Skywatcher 130PDS - Testing Testing

[Carole]

Well as stated in my NGC1333 thread, I sold my 150P to Duncan, and bought a smaller 130PDS as I wanted a larger FOV.

Had a baptism of fire as I have only ever had to collimate the primary mirror previously, but the secondary was quite a bit out on this.  After a big hunt for my hex keys I finally managed - after some blood and sweat - to collimate the scope today.  Hope I have done it OK, now need to do a star test, but because it's a new scope of course there are clouds.

With reducing coma corrector I should be able to image at f4.5.

Carole

[Ivor]

So we all know who to blame for the cloudy and wet weekend then!

[Carole]

 

[Carole]

Well I think I have broken the myth as there have been some clear skies in the last couple of nights enabling me to test out the collimation and focus.  I was concerned after using the 150P how far the focusser would protrude into the OTA, and so until I got a coma corrector that would fit, the only way I could test it was without a coma corrector at all.
N.B. The skywatcher one I bought has completely different threads, and required various adapters to make it fit - more space taken up and more expense.

This is my test shot of the defocussed star to check my collimation.  Should have defocussed the other way, so the focusser was not in the way, but you live and learn.

5 secs Atik460



Anyway I managed to find a 2nd hand Baader coma corrector with threads that match my EFW, and FLO have agreed to refund the skywatcher one I bought that doesn't match.  Last night it was clear, so I imaged M45 just to check how the stars have come out and to see what I could get in the FOV, it was rather near the Full Moon, so not expecting any nebulosity to show up, it was just a star check. 

Resulting stars - they look OK to me.

[Mike]

The diffraction spikes and rings look slightly off centre to me so collimation os obviously very close but slightly off.

Apart from that it is looking very promising.

[Carole]

Can you explain where you mean Mike.  As far as I can see from the defocussed star it all looks symmetrical to me. Is there something I am missing.

PS: Ah, do you mean that the dot in the centre, is on the bottom edge of the horizontal (ish) band which I presume is a defocussed Vane/spike, when it should be halfway up - the same as the band going the other way. If so, I guess I could twiddle with the collimation while the defocussed star is in view to get it spot on, except my hands could be in the way. 

Carole

[MarkS]

Can you explain where you mean Mike.  As far as I can see from the defocussed star it all looks symmetrical to me. Is there something I am missing.

I agree with Mike - slightly non-symmetric rings.  But is the secondary mirror supposed to be exactly central?  My Tak Epsilon is designed with an off-centre secondary so a star test is meaningless.

In your image, you'll notice the diffraction rings are more compressed together on one side (e.g. top left) than the other (bottom right).  But that doesn't necessarily mean a collimation error.  See http://www.goldastro.com/goldfocus/collimation_techniques.php for instance - it explains that symmetric rings don't necessarily mean good collimation and that non-symmetric rings don't necessarily mean bad collimation (my Tak is an example of this).

I'm not being very helpful am I?

Mark

[Carole]

Yes the tutorial I followed showed a slight offset in faster newtonians i.e. up to F5 and this is an F5 scope.
Is this what you mean?

Don't understand why myself, I just follow instructions.

Credit Astrobaby


Slower Newtonians should look like this - same credit:


Mine looked like the top one.  The only thing I don't have is a self centring adapter. So it's slightly possible the 2 screws shifted the eye piece holder which was holding the Cheshire over a miniscule bit.

Carole

[MarkS]

Yes, that's the offset I mean.

The easiest thing is to assume that whatever offset you already have on the secondary is the correct offset and work from there with a Cheshire or laser collimator.

Mark

[Mike]

Can you explain where you mean Mike.  As far as I can see from the defocussed star it all looks symmetrical to me. Is there something I am missing.

What I am referring to is this...



The star is not central to the circular diffraction artefact around it. However, if the secondary is meant to be offset then that would explain it. If so this type of scope must be a bugger to collimate properly.

[Carole]

Thanks Mike.