Observations on Newtonian telescope collimation

Introduction to collimation

Collimation is a word that strikes fear into the heart of many an astronomer. I am a Newtonian owner who has lived through some of the learning curve of collimation, and now feel ready to share a few thoughts with you. I am by no means an expert, I have noted a few points worth mentioning.

New - see section on astrophotographic collimation of newtonians below:

Collimation Basics

I am not going to repeat the basics in detail. Go and search the internet - there are many detailed pages with good instructions to follow.

Suffice to say, the basic principles that you need to learn about are as follows
1. Square the focuser with the OTA.
2. Square the secondary in the focuser.
3. Align the secondary with the centre of the primary.
4. Adjust the primary.

There. Simple. :-) Seriously, go and read around.

Ok, now for some of my thoughts.

1. Focuser slop.

Put your laser collimator into the eyepiece holder and look down the OTA (optical tube assembly). Rack the focuser in and out. The spot of light from the laser moves a bit. This is because an average rack and pinion telescope focuser is not perfect. Whilst being wound up it is at a slightly different angle to the OTA than when it is being wound down.
Now examine your telescope focuser very carefully. More likely than not there are some invisibly tiny hex bolts that adjust this slop from 'very wobbly' to 'I cannot move the focuser'. Twiddle and find a happy medium. You can go a lot tighter if you have an electric focuser. Trying to use a stiff focuser by hand is nightmarishly tricky.
If you are feeling adventurous, disassemble the focuser and wash off all the cheap chinese tacky gunk (laughably referred to as grease) and lubricate the whole thing with white lithium grease.

2. Laser Collimators and laser collimation

Lots of nice things are said about laser collimators, and conversely, lots of nasty things are also said. I've now decided to ignore the nasty things - they don't know what they are talking about. Buy one. Telescope house (well, BC and F) do a nice laser collimator for about 55 ukp, which is miles cheaper than everyone else. I know Telescope House are the sole UK Meade importers and get a lot of flak for the pricing, but this is a bargain.
Make sure whichever laser collimator you buy has a 45 degree slice cut out of it for newtonian telescope collimation.

3. Using a barlow for laser collimation.

This is why laser collimators are not as bad as many think. The idea of a laser collimator of course, is that you shoot the laser down the eyepiece holder, it bounces off the secondary mirror, down to the primary mirror, back up to the secondary and, all being collimated, vanishes up the hole from whence it came.
So whats bad about that?
Well, first of all, that hole in the collimator that the laser comes out of is relatively big. You will be able to make a few tiny adjustments of the collimation screws of your primary telescope mirror and you'll still have the light from the laser going up that hole. Where is the best place we ask ourselves?
Secondly, loosen the set screws and wiggle the laser collimator about. Now tighten the set screws. Laser spot not quite in the same place as before? Yup. Thats why everyone whinges about laser collimators on newtonian telescopes.
Whilst browsing around the internet you will find references to the barlowing of lasers. The barlow laser collimation technique. I admit this was a bit of a mystery to me for a while.

The idea is to remove the inaccuracies of the wiggling laser collimator by transforming the laser from a coherent beam of light to a light source placed an infinite distance away. Or something like that. Anyhow, read on, it works.
This removes the wiggle factor from the equation.

1. Do a normal collimation of your telescope.
2. Find a barlow. I have a 1000mm focal length Newtonian telescope, and get on best with a 2x barlow, but you might want to experiment.
3. Make a mask. This needs to be a little circle of paper or card with a 2mm hole in that is secured over the telescope facing side of the barlow. Personally, I have a barlow I never use, so employ some bluetak and a dremel cutting disc.

4. Put the laser into the barlow where you would normally put eyepiece. Now, look down the OTA. You will see a circle of light on the primary mirror, roughly over the centre mark. If its not over the mark, wiggle your barlow a bit.
5. Now find a small mirror. I use a CD. Something by Pink Floyd works well. Something you can fit into the top of the OTA without touching the sides or the secondary. The idea is to try and view the face of the paper/cutting disk mask that is hidden up the focuser.
6. This mask has the reflected blob of light on it. Look very carefully and you will see the shadow of the primary centre marker.
7. Now adjust the primary mirror so that the dark ring is centred on the hole in the mask. This will involve either an assistant, or a lot of running back and forth. Or rig up a webcam, or something.

