One very long diff spike?

[BrendanC]

Hi all,

Here's my latest shot - IC 1318 - with one huge diff spike off Sadr. What causes this? Is the secondary not quite aligned properly? 

Edited March 2, 2022 by BrendanC

[Captain Scarlet]

The "silvering" on skywatcher secondaries typically have a flat edge on one side where a tool has been used to grip the glass slab during the aluminizing process, leading to a small part of the edge of the mirror missing its aluminization. Ideally of course the edge of the secondary is elliptically-curved all the way around. The ones I've seen have had that flat edge aligned with the long axis of the mirror. That single straight edge will produce its own spike. It most often manifests as a slightly fainter "fifth spike" at a slight angle to a main spider-spike. In your case, it seems that it's exactly aligned with a main spike, augmenting it. If that is case, it suggests your state of collimation is pretty good!

Cheers, Magnus

[BrendanC]

Very comprehensive answer, thank you!

I've collimated the heck out of this thing with a Concenter and Cheshire, so I'm glad that at least that's not the cause.

What would the fix be though? Would perhaps painting around the edge of the secondary make a difference? I've flocked opposite the secondary, and painted over the bright bits I can see such as screwheads, so if painting around the primary might help, where it's silvered, then I might give that a go. I just worry about doing irreversible things to it.

If this is only going to affect unusually bright stars however, then I might leave it.

 

 

Edited March 2, 2022 by BrendanC

[Captain Scarlet]

If that's what it is, there's nothing you can do in terms of DIY adjustments. It's a straight edge on the mirrored face itself. The only way around it is to, say, upgrade the secondary to one which is silvered all the way to its edge. I reckon that would cost you in the region of £100 for a 1/10 secondary. I actually did that for my 70mm SW mirror and had to wait a long time for it to arrive.

[BrendanC]

OK, thanks anyway. Looks like I'll live with it when shooting very bright stars, and/or fix it in post processing.