I have bobs knobs on the sct's and the newtonians. For the newtonians, I use a laser for the secondary and a cheshire for the primary. For the sct's I defocus and try to get the donut as even as I can.I wouldnt dare fiddle with the Rasa at the moment. cheers Paul
Yes indeed! Laser collimators can be very, very bad for collimation!
Check by rotating in 45 degree increments around a full 360 degrees while pointing it to a piece of paper on a wall 5 metres away. Mine scribed a circle very close to 75mm in diameter! IE the laser was 37mm in error (half the diameter of the circle it scribed) over 5000mm. So in my Newtonian's focal length of 1000mm that was an error of 7mm. I repeated the test several times - in case the 1.25 - 2" adapter which I had clamped to a table had moved during the rotation of the laser. So in my Newtonian, moving the secondary mirror so the laser was in the middle of the primary doughnut, was 7mm out ... like pointing it to some bit outside of the doughnut! And worse, it is double that error when adjusting the primary mirror (the laser travels 2000mm for that adjustment) ... like pointing it to some bit outside the bulls-eye target of the collimator.
I picked out the rubber glue holding the collimators three laser adjustment screws, but as per information I found on the web, they are very hard to adjust! It took me all morning adjusting then re-testing the rotation 'error', before realising the best I was ever going to achieve was around 10mm circle of error. It then took several hour more trying to get back to around a 10mm circle of error.
In the end I gave the laser collimator away and just use some tips from somewhere on the web which worked for me to this day. I'll post them up next.
Tip 1. Secondary mirror does not affect collimation much. Secondary mirror adjustment does NOT have to be millimeter precise.
Why? The secondary mirror is NOT designed to be the exact same size of the light cone coming from the primary mirror. It is slightly bigger. Bigger so it can allow for movement of the light cone as you adjust the primary mirror, and also to allow adjustment of the position of the secondary mirror along the axis of the scope. It also allows for the problem where the spider vanes aren't perfectly centered in the aperture. And even allows for the focus tube to not be exactly 90 degrees to the tube axis.
Tip 2. Use a solid colored card within the tube behind the focus tube to center the secondary mirror behind the focuser.
Put a piece of solid colored paper or card inside the OTA opposite the focus hole. It REALLY helps you to 'see' the secondary mirror and its image! Point the tube towards a brightly lit wall. The tube does not have to be on its mount. It can be lying on a blanket on top of a table. The wall color has to be different to the color of the card (white wall is best)! To position the secondary midpoint across the focus tube hole, get a friend to wind the secondary up and down the axis of the tube. You should stand back from the empty focus tube, and using one eyeball, sight down to see the 'image' of the secondary mirror standing out against the solid paper or card background. If your eyeball tells you the 'image' on the mirror looks equidistant within the drawtube opening, then that will do. Once that is done, do not ever adjust that bolt ever again!
Tip 3. Continue with the solid color card and single eyeball to allow you to 'see' an equal amount of each of the primary mirror rubber 'holders'
Same as before, using one eye, look down the empty focus tube, and get to a distance out from the focus tube, where you can see one or more of the primary mirror holders. Hold at that distance out of the focus tube, and continue to use one eye while adjusting those three small secondary mirror angle screws (it is ok to also rotate the secondary if you have to). You want to adjust the three screws so you can 'see' all three primary mirror cell holders. They need to look like they protrude about the same amount in the image you 'see' on the secondary. Remember there is no need to be absolutely millimeter perfect about this as it is the secondary mirror!
NOTE You can tighten down those three secondary adjustment screws quite tightly! Mine are tightened so much they have 'bored' into the metal on the back of the secondary. As a result, my newtonian has done many 1000 km trips on bad potholed outback country roads and at the destination the secondary mirror did not move one bit! LOL.
Tip 4. The primary mirror ... use the plastic collimation cap ... and check it everytime you use the scope (unless you have a permanent observatory)!
Adjust the primary mirror three bolts so you can see the reflection of your pupil in the doughnut ring. Using a collimating 'cap' you know you are collimated! Why? Because it is an absolutely straight path of photons of light between your eyeball and its reflection within that doughnut. Those photons are travelling in perfect straight lines. They do not lie.
If you are doing Astrophotography and want NASA APOD quality ... then ok, you MAY need to perfectly centre the secondary across the aperture. And may need to check the perfect centering of the primary doughnut. And may need to check the axial centre of the focus tube and whether it is hitting the perfect centre of the secondary. Just like you would also have to check the perfect distance of sensor from focus point. And check the sensor is perfectly 90 degree's to the light path. And so on.
But for most of us ... the tips above will work perfectly fine ... 1. Use the solid color card tip to easily use one eye to center secondary across the drawtube. You only have to do this once. 2. And to easily use one eye to get the three primary mirror holders visible about the same amount in the image through the drawtube. Then tighten the secondary down hard! Usually only have to do this once! 3. Use the plastic collimation cap to see your pupil reflected in the centre of the primary mirrors doughnut. Very easy to do ... everytime you use the scope!
__________________________________________ Ken James - Snake Valley Australia CAMERAS: Samsung SCB-4000, Mallincam Xtreme & Xterminator, 3 different Revolution Imagers, 2 ToUcams, SX-5c, Canon 350d, 2 different IMX224's (an RI & RT) and an RT178. Broadcaster on NSN as 'Snake Valley Australia' HERE --> www.nightskiesnetwork.com/ - Video Astronomy Website: ballaratman.wix.com/videoastronomy