Some collimation advice please...

[Blackheart]

Hi all

I've had a 12" SW Flextube for a year now and I really like it. However, I've been frustrated by the poor views of planets (never as sharp as I think it should be!). I've always thought it was due to poor seeing but I am beginning to suspect other culprits namely poor collimation and thermal boundary issues. I have rigged up a Heath Robinson cooling fan which has deffinitely helped but still the image will not sharpen up. I can see cloud bands and the odd hint of a dark patch within the bands but never (in two years) have a seen a sharp image

I am only using the old film cannister technique to collimate and I get it as close as I can but when I first got the scope I loosened the central screw holding the secondary mirror. Only a little as I thought it needed to be loosened before you could adjust it. Doh!

Now, when I look through the draw tube I'm not sure if there is anything wrong or not. I wonder if any of you could take a look at the attached image and see if you spot anything glaringly wrong. I know the collimation is not perfect and bear in mind this is taken with a compact camera held to the eyepiece holder.

However, I am sure the mirror should show concentric circles and mine does not. I have indicated the dark area where it appears that the mirror is out of aligment. Would adjusting the main screw ammend this, and would that make a difference to collimation?

Any help would be greattly appreciated. And if anyone has a spare laser collimator....

thanks

BH

[CanesVenatici]

Isn't the secondary normally offset in a fast Newt? See here:

Astro Babys Guide to Collimation

Have you tried setting up the collimation with a star test, which has to be the most accurate way?

[nicnac]

Looks slightly out to me, a cheshire is a good investment for 25 quid at FLO

[nicnac]

The secondary can be offset in a fast reflector but it doesn't have to be, it can be a pain in the rear to offset just right, stick with consentric circles, you can go wrong with that

[Stephen]

Looks slightly out to me, a cheshire is a good investment for 25 quid at FLO

Seconded, on both counts

I would expect there to be some offset however.

[hes not well again]

One problem might be that the focal centre of the mirror is not in the geometric centre of the primary, if you have set up the collimation as well as possible and still think its not right then a star test is the best answer. If you point the scope at a star and defocuse it you should be able to see the 'airy rings' and within that the shadow of the secondary and the spider. If the rings are not central then put your hand in front of the scope and move it until its by where it is out. The screw for the primary that is inline with your hand is the one you should adjust. Good luck.

[Stephen]

The secondary can be offset in a fast reflector but it doesn't have to be, it can be a pain in the rear to offset just right, stick with consentric circles, you can go wrong with that

The offset is built into the SW secondary holders though is it not?

[Blackheart]

Thanks guys

I had not heard of the offset in fast Newts. that explains it and also puts my mind to rest that I didn't muck things up! I have been reading through Astro Baby's guide and that is excellent. I will just have to invest in a decent collimator and wait for good seeing.

Clear Skies and thanks for the rapid response

BH

[nicnac]

Hmm, not sure if the offset is "built in", not so with my TAL, the offset doesn't really make much a difference when viewing, in my opinion anyway, i line em up n get going and my views are great

[Stephen]

Hmm, not sure if the offset is "built in", not so with my TAL, the offset doesn't really make much a difference when viewing, in my opinion anyway, i line em up n get going and my views are great

I've not owned a TAL, but I'm reasonably confident it is built into SW

[Jason D]

However, I am sure the mirror should show concentric circles and mine does not. I have indicated the dark area where it appears that the mirror is out of aligment. Would adjusting the main screw ammend this, and would that make a difference to collimation?

Any help would be greattly appreciated. And if anyone has a spare laser collimator....

thanks

BH

No, not all circles will appear concentric. That dark cresent in your photo is expected. Do not worry about the offset. Just follow the proper collimation procedure and the secondary mirror will automatically end up with the proper offset. I am attaching a photo from my collimated scope.