Collimating At Room Temperature

[ultramol]

Yesterday I gave my scope a collimating makeover, the works, everything possible I could do I did. This was done in the living room at a comfortable temperature. I left it bang on. Today I checked the scope and the collimation was miles out. The only difference was I have not had the central heating on today. Is it normal for everything to move as much as it did just because of the temperature difference, if so, where is the best place to collimate your scope ?

[Dave In Vermont]

I don't have the answer to this one. But I wanted to say that I'll be watching this thread with great interest! I actually enjoy collimating scopes, but I'd never considered temperature and/or humidity variations before.

Very interesting - thank you for asking this!

Dave

[Stu]

I don't know the answer, but always do my collimation when the scope is at or near outside temperature. Lots of elements contract as the temperature falls so it wouldn't surprise me that things shift.

I think it's valid though to do a basic setup and collimation indoors if that's convenient, and tweak it fully once outside.

[brantuk]

I thought it was common knowledge that a scope can need collimation tweaks during a session due to temperature changes over the night. It's usually when starting in the mild and ending up in a sharp drop an hour or two later. But I agree it's best to start out with a check at the observing site when you begin, and do the odd star test now and then to ensure it's still on song.

[ultramol]

what got me was the amount that it was out of collimation. The secondary mirror,the primary mirror. I thought someone had been in overnight with an Allen key and a screwdriver. I used a camera in the focus tube, a collimating cap and a laser to collimate and it was perfect. Now it is miles out. I can understand it being out a bit but it was a long way out.

[brantuk]

Yes - if it moved out by that much then maybe there's some slop in the whole system somewhere. I would check the spider is attached firmly and the secondary mounted centrally and firmly on it (with no tendency to rotate). Remove any slop out of the focuser. Ensure the primary is locked in place so it doesn't tilt when slewing - some folks have been know to fit stronger springs. Ensure collimation screws on the secondary aren't pitted or subject to slip. If it's a truss tube ensure trusses are firmly attached both ends. Then collimate again and check everything holds in place when using the scope. Hth

[Scooot]

If I understand correctly you haven't used the scope or moved it inbetween collimating it? If so I don't think a temperature change should cause the large difference you mention. Whatever it is I also suspect some slop in the system. Perhaps once you've finished collimating you could try removing the laser and then putting it back in with it rotated, or rotating and lifting the scope and then rechecking it. A Cheshire would be handy with which you could double check it, maybe the lasers out. Hope you work it out.

[ultramol]

Thanks for the pointers guys. I am going to get a cheshire although a collimating cap works well. My laser is slightly out but not by much. I also use tape around my camera and laser to shim them to make sure slop is at a minimum. I have just noticed one of the vanes was twisted slightly. Dont remember seeing this yesterday so I will start from scratch again and see what happens. It is constantly raining here so I have nowt else to do with my scope....

[OneEyedSam]

It could be that your laser is not collimated, this I'll result in inconsistent results each time you put the laser in.

try turning the laser in the focusser. You may see that the red dot on the mirror moves in a circle, it shouldn't.  

Sounds likely, temperature differences might cause mild devaition of you collimation, but not for it to be a mile out

[OneEyedSam]

Just saw your last post....could be the spider as you suggest !

[CraigT82]

You haven't got kids have you? Might wanna interrogate them if you do! 

[Charic]

I suspect the Laser to be at issue! either out of collimation itself, or  too much slop in the focuser, therefore you may see the  error when you re-insert the laser?

If your confident with collimating, set everything as you do, but when it comes to aligning the primary, try using a Barlow if you have one. Then leave the scope the same amount of time, then check again,  to see if there's still an error?

Here's a .pdf regarding  Laser Barlowing.  https://www.cameraconcepts.com/barlowed%20laser%20collimation.pdf  Its for this reason, that I only use my Laser with a Barlow to check the primary is still correct? I trust and prefer the Cheshire  as my primary tool for collimating, although I think I can get just as good a set-up with a dust cap with a 1mm hole!

[Mr Spock]

Any collimation done under conditions not resembling observing conditions will only be 'near enough'.

The only true way to get your collimation spot on is to do a star test once the scope has sufficiently cooled. Even then you might find (with a Newt) you have to re-collimate when the scope is moved around.

[Charic]

I hardly need to collimate my scope often as  it  appears to be just perfect, why I don't know! maybe because I handle it with care, but as Michael points out, the true test is to carry out a Start test, just de-tune the focus and check for those concentric rings?
On my Celestron I could  adjust the collimator knobs  whilst viewing, as the OTA was so short, not a chance on my Skyliner, so help is required?

[LukeSkywatcher]

I'm a bit late to the party here but this is a very interesting topic. Its one i have never seen before about collimation....but i see the logic in it. Collimation as we know is all about aligning the primary and secondary mirrors on a scope.

Certain materials (wood,plastics and i suppose even metal) expand and contract due to temperature change

I doubt glass does, so its what is holding that glass mirror in place that will expand/contract.

In saying this, its probably best to collimate outside when observing and i imagine if the outside temp fluctuates a degree or two over an observing session that minor tweaks may be needed.

Very interesting topic.

[Mr Spock]

I doubt glass does, so its what is holding that glass mirror in place that will expand/contract. 

Not true. Borosilicate glass and 'pyrex' do expand and contract with temperature change - about a third as much as carbon steel. This is why larger, more expensive mirrors are made of Zerodur as it has a lot lower coefficient of thermal expansion.

[LukeSkywatcher]

33 minutes ago, Mr Spock said:

 

 

Not true. Borosilicate glass and 'pyrex' do expand and contract with temperature change - about a third as much as carbon steel. This is why larger, more expensive mirrors are made of Zerodur as it has a lot lower coefficient of thermal expansion.

I had no idea. I know a lot of scope mirrors are made with pyrex (the same glass used in cooking utensils which have to be able to expand/contract with heat). I stand corrected.

[baggywrinkle]

Not just the glass that can expand and contract, the screws for adjusting the primary and secondary mirrors are made of metal which can alter even more.

[todd8137]

20 hours ago, baggywrinkle said:

Not just the glass that can expand and contract, the screws for adjusting the primary and secondary mirrors are made of metal which can alter even more.

It  would need a massive drop to efect the screws