Thursday, 16 May 2013

Collimating My GSO RC8 telescope


Hi all, 
I thought it would be a good idea to start a Ritchey-Chretien (RC) telescope collimation discussion!  There are a lot of folk out there quite confused about how to best get these scopes collimated so here's an attempt to share some ideas.

I have recorded some videos of how I have gone about doing this on my own Altair-Astro re-badge of the GSO RC8 design.  In the US these are sometimes marketed as Astrotech and Astro-Physics RC scopes.  Teleskop-Service in Germany also rebadge them. What I discuss here is equally appropriate to the 6 and 10-inch versions of this scope too.  I understand there is also a 12" version too.  My scope is a 8" (200mm) model.

What I want is a RC scope collimation technique that I could perform during the day, at my leisure, indoors, with no artificial stars etc and would give me a high degree of accuracy - maybe not 100% perfect but somewhere approaching that and giving me a starting position to just do a quick tweak when under the stars. Clear skies in the UK and NW Europe are so rare just lately that when we get one I want to be observing and imaging and not spending half the precious time collimating scopes!

I have read some scary experiences about collimating RC telescopes.  This initially made me nervous and afraid to tinker with mine.  However, I found the process straightforward enough as long as a careful, iterative approach is adopted and you have the correct tools.  Don’t lets forget, all we are doing here is turning a few screws and adjusting mirrors slightly.  We are not splitting the atom!

The Cheshire EP method.  

Please read on and don't dismiss this technique!!! It works superbly well and gives highly accurate results. The idea with the Cheshire technique is to centre the dot from the Cheshire in the middle of the central rings by adjusting the secondary mirror.  Once this step has been done you need to ensure the central rings are concentric by adjusting the primary mirror. 

TIP: Do this outdoors on a bright day with the Cheshire 45 degree surface facing upwards (towards the sky) and the tube parallel to the ground.  It is much easier to see the shadows and reflections that way.  We want the concentric rings' reflections that you will see (as long as you are in good light) as concentric as is possible with a central dot in the centre.  Only tiny, tiny tweaks are required to do this.

Repeat these two steps perhaps three or four times until you converge on a collimated scope.  It sounds complicated and kinda scary but really, it isn't.  Its very straightforward and a quick procedure to perform.  Indeed, after playing around with numerous methods to collimate an RC scope (including the one below), the Cheshire is the one I now use because it works so well when out in the field setting up and needing to quickly tweak the scope. 

Once you get the mirrors correct as above, if you stand 3 or 4 metres (12 feet) away from the front of the tube, adjusting your viewing angle so your eye is aligned with the optical axis (by adjusting your eye position) you should see very close to a perfectly centralised series of multiple reflections, all centred around the central axis of the scope.

Once you get to this stage, use the laser collimator (an accurate one - I use my Howie Glatter) to ensure the laser is projected as close to the central secondary marker as possible and is reflected back into the origin (i.e. back into the collimator body by the secondary mirror bending the laser beam back onto itself).  I use the Teleskop-Service adjustable tipping plate to do this since I don't want to fiddle with the primary collimation screws now that I have the alignment of the mirrors correct!  Once this is done you need to use the Cheshire again to check you did not disturb the ring pattern and the centralised dot.  This iterative approach is fundamental to getting the RC scope design properly collimated.

Once this is done you are finished!  It really is straightforward and takes 5 to 10 minutes once every four months or so - these scopes hold collimation very well indeed!  The GOLDEN RULE when adjusting the scope is that tiny tweaks only to the screws are required. TINY!!! Literally 1/32 of a turn or less.  If you go at it ham-fisted you will wreck the collimation completely (although still straightforward enough to get it back if you do).

It wants to look something like this - with a central dot and concentric shadows surrounding it.  Note my scope looks better than this - it was difficult to take the photo with the camera perfectly on the optical axis!

