Problem collimating an ƒ/4 Newtonian

[Tailspin45]

I understand that collimating a short tube Newtonian (Orion 4.5" Starblast 450mm ƒ/3.95) can be tricky, so I read numerous articles and watched several videos.

Nevertheless, I think I'm doing something wrong.

The secondary seemed to be too close to the primary and, thus, not centered in the drawtube. So I moved it back and made sure the face of the secondary was at right angles to the collimating eyepiece.

I then adjusted the tilt of the secondary to center the image.

Then I adjusted the primary to center the ring on the primary, so everything looked right. 

But when I defocus a star (in SharpCap using an SvBONY SV305 camera), I barely see Airey rings and the image is lopsided. Viz:

Not sure these help, but

 

 

Edited June 8, 2020 by Tailspin45

[alacant]

Hi

The secondary mirror is intercepting only part of the light from the primary. The most likely cause is that the secondary mirror is rotated,  tilted or too far away from the primary or not centred. Or said in any combination. Or something else. E.g. I can see the spider fixing screws at 10 and 11 o'clock, but nothing dianetrically opposite. But best not to guess. 

You'll need a cap with a centred hole and Cheshire sight tube equipped with cross hairs. They are described in this procedure.

That should get you close. If not, post again with views through each of the tools.

Before you start, be sure to read the collimation myths.

HTH

 

Edited June 8, 2020 by alacant

[Tailspin45]

53 minutes ago, alacant said:

You'll need a cap with a centred hole and Cheshire sight tube equipped with cross hairs. They are described in this procedure.

Thanks! I have both, and used them. But not very well, apparently. I'll review the linked procedure.

It occurs to me that the defocused image will look very different in an eyepiece, without a camera and software in the mix. I'll see what that shows.

Edited June 8, 2020 by Tailspin45

[alacant]

30 minutes ago, Tailspin45 said:

 I have both, and used them

Ah, ok. It's probably almost there then.

Post a shot through each. That will help diagnose. That and a frame of a star field -anywhere in Cygnus is fine- would help nail it.

Cheers

Edited June 8, 2020 by alacant

[Tailspin45]

Well this is embarrassing. When I collimated my scope yesterday I left the piece of paper in it that blocks the primary to avoid distraction when adjusting the secondary.

No wonder the stars looked weird and plate solving gave up time after time. It didn't completely cover the mirror and was curled so some light got through but that sure explains the crescent stars!

I went through the whole procedure you recommended and immediate discovered that the secondary vanes were about 5mm off center L to R but okay top to bottom.

The procedure recommended creating a card with a hole in the middle, but when you put the card over the tube you can't see anything through the hole, so that's not much help. But calipers did help and I centered it to within 1mm or less, I think.

The rest of the process was straightforward except judging roundness of the secondary (making sure it is at right angles to the focuser), but I eyeballed it the best I could.

Next chance I get I capture some defocused stars and see how I did.

Thanks for your knowledge and advice.

[alacant]

7 hours ago, Tailspin45 said:

left the piece of paper in

It happens to us all. Me, worse!

7 hours ago, Tailspin45 said:

creating a card with a hole in the middle

You use the hole in the card simply to look at the position of the centre screw on the secondary holder to judge whether it is centered, make any adjustments (in your case L to R) and then remove it for the rest of the procedure.

I think you'll have it near perfect.

They're great portable telescopes. A guy regularly travels here with his as carry on luggage. He fitted a 2" focuser to be able to use a coma corrector. The wide colour free field of view beats any refractor.

Cheers and good luck. 

[andrew s]

All Newtonian owners  have all been there or done something similar.

Regards Andrew 

[iapa]

Been there, done that and worse.