or overfills the viewing window one needs to be quite close to good collimation to find the low contrast shadow. In practice I found this meant that I needed to first collimate the primary using the return laser beam without the Barlow, followed by a final collimation with the Barlow lens in place.
I first started using the RVLB technique 6 or 7 years ago when I combined an Orion LaserMate Deluxe with a Barlow lens to collimate my then Orion XT10i. The idea was that it not only allowed the accuracy of Nils Olof Carlin's Barlowed laser collimation while adjusting the collimations knobs at the back of the scope, but it was something I could try without buying anything in that I already owned a Barlow lens and a laser with a rear view mirror. I wrote up the idea and posted the link to the site you are currently reading on the Orion XT yahoo group I was active in at the time.
Between then and now I moved from the XT10i to an Obsession 15 and for the last few years an Obsession 20 f5 which I have been collimating using the RVBL technique and a Hotech laser.
More recently I bought one of the Glatter 2" lasers and a tuBlug and the difference is like night and day. The brighter higher contrast center spot shadow is much easier to see and fits well within the generous 45 degree viewing window of the tuBlug meaning that there really seems to be no need for an initial primary collimation step without the Barlow. While these really are the same idea, the tuBlug is a much better implementation. It is much easier to use and one can see clearly what is happening.
If you already own a laser with a rear view window and a Barlow lens, by all means use them together to get the improved accuracy that comes with using a Barlowed laser and the convenience of seeing the effects of primary adjustment while at the back of the scope. If you are looking to buy something and can afford it, the Glatter products are great. They seem to be a well-designed system that is easy to use and are precisely machined assuring a good fit and reproducible collimation
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