Tricking up your GSO laser collimator

[mental4astro]

Like many enthusiastic amateurs, I bought a GSO laser collimator. What I didn't know was that this little devise lacked a bit of collimation itself.

No matter how carefully I went through the collimation process, a tiny comet tail remained on the brightest of stars. Was there something wrong with my scope? But this same "tail" appeared in another Newtonian I have too.

The other week, I used a mate's GSO collimator. The beam on this one was the size of a 5cent piece! Though mine was slightly ovoid in shape, this one was way too big.

As I don't want to spend over $250 on a you-beaut one, I looked into how to pimp this one to perform better.

As it turns out, it's EASY!

Tightening the beam requires nothing more than a 10mm X 10mm bit of masking tape and a sharp pencil.

Tightening the beam:

* With a suitable hex key, remove the screw holding the frosted diagonal (pic. 1)
* Place the bit of tape over the hole that sits closest to the laser
* Shining a light through the diagonal so you can see the illuminated hole shining through the tape, use your pencil to pierce the tape right in the centre. Make the hole the same size as the diameter of the lead, about 2mm. Any smaller and the diffraction pattern that results makes the beam to big and useless (pic. 2)
* Replace the diagonal into the collimator

Collimating the laser:

* Hammer four 50mm nails into a block of wood to form two V-shaped cradles for the collimator to sit in (pic. 3)
* Number the three collimating screws on the laser, so you know which one you've tweeked
* With the laser in the cradle, and the cradle clamped in place so it doesn't move, shine the laser onto a card at least 3m away - turn the laser in its cradle, if the beam stays in the one spot, it's collimated, if it traces a circle pattern, it needs collimation
* Do quarter turns to trace the pattern, then tweek the collimation screws and seek to make the beam stop tracing the pattern

The final picture shows the pattern my laser traced initially and as I tweeked it until it stayed bany in the middle. My laser was 4.8m from the card, and the angle of deviation was 0.18degrees - Bugger all, but this was making all the difference.

The only other thing I had to do to make sure of the best collimation for my big dob was mark a tiny centre spot on the secondary mirror with a permanent marker (off-setting needs to be taken into account). The laser is aimed at this spot. This will also show if the secondary needs to be raised or lowered, or if the focuser is not square with the secondary.

Clear skies,

Mental.

[AG Hybrid]

^^^ This ^^^

Very good Alex :) . I'll apply this to my own collimator, although it seems to be quite spot on. But sometimes I do wonder about how you have the time to think about these sort of things while having small children in the family.

[multiweb]

:lol: Did that too at the beginning. Sooo confusing hey? :)

[michaellxv]

Thanks Alex.
Now the other problem I have is that mine sits quite loose in the EP holder and so sags offline. Do I need anything more fancy than a few turns of electrical tape to fix this?

[mental4astro]

Michael, that's where the dot on the secondary helps by giving something to aim at.

Not sure about the secondary? Easy. Get yourself a Kodak film canister with its lid & make a 2mm hole dead centre in the lid & base, and use it in the focuser to check for the dot on the sec. A 2mm hole is just right for this purpose.

Oh, & every time you open the back of the collimator wherf the batteries are housed, you need to recollimate as the spring will shift the laser.

For me it's about problem solving. I can throw money at things, but finding a solution is more satisfying.

[pmrid]

Hi Alexander, this is an oldish thread but spot on a problem I am having. I'm going to try to re-align my Baader laser as a first step to getting collimation on my large newt right.
I have already centre-spotted it. There is a 5mm offset on the secondary (away from the focuser) and I note that your advice is to allow for this offset when centre-spotting the primary. Or do you just aim the laser 5mm below an accurately placed centre-spot?

Peter'

[mental4astro]

Hi Peter,

If the secondary has been off-set in the vertical axis, then the mark is placed on that "off-set centre". If there is no vertical off-set to the secondary then the mark is placed dead centre of the secondary. You don't want more than one mark on the secondary. Any more and it starts to get confusing.

The secondary on my big dob I off-set in one axis only, that one being away from the focuser. I didn't bother with the vertical one, so the mark is placed dead centre for this secondary mirror. From your thread reply, this is the situation with your secondary too, so the mark you've done is all that is needed, :thumbsup:.

The off-set I had calculated for my dob when I was designing it, if I remember correctly, was to be some 3mm down towards the primary. If I HAD off-set in the vertical axis, then the mark I would have place on the secondary would have been 3mm above its geometric centre as it is only the geometric centre that has been lowered, BUT 4.24mm along the plane of the secondary taking into account that the mirror is at 45* to the focuser (remember Pythagoras from school?). It is important to keep this in mind or you will not achieve true collimation as the mark on the secondary will not be in its correct place.

Alex.

[pmrid]

Many thanks for thatclarification. Now to put it into pracice .....

