As an owner of several Newtonian design OTAs in apertures from 6" to now 12" in FL from F6 - F4 I can tell you... Forget trying to properly and accurately collimate the shorter focal length with just the standard cheap tools. The larger the aperture and the shorter the FL the more accurate your collimation tools HAVE to be. Pretty much forget trying to use a standard cheap or even dolled up laser. You will need to see and correct a lot closer than a laser can possible do on its own. Lasers themselves also must be properly and accurately collimated to even begin to get the job done "somewhat" accurately. The secondary mirror in truth when in the size range of the short focal length secondary diagonal mirrors can be turned slightly "out of round" to the Primary and the focuser tube and a laser will show centered collimation.
There are methods of using a "barlowed" laser for collimation that are a way to get much better accuracy from a laser collimation. It can be a little complicated for new users and if not done strictly right can still not properly collimate a large aperture short focal length Newtonian OTA. For a Cheshire collimator if you have the faster F4 & down FL from 8" (200mm) and above OTA then your Cheshire needs to be a native (not 1.25" "adapted" to 2") 2" visual collimator. The 1.25" - 2" cheshire adapter is just one more place for slight misalignment errors to occur... remember on larger aperture/shorter FL OTAs even a slight mirror error can be detrimental to you end game collimations.
1) a sight tube or combination too to position the secondary
2) a laser or sight tube to align the secondary tilt
3) a Cheshire, combination tool, or barlowed laser to align the primary
4 (mandatory) an autocollimator to eliminate all the residual errors after steps 1-3.
Some good tools for the above:
1) Astrosystems LightPipe, Catseye TeleCat, Catseye TeleTube
2) same tools as step 1 plus Glatter laser or Farpoint laser
3) Farpoint Cheshire, Catseye Black Cat, any barlow, or Glatter Tublug
4) Catseye (first choice) or Farpoint (2nd choice)
Right here Starman1 has nailed it, forget the cheap 1.25" lasers, Cheshire & such "collimators", get something proven and made to do the right job on mirrored optics. The Cats Eye Teletube can quickly guide you to the proper offset for your secondary and spider... I've owned 2 Orion Newts an 8" F5 & a 10" F3.9... neither one had a built in offset in the secondary spider, but they were setup that way via the spider adjustment nuts and threaded studs on the ends of the spider vanes as well as the actual secondary initial alignments via the center adjustment screws. The spider and the secondary setup can both be adjusted or changed using those nuts & screws (most of the time erroneously so by the "new" Newtonian owner). If used correctly, which isn't hard, the Cats Eye 2" Comboset XLKP Kit can help you to get -everything- quickly and accurately aligned and collimated, but do spend a little time first understanding the whole offset thing... In your 8" its not much at all and can usually be corrected out with secondary till adjustments. Do a "Google" on "Newtonian secondary offset" to find a boat load of instructions and answers. Once you drop the Cats Eye Tele-Tube in the focuser in the proper instructed setup turn it to run parallel to you spider vanes you will see quickly your spider offset from center against the cross hair of the Tele-Tube. Same on using the secondary collimation and center screw to do an offset that way (the best way IMHO).
Here is just one of the many images I have taken of my 10" F3.9 Newt at collimation via the Cats Eye System. If you own Newtonians and want "perfect" collimation fairly quick its the way to go once you learn how to use the setup. You do not have to change out your Primary center mark either... I do this with the correctly centered marker I placed after the refigure years ago... Its on of those "ring hole reinforcement rings" and it works with the Autocollimator just fine.
Edited by The Mad One, 07 February 2019 - 10:54 AM.