Just thought I'd share something I've been slowly discovering over the last few imaging sessions in the hope it might help someone.
It's worth checking focus regularly. I do it every 3 or so avi's, as I've found that the focus can change, even just slightly, as the planet rises higher (and as it goes lower, I imagine).
This is most likely due to the amount of crud in the atmosphere that you're imaging through, changes at different altitudes. It (focus) may also be affected by your mirror temperature, so as the ambient temperature drops, your mirror might change shape slightly and so the focal point might be slightly different.
Lately i've thought that the seeing was deteriorating, but after adjusting focus i've been able to snap it back in and keep imaging. The focus had changed over the last half hour and made the raw captures look soft.
Also as I turn the camera after each platform reset to get the belts horizontal again, focus needs to be adjusted.
So don't think that if you focus once at the start of the night, that's it. Keep checking regularly to ensure you're capturing the sharpest data possible.
If you just can't get snap focus, seeing and/or tube currents are the obvious culprits.
Have you noticed if it tends to be in one direction, or is it random? If it consistently requires outfocus adjustment during the night as the ambient temp drops, it might be the focus change is due to temp drop driving dimensional change in telescope structure, particularly slight shrinkage in tube length.
hmm good question Ian - I *think* it is outfocus, but can't be sure because with my motorised focuser I'll try one way and then the other, until I find the right point. And I don't have any indication of where it was, so I can't tell if i've ended up outside or inside where it was. I need a digital readout!
It make sene really, the amosphere (which act like a lense (notice the halo you see around the sun and moon at times)) will have different refractive indexes as you move from the horizon to the the zenith, brought about by air pressure, moisture content and difference in thickness of the atmosphere.
Here I was thinking I was over-fussy in this area....I refocus between every AVI just to be sure I have the best focus available to me at the time. I couldn't stop myself from doing it if I tried lol! It just seems the right thing to do!
![[1ponders]'s Avatar](../vbiis/customavatars/avatar45_9.gif){kind=avatar}
