Do I need it?

[Demonperformer]

No, I'm not talking about a new piece of equipment. I'm beginning to understand that the answer to that question is always yes:). I'm talking about [gulp!] collimation:eek:. As a relatively new owner of an SCT, this word still fills me with fear and trepidation.

OK, to prove I'm not a complete Philistine, I know that at some point, with constantly moving the scope in and out of the house, I will need to tackle this. I have also become convinced that the use of Bobs Knobs will make the job immeasurably easier. But therein lies a problem. In replacing the current screws with these marvels of engineering genius I could quite easily lose what collimation I currently have. I don't want to make things 90% worse in an attempt to make them 2% better.

Which brings me back to my original question. I've read articles and seem little diagrams of out of focus stars. Probably due to the wonderful clarity of the skies where I am, all my out of focus stars look like constantly moving wavy lines. Hmmm.

So this morning, while playing with my new camera, I decided to swing the telescope round to Sirius, and take a piccie of it out of focus. To my untutored eye, the central black bit looks reasonably regular and so I am inclined to put off risking the loss of quality my first collimation attempts are likely to produce. But as I lack any experience to compare with ...

So come on all you SCT experts. How much image quality am I actually losing by having my scope in its current collimatary [is that a word?] state? If it were in perfect collimation [i know, a practical impossibility] would my seeing improve 1%, 10%, 100%, ...? Or is a photo like this of no use in answering that question?

Thanks for your input.

[Beyond_Vision]

It's worth doing this on a much higher star as Sirius is low down and you will get much more atmospheric distortion. It looks reasonably ok the rings should be concentric.

Regards

Kevin

[Demonperformer]

Thanks for the advice Kevin. If it stays clear tonight I'll give Deneb a go.

[mcrossley]

For rough collimation it looks OK, however to check it further you need to be much closer to focus, with just a few rings showing at the centre. As Kevin said use a star much closer to the zenith.

The final tweak on an in-focus star to centre the first diffraction ring isn't possible unless you have a night of steady seeing and use a star near the zenith. If you can do it that final tweak will bring out the lower contrast details on the planets.

[Demonperformer]

OK, hopefully I have successfully taken on board the previous comments. A couple of pictures of Capella this morning. Altitude 65 degrees. Closer to focus. Tried to get one even closer, but when image processed looks like an odd-shaped blob [amazing how many of my star pictures end up looking like that:o].

Any additional comments on my need [or lack thereof] for collimation at present are appreciated.

Thanks.

DP

[brianb]

Doesn't look too bad.

The 8" Meade LX90 I had was way out of collimation as delivered, but after adjusting it I dodn't need to do it again. My current CPC1100 was very nearly right on delivery, I've only given it a tiny tweak to get it spot on & not had to repeat the exercise. You can only do the final tweak when the seeing is abnormally good and you won't notice a slight lack of accurate collimation when the seeing is pants; this is more likely why your star images are odd-shaped blobs than inaccurate collimation.

As for Bob's Knobs - you most certainly will wreck the collimation when you fit them, and many people seem to report that the scope doesn't hold collimation as well after fitting Bob's Knobs. Personally I'd rather use a tool - CAREFULLY - than grope round the front end of the corrector plate with sticky fingers in any case. The number of times the job needs to be done, you can take your time & take care.

[Peter Drew]

Collimation looks reasonably OK to me. Apart from using a high altitude star for the test as already recommended and make sure that the scope has had time to reach thermal equilibrim as the temperature "plume" from the internal shade tube can play havoc with the test. I have SCT's with and without "Bobs" type adjusters and find the former much easier to use. I'd sooner scratch the corrector with my fingernails than an Allen key! If you do fit knobs remember to undo and replace the original adjusters one at a time and point the front of the scope downwards, apart from any dropping inside issues the secondary unit can tip away towards the primary mirror making it difficult to engage the first knob. The sooner you get familiar with collimation the happier you will be, plenty of good advice on SGL if you get stuck!. Good Luck.

[David Galvin]

As Peter says and trust me he knows!

My avatar pic (by my name) shows Polaris at X300 and it appears pretty concentric but I have found under further investigation that it is slightly out. I am waiting for a night of good seeing to improve it.