Confused by laser collimator - beam is not centered on the secondary

[adarshajoisa]

Ouroboros said:  "By 'skyward' I assume you mean to the left of centre of the secondary when looked at through the focuser.  

But it should lie on the centre line of the secondary. On the image above the spot appears to be somewhat above that centre line." 

   In the picture, left is towards the open end of the telescope and towards the sky, so that is what I meant.  It looks like the outgoing and incoming laser beams are hitting the secondary just above (in the picture) the major axis of the secondary. Ideally, the beam impact should be on the major axis. They are hitting skyward of the minor axis, as they should. 

 It looks like the center of the secondary has been marked. That's not necessary or advisable, in my opinion.

The center of the secondary has not been marked, Howie. I think it's just a reflection of the oneplus logo from my phone But yes, the beam ideally should have been on the major axis, but is above it (in the picture). I think it's because the focuser is not attached parallel to the beam coming off the secondary. It seems to be slightly off. I realized this because, if I take the collimator just out of the focuser and tilt it a little, the beam hits the major axis. I think the focuser might have to be adjusted for the tilt. ( To clarify, the focuser is tilted not towards or away from the primary, rather parallel to the primary, along the circumference of the tube, if you know what I mean).

[adarshajoisa]

Another thing which led me to this conclusion is that if I first correctly center the secondary using a collimation cap, and then start collimating using the laser collimator, once I adjust the mirrors such that the beam hits the center dot and reflects back to the center of the collimator, the secondary seems to be very off when I look through the collimation cap later on. Only about 75% of the primary is visible through the cap. If I center the secondary through the collimation cap and use the laser collimator, the laser beam hits the secondary off either axes as shown in the photo, and it hits the primary way off the center spot.

[beamer3.6m]

Have we worked out if the scope produces good views as it is? If so just leave it alone.

[adarshajoisa]

Have we worked out if the scope produces good views as it is? If so just leave it alone.

Yes, the views are really good as it is. And yes, I guess I'll leave it alone until someone more experienced can have a go at it

[Moonshane]

The secondary position, as long as it looks about right, has no perceptible impact for visual astronomy. The accuracy of the primary adjustment matters far more.

[Ouroboros]

I would have thought that adarshajoisa is right to be concerned that he can only see 75% of the primary mirror through a collimator cap.

[adarshajoisa]

I would have thought that adarshajoisa is right to be concerned that he can only see 75% of the primary mirror through a collimator cap.

Yes, So I have adjusted the secondary such that the entire primary is visible. This throws the laser from the collimator way off the mark, but until I can do a star test or get a second opinion from someone more experienced, I want to keep it that way. I get reasonably good views with this setup, and I haven't had good enough conditions to do a star test yet.

[jetstream]

I was waiting for someone to mention that a laser is not designed to set the secondary .... no offence to laser lovers and yes there are advanced methods that can work for this purpose.

A really inexpensive, good tool is a combination Cheshire/sight tube that will satisfy your collimation OCD I use a similar set of tools that satisfy my collimation OCD- actually the Cheshire has reduced my OCD, as they work so well!

[Moonshane]

Good point Gerry. 

[Lowjiber]

If it ain't broke, don't fix it.

With crisp views and a star test that is good, your scope is just fine. 

[adarshajoisa]

Hi, all!

Firstly, I apologize for succumbing to my collimation OCD against many of your excellent advice! 

After fiddling around with the scope, and trying out various things, I confirmed that the issue was being caused by an incorrectly attached focuser. I disassembled the focuser assembly from the telescope, and set it back into a correct position by keeping a small piece of folded, laminated piece of cardboard I found in an old box ( laminated to prevent it getting drenched in dew ). I'll attach a couple of photos below to show you how the focuser was fixed:

As Mr. Howie Glatter pointed out, the laser beam ideally should have been on the major axis, even if it's offset along it. With the focuser attached this way, the laser is now centered on the Major axis of the secondary, and I'm able to collimate it with a laser very accurately. The piece of cardboard is essentially lifting one side of the focuser assembly by about 3mm. I tried out several pieces of cardboard of varying thicknesses, until I arrived at a fairly accurate position for the collimator. After fixing the focuser position, I collimated using a collimation cap first to position the secondary, and then a laser collimator to fine tune the secondary position, and to collimate the primary. I later check the collimation using a collimationi cap, and everything seems to be nicely centered. 

I have been unable to get hold of a Cheshire EP. I couldn't find any store here selling it, and the only option for me is to import it from the US (which will make it very, very expensive, considering shipping and the ridiculous customs charged by Inida). One of my friends has a Cheshire, and hopefully, I'll be able to use it this weekend.

I'm also waiting to star test the telescope as soon as possible. There's been a cyclone here for the past 4 days, and it's been totally cloudy, windy and wet!   But with the results looking good through my collimation cap and the laser collimator, I'm very hopeful now!   Can't wait for clear skies to go explore with the new, collimated scope!

P.S., I've extensively tested the laser collimator since I last posted here, to make sure it's collimated fine. It seems to be collimated really well, and I don't think it'd introduce any errors in collimation.

