Question on diffraction spikes

[AstroMuni]

I noticed that the diffraction spikes from my 130pds seem to flare outwards rather than the classic tapering as they move away from the centre. Attaching a zoomed in view. Image taken with ASI533mc pro having an UV/IR cut filter + Baader CC. EDIT: I focus manually so could it be focus issue?
What could be the cause?

 

Edited December 28, 2022 by AstroMuni

[inFINNity Deck]

There is a fair chance that it is caused by a deformed spider  (i.e. the vanes are not aligned by which they no longer form straight lines across the aperture). Normally this only occurs in one direction, but theoretically it could happen in both. You can check this with a straight ruler.

HTH

Nicolàs

 

 

[AstroMuni]

Its a brand new OTA so should hope its not deformed 😞

[Astronomist]

the vanes could be slightly twisted relative to the optical axis, so that they have an apparently greater thickness at one end, if so probably a fairly easy fix

[Whistlin Bob]

That is odd. Like @inFINNity Deck my first thought would be to look at the spider- or more specifically, to measure it precisely. I had some problems with non aligned spikes and it was simply that they were loose, but then when I tightened them (only a little bit), the secondary was slightly off centre. The answer was to get some cheap callipers and measure the distances precisely- which left me with beautiful perpendicular tapering spikes.

[Adam J]

5 hours ago, AstroMuni said:

I noticed that the diffraction spikes from my 130pds seem to flare outwards rather than the classic tapering as they move away from the centre. Attaching a zoomed in view. Image taken with ASI533mc pro having an UV/IR cut filter + Baader CC. EDIT: I focus manually so could it be focus issue?
What could be the cause?

 

Looks like twisted spider. Just rotate them till they are perfectly edge on to the optical axis. 

[vlaiv]

It is "twisted" spider support such that on one side (say closer to secondary) presents thin profile to aperture and then it twists towards the tube wall presenting thicker section.

Here is simulation:

I created one spider support so that it presents "twisted" profile to aperture (like a triangle) - and that spreads associated spike away from the star (horizontal edge creates vertical spike and vice verse).

[inFINNity Deck]

Indeed a twisted spider could cause this pattern as well. What I meant was what I show here below. When the spider is not properly mounted, you may end up with the following situation (the image at the centre is generated from the left one using ImageJ, the one at the right is an actual astro-image showing this issue):

 

Here the vertical vanes of the spider are still properly aligned, but when these get out of line, the issue will show up along all four spikes. In above example the diffraction pattern shows two spikes in vertical direction, but when the angle between the two opposite vanes gets closer to 180 degrees, they will merge into one diverging spike.

Nicolàs

 

[AstroMuni]

Thanks all. I will check out the spider for both possibilities. I have used this scope only a couple of times since I got it from FLO last month (blame the clouds) and I was surprised to see these strange spikes.

Edited December 29, 2022 by AstroMuni

[AstroMuni]

I have checked with help from a friend and one of the vanes is not perpendicular(?) at the end where its screwed into the secondary support. Another vane is slightly bent. 😞 I could try and straighten the bent vane but not too sure about the other one as that would mean trying to remove the tiny screws that hold the vane. @FLO can you advice on what can be done about this? The scope was purchased on 12th Nov and has been used only a couple of times since then.