Collimation (yet again!) for an SCT

[ScubaMike]

OK, I'm sorry alright. I know this has been covered a gazzilion times before and I have been through those posts. Even with all that reading, I still need some assistance.

Scope is C9.25 SCT. quick two star align and swung to Vega to collimate. I don't really have much in the way of high power eyepieces, so I barlowed up my Baader zoom (used at 8mm setting).

I clearly got the fretzle(??) rings and managed to nicely centre them, so that I had a series of concentric rings. Additionally it looked like there were "spokes" from the middle, evenly distributed round the out of focus star. Kind of like an alloy wheel view.

Though I had cracked it. Tried to bring Vega to focus and instead of a point of light, I get a view a bit like looking at a globular cluster. No sharp point of light.

I am really struggling with this and am beginning to wonder if I should have stuck with a Newt.

I hope someone will see what it is I'm doing wrong and make me fall in love with my scope again.

//EDIT: Scope had been out for about 90 mins before I tried to collimate //

Thanks

Mike

[dweller25]

That sounds normal for an SCT to me. Fitting cooling fans will help tighten the image up by removing the boundary layer from the primary mirror surface - quite a job though.

My 8" Newtonian shows the same effect but once I turn the fan on the image tightens up - still not as good as a refractor though.

[Psychobilly]

Have you got a webcam or other "video" camera that you can use on the SCT?

If you have then take a look at Metaguide...

MetaGuide

I used to use it with an SPC900 but now use a DMK21AU04AS running at 60fps and it makes life so much easier - especially if you can guide the scoep at the same time as it automatically re-centers the star image as you make tyhe colliamtion adjustments..

Oh - and a set of Bobs Knobs...

Home

Peter...

[Merlin66]

Mike,

Sounds good to me!

Using a 8mm with a barlow would give magnification of around x587,

this is very excessive both with the scope and the usual atmospheric conditions!!!

What you are seeing is all the atmospheric movement in an overly magnified bright star image.

Try it again but start with 20 or 30 mm eyepiece, just inside and out focus you should see a bright doughnut of light with a central dark circle. Change to a 10mm and look for the same - if it looks OK - you're there... if not adjust the secondary screws to make the dark disk central...work a bit closer to the focus and check that the image is symetrical...

Vega is OK to start with, but a bit bright for fine collimation...a 3rd magnitude star is much better.

Onwards and Upwards

[ScubaMike]

Have you got a webcam or other "video" camera that you can use on the SCT?

If you have then take a look at Metaguide...

MetaGuide

I used to use it with an SPC900 but now use a DMK21AU04AS running at 60fps and it makes life so much easier - especially if you can guide the scoep at the same time as it automatically re-centers the star image as you make tyhe colliamtion adjustments..

Oh - and a set of Bobs Knobs...

Home

Peter...

Thanks Phsycobilly,

I tried bobs knobs, but just couldn't get on with them, couldn't get my fat fingers to gain any purchase on them, so gave them away. Replaced with some stainless screws with allen key tops - much easier (and only 50p from local hardware shop ), I can leave the tight fitting allen key in the socket so no more pointy screwdrivers near my front lens.

I'll have a look at Metaguide as I have a couple of SPC900's.

Regards,

Mike

[ScubaMike]

Mike,

Sounds good to me!

Using a 8mm with a barlow would give magnification of around x587,

this is very excessive both with the scope and the usual atmospheric conditions!!!

What you are seeing is all the atmospheric movement in an overly magnified bright star image.

Try it again but start with 20 or 30 mm eyepiece, just inside and out focus you should see a bright doughnut of light with a central dark circle. Change to a 10mm and look for the same - if it looks OK - you're there... if not adjust the secondary screws to make the dark disk central...work a bit closer to the focus and check that the image is symetrical...

Vega is OK to start with, but a bit bright for fine collimation...a 3rd magnitude star is much better.

Onwards and Upwards

Thanks Merlin,

I am getting the doughnuts as decribed, but the only way I could get the multiple rings (fretzle rings ??) was to ramp up the magnification really high. I'll try on a fainter star, with lower power, and see if any easier. I gave up after about 2 hours of trying to get rid of the "globular cluster" effect I was seeing. Maybe I was fretting over nothing?

Regards,

Mike

[ScubaMike]

That sounds normal for an SCT to me. Fitting cooling fans will help tighten the image up by removing the boundary layer from the primary mirror surface - quite a job though.

My 8" Newtonian shows the same effect but once I turn the fan on the image tightens up - still not as good as a refractor though.

Hi Dweller,

This sounds interesting. Do you have any pictures of your set up? would a simple computer fan suffice? somehow mounted to gently blow across the front lens? or do you mean a fan which somehow blows "inside" the tube?

Is this to speed up the cooling down process?

If blowing internally, I would envisage some kind of filter arrangement?

regards,

Mike

[Merlin66]

The Airy rings (?!) should be visible with a perfectly focused star image...does require good seeing.

As you approach the focus, going from a small doughnut to a pretty sharp spot you'll see some diffraction rings which again show the mis-collimation.

I think you're fretting over nothing. Just enjoy the view!

[Merlin66]

Something like the Lyman cooler - to quickly cool the interior of the SCT - yes it comes with filters!

Lymax's Earth, Sky, & Astronomy

[ScubaMike]

Thats the one Merlin - Airy rings - couldn't for the life of me remember the name.

I seem to be getting these with a point in the centre and concentric rings, but as I go to focus - i don't get a point source, just what looks like multiple stars in very close proximity.

This may just be my unfamiliarity with the collimation (I used a laser collimator on a newt before, so this is the first time using a star), or maybe less than perfect seeing conditions.

All is not lost, if I continue to have problems, I'll arrange to take the scope & mount along to my local astro club one night when they are at the Obs, and get someone who is experienced to go through the process slowly (so that it sinks in!).

Thanks again for all your quick responses.

Mike

[todd8137]

hope you git there with it in the end