[Dave In Vermont]

Very good news, and a good approach to correct the problem! Once your monsoon makes tracks for the Pacific and El Niño - do a quick star-test to fine-tune. If you want. Or leave it in peace if your happy with the views it gives you now. Squaring the focuser has likely solved a plethora of problems. Congratulations!

I'll throw your photos of the fix in my How-To file for anyone in the future who is finding themselves in a similar predicament.

To clear skies!

Dave

[Peter Drew]

I'm sorry, but the photo doesn't make things look right to me. The focusers are all mass produced which these days equates to all being the same, 3mm tilt is quite a large amount of difference, the same effect would be gained by a very small rotation of the secondary by comparison in my view. I could be completely wrong but as a retired professional telescopemaker this would concern me. Good luck in any case. 

[adarshajoisa]

I'm sorry, but the photo doesn't make things look right to me. The focusers are all mass produced which these days equates to all being the same, 3mm tilt is quite a large amount of difference, the same effect would be gained by a very small rotation of the secondary by comparison in my view. I could be completely wrong but as a retired professional telescopemaker this would concern me. Good luck in any case. 

Peter,

I tried a few alternatives before adjusting the focuser as a last resort. Let me try to clarify:

Firstly, I tried rotating the secondary in different ways. At the only position where the whole of the primary was visible with edge supports, the secondary looked elliptical from the collimation cap. Two opposite edges of the secondary showed areas outside the primary mirror - just like embedding a smaller circle inside an ellipse. So, I assume it was tilted away from the focuser. If I first turned the secondary by looking through the collimation cap so that the secondary looks circular, only about 75% of the primary was visible. When I put a laser collimator in the focuser ( I tested the laser collimator to make sure it was perfectly collimated), the laser spot on the secondary mirror was off center, outside either of the axes of the secondary mirror. You can see this in the photo I attached with my first post in this thread. This, consequently made the laser spot fall about an inch away from the center of the primary. 

Also, the secondary wasn't centered when I looked through the focuser. It seemed off to one side of the tube by quite a bit. I double checked the length of each of the spider vanes, and also tried adjusting the spider vanes to center the secondary in the focuser, but the adjustment screws don't allow such a large range of motion, and any adjustment I make would take the secondary off the center of the tube. 

So, the secondary rotation and offset, both didn't solve my problem. Using the laser and a bit of geometry, I figured out that the issue is likely being caused by an incorrectly fixed focuser. I've attached a rough sketch I did in Paint for your reference below.

In this picture, you're looking down the tube towards the primary. The star at the center is the center of the tube, and of the secondary holder. The thick line passing through the center is ideally how the focuser's axis should be, and the thin line you see, is the actual focuser axis in the telescope. Any rotation of the secondary mirror to make the laser beam hit the target, will be in such a way that a large portion of the light from the primary doesn't reach the EP ( only a part of the primary would be visible in the collimation cap). And if I rotate the secondary such that the entire primary is visible, the secondary is no longer circular, but elliptical, and the laser beam from the collimator exits the scope enitrely instead of reflecting back into the focuser. 

What  I did by adding the cardboard piece, was to tilt the focuser such that its axis lies in the thick line in the picture. This ensures that the secondary is circular and centered in the focuser, the primary is fully visible, with the center spot being at the center of the secondary shadow ( Basically, everything looks like concentric circles when viewed through the collimation cap), and the laser hits the center of the secondary, the center of the primary, and back into the bulls eye nicely. 

I'm in no way as experienced as any of you in making or using telescopes, and I just tried logically eliminating each possibility until I arrived at a working solution. Honestly, I am as surprised as you are as to why a mass manufactured scope has such a defect. Note that the focuser unit itself doesn't seem to have any issue. It's only the way it was attached to the tube that was slightly off. 

Please let me know if something wasn't clear, or if you think I've missed something, and there's a simpler solution! 

Thank you!

[adarshajoisa]

Also, this problem is only in the new 8 inch skywatcher that I recently acquired. My older newt seems fine - the focuser, secondary and the primary are in perfect alignment.

[adarshajoisa]

Very good news, and a good approach to correct the problem! Once your monsoon makes tracks for the Pacific and El Niño - do a quick star-test to fine-tune. If you want. Or leave it in peace if your happy with the views it gives you now. Squaring the focuser has likely solved a plethora of problems. Congratulations!

I'll throw your photos of the fix in my How-To file for anyone in the future who is finding themselves in a similar predicament.

To clear skies!

Dave

Thanks, Dave!

The sky seems to be clearing a bit today. With any luck I might get to do a star test. If I do, I'll write a post on how it went, and if the fix actually worked! Keeping my fingers crossed till then!

[Peter Drew]

Thanks for taking the trouble to clarify your attempts at a solution,  your "fix" is the obvious solution and if it works, good news. It's just for the reasons we have both posted that leaves me surprised as well.  

[Thalestris24]

Hi

This (long!) video is worth watching for a way to adjust the focuser to get it spot on. However, I'd say only really worth the time and effort for a scope used for imaging.

Louise

[adarshajoisa]

Hi

This (long!) video is worth watching for a way to adjust the focuser to get it spot on. However, I'd say only really worth the time and effort for a scope used for imaging.

