Collimation questions.

[Malcolm]

I have an Orion Deluxe II Laser. After I've done the procedure, with everything lined up perfectly the stars have tails, and separates to a double image when in-out focus rack.

So as my Cheshire got broken when some clown picked up my eyepiece case to move it when it was still open and everything spilled out the cross-hairs broke I made a collimation cap. I put in a sheet of white paper and I could see that the primary wasn't concentric around the secondary and only three clips visible out of the six.
So I figured that the secondary was possibly too far in as I could only see the clips on the focus tube side of the view. Undid the hex screws and gave the centre screw a couple of turns and now the view is concentric with all the clips visible.

Fit the laser and the beam is off centre, so I adjust the beam with the hex screws, all good. Put the collimation cap back on and the the view is no longer concentric and I'm back to seeing only three clips! I can see that the primary mirror donut is obscured by the reflection of the collimation cap perfectly, but the laser tells me the primary is fractionally off so that got a minor tweak to put the return beam on target.

The cap still shows black dead centre of the donut but the primary is not concentric in the view, I can clearly see three clips, and the remaining three are just there with the focus tube racked all the way in. The primary is definitely not in the centre of the view. The secondary shows as concentric through the cap.
What exactly is going on? I can't test this again until tonight, but I suspect it will be off.

I took a pic through the cap as well.

[bojan]

Not necessarily...

It seems your secondary or your focuser may not be positioned properly.


I would suggest you to start with the focuser - remove the secondary and see if laser beam is pointing at the place exactly opposite the focuser... and adjust that.

Then put the the secondary back in place (make sure it is offset away from focuser to avoid vignetting) and adjust the laser beam to point to the centre of the mirror.. then adjust the returning laser beam such it is hitting the secondary at the same spot by tilting primary.

[Malcolm]

Hmmm, adjust the focuser? There's no adjustment possible as it's firmly secured to the OTA by four screws, and it's never been moved at all. I don't understand that at all, and how would you determine the exact centre on the inside of the tube?

[bojan]

It is just a suggestion..

I had that problem (with my DIY tube). Not likely, but possible.

Adjustment is possible by inserting spacers under focuser, if necessary.


If you rotate the secondary by 90°, the beam shoud graze the mirror surface and hit the tube at opposite side (exact expected place can be determined using measuring tape from outside... it should be simmetrically spaced from focuser.

[Malcolm]

OK. I doubt it's the focuser. I'm not keen on fiddling with secondary much more. I have no idea why it's off all of a sudden. It's been perfect for so long.

[mental4astro]

One reason for this misalignment is the secondary may not be square to the focuser. The secondary can be rotated around in the spider, and everything may look OK for the laser, but the tell tale sign is the mirror is not centred and the stars have little tails, all that you are seeing.

I had this problem too. Took me a while to figure it out when the laser was saying everything was "OK". I wasn't using a Cheshire either, and it made things worse.

If the cross hairs of your Cheshire are busted, you can is still us it to work this out. The cross hairs make things easier but not essential.

I am guessing that your Cheshire is like one of these?

233539

If it is, let me know. I can work out a plan to help you out. There's a couple of ways to do this.

Alex.

[mental4astro]

Looking at your photograph at bit more carefully, I am assuming the pic is through the a collimation cap? For what I am seeing is the secondary is not cocentric to the cap. The lower right edge of the secondary is obscured by the edge of the cap. The reflections seen in the secondary tell me the same story.

Despite the crosshairs of the Cheshire being busted, the tube itself if what makes it critical for centering the secondary. Don't worry about any offseting of the secondary for now. Unless you are doing photography it is not that significant, and for now it is only going to make things more complicated.

Collimating a Newt is not hard. But because of the various elements that are involved, it can make keeping track of everything very difficult and all the reflections only add to the confusion.

Are you familiar with the Collimating page from AstroBaby? It is a really good guide to the processes involved, trouble shooting, analysing optical issues that star images give, and the differences between fast f/ratios slow f/ratios throw up in the collimation process.

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

When you are all well and truly ready to take to the scope with a hammer, this is when you walk away and do something else for a while. Frustration feeds on itself, and won't help you sort out the problem at hand. And Collimating a Newt can make a hammer look mighty appealing! :bashcomp:

Have a little read of AstroBaby's page, even if you know it all already. It will help calm and re-focus your attention to the job at hand. We here while wanting to help are also limited in how well we can guide sometimes.

Like I said in my earlier post, crosshairs are not essential, so don't turf out the Cheshire as it needs to be used exactly the same way for everything - centering of the secondary to the focuser, squaring the secondary (rotation), and for tweaking its alignment to the donut in primary's centre - used exactly the same way as a collimation cap. A laser will do none of these. A laser will only help with the very last fine tweaking of the secondary's collimation screws, but ONLY the very last fine tweaking!. The laser will only serve to screw things up even more if the secondary is not right. Leave the laser to deal with that very final tweaking of the secondary as it's sole purpose is really JUST FOR THE PRIMARY.

Alex.

