collimation in my 8 inch dob

[breammaster]

Just wanted see get your opinion on the collimation in my saxon 8 inch dob. I bought it second hand and haven't adjusted collimation yet. Would prefer not to if it won't make much difference To the view. The thing i am noticing is that the reflection of the primary mirror in the secondary seems to take up the entire secondary surface. there is no black space around the primary mirror reflection. My 5 inch dob had that narrow black space around the circumference in the primary reflection on the secondary. Is this normal? Will it effect views?

[barx1963]

Short answer, it looks fine. You can see all three mirror clips and all appears centred so it is pretty good. Are you using any collimation tools?

Malcolm

[breammaster]

Thanks Malcolm. I just have a collimation cap I made from a film canister. And just the usual screw driver and Allen key. I did notice that with my 5 inch dob, if I made the tube length shorter, it would make the primary mirror fill up more of the secondary mirror, hence I am assuming that the primary in my 8 inch is at its closest distance to the secondary mirror. Not exactly sure if I can adjust this distance but I dont want to attempt it for fear of breaking something .

[barx1963]

I am certainly not a collimation expert, there are people on here who know far more than I do. But as I understand it, what you are really talking about is the size of the secondary mirror. Obviously if you put a larger secondary in you would see the space surrounding the image of the primary. If you get a larger secondary it has 3 effects. Firstly it will block some of the light reaching the primary, making the overall image slightly less bright (a negative). Secondly it will slightly reduce the amount of contrast in images (a negative). Thirdly it will make the area of 100% illumination at the focal plane larger (a positive).
Odds are that none of these effects would be visible or detectable anyway unless it was a much larger secondary so I wouldn't worry about it.
If you can see all three mirror clips while using the collimation cap then I reckon all is good.

Cheers

Malcolm