Help! Collimation woes.

[CJ (Chris)]

Hi all.
I recently picked up a used Orion XT10i. I managed a quick session on it between the clouds up here and the collimation was ok. I knew the primary mirror was rather dusty so, with all the cloud, and because it was niggling away at me, I decided I would dive in and give it a clean. I carefully cleaned it and rinsed it with distilled water, and, pleased with the result, popped it back in the tube. I aligned it with a laser collimator but on my next session found the stars to be quite blurry. Time to learn the whole collimation process I thought to myself. I made myself a collimation eyepiece by popping out the lens from an old H8 ep that I dont use, put some paper opposite the focuser and blocked off the primary with a piece of card. To my puzzlement the secondary was way off toward the tube opening, and a little elliptical too. I've adjusted the secondary to what I think is, by eye, nicely round and central below the focuser. I got a second opinion on that too. The primary has again been aligned by laser. Wen viewing down the focuser, the primary image is rather off centre to the focuser but all the mirror clamps are visible. All well and good I thought. Unfortunately things are not quite coming to crisp focus. Jupiter had a milky, slightly off centre "glow" to it. Bright stars when slightly defocused have a kind of spiked double comet tail effect, both tails going in the same direction but about 30° apart. The eccentric aura round Jupiter was bulging in the same direction too.
So in summary:
To my inexperienced eye, it looks to be collimated ok but doesn't pass the star test.
It was ok before I took out the primary despite the secondary being noticably off.
My questions are
Does that comet tail effect point to a specific problem?
How could it have been passsing the star test with the secondary being off centre to the focuser?
Have I screwed up the mirror somehow?

Any answers, comments or suggestions would be very welcome.
Thanks.

[glend (Glen)]

When you remove a mirror it has to be put back in exactly the same orientation in the carrier (if removed fom the carrier) and if the entire carrirr was removed from the tube at lrast one screw shoukd be marked at a reference to align when reasembling.
Get yourself a Cheshire eyepiece, its a good investment fir aligning. The problems that arise with lasers is that the laser has to be collimated itself and many focusers may have clearance slop that can allow a laser to lay over slightly. I use a Chesire but also some times check the secondary with the laser, and I shim the last so it can't flop in the focuser.

Recommended reading - Astrobaby's articles on Collimation.

[geolindon (Lindon)]

g day Chris,
you probably already have this

http://www.astro-baby.com/collimatio...on%20guide.htm

[barx1963 (Malcolm)]

Chris
Given you have removed the mirror completely, most likely cause is pinched optics. It take remarkably little pressure on the surface of the mirror from the mirror clips to deform the mirror surface to the point where the image is seriously degraded. If all three clips are too tight, stars turn into little triangle, which is not quite what you describe but still worth checking out.

Regarding Glens comment about putting the mirror back in the same alignment. The main reason for doing this is that the clips can leave marks on the mirror surface so you make sure the marks are put back under the clips simply so you do not get any new marks. Rotating the mirror should not have any effect, I have rotated my mirror in my 20" several times with no ill effects. The cell itself should only go in one way, although it may be different with Orion's, certainly the GSO and SW newts I have disassembled mus go back in the same orientation.

Cheers

Malcolm

[Renato1 (Renato)]

It would have been better if you hadn't moved the secondary mirror, as the secondary is meant to be slightly offset in short focal length telescopes. But maybe it doesn't matter - since you can easily see the entire primary in it, the secondary may be a bit oversized.

To get the position of the secondary correct, it is more useful to have a long tube with a peep sight in one end and with cross hairs at the other end. You can easily improvise, using your current eyepiece without the lens, getting a Barlow and unscrewing the lens from the bottom, and temporarily sticking some hairs across the open base to make cross hairs. Use the eyepiece in the Barlow tube with the cross hairs to make sure the secondary is in the right position. After you've done it once, you never have to do it again.

When you throw a star out of focus, do you see concentric rings around a dark dot? If your telescope shows this, and the dot isn't dead centre, you have to adjust it to make it so.

