Hi Rally,
No you've made an error there somewhere. Coefficient of expansion for 2mm aluminium works out to about .35mm for 20C. You must've missed a few decimal places there. But your point still stands.
On the CDK17 the digital readout goes to 3 decimal places for accuracy.
Usually a move of .020 (I think its in inches though = .025mm for every .001 inch) will cause focus to be noticeably improved or worsened at 2939mm focal length. So .375mm tube increase over 20C = around 100 units per 1C on my CDK17 focuser. That definitely shifts the focus quite a bit (100 units on the focuser that is).
So .375mm (as small as it is) is significant at long focal length and will definitely be noticeably out of focus for every 1C shift.
Don't forget these expansions are magnified by whatever focal length you are using and in the case of the 12 inch its pretty long.
Now we see why the top manufacturers go for the CF tube or the temp compensating cell (Tak BRC250 which has rods that match the expansion of the alum tube when they used Al. Now they are shifting back to CF).
Although there must be people on this site using an Al tubed 10inch RC that will know how much it shifts with temperatures in case this is overstated.
Greg.
Quote:
Originally Posted by rally
If my calcs are correct ? a 20C drop in temp over a 750mm aluminium section will change its length by less than half a micron ! (0.0003 mm)
If the contraction is uniform, it is barely going to show up and is probably well within the noise.
If the contraction is not uniform due to points being constrained by brackets that have different thermal expansion rates (eg different lengths and geometry, dovetails, mounting fixtures or different materials) or the circular diameter of the tube affecting things then bending might occur and that may or may not affect things more so by way of collimation and tilt.
Does the primary/secondary distance change the focal length on these OTAs?
I would expect that anyone is who imaging across that temperature change will have probably changed filters and/or performed a refocus anyway !
Not that most focussers are reliably able to reposition within the 1/10 micron range anyway - there is more flex and backlash and minimum focus increments in the system to worry about that this.
So I doubt that simple thermal movement on its own is likely to be an issue.
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