[Malcolm]

I wish I hadn't turfed out the Cheshire a few weeks back. It's long gone now.
The pic was taken through the cap, I'll give it another go.

[mental4astro]

Argh, bugger! :doh:

All is not lost. The cap will work, though it can be more tricky to get the secondary right with it.

I've done a couple of tricks with a couple of caps I've used over the years. The easiest is to paint a white ring around the hole that faces the secondary mirror. This makes identifying the cap and the hole a lot easier. Another is a ring of reflective tape around the same hole.

The good thing is that long Cheshire eyepieces are not expensive. I too got rid of mine when I thought a laser would cover it all. Didn't I learn that lesson... :rolleyes:

[Malcolm]

I don't have any white paint, but I'll try a bit of tinfoil, I'm pretty sure I can get a small bit of my hat. :-) Meanwhile, I just hauled it outside for a test and it's no better. :-(

I'm familiar with astrobaby, and gary seronik's page on collimation.

I re-read astrobaby page and using a sheet of paper to baffle the tube, I adjusted the secondary to a perfect circle with all six clips visible. As I don't have the cheshire for the next stage I put in the laser and it's a good 2cm off the centre donut. If I adjust the secondary to put the beam on target, I lose sight of three clips on the right hand side of the view through the cap. Where's my hammer?

[Karlzburg]

I found this a few weeks back and found it pretty simple.

https://garyseronik.com/a-beginners-guide-to-collimation/

Sorry just saw your comment, it didn't show up when I was looking at the page

[Karlzburg]

Go the grinder at least you'll have pretty sparks

[Malcolm]

I think I might have solved it! After careful study of the view through the cap, and looking at pics on the intertube I came to the conclusion that the secondary was not centered under the focuser. There was more gap on the right than on the left. So I undid the centre screw and moved the secondary down the tube by about 4-5mm. Using a paper baffle to block the primary reflection and a piece of paper behind the secondary I got a perfect circle, centered under the focus tube with all six clips clearly in view. Removed the paper baffle blocking the primary reflection and the donut was off centre. I put in the laser and to my astonishment the beam was only just outside the donut, right on the edge. So a minor tweak of the secondary was all it took to put the beam on centre. Primary was well off the return bullseye so I adjusted that, then the cap went back on to find that everything looked as it should with donut reflection bang on centre. Woo! No idea how it could possibly have got so far out of wack. Meanwhile, I've bought a Cheshire on eBay, and roll on tonight's dark for a look see. :-)

[bojan]

Yep...

It may also be a bit of wrong secondary offset...
Because the light beam reflected from primary is conical, for it to pass the secondary unaffected by vignetting, it needs to be positioned a bit away from focuser (2~4mm, depending on f/ ratio and other factors).
I intend to use my Newt for AP, so I will replace existing secondary with larger one - and adjusting the correct offset will be even more important part of colimation process.

[mental4astro]

:thumbsup:

Good stuff, Mal! :)

It's very easy for things to get so out of whack! I still think your initial problem came from a secondary that wasn't square to the focuser - if the secondary wasn't centered but still square to the focuser, it is still possible to achieve a properly collimated situation. What not centering the secondary does is not maximize the amount of light going into the eyepiece. Not having the secondary square is the source of your collimation troubles.

So, by taking time to centre the secondary, you also set it square to the focuser, and everything fell into place! How about that! :D

Bojan mentions off-setting the secondary. If you are doing photography, this is more important than for visual. The percentage gain for visual isn't all that significant, and for most observers more of a pain in the bum than a help. Photo is much less forgiving, especially with fast f/ratio Newts! If you really want to off-set your secondary, the minimum you should do is set it back from the focuser anywhere between 1.5mm and 3mm depending on the size of your Newt. The other axis that can be off-set is the long axis, and the sec lowered towards the primary again between 1.5mm and 3mm, depending on your Newt. There are various formulae provided on the Net for the exact amounts for your Newt, just do a search for "Newtonian secondary mirror offset" (https://www.google.com.au/search?q=newtonian+secondary+mirror +offset&spell=1&sa=X&ved=0ahUKEwirjYa8nLTdAhVOAogKHfIlAs 0QBQgmKAA&biw=1366&bih=631)

On my big 17.5" dob, I put a little permanent marker dot on the secondary, along the long-axis where the off-set will leave the laser pointing at, which is a little higher above the sec's centre. However, I don't bother too much about it. I leave the secondary centred in the Cheshire. BUT, I do have the secondary set back from the focuser by 3mm. It does alter a little the co-centricity of the reflections that are seen, but that's not a problem for me.

My big dob is my only Newt I bother to have off-set the secondary.

If you do intend to do astrophotography with your Newt, DO look into offsetting your secondary as Bojan mentions.

Alex.

[Jason D]

In most cases you do not need to offset your secondary mirror away from the focuser to achieve perfect collimation. There are three main reasons why the away-from-focuser offset might be necessary:
1- To reduce tracking error -- assuming your scope is equipped with a tracking system and that you are doing long exposure astrophotography. For visual, the error reduction is insignificant.
2- To accommodate a front corrective plate -- assuming your scope has one.
3- To avoid front aperture vignetting -- assuming the OTA opening it too tight.
Above three reasons do not apply to most Newtonan reflectors used for vsual observation; therefore, away-from-focuser offset is not needed in most cases.

