Collimation Problem

[petejw]

Hi

Can anyone help with a collimation problem.

I have a 200pds Skywatcher and I am trying to collimate the scope but I am having problems centering the secondary under the focusser.

The secondary looks to be circular and positioned correctly to the optical axis but I do not understand how to align it in the axis 90 degrees to this - see photo

The mirror is central to the tube on the spider, so the only thing I can think of is that the focusser is not pointing directly at the centre of the tube, so the focusser mounting itself needs adjusting - Is this correct or am I missing something.

I have taken a few photos and an example is attached ISO 800 30 seconds.

Any help would be greatly appreciated.

Pete

[alacant]

10 minutes ago, petejw said:

the axis 90 degrees to this

Hi. Spider vanes? They should position the central up-down bolt of the secondary centrally in the tube. HTH.

[alanjgreen]

measure the 4 segments of spider vane. Are they all the same length?

You can adjust them by loosening the screws on the outside of the tube.

 

Edited February 3, 2017 by alanjgreen

[petejw]

Hi,

Thanks for the quick replies

I've just re checked the vanes on the spider, they are all showing the same distance from the centre of bolt on the secondary mirror mount.

Pete

[barkis]

Is you scope a fast f/ratio? If so, F5 and faster,
 the secondary will need a slight offset.
both a little away from the focuser, and a little towards the main Mirror.
The pds Scopes are primarily for Imaging, so that offset should be applied.
 

[petejw]

Yes, its an F5, But isnt the offset towards the mirror for the PDS?. ("X" axis), my problem is the "Y" axis

[Ricochet]

If you want to check the focuser is pointing at the right spot you need to remove the secondary. You then measure around the inside of the tube from the edge of the focuser both ways to find the midpoint and mark that with a line. You then measure the distance from the centre of the focuser to the end of the tube to get the right distance to turn your line into a cross. You can then check with a Cheshire or laser that the focuser is actually pointing at that spot. If it doesn't some washers can be inserted between the tube and focusser to correct it. 

However, that might not be necessary. You can probably fix your collimation with the secondary screws. From the same perspective as your photo one or two of the secondary adjustment screws can be considered as above the centre and one or two as below. In order to move the secondary down you need to loosen the "bottom" screw(s) and tighten the "top" screws. 

[petejw]

If I adjust the secondary screws and move the mirror as you suggest, wont the second part of collimation, where I align the secondary mirror with the main, put it out again?

[Ricochet]

No, you should be able to make the minor tweaks needed without pushing the secondary off centre again unless the focuser is a long way out. 

[petejw]

OK, I'll give it a go

[John]

As I unserstand it, the secondary offset is built into the way that the secondary holder is designed.

 

[Ricochet]

I should probably add that once you have centred the mirror you might find you have a rotational issue. In this case you have to physically hold the secondary stalk, loosen the centre bolt, twist the stalk so that the rotation is corrected and then tighten the centre bolt back up again. 

[petejw]

Well, what a pallaver. Finally got the collimation looking ok, took the spider out and checked the focusser alignment, decided seeing as I had gone that far and weather was rubbish i'd take the OTA apart and nip down to Wilko and get some flocking. It took my son and I about 1 1/2 hours to apply it and I must admit, it looks great. Put the OTA back together adjusted the collimation bolts and finally got it looking OK.

Photo, all yellow stripes are same length, blue circle is photoshopped onto image to check "circle" of secondary mirror seen through eyepiece. The secondary can just be seen showing around the edge of the blue circle.

The focusser alignment incidentally was OK, unfortunately by the time I'd picked up Ricochet's post on the potential rotational issue, I'd already removed the spider and secondary, but I think it was this tip that allowed me to collimate in the end.
Thanks everyone for your suggestions and thanks to various guides and tips on this site regarding the flocking, it couldnt have gone smoother.

Off topic, but here is the flocked scope, now waiting for some clear skies so I can do some star test.

Pete

Edited February 5, 2017 by petejw

[Ricochet]

Well done. I'm glad you got there in the end. 

[Dave In Vermont]

Astro_Baby's collimation-guide shows the off-set in fast Newtonians. Most instruction-guide do not - serving to compound people's confusion. Here's a copy of her guide to tuck-away for safe-keeping:

Astro Baby's Collimation Guide.pdf

Enjoy! Collimation becomes like riding a bicycle - the first-time can be a pain. Then it becomes simple.

Dave

[Usman]

Hi,

I was trying to collimate my new Skywatcher Heritage 130P for the first time yesterday and after I got my secondary mirror centered under my focuser, I tried to use the three alignment screws to align it to the primary mirror. Each time I tried that, though, I realised that the secondary mirror would move with respect to the focuser. I tried to hold the secondary holder with my fingers to avoid any rotation but realised that the holder itself was swiveling on its stalk. I tried tightening up the central screw as well as the alignment screws but there's still a significant amount of play in the holder (as shown in the attached photographs) and it doesn't take more than a slight touch to change its position. Can you please let me know if I am making a mistake during the collimation or whether there's a problem with the telescope holder itself? I might be wrong but shouldn't the back of the holder, where the central screw's located, always be perpendicular to the plane of the open end of the tube?

Thank you.

[Ricochet]

13 hours ago, Usman said:

the holder itself was swiveling on its stalk

This is not right. I believe that the stalk is threaded both ends and just screws into both the secondary holder and the side of the telescope tube. Check when the secondary support swivels if it is turning on the stalk or if the stalk itself is actually rotating as well so that you know which joint is faulty. Once you have determined that, turn the loose part to tighten it up. If you can get one full rotation you may be fine, but if it is always loose in the correct orientation then I would suggest going back to your retailer and see what solution they suggest (probably replacement). 

 

Edit: looking at photos down the barrel of the H130p it appears there is a nut where the stalk enters the tube side wall. If this nut is not fixed to the wall of the tube as the mounting point then it is a lock nut and you should tighten the nut to stop the stalk from turning. 

Edited October 8, 2019 by Ricochet

[Moonshane]

Sometimes taking a newt apart promotes understanding of how it all fits together 😊

 Well done 👍

[Moonshane]

Ps

For others and future reference a couple of things to consider:

- ensure the secondary adjustment screws are all extending about the same length

- your tube may not be circular. My Orion Optics UK newts are not round (more a slight oval) so each axis has different spider arm lengths.

[Usman]

12 hours ago, Ricochet said:

This is not right. I believe that the stalk is threaded both ends and just screws into both the secondary holder and the side of the telescope tube. Check when the secondary support swivels if it is turning on the stalk or if the stalk itself is actually rotating as well so that you know which joint is faulty. Once you have determined that, turn the loose part to tighten it up. If you can get one full rotation you may be fine, but if it is always loose in the correct orientation then I would suggest going back to your retailer and see what solution they suggest (probably replacement). 

 

Edit: looking at photos down the barrel of the H130p it appears there is a nut where the stalk enters the tube side wall. If this nut is not fixed to the wall of the tube as the mounting point then it is a lock nut and you should tighten the nut to stop the stalk from turning. 

Thanks for your reply. At first I thought I could tighten the nut on the tube wall but the stalk itself isn't rotating even a little bit - it's just the holder which is swiveling from one position (as shown in the first photo) to the one shown in the second photo (so it's not making a complete rotation otherwise I would have tried to tighten it that way). I tried checking if there was a nut on the holder which could fix its position relative to the stalk but there isn't. As far as I can tell, the only way to completely fix the position of the holder is to use steel epoxy to 'weld' it to the stalk but I don't want to do that on a brand new telescope