However, my f/4.5 dob doesn't show the concentric rings, so I made a an aperture mask for it with circles cut out that fitted where the mirror was clear (between the secondary and the primary's outer edge). Then the concentric rings around the dot were easy to see. But even when concentric, there is a slight flare to one side in the direction of outer edge of the primary. It is then a matter of rotating the mask around, making sure that the out of focus image in each of four positions around the diameter is the same (i.e. each is concentric with its little flare the same length pointing to the outer edge of the mirror).

If you get say three images correct, but one not right - suspect pinched optics on that side.

This aperture mask technique (which you make up with cardboard or plastic from a folder) always shows where you are out. I remember looking through an 18" telescope and saying it was way out of collimation. The owner replied that it couldn't be as his laser said it was perfect. Fortunately he had an aperture mask handy which I used to show him how hopelessly it was out of collimation.
Good luck.
Renato

[CJ (Chris)]

Thanks for the replies guys.
You're right Lindon. I do have that guide and I watched several Youtube videos too.
https://www.youtube.com/watch?v=8ROvNH5uwDo
https://www.youtube.com/watch?v=YAVGcGEBmCE
https://www.youtube.com/watch?v=8G98RTP6jbY
Plus a few others. They're what gave me the confidence to have a go. A little knowledge.......
Those Astronomy and Nature blokes aren't too bothered about dropping things down the tube!

Renato, I'd read about that offset secondary on faster scopes somewhere, but the Orion manual shows it as central in the assembly instructions. Those instructions are maybe a bit of a one size fits all approach as they do apply to several different focal ratio scopes. More reading to be done. That aperture mask process sounds worth looking into (hoho). I presume it has just the one hole?

Glen and Malcolm. When I removed the mirror carrier I marked its position but, I must confess, I did not mark the mirror position. Doh!
I will remove it again and look for the earlier mirror markings. I don't think I overtightened the clamps but I'll loosen them off anyway. I either got lucky putting the whole assembly bck together or there is no specific orientation of the various parts. I'll check it all again.
It's all good practice!
Thanks again.

[Jason D (Jason)]

Originally Posted by CJ

Glen and Malcolm. When I removed the mirror carrier I marked its position but, I must confess, I did not mark the mirror position. Doh!
I will remove it again and look for the earlier mirror markings. I don't think I overtightened the clamps but I'll loosen them off anyway. I either got lucky putting the whole assembly bck together or there is no specific orientation of the various parts. I'll check it all again.
It's all good practice!
Thanks again.

There is no reason to place the mirror exactly in the same orientation. Mirrors are symmetrical and any orientation will do just fine.

Jason

[Renato1 (Renato)]

Originally Posted by CJ

Renato, I'd read about that offset secondary on faster scopes somewhere, but the Orion manual shows it as central in the assembly instructions. Those instructions are maybe a bit of a one size fits all approach as they do apply to several different focal ratio scopes. More reading to be done. That aperture mask process sounds worth looking into (hoho). I presume it has just the one hole?

Yes, one hole does the trick if you can rotate the mask around.

The mask is also useful by itself, as it simulates an apochromatic refractor of size (radius of big mirror- radius of small mirror). On some really lousy nights when everything was very blobby, I'd use my aperture mask to get nice sharp views of open clusters.
Regards,
Renato

[Jason D (Jason)]

Originally Posted by Renato1

However, my f/4.5 dob doesn't show the concentric rings, so I made a an aperture mask for it with circles cut out that fitted where the mirror was clear (between the secondary and the primary's outer edge). Then the concentric rings around the dot were easy to see. But even when concentric, there is a slight flare to one side in the direction of outer edge of the primary. It is then a matter of rotating the mask around, making sure that the out of focus image in each of four positions around the diameter is the same (i.e. each is concentric with its little flare the same length pointing to the outer edge of the mirror).

