How good is my 150p collimation

[AstroNebulee]

I'm hoping some very helpful SGL members can help me out with a question regarding the collimation of my skywatcher explorer 150p, it's just the secondary I'd like you to look at for me please, I've rake a photo to show you and racked all the way out, it looks a little off centre to me but is this the offset coming into play here  I've placed a piece of paper in between the primary and secondary. Clear skies and thank you for helping with another collimation issue 🙄

[NGC 1502]

Assuming that your camera is central on the focuser -

Your secondary mirror is not central with the focuser, it’s not far off but not quite right.  The problem would be easier to see if the white paper was covering the whole area opposite the the focuser.  If you have a Cheshire collimator that will help, if not a basic collimation cap can suffice with care.

The secondary needs to be shifted a bit closer to the tube wall, or “up” in the orientation of your photo, and a nudge further towards the primary.

You should be able to shift the secondary by adjusting the support vanes or using the adjustments available on the central hub. Adjusting the centre bolt will shift the secondary closer the the primary and the 3 outer bolts to fine tune. You may need to rotate the secondary on the central hub.

Once that’s done you could remove the paper, tweak the 3 secondary adjustments to point the secondary precisely at the primary.  Getting the reflection of the 3 primary mirror clips centred is your best guide for that. Whilst you’re doing this bit ignore the reflection of the secondary and support vanes.

When you’re happy with the above, the secondary is sorted.  Then you can use the adjustments on the primary to centre the reflection of secondary and support vanes.

The above is far harder to describe than actually carry out 🙂

Fine tune on a slightly defocused star. For a non driven scope Polaris is good.

Hope you sort it, Ed.

 

Edited November 26, 2020 by NGC 1502
Extra info added.

[Pixies]

As Ed says - you need to make sure your camera/eye is central. So a collimation cap will help. A Cheshire moreso, as you can get the outer 'circle' closer to the secondary and check concentricity that way.

If you are just using a collimation cap, you can use 'Mire de Collimation' to check how everything is centered against your smartphone pics:

 

But it's quicker/easier with a long Cheshire acting as a sight tube:

[AstroNebulee]

Thank you both for your guidance and detailed write ups, I agree the secondary does appear to need to be shifted up the focus tube a little. I will try adjusting the spider vanes first to do this step. I have a long Cheshire collimator so will try using that along with mire de collimation, could I use it with my dslr in live view on ATP tool, I'm sure I've read here that can be done whilst adjusting the secondary. I will post more photos of my progress to see how I'm doing with it. Sorry it's another collimation question but once it's positioned prop I should only need minor tweaks in the future. Sometimes you can get in a pickle with collimation and go over the steps countless times and with talk of an offset on faster newts always got me thinking is it correct. The only adjustments I have ever made on this is the 3 adjustment screws on the secondary and primary mirror, never moved the central screw or spider vanes and had this scope since May. Clear skies and thank you do much again 🔭📷🌌

[Pixies]

When adjusting the spider vanes, you need to make sure that the central screw in the secondary holder is equidistant from the tube edges in all directions. You don't need to use them to align the secondary in your scope.

As long as the base of the mirror is parallel with the part that the adjusters are in (I'm not sure what you call this bit) it should be reasonably aligned with the focuser, once you have the distance up/down the tube correct. Then you fine-tune it's position under the focuser using the 3 adjusting screws.

The 3 adjusting screws should be roughly at the same level. If they are very different, it might be worth trying to get them level before going any further.

[AstroNebulee]

1 hour ago, Pixies said:

When adjusting the spider vanes, you need to make sure that the central screw in the secondary holder is equidistant from the tube edges in all directions. You don't need to use them to align the secondary in your scope.

As long as the base of the mirror is parallel with the part that the adjusters are in (I'm not sure what you call this bit) it should be reasonably aligned with the focuser, once you have the distance up/down the tube correct. Then you fine-tune it's position under the focuser using the 3 adjusting screws.

The 3 adjusting screws should be roughly at the same level. If they are very different, it might be worth trying to get them level before going any further.

Thank you pixies, I will measure from the central screw to all edges of the tube first before moving anything just to check it's central in the tube. I think as you say it could be a case of getting all the adjustment screws the same length first. Then moving the secondary. Just need to nudge it up the focus tube and towards primary a bit. 