Peter

[alistairsam]

Hi Alex,

I'd like to spot my secondary as well, can you explain how you calculate the offset along the secondary plane from its centre?
I do understand the concept of the offset, but not sure how you measure and spot the secondary accurately.
and the other thing, how do you measure the offset from the secondary holder? I bought mine second hand so no idea if it has.
mine is the 8" F4 with the 63mm secondary.
btw, have you tried adding a 1.25" barlow to the collimator? since the angled glass is frosted in this collimator, I'm not sure if the primary spot will be visible in the diagonal of the collimator. but worth a test.

edit: slightly off topic from the laser, but just struck me that if the secondary is offset, then do you still centre the secondary with a sight tube? i'm guessing not, and that would explain my collimation woes.
I'm using a 2" extension tube and then a 1.25" adaptor as my sight tube with a paper underneath the secondary to improve visibility.


thanks

[mental4astro]

Off-setting a secondary mirror is not critical for a visual instrument. The 5% gain of light is not perceivable to our eyes, especially at the low light levels we view at. The purpose of it is a get a fully illuminated field into the focal plane. For photography purposes, 5% is a lot of light.

If you want to off-set a secondary, there are plenty of sites that can help. It is not difficult, but involved as it requires dimensions such as the diameter of the OTA, the height of the focuser, the position of the focal plane relative to the centre of the path of light through the OTA. This to optimise the illumination of the field.

Many commercial OTAs don't bother with off-setting the secondary.

Locating the centre & marking the secondary mirror

To place the centre spot on the secondary, you will need to remove it from the spider. You will also need a set of vernier calipers, a ruler, a clean sheet of paper, some sticky tape, a sharp pencil and a fine permanent marker.

* Use the vernier calipers to accurately determine the size & position of the minor axis and the major axis. Tip: The minor axis will give you the smallest dimension of the secondary, and the major will give you the largest. Make a mark on the side of the mirror to where each point lies. The size of the secondary's axis will be used in any off-set calculations you do.

* Lay the secondary face down onto the paper and trace around the mirror to get its outline. Also note on the paper the position of major and minor axis positions.

* Use a ruler to join the corresponding axis points and you should have the position of the secondary's centre.

* Any off-setting in the vertical axis can now be placed on the paper. Be mindful of the orientation of the mirror - the off-set will be above the centre of the secondary.

* If you do not off-set, use the sharp pencil to pierce the paper at the centre point. If you do off-set, pierce the paper at the position of this off-set. Place the mirror back over the traced pattern and carefully tape the paper in place. Take the permanent marker and make a mark on the mirror through the pierced hole. DONE!

Put the mirror back into the spider and carry on as normal.



Note, don't remove or replace the secondary mirror with the OTA pointing up!!!!!! You WILL drop something onto the primary if you do, screw, nut, secondary, screwdriver. Get the message.

I'll upload a picture of my marked secondary mirror later on today.

Now, if you are not sure if your existing secondary is off-set or not, you can mark both centre and off-set positions, replace the secondary mirror and check which of these spots is centred in the focuser. The one that is not can be removed with a good quality cotton bud and usually metho. You can practice this first on another piece of glass, marking it with the marker you've used and practice removing this spot first a few times before you attempt it on the secondary.

*This is the process I used. There is a chance that damage can be caused to your instrument if you choose to follow this method. That is a responsibility and risk you need to take on yourself.*

Sam, no I haven't tried a barlow with the collimator.

In the dark you won't be able to see the spot on the secondary. This can be overcome by holding something white below the secondary and moved around until you see the reflection of the white surface in the secondary. The mark will then be easily visible and the laser used accurately. Just as you do Sam.

[alistairsam]

Thanks for the explanation Alex, clearer now.
Will give it a shot.

[GeoffW1]

Hi,

Yes true, and even turning it on with big muscles does that as well! You are best to try to turn it on with about the same tension on the ON/OFF screw as you were using when you collimated the laser unit.

I could watch the laser dot go right off centre just from giving the ON/OFF screw an extra turn or 2.

Cheers

[Jason D]

Actually there is a simplified mathematical formula. The offset along the secondary mirror surface is:
( secondary-minor-axis-length * sqrt(2) ) / ( 4 * F_ratio ) above the geometric center along the major-axis of the secondary mirror. See first attachment.


For example, for your scope:
(63mm * 1.414)/(4*4) = 5.6mm above the geometric center




No, the offset is what will make the secondary mirror appear centered under the focuser. If you do not apply the offset, then the secondary mirror will appear higher in the OTA. Not intuitive but the reason can be explained graphically – see 2nd and 3rd attachments.

In general, most reflector owners do not mark their secondary mirrors. They can get the same outcome by just using a sight-tube. Once the secondary mirror is centered/rounded under the focuser, the offset would automatically be incorporated. I am referring the offset along the OTA axis. The offset away from the focuser has no effects unless you are using DSC.

Jason