Adjusting the spider vanes to move the secondary mirror assembly away from the focuser to offset the mirror can be more harmful since it will worsen the spider vanes diffraction.

If the secondary mirror requires an offset, it would have been done by the manufacturer. If it was not then it is not needed and you should not attempt to do it.

Jason

[bojan]

Well, I am the manufacturer (of my Newt), so... :P

[Malcolm]

Interesting discussion, learnt a few new things. :-)

[Jason D]

Just curious, did you mount your secondary mirror with an offset? If yes, was there a design requirement to do so?

Jason

[bojan]

Yes and yes :)

It is possible (but not easy) to see the designed offset on attached images.
However, on design drawing, the offset from optical axis of the secondary mirror centre is evident

[Jason D]

I see the offset clearly in the photo.

Check my first attachment.

Refer to the left-hand diagram. It represents a reflector with its secondary mirror mounted with the proper offset away from the focuser. This is the typical diagram most have in mind when they think of Newtonians.

Refer to the middle diagram. It represents a reflector with its secondary mirror mounted without the proper offset away from the focuser. Many would claim this is a problem because it will miss part of the light cone. I presume this is the issue you had in mind for a secondary mirror mounted without an offset, correct? This is a common misconception. There is really no optical issues because after the proper collimation steps are completed the primary mirror will end up tilted forward towards the focuser automatically -- as shown in the right hand diagram. Now the central star in the FOV is no longer coincident with the OTA axis which has no impact on the optical alignment.

The second attachment illustrates better how both secondary mirror mounts are equivalent from an optical perspective.

One thing to watch out for is to ensure the now tilted light path does not get eclipsed by the edge of the front opening of the scope. Unless the OTA opening is within few millimetters from the primary mirror size, this is not a concern.

I have my secondary mirror mounted without an away-from-focuser offset and I do not have any optical issues with this setup.

Jason

[bojan]

Yep.. all clear.
As demonstrated on your drawings, I also wanted to reduce misalignment of light beam relative to tube (truss frame) axis... because this scope will be mounted on EQ6 and used for AP, and misalignment such as this potentially reduces pointing accuracy somewhat (it is possible to compensate that with Bartels software, but it needs to be done every time collimation was changed… not very often).

[Jason D]

Do you still believe that an away-from-focuser secondary mirror offset is needed to avoid vignetting? That is what I understood from your above statement.

Jason

[Malcolm]

Well, it's still up the creek! Took it out for a test and I can't get a star to a pinpoint. The secondary shows as a perfect circle, in the center of the focus tube. The donut is aligned with the spider vanes in the center. The laser says it's collimated. See the attached pic for what I see when I rack the focus in and out.

[bojan]

Yes, in specific circumstances (narrow tube for example), as illustrated on your drawings above.
And to have mechanically correctly designed telescope system.

[Jason D]

Above is a quote from the first reply to the OP which happened to be yours. You asked the OP to offset the secondary mirror away from the focuser to avoid vignetting which later you explained to be light cone vignetting. You did not question the OP to find out if such an offset is needed. I can tell
from the OP's photo that the secondary mirror is mounted centrally on its stalk without an offset. Asking the OP to move the secondary mirror away from the focuser unnecessarily will cause opposite spider vanes to be at an angle with respect to each other which will worsen the diffraction spikes. In addition, suggesting that the away-from-focuser secondary mirror offset is needed to avoid light cone vignetting is incorrect.

Jaosn

[bojan]

I am not sure why you insist I am in wrong here.

Yes, of course this statement was mine, the post is here to stay for all to see for all times, and I do not see any problem with it.

All I did in this thread was I gave an (free!) advice in attempt to help clarifying the cause of the problem (based on my own experience with Newtonian scope design), in an evolving discussion which at the end produced some results (and I never intended to claim at any time that my contribution is crucial for resolution).




I disagree with this - offset may be needed to avoid the vignetting in case the scope tube is too narrow, situation clearly illustrated with your third drawing, and this is what I wanted to avoid in my design. Plus I wanted the optical axis of my scope to be paralel with mechanical axis of my tube (frame), also illustrated with your own drawing.


This offset is of course meaninless with open designs (and oversized tubes, used in Dobsonians).

[Malcolm]

If I can get a word in...... turns out I have a pinched primary.

[bojan]

Elimination process is obviously finished now :thumbsup:

[Jason D]

You were clearly not thinking of the front UTA/OTA opening when you referenced vignetting. You were clearly referencing vignetting introduced by any secondary mirror mounted without an offset. I decided to contribute to this thread to explain this is incorrect. There are reasons when a secondary mirror offset is needed but what you have stated in the above quote is not one of these reasons.

OK, I will stop now. This exchange is becoming extremely uncomfortable.

Jason