I am unsure if I understood the idea above. When star collimation is executed correctly then the defocused star rings will look concentric regardless of F-ratio. The key here is "executed correctly". Star collimation should be done at high magnification -- at least 25X the aperture in inches. So, for a 10" dob, a minimum of 250X magnification is needed. The star needs to be defocused by a small amount -- enough to show 3-5 rings. The secondary mirror offset will be invisible using this setup. When the star is defocused too much, then the results will be inaccurate.

Originally Posted by Renato1

The owner replied that it couldn't be as his laser said it was perfect.

Typical laser collimators can only be used to achieve axial alignment between the eyepiece and the primary mirror. They can't be used to center/round the secondary mirror under the focuser. After all, the laser does not interact with the secondary mirror edge -- only with a tiny area on the secondary mirror surface.
Many posters complain about how after completing collimation via a laser collimator, they found out their secondary mirror is off-center. Then they blame the laser collimator.
Centering rounding the secondary mirror can be done by a sight-tube or by a laser collimator with a special holographic attachment.

Jason

[Renato1 (Renato)]

Originally Posted by Jason D

I am unsure if I understood the idea above. When star collimation is executed correctly then the defocused star rings will look concentric regardless of F-ratio. The key here is "executed correctly". Star collimation should be done at high magnification -- at least 25X the aperture in inches. So, for a 10" dob, a minimum of 250X magnification is needed. The star needs to be defocused by a small amount -- enough to show 3-5 rings. The secondary mirror offset will be invisible using this setup. When the star is defocused too much, then the results will be inaccurate.

Jason

Hi Jason,
All I know is that when I collimated my C8 and my 114mm reflector, I had no problem getting concentric circles with pretty much any magnification.

With my 14.5" f/4.5 dob, I couldn't and still can't get concentric rings with normal magnifications - at the time my highest power eyepiece was a 6.4mm plossl which gave 259X. True, I didn't bother with a Barlow to try get 400X or 500X or higher.
Regards,
Renato

[gb_astro]

CJ Chris what laser collimator are you using?

Your Orion XT10i at f4.7 really needs a barlowed laser for decent primary mirror collimation.

Likewise Renato with your f4.5 dob.
Your aperture stop is just reducing your f-stop down to f10 or so where collimation becomes less critical.
If you are not seeing concentric rings at f4.5 the scope is just not collimated correctly.

gb.

[CJ (Chris)]

Hi gb.
It's one of these
http://www.bintel.com.au/Accessories...oductview.aspx
I'll try it Barlowed. Thanks

I checked the mirror clamps and they are really not tight. Possibly a bit loose.

Cheers

[Renato1 (Renato)]

Originally Posted by gb_astro

Likewise Renato with your f4.5 dob.
Your aperture stop is just reducing your f-stop down to f10 or so where collimation becomes less critical.
If you are not seeing concentric rings at f4.5 the scope is just not collimated correctly.

gb.

My Cheshire eyepiece says it's collimated, and after it has cooled down, the telescope has delivered more detail on Jupiter than I've ever seen in any other telescope.

And my aperture mask has a 6", a 5" and a 4" hole cut out of it, corresponding to f/11, f/13, f/16, and each gives me a perfect bullseye pattern on a defocused star and very slight flare on the outer ring in the direction of the mirror's outer edge.

How can it not be collimated correctly?

The only difference from it and most other telescopes, is that the secondary isn't held in four spider vanes, but on a hemispherical arc/vane.
Regards,
Renato

[gb_astro]

Originally Posted by CJ

Hi gb.
It's one of these
http://www.bintel.com.au/Accessories...oductview.aspx
I'll try it Barlowed. Thanks

I checked the mirror clamps and they are really not tight. Possibly a bit loose.

Cheers

I don't think that Orion collimator is going to work with a barlow.
The target for the return beam has to be in front of the barlow.
With your one the beam will be going back through the barlow again messing up the mirror centre spot shadow pattern (I think).

https://www.cameraconcepts.com/barlowed%20laser%20collimation.pdf

(Looks like you have to copy/past this entire line to get the link to work)

gb.

