Yet Another Collimation Question (YACQ)

[snarkyboojum]

UPDATE: Eek, I meant to post this in the Beginner's Equipment Forum, but don't seem to be able to delete or move it.

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Hi peeps,

I'm probably obsessing over the collimation of my reflector (250mm diameter, 1250mm focal length, f/5), but who doesn't :)

Part of the issue is that I'm getting really crappy seeing conditions from the backyard at my new place (I've moved right next to Sydney airport - yeah, bad move) and so I'm not happy with the detail I'm seeing, even on planets like Jupiter and Saturn - though I can see bands on Jupiter and make out some bands on Saturn and just spot Cassini's Division.

I'm trying to make sure my scope is collimated properly though to rule that out as a possible reason for the poor detail I'm seeing, and I'll be making a trip to properly dark skies soon to see what I can see with much improved conditions.

I'm using a long tube Cheshire collimator. I'm pretty happy with the secondary placement and then getting the primary tilt right, BUT I have a bit of an issue with the secondary not being perfectly circular though the cheshire tube once I'm done. I see a slightly elliptical shape for the secondary - and this seems to indicate to me that the secondary isn't tilted in such a way to the sight tube that it appears circular, i.e. it's not perfectly perpendicular (I hope this all makes sense). I've played with the tilt on the secondary for ages, and it always becomes slightly elliptical when I've lined up the centre of the primary. It's like some kind of offset.

Can this happen? Does it matter? I'm really not sure, some images I see in collimating instructions seem to show the secondary as slightly elongated, and some perfectly circular.

Thank you to the collective collimating brains, in advance.

[Paul Haese]

From your description you need to rotate your secondary. Slightly undo the centre bolt and then rotate the secondary, then go through the collimation process again until you have good collimation and a circular secondary.

[mental4astro]

As Paul says, sounds like your secondary needs to be rotated to face the focuser a little more.

The image of the secondary needs to be circular.

I see that you may not be too far from my place here in Sydney. I'm in Maroubra. If you like, I'd be happy to show you and take you through the collimation process. It can be a little confusing to start with as there is a sequence of steps involved, and the reflections one sees in the secondary mirror can be misleading if not understood. I'll send you a PM.

Alex.

[snarkyboojum]

Thanks guys. I can't for the life of me see how rotating would fix it - it seems to be an alignment issue, i.e. would need to tilt towards the focuser so the optical path is perfectly perpendicular. I could well be wrong though :)

Thanks for the PM, Alex.

[floyd_2]

To reduce the number of reflections that you're looking at to adjust the secondary, you could put a white card between it and the primary (just down the tube a little from the secondary), and also one behind the secondary opposite the focuser. That should let you focus on just the outline of the secondary with good contrast to get it spot on with the bottom of your cheshire (ie. perfectly round) without any other reflections to muddy the waters. Once you have that squared away, you can start work on your primary.

It might be worth checking that your focuser is perpendicular to the axis of the telescope too, and then shim it if it isn't. You can use the cross hairs on your cheshire (if it has them) for this. I used to use a sight tube for this, so I'm not 100% if the newer do-it-all cheshires will work the same way. If you're not removing the secondary, place a card just in front of it between it and the focuser. Draw a spot on it, and align the card such that the spot is right under the cross hairs. Now rack the focuser in and out. It should stay on the spot. If the cross hairs move away from the spot as you wind in and out, you're focuser isn't perpendicular to the scope and will need to be shimmed.

You could also measure your spider vanes to make sure that your secondary is in the centre of the UTA / tube and adjust if there are problems. It's certainly more finicky to fine tune the secondary compared to the primary IMO, but worth the effort.

[snarkyboojum]

Ah ha! That paper trick worked. I've been focusing on the reflection of the secondary it seems. When I put paper between the secondary and primary, the secondary does indeed look very circular :)

[mental4astro]

:thumbsup:

Awesome stuff! :)

Be patient with the planets. Surface details are low in contrast, so you need to give your eye time to adjust.

If seeing is dodgy, be patient again, waiting for those fleeting moments of clarity, and like a veil being lifted a wealth of detail can be seen until the veil is all too quickly replaced.

With the planets too, a dark site is no help. All a dark site could help with is a different location that may have provide better seeing conditions. It is possible to improve your chances with better seeing conditions and it takes careful and considered site selection. There are some spots up in the Blue Mountains that routinely provide the superb seeing and transparency. These are few not for lack of spots butf for lack of access. Local microclimate is the deal clincher here, and understanding how this works with the local geography is the key. The added bonus of careful site selection is it can provide a location that is totally dew free, not by chance either but deliberate selection. You don't set up on a lush grassy field in a valley. Nor on agricultural land. These are all dew traps. Where we go we've only had dew be problematic twice in seven years because the wind was the sea breeze from the east. Pretty good track record though.

