29.05.2021 Main tube 2cm too long! Imaging new irregular AR.

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Saturday 29th 61F, grey, cool start then sunshine.

I was struggling to get a decent image on my monitor again. Can the seeing conditions be this bad all of the time? I whipped out my tape measure and did some checking. 

My 6" f/10 H-a refractor's, main tube length is 2cm longer than the maximum permitted. It is 121cm long instead of 119 absolute maximum. Which means I cannot reach the PST etalon's 200mm magic number inside focus for best performance. 

The new FT focuser was longer than the old Vixen. Somehow I did not shorten the tube for this. Or badly miscalculated when I was making and fitting a new [turned] back plate.

I have removed the OTA from the mounting and have it resting [minus optics] in the jaws of a B&D workbench. It is ready for sawing a ring off the tail end after lunch. Think of a number. Make the new tube length to match the middle of the focuser range? Not that the focuser should be used. Except for making very small adjustments to fine focus.

I have allowed the focuser to show 30mm when the PST etalon is spaced at the correct 200mm inside focus. 116cm to the back plate.   I needed a longer 2" extension, than before, to bring the camera to focus.

15.45 62F. Does the new image show any difference? Certainly better than the featureless mush from this morning. Probably the seeing isn't good enough for any better results at the moment. I shall persevere until late afternoon in the hope of better seeing.

I could rack the focuser out to match the earlier PST setting. Just to see the result. While I could still focus [with a shorter extension] the sweet spot became hopelessly lop-sided. 

Returning to the correct etalon spacing [and refocusing using the longer extension] provided a much more evenly lit image. Interesting. I thought the H-a detail would be worse at the incorrect spacing. At the correct spacing it was much easier to tune the etalon.

17.20 63F Still making adjustments to tuning, spacing and focus. The weird AR is plodding westwards. The seeing conditions are very slowly improving. Though with irregular de-focusing.
 

18.44 Still here. Capturing the new AR as the seeing settles. Now using the 2.6x GPC for more scale. The magnetic lines are beginning to show themselves. I am deliberately darkening the surface background to increase the contrast with the brighter AR.

18.50. 62F. Last image today. The trees were already blocking the sun. The light level drops like a stone and flashes dark and light if there is any wind in the trees. 

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