[barx1963 (Malcolm)]

Originally Posted by CJ

Hi gb.
It's one of these
http://www.bintel.com.au/Accessories...oductview.aspx
I'll try it Barlowed. Thanks

I checked the mirror clamps and they are really not tight. Possibly a bit loose.

Cheers

The mirror clamps need to be loose, in fact not touching the mirror surface at all, the simplest way to check is you should be able to slip a piece of paper between the mirror surface and the clip, if you cannot it is too tight. The clips are only there to stop the mirror falling out if the tube is moved beyond the horizontal or it is bumped during transport.

Malcolm

[gb_astro]

Originally Posted by Renato1

My Cheshire eyepiece says it's collimated, and after it has cooled down, the telescope has delivered more detail on Jupiter than I've ever seen in any other telescope.

And my aperture mask has a 6", a 5" and a 4" hole cut out of it, corresponding to f/11, f/13, f/16, and each gives me a perfect bullseye pattern on a defocused star and very slight flare on the outer ring in the direction of the mirror's outer edge.

How can it not be collimated correctly?

The only difference from it and most other telescopes, is that the secondary isn't held in four spider vanes, but on a hemispherical arc/vane.
Regards,
Renato

Renato what is the appearance of the out of focus star image with no aperture stop on?
You are placing the test star very near the center of your eyepiece's field of view?

gb.

[Renato1 (Renato)]

Originally Posted by gb_astro

Renato what is the appearance of the out of focus star image with no aperture stop on?
You are placing the test star very near the center of your eyepiece's field of view?

gb.

The star is dead centre and it becomes a concentric doughnut without any rings.

Believe me - when I first got it I was scratching my head as it didn't do what my other reflector, refractor and C8 was doing - they all gave me rings. I immediately had to order a Cheshire.
Cheers,
Renato

[CJ (Chris)]

Thanks for fixing that link gb.
I can just make out the circle of the centre mark. It was a little off compared to the non barlowed laser. The heavy clouds have returned for a while so no further testing until I can see some stars!
Cheers

Originally Posted by gb_astro

I don't think that Orion collimator is going to work with a barlow.
The target for the return beam has to be in front of the barlow.
With your one the beam will be going back through the barlow again messing up the mirror centre spot shadow pattern (I think).

https://www.cameraconcepts.com/barlowed%20laser%20collimation.pdf

(Looks like you have to copy/past this entire line to get the link to work)

gb.

[gb_astro]

Originally Posted by Renato1

The star is dead centre and it becomes a concentric doughnut without any rings.

Believe me - when I first got it I was scratching my head as it didn't do what my other reflector, refractor and C8 was doing - they all gave me rings. I immediately had to order a Cheshire.
Cheers,
Renato

Not seeing any rings at all would not be a collimation issue here but an indication of an optical problem with the scope.
I'm no expert but a complete lack of rings is usually associated with excessive surface roughness of the mirror or an eyepiece that is very bad at your f-ratio.

Adding an aperture stop to a scope will suppress a few of it's inherent aberrations so that could explain why you are seeing the rings at the higher f-ratios.

Hope I'm wrong though...

gb.

[Renato1 (Renato)]

Originally Posted by gb_astro

Not seeing any rings at all would not be a collimation issue here but an indication of an optical problem with the scope.
I'm no expert but a complete lack of rings is usually associated with excessive surface roughness of the mirror or an eyepiece that is very bad at your f-ratio.

Adding an aperture stop to a scope will suppress a few of it's inherent aberrations so that could explain why you are seeing the rings at the higher f-ratios.

Hope I'm wrong though...

gb.

Visually the telescope is great, and i mainly use a Nagler or Meade Ultrawide in it. The only difference is that I don't have diffraction spikes on stars from spider vanes, but instead have sort of a glow around stars from the arc vane.

The other odd thing is that I keep reading that if I use huge exit pupils, I should see the image of the secondary in the eyepiece. Yet with a 9mm exit pupil, I really have to strain to make out slight darkness in the centre of the image.
Cheers,
Renato