Alex.

[floyd_2]

Out of interest Adrian, have you done a star test on your scope as yet? I'm interested to know how the star test looks (focused, inside of focus, outside of focus). There are a lot of resources on the web about star testing, but you could take a look at this one to see if it helps:

https://www.bbastrodesigns.com/JoyOfMirrorMaking/StarTesting.html

[Wavytone]

Hi Adrian,

First clue - weather map. Bookmark http://www.bom.gov.au/australia/charts/synoptic_col.shtml because seeing is usually best when there is a big, fat high pressure system - labelled "H" - smack over Sydney. This is why Saturday night will be hopeless, but by Monday or Tuesday that H currently west of Perth will be over Sydney.

Second clue: Isobars (lines on this map) close together = lots of wind = poor seeing. Isobars far apart = calm, and (maybe) good seeing. A cold front (curve with pointy bits) means seeing will be utter crap.

Third clue: Sources of heat nearby:

a) domestic houses with dark tile roofs - these get hot in the sun. They spend all night cooling... by creating thermals that destroy your seeing.

b) nearby buildings get very hot in full sun (summer) and spend all night producing thermals to cool down. if you have lots of big-box buildings nearby its basically a catastrophe seeing-wise.

c) in winter, thanks to air-conditioning systems, buildings are still warm and spend all night creating thermals as the cool down, not not as badly as summer.

And guess what is near you...

Fourth clue - large areas of tarmac. its black, it gets hot in the sun, and you know what that means - more thermals. And guess what is right near you...
And thats before thinking about whether you are looking through jet exhaust.

Fifth clue: local terrain and microclimate. The seeing you will experience is most affected by the first 1000 metres of air immediately above you - and in particular what lies upwind of you. If the local airflow over your site is turbulent you will have bad seeing, period. For a frequent chance at good seeing you have to find a site which has a prevailing breeze with laminar flow over the top. I only know of 2 such locations; one being the NSAS site at Terrey Hills, the other being Shipley Plateau west of Blackheath.

Best way I can describe this is to imagine your locality reduced in scale to the size of a desktop, then imagine the air as being like a river of water flowing over that. If it would flow smoothly, great. if its turbulent, poor seeing.

[snarkyboojum]

Yep, as you rightly point out it's pretty obvious my current location is a disaster for good observing.

I appreciate the detailed run-down though. That's a pretty comprehensive list of disasters :)

Cheers!

[snarkyboojum]

I haven’t recently, but I will give it another shot on the next clear night in Sydney. I’m taking the scope out bush next week so will have no excuses for lack of dark clear skies.

Also recently bought a ZWO ASI120MC-S, so should have some photos of what I’m seeing too.

Thanks for the link. Will check it out and let you know how I get on.

[snarkyboojum]

Sounds like you have this all down to a fine art. I’m slightly jealous.

Thanks for your help, Alex.

[Fleegal]

Hi Adrian,

I am new to this forum. I have a 16" lightbridge dob. I have never been happy with collimation. Owned for 12 yrs and found it not much better than my previous 10" f6.
I could never get the secondary circular although the laser hit the centre and returned perfectly. I made myself a 2" extension tube, crosshairs etc and found that I only saw 1/2 the primarý reflecting when the secondary was circular. The sight tube was grossly out of alignment. On squaring the sight tube, I was then able to align the secondary so that I now have a perfect circle reflecting the entire primary. WOW!!! What a huge improvement not just in brightness but sharp as a pin images. I now have a new scope, an entirely different telescope. After reading several collimation articles I realised that I had never done it right, just relying on the laser. Starting with the focus tube and working forward from there. I've been a casual astronomer for nearly 50 yrs, starting with a 2 inch tasco refractor, 10 inch f6 eq, and now my 16 inch dob. Hope this helps someone. Also was using my 1980 plossls. Now purchasing some much needed upgrades. Cheers guys.

Tony:lol:

[snarkyboojum]

Hi guys,

Here's Saturn and Mars from my backyard on a not particularly clear night under jets taking off from Sydney airport :P Still futzing with collimation, but the tips in this thread have helped a great deal.

Adrian.

[Fleegal]

I put some graph paper behind the secondary to make sure the mirror is parallel to the telescope tube before squaring it with the sighttube. This helped.