The objective of this post however is to discuss a novel method that Jefferey Jongmans from the Netherlands introduced me to and we have discussed at some length. My advice is to read this thread entirely and take your time and don’t rush.  Do the procedure when you have plenty of time in a spare room or somewhere where you can draw the curtains/shutters to make it dark easily. Don’t be tempted to take shortcuts or skip the iterative nature of this method.

In my case I have purchased a very accurate Howie Glatter laser collimator (which I have also reviewed on my blog) and a circular holographic attachment that screws onto it.  This tool is very useful for collimating an RC scope as we shall see.  It will also be very useful for my other scopes too so I regard this investment as one that will reap dividends on my other scopes for years to come.  

I have also bought a focuser tipping plate for my RC8 scope so that the focuser can be adjusted independently from the main mirror.  Recall that on the GSO RC design, the focuser is permanently attached to the mirror.  As such on the scope as purchased one cannot adjust the alignment of the focuser independently from the primary mirror.  In a perfect world, with a 100% perfectly built scope the mechanical axis of the scope (focuser, secondary and primary) would be precisely aligned around the centre axis of the scope.  Ahhh, perfect worlds, wouldn’t that be nice?  I’d have several Ferraris, my own 40m yacht off St Tropez and my own premiership football team……  Alas, we don’t live in that perfect word and these scopes are built to a budget - its amazing they can be built as good as they are for such little money.  If the focuser and primary are not 100% perfectly mechanically aligned then unless we can separate them in some way to allow independent adjustment we can never collimate this design of RC scope properly. The tipping plate provides this decoupling of the focuser and the mirror and allows collimation of the focuser, secondary and primary completely independently.  As we shall discover later, this independence is very important.  Indeed, even if you use the Cheshire method, it is very useful to be able to separate the focuser and primary for collimation purposes.

What you'll need:
1. Cheshire Eyepiece (and if its a 1.25 inch one then ideally also a centering adaptor so that it fits snugly in a 2" focuser).
2. Howie Glatter Collimator with holographic attachment (or equivalent other brand).

STEP 1
Firstly, ensure your scope OTA is approximately parallel with the floor on your mount or a table, facing a plain wall in a well lit room (or outside of course).  You do NOT want the tube vertical and risk dropping an allen key/wrench onto your primary! 

Now, the very first thing to do if the scope is considerably out of collimation is to get it back to something resembling collimation with a Cheshire EP in a bright room or outdoors.  You want the dot to be in the centre and the rings that you see in the centre of the FoV of the Cheshire as concentric and as circular as you can.  The rest of the entire procedure is to refine what you have achieved in this very first step!  Indeed, if you are accurate enough with this step you don't need the rest of the procedure, as mentioned.  I have found I am now accurate enough with the Cheshire Eyepiece not to need to do the holographic procedure.

With an ACCURATE laser collimator inserted into the focuser check that the laser falls onto the centre marker spot of the secondary mirror – you will see this reflected off the primary by slightly standing to one side when peering into the OTA.  I cannot emphasise enough the importance of an accurate laser collimator in this method; if its inaccurate you are worse than wasting your time – you are putting errors INTO your scope!!!  As mentioned I use the Howie Glatter 2” version with the 635 nm laser.  Expensive?  Sure. Worth it?  Absolutely in my opinion.  If you want quality then get your wallet/pocket book out! Rotate the collimator in the focuser and make yourself happy that the collimator is indeed accurate; it should stay on one spot as it is rotated.  I have found that only LIGHTLY tightening the focuser EP retaining screws ensures you don’t compress the collimator body in the focuser on one side and skew the result in one direction.

