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COLLIMATION DILEMMA

ultranova

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ultranova

ultranova

Posted
Posted

Hi Guy’s got a bit of a dilemma, I purchased the 130 ds newt a week ago or so

From FLO, great service on that buy the way, my dilemma is this,

I have done the usual, on the collimation using a Cheshire / made sure the secondary is bang in the middle, Checked the focuser is ok / then made sure I had a round disc looking down the Cheshire and it was bang in the middle / from then on it was plain sailing, in the end I was please how quick and simple it was, Eyeing through the cross was spot on with the primary mirror and everything looked good.

Until I put my infinity xl on it, it wasn’t miles out, and I did expect to see that, but when I set it up spot on with the xl infinity it shows when looking through the Cheshire that its about 1/3 rd of the primary spot out, so my question is has anybody else seen this, and what one do I trust, the xl infinity or the Cheshire.

Any help would be appreciated

Thanks Paul   

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ronin

ronin

Posted
Posted

Well FLO page says:

Note: An autocollimator compliments a traditional Cheshire/Sight-Tube, it doesn't replace it. The Cheshire/Sight-Tube is still the best tool for positioning the secondary mirror under the focuser.

So on that basis take the Cheshire as the "datum" at this time. But as John say's do a star test and if the star test is out you could then set it up using the Infinity and then repeat the star test. The purpose being to determine which of the instruments gives the best star test result. Then you use whichever produces the best in the future. Takes longer to determine but you have a result to follow.

Do either of the 2 tools have or require set up or collimation themselves ?

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Charic

Charic

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Posted

Use the one you feel most confident with.

I can set up my scope  confidently using the  35mm film cap method, a laser or the Cheshire, and all will give good results, the final  Star test proving satisfactory, but compare all three tools and the problems arise, or so it would appear?
Each tool I find will differ slightly  in its method of execution and  how my  eye is aligned to the tool in question, excepting the laser.

If I have reason to disassemble my scope, I would complete the task today using my Cheshire tool. The laser is only ever Barlowed in the field as a rough guide after transportation, there is no need to use the 35mm. But I can still  collimate my scope from scratch, and achieve a perfect  Star test,  using either of the tools in my armoury.

Go with what you trust, comparing just throws doubt, is it the XL, is it the Cheshire?

Why not  work backwards?  Collimate  your scope with Polaris alone, the Star is almost stationary, giving you plenty  time to complete the task. Once your satisfied, check your tools to see which compares.
There still may be a slight correction needed for one of the tools,  as seen by the observer, but  these tools are not 100% scientifically accurate, their just a guide tool,  and its only down to the end user,  just how close to  accurate your scope is collimated, but the true and final test is  having clear and concentric rings  in your Airy disk. 

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gazza

gazza

Posted
Posted

have you checked that the centre donut is actually in the centre? I just bought a 130 and the donut was several mm out so I replaced it.

Have seen quite a few newts where this was the case, so I always check.

Also collimation is an iterative procedure.:

align cheshire

align using autocollimator

go back to cheshire 

go back to autocollimator.

If the scope was out it could take 3-4 iterations.

Also make sure that when adjusting secondary that all you do is adjust the tilt forward/back in the direction of the focuser. The secondary should only need this and a twist by hand in the rotational direction. If you adjust all the screws on the secondary you will likely never get spot on collimation as this puts the secondary mirror off centre and will result in a tilted field - only important if imaging though. This is often why people using correctors se not round stars in part of their field..

cheers

Gary

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Jason D

Jason D

Posted
Posted
On 2/18/2017 at 13:07, ultranova said:

I did manage a star test tonight, fairly please with it, the secondary looked bang in the middle as far as I can tell.

Just curious, did it look something like the following photo?

star_collimation_20.jpg

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ultranova

ultranova

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On ‎18‎/‎02‎/‎2017 at 23:28, gazza said:

have you checked that the centre donut is actually in the centre? I just bought a 130 and the donut was several mm out so I replaced it.

Have seen quite a few newts where this was the case, so I always check.

Also collimation is an iterative procedure.:

align cheshire

align using autocollimator

go back to cheshire 

go back to autocollimator.

If the scope was out it could take 3-4 iterations.