Louise

Thank you for that link, Louise! I'll check it out. I haven't had a chance to try out my newly adjusted scope owing to bad weather since a few weeks. If I'm dissatisfied by its performance, I'll give it a try

[Charic]

IMHO unless that telescope has been dropped or damaged in some way, deforming the tube where the focuser is mounted, physically changing the alignment of that particular focuser, to that scope,  then as Peter mentions, something here is not quite right?

The focuser / OTA system is mass produced ( as already stated ) and no-where in the instructions, does it say remove the focuser assembly and start aligning in a way that suits you, because the mirror geometry is somehow out of alignment.

Yes your fix may have aligned what you perceive to be accurate, but using a Laser alone is a pure waste, unless that laser is 100% collimated, and rock solid in the focuser. Lasers can and will cause many issues, and I believe this to be the case, in this incidence.

I have a cheap laser,  collimated to as good as I can fathom, but using the laser is still  fraught with inaccuracies.

I only use the laser  with a Barlow to check the primary mirror when away from home. As long as the reflected spot is centred, that's as good as I need or will achieve.

I can achieve  good collimation just with the dust cap alone. The secondary mirror which is always corrected first, is fully centred with the focuser, then the tilt/rotation if any, is acquired to see the primary, followed by a Star test. Job done.

Of all the tools available to me, and knowing their limits, I will always use the Cheshire tool to align my secondary mirror to the focuser assembly. once done, I continue to use the Cheshire until complete.

If I waned to check/confirm with the laser, I know the reflection will be ok, when Barlowed, but I doubt the laser spot, without a Barlow, will be mm perfect, dead centred within the primary mirrors centre spot. 

Your fix may have a solution that works, and  these mirrors can still  be out  of  true  axial alignment  with the OTA and focuser assembly, yet still work, which is what I believe is happening here, because of the offset you have added to the focuser?

I would be inclined to re-build the scope, with everything squared to its own mount, i.e. the focuser  to the OTA, the secondary  butted against the spider assembly and centred to the OTA,  and the primary squared and butted against the mirror cell, then start  with centring the secondary.

By all means,  just leave everything as it is, if its working, but If  I was buying  your scope! I would have some doubts as to why there is a bit of paper sticking out of the base of the focuser  assembly, knowing something is wrong with this set-up and probably walk away?

[Dave In Vermont]

I would simply stop speculating and take the scope outside on the next clear night and conduct a star-test. If that checks out okay - finis! Done.

I always - as I have the requisite gear - use a laser and my Cheshire Sighting-Tool (Orion-USA). They have always agreed with each other. And the star-test confirms them. I even have a Hubble Optics artificial-star for rainy days.

Mañanas -

Dave

[adarshajoisa]

I finally found out the issue, and now I feel really stupid!

Picture is a screenshot from Astrobaby's Guide to collimation here (Thanks, Astrobaby! Your guide is a lifesaver!)

It was this! The secondary holder was tilted and I never thought of checking it!   It was a facepalm moment. Once the secondary tilt was fixed, I was able to return the focuser to its original position and collimate it. Then a star test after the scope had cooled for 2 hours at night, and the collimation was done to the best of my ability

I tried splitting a few binaries, and the scope performed quite well. I could clearly resolve both the Epsilon Lyrae binaries ( they're separated by 2.3" and 2.6" respectively ), Rigel, 52 Ori, etc.

I tried it on Jupiter early this morning, and the view was spectacular! I could see a steady, crisp shadow of Europa on Jupiter's disc, and lots of detail on the belts and the polar regions. Just for the heck of it, I pushed the magnification to 350x, and I could even see the Galilean moons as tiny discs! Overall, now I'm extremely happy with the performance!

Charic, good point. After reading your comments, I measured the secondary cage to see if it wasn't circular. It was. So, theoretically, there was no reason for the focuser to be off. After reading Astrobaby's guide again, I realized that I had forgotten to check the secondary holder's tilt, and it turned out to be the culprit. Honestly, I'd be suspicious too if I were buying a used scope and it had a piece of cardboard sticking out under the focuser.

[Charic]

Thats good news, and the fact your receiving great images is proof that its all working well.

Dont feel stupid, its easy to overlook something, the fact you have found the solution must make you smart!

[Dave In Vermont]

What a great ending! I hope you're having a good laugh over all this by now! At least it's better than feeling stupid! Your views thus far would indeed confirm that your optics are quite sound. And your telescope has probably been examined more times than any other telescope on the entire Asian continent!

I wonder if Charic will fly over to make you an offer for it.....?

Once again - Mel (Astro_Baby) saves the day!

Dave

[adarshajoisa]

What a great ending! I hope you're having a good laugh over all this by now! At least it's better than feeling stupid! Your views thus far would indeed confirm that your optics are quite sound. And your telescope has probably been examined more times than any other telescope on the entire Asian continent!

Haha! It's very likely, Dave!

And yes, I owe a lot to Astro_Baby. I've used his guide several times even with my previous scope. It's by far the easiest and best guide to collimation I've seen.