CCD Camera Star Spikes. What am I doing wrong?

[Sabalias]

Hi All,

I was hoping someone might be able to educate me as to what I have done wrong with regard to the attached image and the huge spikes coming off the stars. I'm still trying to get my head around using a dedicated camera (its a QHY 8L-C) and I've been through the calibration for gain and offset settings (12 and 125 or thereabouts) but I am assuming I have made a mistake.....or is it something far more simple (like lens smear or something)? There was no dew on the scope (Evostar 80ED) etc and the camera was set to -15. I tried various exposures (the attached is 2 mins) but they didn't seem to make a blind bit of difference.

Many thanks in advance,

Stu

[tomato]

I can get these spike effects on a refractor if the scope starts to be occluded by the dome aperture edge. The other way this can happen is if a power line or washing line is across the field of view, or branches of tree perhaps?

[Sabalias]

17 minutes ago, tomato said:

I can get these spike effects on a refractor if the scope starts to be occluded by the dome aperture edge. The other way this can happen is if a power line or washing line is across the field of view, or branches of tree perhaps?

Thanks for the reply. I presume you’re referring to the edge of an observatory dome? Sadly I don’t have one of those. I don’t think there were any wires in the way but I’ll have to try a different target next time it’s clear. 

Cheers,

Stu

[The Lazy Astronomer]

I get this if my target's gotten too close to the tree at the end of my garden - the shadows of the branches can be pretty much undetectable in the image until it gets properly into the tree. The only thing that gives it away is the diffraction spikes.

However, when it happens to me, the spikes are the same or similar on all bright stars, but yours are in different orientation in different areas of the frame... 🤔

Edited February 21, 2022 by The Lazy Astronomer
Typo

[tomato]

Yes, the fact that the spikes have different orientations rules out a single wire, so it’s more likely branches of a tree. Was your scope pointing near to the horizon? You can check if this is the cause by imaging somewhere near the zenith.

[Sabalias]

I also wondered about that and I am certain that it wasn’t pointing near the horizon and there were definitely no branches in the way.

Perplexing. 

Edited February 22, 2022 by Sabalias

[Sabalias]

I guess my worry is that it might be down to small scratches on the camera glass. Please tell me it’s not! If it is then I think I might be able replace the front end but I’d rather avoid that.

Edited February 22, 2022 by Sabalias

[MalcolmM]

I had a very similar problem. Don't know how to link another thread but if you do a search on "Star spike question" you should be able to find it. I put mine down to a washing line.

[Sabalias]

Hi Malcolm,

Now that’s a coincidence - I was looking at your thread just as the alert for your post came in 😂.  Though there is a phone line in the vicinity of the field of view I don’t think it was intruding but also I have spikes in all sorts of directions which is very strange. I need to try a different object next clear night just on case.

Stu

[tomato]

Those scratches don’t look good, they could explain the different orientation of the spikes. They will be present in every image however, if they are the cause.

It looks like a lens, but dedicated Astro cameras don’t normally have a lens associated with them? If it is just a window, it shouldn’t be too hard to replace.

[Sabalias]

6 minutes ago, tomato said:

Those scratches don’t look good, they could explain the different orientation of the spikes. They will be present in every image however, if they are the cause.

It looks like a lens, but dedicated Astro cameras don’t normally have a lens associated with them? If it is just a window, it shouldn’t be too hard to replace.

Yes, I need to see if the same problem happens on a different night pointing in a different direction. The glass is a filter built into the front end of the camera which, I believe, I can replace (need to check with a specialist).

Stu

[Sabalias]

Ok, so I've managed to find a few minutes to look at some of the other subs and it does look as though there is a line intruding on the field of view, so that would definitely explain some of the spikes (which I believe should be perpendicular to the line). Unfortunately it doesn't explain why some of the other spikes exist; that said I have been able to source a replacement front end so a small job for me to undertake sometime this week will be to release the resin sealing the unit, and replace the front end. Thank you all for your helpful input 👍

Stu

[7170]

Do you have any filters in the light chain by any chance?

[Sabalias]

8 hours ago, 7170 said:

Do you have any filters in the light chain by any chance?

Only the filter on the camera nosepiece (replacing this) and the FF.  I got out again last night and took some more shots of Pleiades - most of the spikes had gone so there had been something crossing the light path on the previous night; however, some are still present and move when I rotate the camera so I put it down to the larger of the scratches. 

Stu

[Sabalias]

Just a quick update. I changed the nosepiece on the camera (and cleaned the sensor while I was at it) and the difference is startling. No spikes and no dust spots either (I thought the dust spots were in the scope or FF but they weren’t. 

Anyway, thanks to everyone for their help 👏

Stu