8. Now the magic bit. Whilst looking in your mini mirror at the centre marker shadow neatly centred on your mask hole, reach around with your third hand and wiggle the laser/barlow/eyepeice holder assembly. You'll see the blob of light move, but the ring of shadow from the primary centre marker remains steady! Clever, eh?

Note: You might not need the hand mirror, with some setups you can see everything you need by looking in the primary and seeing the donut relfected in the secondary.

4. Camera Collimation

I'm a bit of a focal length nutter. I like to operate my SC1 modified toucam webcams on my 8 inch newtonian with focal reducers. If your CCD is in your fancy homemade, aircooled, peltier boosted hyper box it needs to be level and centred. The more you fool about with focal reducers, the more critical it becomes.
I had nasty shaped stars in my focal reducer images, and knew that my camera needed collimating!
It occurred to me that the laser might be good for this. So, I fiddled with my embarrassingly large collection of T adapters, extension rings etc etc until I figured out how to mount the laser pointing down into the camera.
Then it was simply a case of moving the CCD about until it falls under the laser. The laser appeared to do no harm to the CCD. Mind you, don't blame me if the CCD melts.
Then the clever bit. Look in the 45 degree slot on the laser. Wiggle it about a bit. There will be a number of dim spots and one bright spot, which is the laser reflecting off the glass plate on top of the CCD.
Adjust your CCD until the spot vanishes up the hole in the middle of the collimator. The more extension rings you use and therefore the further the laser is from the CCD, the more accurate this is.

The really elegant part being you have made the CCD perpendicular to the axis that light normally enters the camera. Not level to the box, or lid or whatever. Its lined up to that optical axis you have been struggling to get collimated for the last dozen paragraphs.

5. Star Collimation

I should add at this point... there is no substitute for real star collimation. Looking at airy rings around slightly defocused stars in a stupidly powerful eyepiece during periods of perfect seeing is the best, and some might say only, way of getting perfect collimation.
However, this is not something I have mastered yet.

This is a great place for collimation information

5. Collimating for astrophotography

At first glance newtonian telescopes appear fantastic for astrophotography. Lots of inchs of aperture for not very much money. The reason they are not perfect is that compared to expensive refractors they do not have a large illuminated field. Also, they exhibit off-axis coma. The coma is inherent in the optical design of fast newtonians, and gets worse the faster they are. To understand how factors such as secondary size effect these things look at this spreadsheet.

The point I am trying to make is that on most newtonians the "sweet spot" with good illumination and low coma is very small. Which means that you need to make sure the light cone is collimated to hit the ccd chip squarely.

It is my opinion the part of the collimation that effects this most is adjusting the secondary so that is is centred when you look down the focuser. The secondary rotates in it's holder. If the secondary is not rotated so that is it pointing direcly square with the focuser, the light cone will miss the centre of the ccd. Most collimation instructions skim over this section, telling you something about making it look centred when you look down the focuser. I think this is very hard to do.

This first thing to do is make sure the secondary hangs down the tube the correct amount. Use a ruler to mesure the distance from the top of the OTA to the centre line of the focuser, and make sure that the centre of the secondary is at about the same position.

Next, look at the secondary holder. Usually they have three screws for adjusting the tilt of the secondary 120 degrees appart. You must make sure the spider that holds the secondary is correctly fitted. You should have one screw pointing at the focuser side of the tube.

Now, the key is to rotate the secondary into position, and only use the front screw (nearest the focuser) to adjust the tilt. First make sure that the 3 screws are extended the same amount. Then put the laser into the eyepeice holder. The red spot will point somewhere onto the mirror. Slacken the three screws so that they just hold the secondary in position, and rotate the secondary so that the laser spot falls on an imaginary line between the primary centre spot and the wall of the ota in line with the focuser.

Now adjust the tilt of the secondary so that the laser spot hits the primary centre spot. You must tighten and loosen either the focuser side screw on its own, or, the two other screws together. If the spot moves off the imaginary line on the primary, do not use the back two screws to tilt the secondary, you must rotate the secondary until the laser spot is back on line, and then try to work the laser back onto the centre spot again using either the focuser side screw or the other two together. Once the laser spot reaches the primary centre mark, your secondary is correctly aligned, and you can carry on normally with tweaking the primary.