If the laser is not on the centre spot (ideally in the centre of the centre spot) adjust the COLLIMATOR TIPPING PLATE so that the laser moves to the centre.  DO NOT adjust the primary or secondary at this stage. It may take some time and practise to get this right.  When you’ve done this rack the focuser in and out a few times and you may see that the laser spot does move very slightly from the dead centre but stays inside the centre spot.  Don’t worry about this too much – get it as accurate as you can.  Keep your eye on the collimator reflection back into the collimator body by the secondary mirror folding the laser beam back into itself.  This is the situation we want in step 1.   First, ensure the laser is as accurate on the centre of the secondary centre spot as you can realistically get it.  Take your time and get this right.  Now, be aware that the exact optical centre of the secondary may not be the exact physical centre of the mirror (i.e. where the centre spot is located).  It all probability it will be but it may not be (in my RC8 scope the two match precisely).  So in order to get the laser beam to bend back onto itself (and re-enter the collimator body) its possible that the laser may need to hit the secondary mirror slightly off the exact center of the secondary physical centre spot to achieve this bending of the laser onto itself.  Our objective at the end of this step is the laser is as close to the centre of the secondary collimation spot as we can realistically make it for all focuser positions with the laser folded back onto itself.  You may have to compromise getting it 100% perfect in one focuser position in order to get it 99% right for all focuser positions.

TIP – you may find that with some focusers at the very ends of the focusing action (i.e. when the focuser is all the way in or all the way out) the focuser “droops” very slightly.  Don’t worry about this - just don’t do your adjustments with the focuser in those two positions – and naturally, don’t do your imaging with the focuser like that either!  Ideally, try to find the position that the focuser will be in on your setup with your camera. 


First Step - Aligning Focuser with the Secondary Centre Spot


STEP 2
Screw the circular holographic attachment onto the Howie Glatter collimator and insert into the focuser.  Now look into the front of the OTA tube.  You are looking for a set of rings from the collimator reflected off of the secondary and back onto the primary mirror.  In other words there should be a set of concentric rings visible on the surface of the primary.  It may not be easy to see these rings and, paradoxically, the cleaner the mirror the harder it will be to see the rings.  The rings are reflected off of the microscopic dust on the mirror surface that will be present even on a very clean mirror. Adjust your eye position by hiding the bright central beam behind the secondary when viewing from the front.  Can you see the concentric rings?  They are faint so if not look again and keep trying!  They will definitely be there.  You may need to darken the room.

When you have found the rings you want them to be as concentric as you can possibly get them and equidistant from the centre and to the mirror edge.  Rack the focuser in and out to try to get a ring near the edge of the mirror – this makes it easier to judge whether or not they are concentric.  If they are not, and it will be most obvious if they are not, you need to adjust the three collimation screws on the secondary.  Using VERY small adjustments, only a 1/32 of a turn or less, adjust these three screws to get the rings concentric on the primary’s surface, checking after every tweak. It can be tricky to do this and see the effect on the rings simultaneously. 

Personally, I have found that combining this ring viewing method and a Cheshire EP to be highly effective.   Get the dot in the centre by adjusting the secondary by using a Cheshire EP then swap for the laser holograph and check the rings again by viewing from the front of the OTA.  It is surprising how little adjustment of those screws is needed. Very small tweaks are the difference between being collimated and not collimated.  Take your time and enjoy doing this precision part of our hobby!  That said, don’t be obsessive either.  You want it as accurate as is realistically possible within your capabilities.  You can get 98% there in five minutes say but that final 2% can take hours!  Don’t bother!  When checking the concentric rings get them as good as you can.  They may not be 100% perfect but do the best you can.  For example, if the rings are one or two mm from being perfect then don't bother adjusting - that is accurate enough.  It isn't the Hubble Space Telescope we're collimating here!

At the end of this step our objective is as near as possible to concentric rings on the primary surface with the laser central beam folded back onto itself back into the collimator body.  Do not move onto step 3 until you have achieved these objectives.

Second Step - Ensure Perfect Circles reflected onto Primary


Should look something like this

STEP 3
You may have noticed the rings on the primary mirror we discussed in Step 2 are also projected from the primary out the front of the OTA onto an adjacent surface.  In this third step we want to project those rings onto a wall about two metres (6 feet) or so away.  The scope needs to be approximately parallel to the floor. We are looking for concentric rings on the wall with the shadow of the secondary absolutely centred.  Make some accurate circles on a piece of paper and tape it to the wall or use a ruler to check.

