Collimation ok?

[Demonperformer]

Was out a few nights ago doing some planetary imaging with the 8SE ... total disaster. However, I also took some collimation shots of Altair (about 45 degrees above the horizon). I have never touched the collimation of this scope, but have a set of Bobs Knobs ready to install when they are required. Until then I don't see the fun in messing up the collimation installing them. Once I start down this road, I have visions of ending in a worse position than where I start.

This looks reasonable to me, but my only concern is in the five to six o'clock position where there (may) be some flattening of the two thin rings towards the centre. I just don't have sufficient experience of collimation to know if this is anything about which I should be concerned. So, what do people think? Collimation OK, or grasp the bullet and install the BKs now?

[John]

I think the secondary shadow is offset a little towards the bottom edge of that diffraction pattern. The diffraction rings seem more tightly compressed together below the secondary shadow to my eyes.

[Mr niall]

12 minutes ago, John said:

I think the secondary shadow is offset a little towards the bottom edge of that diffraction pattern. The diffraction rings seem more tightly compressed together below the secondary shadow to my eyes.

Yep that’s what I thought, you’ve actually lost a ring in compression there. What’s it like the other side of focus? With my recent ventures in collimation I’ve noticed seeing and tube currents can cause this effect as often as not anyway.

It still looks pretty good to me though all things considered it’s only a smidge out (or is the right answer “it’s a disaster - good thing you bought those bobs knobs you’d better get using them immediately!!” ?)

[Peter Drew]

It could certainly do with making a little better, SCT's are very sensitive to collimation for the best results. Bobs knobs are not essential but they do make the process of collimation physically easier. With a 8SE you can reach the BKs from the eyepiece position rather than fiddling from the front with an Allen key. Most important to make sure the star is centered in the eyepiece and use Polaris if possible so that it stays there. Also important to judge the collimation with the focus knob turned counter clockwise as this advances the mirror against gravity, subsequent focusing when eventually observing should also be arrived at by the same direction.  ? 

[Demonperformer]

Thanks for the replies. I had not considered the direction of travel of the focusser and could not confirm which direction I used.

I think before I do anything too drastic, I will redo the test making sure I use the correct direction.

Once aligned, stars tend to stay put, but I take your point about Polaris moving less than most.

Thanks.

[Ben the Ignorant]

1 hour ago, Demonperformer said:

I have never touched the collimation of this scope, but have a set of Bobs Knobs ready to install when they are required.

They are required now. The bottom of the pattern is narrower than the top, and a bit blurry. As Peter says, Schmidt-Cassegrains are very sensitive to collimation errors; once the job is done right sharpness you didn't think you could have pops out of nowhere.