Fine Tuning Newt Collimation

[kbrown]

Hi,

I've just started to systematically go through and fine tune the entire optical train of my 10" reflector (Old Skywatcher 250PX with steel tube). The reason being because I have taken the scope apart pretty much completely in order to re-coat the primary, flock the tube, change the focuser to a better one plus add a DIY dew heater to the secondary mirror. After getting the OTA sorted out  I want to eventually check the collimation of the imaging train too which consists of a Coma Corrector, Filter Wheel, Filters of various thickness and two different CCDs (Atik 383L+ and ZWO ASI183MC Pro). But that's another beast to tame...

Ever since I've put the OTA back together again I haven't been able to achieve perfect focus. Initially the biggest problem was slightly triangular shaped stars which was due to the primary being held in the cell a bit too tight. I've now loosened it to a point where it's barely touching the mirror clips. I was still getting very slightly triangular (at least not round or perfect oval) stars so I thought a more thorough inspection is in order. I'm by all means not an expert so I was hoping to get some tips and comments on how to improve things.

Starting with the OTA, here's what I've done so far:

• In order to position the secondary mirror in the centre of the tube (on the main optical axis), I printed out and laminated a template that allowed me to check and adjust the spider vanes visually. It's not the most accurate thing in the world but the best I could come up with. The template fits snugly on the inside of the tube and has a hole in the middle so you can easily see when the depth adjustment screw is centred.
• I then took my guider camera (QHY5L-II) and put that in the focuser together with a small CCTV lens so I could easily see what's going on inside the scope.
• Using the live video feed in SharpCap with the circular reticle turned on I adjusted the depth (towards/away primary), position and rotation of the secondary as good as I visually could. It seems like the secondary should be slightly closer towards the primary but I could not do that as my tilt adjustment screws (not original) are not long enough. It's not a massive error but I will fix it once I receive replacement screws.
• I then swapped the camera to a laser collimator and adjusted the tilt of the secondary until the red dot was in the centre of the primary.
• Then it was time to adjust the primary until the beam was reflected back to the centre of the collimator.

After the above procedure I popped the camera back into the focuser to see what's going on and here's what I see:

https://www.dropbox.com/s/cn3cyu4xcr2qtkb/Capture_00001.png?dl=0

https://www.dropbox.com/s/g8ky8j8kc0ujbr8/Capture_00003.png?dl=0

I was surprised to see the reflection of the primary being offset from the centre of the secondary. I have read about some fast Newtonians having an offset collimation by design but I do not know whether mine does or not (it is F/4.7). How can I find out?

Any other comments on my  methods so far are also welcome. I'm now thinking of taking the scope out and do some visual star testing without the imaging train in the focuser to see where I'm at. Again I'm not an expert in star testing so any tips on what to do and what to look out for are welcome as well.

Cheers,

Kari

[johninderby]

Best thing for setting up a secondary properly is to get a Concentre. No other method comes close in accuracy and it is so easy to use. It’s a simple device but I wouldn’t be without one. 
 

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5506_TS-Concenter-2--colimation-eyepiece-for-Newtonian-Telescopes.html

Edited January 27, 2020 by johninderby

[kbrown]

I repeated what I said in my first post a few times and the results improved a bit every time. Last night I did a star test with only a 6mm eye piece in the focuser. I must have done something right as it seemed bang on. Well defined airy disk with the rings symmetrically collapsing to a point when approaching focus from either side.

Edited January 29, 2020 by kbrown

[kbrown]

Last night I started having a closer look at my imaging train. I have a Skywatcher coma corrector attached to an Opticstar filter wheel. The CC requires 55mm back focus but this presents a bit of a dilemma. I have 1mm and 2mm thick filters which will lengthen the back focus distance by roughly 0.5mm and 1mm respectively so I'm not sure at what distance I should place my CCDs. At the moment I have it at 55.75mm hoping that would cover all combinations. Any insights on this? Haven't been able to test my rig yet. 

Edited February 3, 2020 by kbrown

[spillage]

Don't forget to add 1/3 thickness of the filter into your sums. I used a digital caliper to get to 55mm then just added/removed spacers based on the image produced until I was happy.

If your first image is not right throw in a good couple mm of spacing as this might help you see which way to go.