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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.
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