[Pixies]

If you  have trouble getting it level, one thing you can do is:

• Wind all the adjusters out and tighten the central screw until the secondary mirror is brought right up to the end of the tube as far as it can go and is flush with the end assembly. now you know it is parallel!
• Now, bit by bit, loosen the central screw and take up the slack by tightening all 3 adjusters by equal amounts - thus moving the secondary back down the tube. As long as you keep tightening the 3 adjusters by small equal amounts each time, you know the mirror base will remain parallel.
• Keep going until the mirror is at the correct distance down the tube, and do a final rotational adjustment so that the secondary appears completely concentric in the Cheshire.

Make sure you don't over-tighten the 3 adjusters at any time. They just need to be finger-tight.

Once the above is done, you can remove the paper and start the final tilt-adjustment of the secondary to align it with the primary. Then it's just the standard alignment of the primary.

 

[NGC 1502]

The hardest bit is sorting the secondary. Once that’s done it usually stays ok for a long time, unless the scope gets a hard jolt.

You may have to give the primary a tweak now and again, that’s relatively simple.

I know that fear of collimation can put off some from buying a Newtonian reflector.  That’s a shame, because once mastered you will be glad you bought a reflector, definitely the “best bang for the buck” there is 👍

Edited November 26, 2020 by NGC 1502
Correction

[AstroNebulee]

Thanks pixies, I will do that thank you, then at least I'll know it's paralell, thinking of getting some bob's knobs to help collimation but are they worth it. I agree NGC once the secondary is spot on it'll only ge the odd tweak to the primary needed. Newts are indeed best value for money aperture wise, Clear skies 

[Pixies]

Thumbscrews like Bob's Knobs make it much easier to adjust the secondary while you are looking down the cheshire. They help avoid over-tightening too.

You don't need to get something expensive like Bob's Knobs. Suitable thumbscrews are available as long as you know what size you need. If you are going to bring the secondary right back up the tube, take the opportunity to remove one of the adjusters and measure it. I think it might be an M4 bolt. You will need to work out the appropriate length, as they will need to be longer than the existing ones. I have a bagful of M4 x 40mm if you find this would be a suitable size - but they might stick out a bit (although that's not too bad a thing). Like these: https://www.amazon.co.uk/Thread-Knurled-Button-Thumb-Screw/dp/B0848PGF57

Alternatively, you can find plastic bolt head covers that will snap on to the head of a suitable bolt and turn it into a thumbscrew.

[AstroNebulee]

19 hours ago, Pixies said:

Thumbscrews like Bob's Knobs make it much easier to adjust the secondary while you are looking down the cheshire. They help avoid over-tightening too.

You don't need to get something expensive like Bob's Knobs. Suitable thumbscrews are available as long as you know what size you need. If you are going to bring the secondary right back up the tube, take the opportunity to remove one of the adjusters and measure it. I think it might be an M4 bolt. You will need to work out the appropriate length, as they will need to be longer than the existing ones. I have a bagful of M4 x 40mm if you find this would be a suitable size - but they might stick out a bit (although that's not too bad a thing). Like these: https://www.amazon.co.uk/Thread-Knurled-Button-Thumb-Screw/dp/B0848PGF57

Alternatively, you can find plastic bolt head covers that will snap on to the head of a suitable bolt and turn it into a thumbscrew.

Wow, thanks pixies, I will measure my secondary adjustment screws and see what size I need, the ones you link to are much more affordable that bob's ones, that brilliant 👍

[AstroNebulee]

Ive had a go at moving the secondary and now i think ive got in central to the focus tube, the 2 photos attached are using my long chesire with a sheet of paper behind the secondary and white paper blocking the primary. 1 photo is racked out and 1 photo is racked in. This is obviously just trying to get the secondary central in the focus tube and do primary mirror following this, just want to check the secondary is central first before continuing. Ive attached an image without the chesire in looking as best as i can down the focus tube. Bear in mind that this image is slightly off looking at it now and couldnt hold the camera steady enough, but putting mere de collimtion on the racked out image the lines are concentric.

Edited November 28, 2020 by LeeHore7
Added content

[jonathan]

Sorry to partially hijack a thread but I'm also trying to collimate my Skywatcher 150P reflector but having a hard time following AstroBaby's guide... it says to use the vanes to adjust the secondary so that it appears as a circle in the focuser, but in order to achieve this I find myself adjusting almost completely to one side (one vane is almost completely unscrewed, the opposite almost completely screwed in), that surely can't be correct as it's definitely not central to the main tube.  Perhaps I've misunderstood the process.  Initially my secondary mirror looked elongated.

