Basic collimation question

[ScubaMike]

Sorry if this has been asked before, but it will only take a second for someone to put me right.

I collimated my 150PL a few nights ago, but subsequently had issues with focus (which I put down at the time to bad seeing).

Re-reading some of the posts on collimation I see that there are three lock screws & three adjustment screws on the end of the tube. But, which are which?

I have three low profile thumbscrews, and three thinner *taller* screws (also three flush screws on the side of the tube which I presume hold the cell in place). Each set is equally spaced round the circumference of the primary end of the tube.

I think that I may have collumated using the lock screws which is leading to distortion (they were pretty tight). Stars looked OK (and star test showed concentric circle when out of focus), but Jupiter wouldn't focus as clearly as it had in the past.

PS I adjusted using the low profile thumbscrews.

[kniclander]

I suspect the low prfile thumb screws are the ones you use for adjustment but you have to loosen the locking screws a little first. My (older) skywatcher 150 has big philips screws for adjusting and small hex screws for tightening / loosening. I don't think I have amnother three though....?

[ScubaMike]

To clarify, there are six thumbscrews on the *end* of the tube (3 lock/3 adjustment), and three more philips screws *at the end* of the tube (ie, on the sidewall of the tube)

[John]

The ones on the side wall of the tube hold the mirror cell assembly in place in the tube - no need to touch those for collimation. Mirror lock screws should not be too tight as they can distort the mirror cell. I don't actually use them at all on my 10" newtonian.

[Keiran]

Take a look at the image under 'B'

SkyWatcher Dob

The big one with the spring under it is the adjustment screw, the other is the locking.

When observing planets, the conditions are important, the image will be badly distorted if the conditions are poor... high ambient air temp etc.

Atmospheric condtions are a big pain in this department...

[ScubaMike]

Just the job Bulldog.

Thats the image I was looking for.

Pretty sure that the low profile thumbscrews are the ones with the springs.

@John, Maybe I should just loosen off the lock screws to leave a gap, and then re-collimate.

Maybe collimating at night would be best too.

I did it during the day, so maybe the change in temp between collimating and using made a difference.

Cheers,

Mike

[John]

To be honest, your F/8 scope is more forgiving of collimation errors than faster newtonians. If you are getting poor images I'd tend to think of seeing conditions or the need for more cooling time before concluding it's collimation.

[damnut]

Hi ScubaMike,

wehn I had a 200mm newt I fiddled with all the collimation screws. The best 10 quid I spent on the scope was taking it to Rother Valley Opts and have them collimate it properly, after that I marked all the screws with a bit of tippex so that I could at least get it back to some semblance of order!!.

Good Luck

Damian

[great_bear]

I did it during the day, so maybe the change in temp between collimating and using made a difference.

For planetary viewing, you need to check that the jet stream <click> is favourable, or your chosen planet will look out of focus.

[todd8137]

hi mike glad to see you give in it a bash ,when i take the dob out side ,because i have to lug it down the garden i always check it befor looking .get a pic up of the bottom of the scope to see what ya on about

[ScubaMike]

This is what I learned today

Undo the lock screws so they are nice & loose.

collimate by seeing how tight the collimation screws are and either loosening or tightening so that the end result is NOT tight adjustment screws (I tried by just tightning and it doesn't work well).

Don't know how to post an image so you will have to take my word for it that the low profile thumbscrews are the collimation adjustment screws and the tall thin screws are the lock screws.