Going H-alpha: Back focus adjustment [again!]


Fleeting sunshine during a 45mph gale found me outside again checking the solar image in H-alpha. I had bored the plywood adapter ring deeper to see how that affected things. It was marginally sharper once I had refocused by sliding the Cemax 12mm in the PST's push fit holder. The latest image of the H-alpha set up is shown alongside at focus. 205mm back focus seems rather short compared with the recommended 225mm from the tail edge of the etalon tuning band.

There were three nice prominences at my 8 o'clock in the field of view. Two perpendicular jets and an arch leaning towards them. Surface detail was slightly improved during brief moments of clarity between heavy clouds. There is a red wash to the background on one side of the solar image only.

The wind was so strong, despite there being two [bare] 10' hedges between myself and the easterly gales, that was I being pushed off my beer crate hop-up. It clouded over much more heavily after lunch and the blue gaps were now no more.

If only I could get some steady sunshine I could check the position of the etalon tuning band relative to the refractor's focus. It really needs a steel rule and a board to measure the distance of the sharpest red focus from the plywood backplate. This would be far more accurate than trying to measure from some arbitrary point on the thick, objective lens.

A black surface [emery paper?] on which to focus the sun's image is best IMO. Because the intense brightness of the sun's image even with the D-ERF in place completely blinds one to the sharpest image.

The briefest of clearing in the thick cloud allowed me to double check the precise focal distance from the backplate. 26cm exactly, with or without scudding cloud. So I need to aim for 6cm to bring the PST etalon 20cm inside focus. I had no choice but to swap out the WO extender to the 40mm x 2" extender. Drilling 120° apart and tapping the holes 5mm for extra thumbscrews should provide some extra stability. The actual focal point for the eyepiece remains completely unknown as I waited in vain for one of the tiny blue holes in the cloud to cross the sun. It even started snowing to add to the misery of standing in a gale at -1C, 30F. Talk about obsessive behaviour!The moment I returned indoors for lunch there were a few seconds of watery sunshine. Grr!

Two more freezing hours chasing gaps in clouds in a bitterly cold gale. With the etalon now fixed at the correct distance the EP is in focus just as it falls out of the PST eyepiece holder. Grr. A quick check and I found the 2" star diagonal would now work instead of the 1.25". Tomorrow is promised to be sunny yet again.

If I can get a few seconds of real sunshine I can confirm if any progress has been made in image quality. Not to mention where the EP will reach focus with the larger star diagonal in place.  The latest image shows the eyepiece at focus using the 2" star diagonal. I'm back to using the bored out [plain aluminium] adapter to hold the ITF/BF filter stack.

Click on any image for an enlargement.

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