Edited November 28, 2020 by jonathan

[Pixies]

That looks great. Now remove the paper and fine-tune the tilt of the secondary, so that the doughnut marker on the primary is directly under the cross-hairs. Then that will be the secondary adjustment finished. Don't overtighten the adjusters, mind. 

[Ricochet]

17 minutes ago, jonathan said:

it says to use the vanes to adjust the secondary so that it appears as a circle in the focuser

No, it does not. You are getting two separate sections confused.

Initially, you adjust the vanes so that the central bolt of the secondary spider is centred within the tube.

When you are trying to make the secondary circular you use the secondary collimation screws. There is an image to show this under the subheading "Adjusting the secondary mirror's tilt" as you follow the guide.

 

[NGC 1502]

6 minutes ago, Ricochet said:

No, it does not. You are getting two separate sections confused.

Initially, you adjust the vanes so that the central bolt of the secondary spider is centred within the tube.

When you are trying to make the secondary circular you use the secondary collimation screws. There is an image to show this under the subheading "Adjusting the secondary mirror's tilt" as you follow the guide.

 

Indeed, part of the process of a full collimation of both the secondary and the primary is knowing which bit to ignore at what part of the process.

For a full on collimation -

When sorting the secondary to bring it in line with the focuser (which should be done before the primary) ignore the reflections of the secondary & spider vanes.  That’s why some recommend a piece of paper inserted to block those reflections.  Then remove the paper, still ignoring the reflections previously mentioned, then point the secondary at the primary using the primary mirror clips as a guide.

Once the secondary is fully sorted, leave it alone.   Then adjust the primary to get the reflection ( the reflections you’ve previously ignored ) of the secondary and support spider sorted.

As previously mentioned, it really is harder to describe than actually do.....no really......🙂

The secondary once sorted usually stays fine, unless something is loose.  

Most often all that’s needed is to tweak the primary.

 

 

[AstroNebulee]

1 hour ago, Pixies said:

That looks great. Now remove the paper and fine-tune the tilt of the secondary, so that the doughnut marker on the primary is directly under the cross-hairs. Then that will be the secondary adjustment finished. Don't overtighten the adjusters, mind. 

Thank you Pixies, I will now remove the paper and get the doughnut of the primary lined up with the crosshairs of the Cheshire.  I have ordered some of those M4 thumb bolts as you suggested as I did remove one of my adjustment screws and is indeed a M4 grub screw, the new ones will do the job admirably and a fraction of the cost of Bob's.  will post an update of the further progress of the collimation to get your advice.  

[AstroNebulee]

Here is hopefully the crosshairs of the Cheshire lined up with the doughnut of the primary so the secondary alignment should be done, now just to adjust the primary mirror, only a slight adjustment needed here.  Below this image is the completed collimation process with the primary now lined up with the hole in the Cheshire,  Thank you to Pixies and NGC for your help and perseverance with me in this process.  It wasn't that bad to do just needed to be brave, Clear skies

Edited November 28, 2020 by LeeHore7

[Pixies]

Cool. You won't need to do the secondary again unless it takes a big whack - or when you replace the adjusters with thumbscrews. Even then, it'll just be the final alignment with the doughnut.

When you get a chance to observe with it, see if you can do a star test to confirm it's all OK.

Normally, you should check the collimation every now and again. I do it each time I set it up, while the scope is cooling. All you do is put in the Cheshire and point a red light torch in the 45deg window of the Cheshire - then you can check the 'dot' is in the doughnut when you look down it. Most times, it's fine and doesn't need adjusting.

[AstroNebulee]

34 minutes ago, Pixies said:

Cool. You won't need to do the secondary again unless it takes a big whack - or when you replace the adjusters with thumbscrews. Even then, it'll just be the final alignment with the doughnut.

When you get a chance to observe with it, see if you can do a star test to confirm it's all OK.

Normally, you should check the collimation every now and again. I do it each time I set it up, while the scope is cooling. All you do is put in the Cheshire and point a red light torch in the 45deg window of the Cheshire - then you can check the 'dot' is in the doughnut when you look down it. Most times, it's fine and doesn't need adjusting.

Thank you again Pixies, the whole process wasn't as bad as I thought it was going to be.  I will give it a star test when I get to see some stars when the full moon has passed , the air quality out there at the moment is quite hazy but a quick view of the moon with my 20 mm ep gave good views, I will defiantly  use my red light headtorch and Cheshire combined to check the collimation.