Going H-alpha. Back focus adjustment.

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The etalon position changes the back focus [beyond the etalon] much like a Barlow lens. So I need to get the etalon positioned 20cm inside focus. Only then will the post-etalon focus lie in the correct position of 225mm beyond the back of the knurled rubber band.

To that end I have obtained an extension of 18mm via T2 threaded parts: The image [left] show the assembled parts and [right] the components laid out.

The 2" adapter has a T2 male thread on top. This accepts the T2 to 1.25" adapter.

I removed a standard 2" to 1.25" adapter with its single clamping screw. But gained a high quality 1.25" compression band fitting with two seriously dimensioned, thumb screws. I also bought a locking ring to conceal the unwanted T2 male thread on the 1.25" fitting for better neatness.

The next image shows the assembled etalon with AOK etalon to 2" adapters. The 40mm x 2" extension tube is probably too long. The TS 2" adapter fits into the 40mm extension.

The new 1.25" compression band firmly holds the 1.25" spigot from the 1.25" star diagonal. The PST filter stack and EP holder is screwed directly into the top of the star diagonal.

The new parts provide a very firm and inflexible support for the heavily loaded star diagonal. Where the earlier 2" to 1.25" adapter, very clearly, did not. The whole star diagonal and filter stack would literally rock up and down.

Until I get some sunshine I have no idea how long the new back focus will be. I'm guessing that  I must discard the 40mm x 2" extension. The TS T2 parts [alone] offer only 20mm extension but T2 spacers are readily available in numerous widths once I am in the ballpark on back focus. There are even adjustable, male threaded components which can act like a simple helical focuser. Which can then be locked, at the desired length, with a female threaded  ring.

I started turning a 2" x 20mm long brass extender in the lathe. Then suddenly realised that it was physically impossible. The 2" OD spigot of the extender must always fit into another 2" bore fitting of matching length. While the 2" bore tubular section of the extender must be deep enough to hold the full length of the following spigot. It is thus impossible to make any 2" to 2" tubular extender shorter than the spigot length plus the minimum safe shoulder depth [X] of material to join the two sections of pipe.

A reducing adapter [from 2" to 1.25"] can certainly be made much shorter. But not a 2" to 2" extender. Attempting to shorten the spigot, or the tubular section, merely weakens the connection though lack of adequate support for the fittings which come before or after. Which is partly to blame for the poor support offered by most 2" to 1.25" adapters. Ideally they should be made to the full depth of the 2" Ø tube in which they are clamped. Though this would add considerably to the weight unless they were bored out to be much thinner. A compression band fitting is a huge advantage in this context. Rather than the single, miserable, little thumbscrew usually offered in cheaper fittings.

Click on any image for an enlargement.

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