Also make sure that when adjusting secondary that all you do is adjust the tilt forward/back in the direction of the focuser. The secondary should only need this and a twist by hand in the rotational direction. If you adjust all the screws on the secondary you will likely never get spot on collimation as this puts the secondary mirror off centre and will result in a tilted field - only important if imaging though. This is often why people using correctors se not round stars in part of their field..

cheers

Gary

Hi Gary, to be honest, I did not check the donut, its a good call though

as we all take it for granted it is in the middle, I will check some time this week

thanks

Paul

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richyrich_one

richyrich_one

Posted
Posted
On 18/02/2017 at 23:28, gazza said:

Also make sure that when adjusting secondary that all you do is adjust the tilt forward/back in the direction of the focuser. The secondary should only need this and a twist by hand in the rotational direction. If you adjust all the screws on the secondary you will likely never get spot on collimation as this puts the secondary mirror off centre and will result in a tilted field - only important if imaging though. This is often why people using correctors se not round stars in part of their field..

Why have I never seen this explained like this anywhere before?!

This makes perfect sense now. Thank you!!

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Jason D

Jason D

Posted
Posted
12 hours ago, ultranova said:

Hi yes it did, I had it on Rigel at the time, then done it on my artificial star,  the

secondary shadow appeared to be bang in the middle.

I need to apologize because my question was a "trick" question for a good cause -- to make an interesting point.

That star testing image was actually taken off a scope with a HORRIBLE collimation. Here is the rest of the photos.

Left most photo is the one I shared earlier (too much defocus). The middle one is for the same exact setup but less defocus. The right most image is for the same exact setup but with the cheshire (Catseye Blackcat with Hotspot center spot)

star_collimation.jpg

The point I was trying to make is that star testing needs knowledge and skills to perform it the proper way; otherwise, you might end up with less-than-acceptable collimation.

It needs to be performed at high magnification and the star needs to be defocused by a small amount -- enough to show 3 to 4 rings. If the star is defocused too much where you can clearly see the diffraction pattern of the spider vanes then the star image can't be used to assess collimation accurately.

The defocused star image needs to look something similar to the following:

star_test.jpg

 

Jason

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andrew s

andrew s

Posted
Posted

I know I am late to this but I don't think the following has been mentioned.

A Cheshire is not effected by how well or otherwise it is square on to the optic axis as it just scatter light towards the primary mirror. An auto-collimator depends critically on if it is square on to the optic axis or not. Thus the two can differ because a) the collimator is not square in the focuser (as mentioned before) or b ) the focuser is not "square" on to the optic axis.

You can use the two tools together to ensure all elements (primary, secondary and focuser) are correctly aligned.

Regards Andrew

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ultranova

ultranova

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On ‎21‎/‎02‎/‎2017 at 12:46, andrew s said:

I know I am late to this but I don't think the following has been mentioned.

A Cheshire is not effected by how well or otherwise it is square on to the optic axis as it just scatter light towards the primary mirror. An auto-collimator depends critically on if it is square on to the optic axis or not. Thus the two can differ because a) the collimator is not square in the focuser (as mentioned before) or b ) the focuser is not "square" on to the optic axis.

You can use the two tools together to ensure all elements (primary, secondary and focuser) are correctly aligned.

Regards Andrew

Again, Thanks guy's for all your inputs,

Just to add another one into the wonderful world of collimation

would it be fair to say that, using ccd inspector would give you

a better overall picture of what you optical axis is doing, or

at least show you that something is out somewhere in the optical train,

ie focuser not square /  primary mirror spot out.

As soon as I get a chance I will check the centre spot as  if I am reading it correctly

the auto collimator is showing this.

Makes you wonder How Sir Isaac Newton got on collimating his Newt,

I bet he didn't have a auto collimator, or a laser, even a Cheshire

probably done the old tweak a bit bit and try it routine, ( nothing changed much

in the past few hundred years if that's the case ), well not for me anyway  lol  :)

Paul

  

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andrew s

andrew s

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Posted
12 minutes ago, niallk said:

Yeah, Newton couldn't just go and print off Astrobaby's guide from the web... :grin:

The only Web in use then was spiders Web used for cross hairs on eyepieces. 

Regards Andrew

PS sorry no experience with CCD inspector. 

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  • 2 weeks later...
ultranova

ultranova

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Just a quick update Guy's,

I did eventually get around to checking the centre spot on

the primary mirror, it was 2.1mm out one way, yet excuse the pun

spot on the other.

just order some reinforcement rings so hopefully will get this little

puppy working to spec.

I cant understand how it can be that far out, you got to love sky watcher

they turn out some good value kit for the money, but some of the most

simple things they just cant get right, oh well, live and learn.

Paul

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