You may think that if step 2 was done carefully to achieve concentric rings then surely step 3 would be unnecessary and that the rings on the wall must likewise be concentric?  This is NOT the case with a RC scope.  This is because the primary mirror could be slightly tipped with respect to the secondary resulting in the wall projection being inaccurate with incomplete or partial inner or outer rings even though step 2 was perfectly concentric.  Remember, with an RC design we are adjusting two hyperbolic mirror surfaces, not just the secondary that we were adjusting in step 2.  These two mirrors need to be collimated with respect to each other.

It is very possible that your scope shows these rings perfectly without any adjustment.  This is because in the RC design the secondary can lose collimation more easily than the primary. If so then good for you and well done!  However, if the rings on the wall are not concentric, or the secondary shadow is not central, or you have partial inner or outer rings, then the primary mirror needs to be adjusted.  Again, only small adjustments or tweaks to the primary mirror collimation screws are required.  Observe the rings on the wall whilst adjusting the primary collimation screws. Get the wall rings as concentric as possible.  Also, ensure that when the focuser is racked in and out the outermost and innermost rings on the wall “come in and out of view” around the whole ring at approximately the same time.    You want the rings to fade in and out all around the edge at almost the same focuser position - this indicates alignment of the two mirrors.  You may not get this 100% perfect but do your best.  You will find that only the inner and outer rings will show this effect – the middle rings will always be visible.  Again, take your time and don’t rush.  Little tweaks are all that are required.


Third Step - Project Rings onto Wall


This is the projection on the wall


Now, in the process of collimating the primary mirror in step3, we will also have slightly spoiled steps 1 and 2 since the focuser tipping ring and focuser itself will have moved slightly as a result of adjusting the primary mirror.  So go back to step 1.  Check again the beam is on the centre spot (removing the holographic attachment of course before doing so).  Then check for the concentric rings on the primary and finally the projection on the wall again.  You will find the second and third iterations take only a couple of minutes now you understand the method.  Before putting your tools away make sure all the collimation screws are tight, particularly the locking ones on the primary.  Then do a final check.  This process may take some time the first time you do it when you are naturally being very cautious.  But trust me, it gets a lot quicker after a few practices!

NOTE.  I have found that it is quite tricky to get all three stages perfect for all focuser positions.  In other words you may find you can get it perfect in one focuser position but it will then move out slightly for different focuser positions.  So I suggest that before you start this procedure, make a note of where your focuser is with respect to your imaging train setup when in focus one night and then perform this three step procedure with the focuser in that position.

Finally you may want to do a star test and a final primary tweak to further refine the collimation.  But this method will get you very close to collimation whilst indoors and you may not need to do such a star test!  Check also with your Cheshire EP and with the "hall of mirrors" method.  It should be almost spot on accurate!  And there’s the thing…. you can still get very accurate collimation with the Cheshire EP alone if you know what you are doing.  But the holograph method makes it so very easy to CHECK the results and allows you to fiddle with collimation screws in real time whilst checking the circular patterns whilst you have the wrenches in your hands.  Brilliant!

It goes without saying that now you have got the scope collimated you DO NOT want to remove the focuser  or turn it!  If you do you, or you need to add another M90 tube extender, you will probably slightly disturb step 1 and as a consequence steps 2 and 3.  Ideally you should collimate again.

I have found that RC telescopes hold their collimation fantastically well.  Since the primary does not move, like it does on a SCT, the collimation tends to stay accurate for many months at a time unlike a Newtonian.

Enjoy using your collimated RC telescope!  They are truly wonderful and easy scopes to own!

By all means feel free to add your own experiences to this in the chat window below.

4 comments:

  1. Hi Steve, a very informative post. I will be trying this procedure over the next couple of days as my Glatter laser should arrive today.

    Mike Millan

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  2. Steve,

    I have the same laser collimator but the attachment I have produces a rectangular pattern. Can it still be used? If so, can you briefly explain what I have to do differently when adjusting the secondary?

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  3. Wish I could find one of howies lasers now since he passed they are a true golden egg it seems :(

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