Is there any way to fix a bent spider vane?

[Windston]

Hi

So I recently did a re-haul of my imaging setup, and the start me off in the world of GEM's, I picked up a really nice Celestron 130SLT OTA that has been hardcore modded for imaging! It is a beast of a little scope!

Anyway, the only problem that I have with it is the fact that I believe the spider vane is a little bent out of shape, giving some really ugly diffraction spikes as seen in the image attached. It is not stacking artifacts or rotation because the exact pattern is present in the RAW data. It is just a bit of an eye sore when imaging some targets with bright stars, like M45, M42 and the horsehead.

Is there any easy way to remedy this without being too destructive? Taking it out and putting it in a vice for a few hours? I just dont want to be sorry that I fiddled with this if I destroy it!

The bending is just noticeable in this image when zoomed in http://i.imgur.com/pWqgUfK.jpg

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Thanks heaps
Dan

[julianh72]

Just wondering - would those two long screws that mount the Guidescope be a bigger issue?

[Windston]

I don't believe so. I dont think that they have a large enough light imprint to make any noticeable difference, and I haven't noticed any. Any shadow on the mirror would be corrected by flats in post anyway! :thumbsup:

But yeh it's not ideal, I will have to get around to cutting them off at some point, or just move to a 80mm when the funds are available to me, after a CCD or DSLR mod though. Its all one long buy list! :P

Dan

[rally]

They look like a potential diffraction problem to me.
There seems to be more than one problem occurring, but this isnt my area of expertise.

[Dennis]

I strongly suspect that those 2 long screws intruding in the light path will have a noticeable effect on the image in terms of producing diffraction pattern artefacts.

I can easily see such artefacts when I am collimating my ‘scope and I use a small Allen key to adjust the secondary. The diffraction pattern from the Allen key is clearly visible on my computer screen with a CCD camera in the eyepiece holder.

(The Allen key is of course attached to a small length of nylon fishing line so I don’t drop it onto the mirror).:)

Cheers

Dennis

[rmuhlack]

Dan, I previously owned that telescope, and implemented some of those mods (shortening the tube and upgrading the secondary). I did not see the diffraction pattern artefacts you are observing. eg http://www.astrobin.com/full/153530/0/. As such I suspect that Dennis may be right in that the long screws are to blame.

edit: collimation may also be out. I previously used catseye collimation gear for that scope.

[Windston]

Ok I didnt know that the bolts would actually impede in the light train that much, I will remove those this afternoon and see if I notice a difference. However that is not my only problem, if I hold a ruler up to the spider, it is clearly bent relative to each other, and that is my main concern. I can provide images of it this afternoon if you like?

Also I know that it is not colmination as it has popped up in multiple sessions after multiple different collminations.

Other than that, it is an amazing little scope, and have really enjoyed using it as my first proper imaging scope!

Thanks
Dan

[julianh72]

It's not so much a matter of "impeding" (blocking) the light, it's that you've got a couple of straight parallel edges in the light path, which will cast their own diffraction pattern, distinct from the cross created by the spider.

Rather than cutting them off - can you just turn them around so there is just a screw head inside the tube, and the thread extension is outside?

Yes, straightening the twisted spider vane would also help a bit, but I suspect most of the extra diffraction artefacts you're seeing come from the two long screws, and any other objects which may intrude into the light path. (I don't know, but could the inner end of the focuser tube, and the "ragged" edge of the paper flocking, also contribute some diffraction artefacts?) The effect of a slightly twisted vane might be to make one axis of the classic "diffraction cross" a bit stronger than the other axis, but I suspect this effect is quite small, unless your vane is so twisted as to make it noticeably wider than the edge-on thickness of the vane.

[multiweb]

Looks like my old 5" newt that I sold to Richard (rmuhlack) couple of years ago. Simple fix: your secondary offset is too big so your vanes are not square and the diffraction spikes don't overlap. You need to recenter your secondary. Here's an old shot (https://onedrive.live.com/?cid=6930D3AD12D5044C&id=6930D3AD12D5044C%21953&parId=6930D3AD12D5044C%211064&o=OneUp) with the same scope of what it will look like when it's collimated.

[Windston]

Ok other than the bolts in the tube, this seems like a very plausible explanation.
It would seem to me that this really matches up with another anomaly with the tube that I have noticed and thought nothing of it. The heavily linearly stretched image attached shows an uneven illumination of the field that I have noticed throughout all of the images. Would this correlate to the mirror droop? It was another thing that I was going to look at fixing once the diffraction spikes are finished, but if it is our real problem then that would be two birds with one stone!

Also with the full resolution image you can see the diffraction spike problem on that mag 6 star.


Thanks
Dan

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[multiweb]

This further confirms my previous post. The light cone is offset and you have uneven illumination. Recenter your secondary.

[Windston]

Ok sure, would this be just adjusting the tension bolts on the spider vane and measuring it with a ruler? Or do I have to take out the secondary and physically move it into place?

Thanks
Dan

[multiweb]

Hi Dan, I'm not sure what mods were done to it after I sold it and if the spider vanes are the original ones. First thing you need to do is center the secondary in the middle of the aperture, looking from the front, so all the vanes are inline and square. Second thing is to make sure the secondary is centered under your focuser. This scope is so small that the secondary offset is very minimal.

Once you've done these two things your diffraction spikes will overlap and you'll get 4 instead of 8.

[Windston]

Ok, I think I have fixed this!

Last night I had a look at the actual measurements between the spider vanes and they where definitely not square! So after I used the adjustment bolts, I managed to get them inline with each other and square. It was cloudy, but I managed to find a very distant bright light and focus on that, and the diffraction spikes where almost perfect! I also think that the field is evenly illuminated now, although I had some trouble testing that.

Now all i have to adjust is the bolts in the tube and it should be performing nominally.

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Thanks heaps guys :)
Dan

[multiweb]

Good stuff. It certainly looks a lot better :)