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Indeed, I’ve heard of two manufacturers coming out with carbon fibre (CF) APO tubes, being A&M and WO. Though the WO models are limited edition. Interestingly, both A&M and WO use TMB lense sets – perhaps one of them negotiated the supply of the tubes. There might be more in the future, but it would appear that this is not the current trend. The high-end APO manufacturers continue to use aluminium. CF does look rather cool though.
I think the snippet of information you provided covers the key points;
A faster scope will have a shorter CFZ and as such will take some refocusing work to remain in the CFZ as the temperature changes. This would also apply to CF tubes, however you would expect the shift would be less over time. I seriously don’t believe you’d avoid refocusing all together especially on fast scopes. To give you an example of the CFZ size, the Takahashi FSQ-ED operating at F/5 has a zone size of only 55 microns. Thus, it doesn’t take much of a contraction/expansion to the focus out. This also depends on how rapid the temperature changes are.
The last sentence makes a valid point – “In any case it may be academic since most of us normally always focus with each filter change and confirm focus throughout the night." This is the reason why I indicated in the previous post that a motorised focuser that can be computer controlled is a major leap forward in acquiring quality data – I wouldn’t purchase an imaging scope without it. I can’t imagine you’ll want to manually refocus the camera due to different filter changes or every hour during an imaging run to compensate for temperature changes. If done manually, you’ll waste too much time validating you’re in the CFZ. However, if you can automate the procedure, life becomes a whole lot easier. Free software like FocusMax is a “god sent” for astro imagers. It will provide repeatable focusing ensuring you hit the CFZ every refocus, typically under 70 seconds. Mine is around 90 seconds, but I tell FocusMax to do more – It first takes an image, plate solve it, then find a 4-7 mag. star (based on the plate solve info) that is +70 degrees to focus on, perform the auto focus routine, then slew back to the original target. As you could imagine, refocusing the telescope every hour to compensate for temperature changes is no longer a tedious task with motorised/computerised focuser. You can refocus the telescope every 20 minutes if you really wanted. Alternatively, you can also calibrate the focuser to compensate for ambient temperature changes. Many possibilities are available.
Don’t let “flashy” APO tube assemblies dictate your final APO decision. Ultimately when buying an APO you’re paying for its optical configuration/performance and associated diffraction limited specs and strehl ratio. I’d much prefer to buy a scope with quality optics that needs to be refocused regularly, than one with mediocre optics that holds focus all night.
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