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Collimation and freezing weather


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Hi, I have a SW 200P Dob and I was out observing last night, orion and its nebula, and pleiades. I have to say i am very new to the hobby.  I had seen the same targets a few weeks ago with great detail. Using my 15mm BST the stars looked sharp but when I switched to my 8mm BST i could not get a good focus on the stars (previous viewing of these targets with 8mm gave very nice sharp views) so i suspected that the collimation might be out (I only had the telescope for 1 month and I had only done 4 viewings). Last night it was around -3C (my telescope was cooled down for 40min). Getting back in the house after my session, I thought I would have a quick look with my chessire collimator and it appeared to be out of collimation. I thought i would collimate it the next day....so i was getting ready to do so...I checked today again and everything looked to be collimated (see photo after the telescope had warmed o/n; sorry no photo from last night). 

My question is, have other members here noticed that at freezing nights their telescopes go out of collimation due to the mirror getting too cold or the OTA itself? Do you collimate outside at such cases and ignore the indoors collimation? Or you do not bother getting out? I am asking since I plan to do some more viewing tomorrow night and it is predicted to be rather cold.

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I didn’t go out last night despite it being clear, because there was very high humidity, about 95% where I am. With low temperatures I think this can cause heavy dew, soaking everything and poor seeing. Could this have been your problem rather than collimation?

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I find that collimation is not affected particularly by the cold but eyepieces that get cold can mist up very quickly from the body heat from your eye. This is not apparent until you try and view something and it is frustrating when suddenly what was clear can't be bought to sharp focus.

For this reason I keep my eyepieces a few degrees warmer than the outside temperature. If I leave one in the scope for a while I try and remember to pop it back in the case to warm up for a bit.

 

 

Edited by John
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49 minutes ago, Pixies said:

Was the scope at different angles when you checked collimation? Sometimes the primary can tilt slightly.

Both days it was horizontal on its base.

 

33 minutes ago, Jiggy 67 said:

I didn’t go out last night despite it being clear, because there was very high humidity, about 95% where I am. With low temperatures I think this can cause heavy dew, soaking everything and poor seeing. Could this have been your problem rather than collimation?

It could have been. i did not check in the dark. I need a red light.

 

25 minutes ago, John said:

This is not apparent until you try and view something and it is frustrating when suddenly what was clear can't be bought to sharp focus.

My 15mm EP and 25mm were both focusing without any issues and it was actually left on the Dob rack but it was the 8mm EP that was giving me issues, thus why i checked collimation; at first i though that seeing conditions might not be great for high magnification.

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The seeing was quite good here (North Somerset) last night despite the cold. There can be very local issues that can spoil that though and it's when you try and use higher magnifications that you notice where the "drop off" occurs.

Lots of central heating plumes in a certain direction, for example, can cause localised poor seeing.

Your collimation looks a little off but not by much.

I usually check and collimate my scope at a 45 degree angle rather than horizontal.

 

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I find it hard to believe your collimation being slightly out (which it doesn’t really appear to have been) would cause the issue you describe, much more likely to be conditions outside 

I’ve never noticed weather conditions affecting collimation but, then again, I’ve never checked immediately after a session, I don’t think I want to either, there’s enough to obsess about as it is!! 🤣

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21 minutes ago, John said:

Your collimation looks a little off but not by much.

I usually check and collimate my scope at a 45 degree angle rather than horizontal.

I agree, it is slightly off. I will try check and collimate at 45 degrees. Thanks for the suggestion.

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1 minute ago, Jiggy 67 said:

I find it hard to believe your collimation being slightly out (which it doesn’t really appear to have been) would cause the issue you describe, much more likely to be conditions outside 

It is reassuring to read that many off you put it down to conditions. Moment of truth will be tomorrow night if i see a similar effect. I was trying to be prepared so not to miss a good seeing conditions as predicted.

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  • 2 weeks later...

I thought I would give a quick update since my last session. Without collimating or touching the optics, the viewing was absolutely fine tonight. So the above suggestions that the bad seeing was due to the conditions seem to be the case. The 8mm EP was giving really nice views tonight.I also did a start test with the defocusing and the collimation was absolutely fine as per chessire confirmation prior to viewing. Thanks again for your suggestions (i still do not know why i thought the mirror might have moved, maybe too tired after the viewing to look carefully).

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The pics you posted showed collimation was very close indeed. Best to make certain the Cheshire is properly clamped in the focuser, just a teeny wobble can affect what you see.  Best collimation check is a star at high power, with the star in the centre of field of view and slightly out of focus just like you’ve done.

All sorts of things can affect the view other than collimation- poor seeing (turbulent atmosphere) or not enough cooldown especially for high power observing with the 8mm eyepiece.

Your observing notes from tonight suggests you don’t have a problem 👍

Ed.

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