Is There a Collimation Issue Here?

[lensman57]

Hi,

I am sorry to bring this up as I am sure that it has been discussed many times before. I have an SW Quattro 8s . I have done the collimation using a tube sight and then a webcam. I have attached an image showing the present state of the collimation .The laser does not agree but I don't trust the laser.  My problem is that the stars do not look right to me. I know for sure that there maybe an issue with the way the MPCCiII sits in the drawtube. I would appreciate any input or guidance.

Regards,

A.G

[Moonshane]

I'm never very good with images down the tube but it looks like your secondary needs turning a little.

[Coco]

I'm never very good with images down the tube but it looks like your secondary needs turning a little.

Agreed the secondary looks like its facing 4 oclock, it needs to be 3 oclock .. 

On an F4 imaging Newtonian it will really show up on the stars

[lensman57]

Agreed the secondary looks like its facing 4 oclock, it needs to be 3 oclock .. 

On an F4 imaging Newtonian it will really show up on the stars

Thank you,

I will try and correct this, I hope that I don't ruin anything else.

Regards,

A.G

[BlueAstra]

If your primary has a triangular or radiation type centre spot, then I would recommend one of these. Also take a look at this page for more information.

[Dave_D]

what's the issue with the mpcc? mine sits quite snugly? although my scope isn't as fast as yours, your secondary offset looks a lot more 'aggressive' than mine. 

dunno how 'correct' a method it is, but i printed a template of the secondary with the offset indicated, sellotaped the tabs i added to the egde of the secondary and set the laser to point at the offset mark, then just a bit of tweaking to align everything. seems to have worked for me though.

[Jason D]

Hi A.G,

Explain your procedure. Did you align the secondary mirror edge against the primary mirror reflection then placed/aligned the reticle on the top? if the answer is "yes" then you are doing it incorrectly.

Jason

[lensman57]

Hi to all,

Thank you all very much for your  input and advice. I have followed the standard procedure of getting as close to a  circle from the secondary under the focuser with all the clips visible and equidistant  into the frame. I believe that the image posted confirms this. I am a bit confused as to the importance of the secondary reflection pointing at 3 o'clock rather than about 4.30 that it is doing in the posted image, comparing this to Astro Baby's instructions my secondary appears to be in a very comparable position. BTW I have no faith in any of these gadgets for the alignment, the pre requisite seems to be a perfectly aligned and centered focuser and then a secondary but that is the main problem anyway, if the focuser and the secondary are reasonably aligned then the rest is a just a simple procedure not requiring complex and expensive tools. 

Regards,

A.G

[Jason D]

Hi to all,

Thank you all very much for your  input and advice. I have followed the standard procedure of getting as close to a  circle from the secondary under the focuser with all the clips visible and equidistant  into the frame. I believe that the image posted confirms this. I am a bit confused as to the importance of the secondary reflection pointing at 3 o'clock rather than about 4.30 that it is doing in the posted image, comparing this to Astro Baby's instructions my secondary appears to be in a very comparable position. BTW I have no faith in any of these gadgets for the alignment, the pre requisite seems to be a perfectly aligned and centered focuser and then a secondary but that is the main problem anyway, if the focuser and the secondary are reasonably aligned then the rest is a just a simple procedure not requiring complex and expensive tools. 

Regards,

A.G

1- Is the reticle movable on your screen and you just happen to align it with the primary mirror reflection?

2- 3:00 vs 4:30 is unimportant

3- "the pre requisite seems to be a perfectly aligned and centered focuser and then a secondary but that is the main problem anyway". I do not know what your source is for this information but it is untrue.  

Jason

[lensman57]

1- Is the reticle movable on your screen and you just happen to align it with the primary mirror reflection?

2- 3:00 vs 4:30 is unimportant

3- "the pre requisite seems to be a perfectly aligned and centered focuser and then a secondary but that is the main problem anyway". I do not know what your source is for this information but it is untrue.  

Jason

Hi Jason,

Thank you your reply, I have readjusted the secondary ever so slightly so I just have to wait for a clear night to test the collimation, I may have to wait a while as clear nights are quite rare up here. I have searched the web for a visual reference for a fast F4 NEWT collimation but what there is,  are mostly to do with the classic F6 ( with little or no offset ) or Fast F5 where the offset is considerably different to an F4. Will keep trying.

Regards,

A.G

[Jason D]

Hi A.G.

Forgive me for being persistent but I am curious to know if you adjust the secondary mirror until you see the whole primary reflection then move the rectile on the top?

Jason

[carastro]

This is an excellent tutorial and should answer your question:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

[lensman57]

Hi A.G.

Forgive me for being persistent but I am curious to know if you adjust the secondary mirror until you see the whole primary reflection then move the rectile on the top?

Jason

Hi Jason,

Yes, I adjusted the secondary till I got a good circle, then adjusted the grub screws untill the 3 clips were equally visible , the reticle is just there to make sure that I have good circles and not ovals and the last stage was to get the primary right. I know that the issue has been raised about the shadow of the secondary pointing roughly at 4 o'clock and I will try and correct his further.

Regards,

A.G

[Jason D]

I am afraid you are not doing it correctly. See attachment. You can raise the secondary mirror and with the right tilt you should see the whole primary mirror. Or you can lower it and again with the right tilt you can see the whole primary mirror. Which one is right? Neither. Only the center diagram will agree with the laser -- the other two will not. Only the center diagram will ensure the focal plane is parallel with the CCD sensor -- the other two will not.

Jason

[Moonshane]

You are right of course Jason but I suspect the OP just left out mentioning the centering of the secondary in the drawtube which he probably does.

[Jason D]

In the initial photo, I calculated the secondary offset to be around 7mm (66mm secondary size divided by ~9). For this scope, the offset should be around 5mm. That suggests the secondary mirror might not be as centered under the focuser as it should be. That might explain why the laser beam was little off.

[lensman57]

many thanks for

In the initial photo, I calculated the secondary offset to be around 7mm (66mm secondary size divided by ~9). For this scope, the offset should be around 5mm. That suggests the secondary mirror might not be as centered under the focuser as it should be. That might explain why the laser beam was little off.

Thank you Jason for taking the trouble of analising the posted image. I am a little confused as to how this centering of the secondary works now. All tutorials that I have read just mention that provided that the draw tube is orthogonal to the tube then by adjusting the secondary till a circle is achieved is sufficient, if I have got it wrong then I would be grateful for any guidance. The image that I posted shows all the componenets in the correct position with the exception of the secondary shadow which is pointing at 4 o'clock but as I am not happy with the star shapes clearly indicates that something is not right here.

Regards to All,

A.G

[Moonshane]

as I understand things, with regard to the secondary alignment to the drawtube the key aspects are 1) that the secondary is centred in relation to the bottom of the drawtube (i.e. there is an equal gap around the secondary when looking down the drawtube - this is achieved by tightening / loosening the central bolt on the secondary) and 2) that the secondary is rotated to present a round face to the drawtube.