20" Skywatcher Dob question

[gaseous]

Last night I gave my Skywatcher 20" dob its maiden voyage, and one thing I noticed was the extreme "diffraction" for want of a better word, on very bright stars.



On normal stars, or at lower magnification, I generally saw just the normal 4 spikes you'd expect, but the attached photo is of Alpha Centauri. While the photo doesn't really do it justice (It was sharp and easily split but my smartphone photographic skills are virtually non-existent), you can clearly see the wild rays coming off it.



Would I be right in thinking this is some function of the ribbed primary mirror design, and is it normal? I was only out for about 3 hours, so maybe the mirror hadn't completely cooled, despite the chilly conditions and moderate breeze, but it hadn't shown any sign of improvement by the end of the night. Any comments/suggestions would be most welcome.

[AstroJunk]

I don't recall the seeing being particularly good last night - You may just be a victim of that. Big mirrors behave disproportionately badly under unsteady sky :(

[gaseous]

Thanks Jonathon - yeah the seeing was pretty average, I've just never seen such a pattern before. It's not like the spikes were blurred or shimmering - they were sharp. I'm gonna tell myself it was the extra light-gathering power......

[raymo]

You are also getting spikes from two stars if that is Alpha C. What did
Beta C [Hadar] look like?
raymo

[Merlin66]

Patrick,
Use a Ronchi grating....it will quickly show any issues.....

[gaseous]

Thanks Raymo/Ken,


Still had significant spikes on very bright single stars - Alpha Cen just had an explosion of them. Ken, where would a chap go to source the gear required for a Ronchi test? I've never even contemplated doing one before.

[Merlin66]

Patrick,
I usually make my own Ronchi gratings....
You can at a push use a piece of fine fly wire mesh...
Can't see any eBay suppliers but:
http://www.ronchiscreens.com/screens.php

and some background..
https://nicholoptical.files.wordpress.com/2016/11/the-ronchi-test.pdf
http://www.jeffbaldwin.org/ronchi.htm
Note: similar results are obtained when using a grating on a telescope while viewing a moderately bright star.

[Wavytone]

Patrick

There’s a vague three-fold symmetry in this spikes. I’d say something is protruding into the light path, aside from the spider - probably retaining clips or fingers on the primary mirror cell, or the secondary cell.

Take a close look at the secondary cell as well, if this has any lumps or bumps that result in a non-circular obstruction in the light path, these will cause spikes.

Another no-no are the ends of bolts sticking into the light path from things bolted on, like the focuser, finder or guidescope rings.

[gaseous]

Thanks guys, I'll need to put my detective hat on. I hadn't noticed the symmetry that Wavy mentioned while at the eyepiece, but it's sort of evident in the photo now you mention it.

[geolindon]

g day Patrick,
BIG congratulations on your new 'scope. :cool:
interesting issue, how does it go on Jupiter?
regards, L

[gaseous]

Jupiter was fine, no issues at all. Very bright, obviously, and the seeing wasn't great, but no spikes as such.