Cause Of Giant Diffraction Spike?

[bobro]

My first attempt at imaging with a SkyWatcher 150PL - there is what appears to be a very large diffraction spike going back to Alnitak and it isn't doing much for the final image. Comments and any way to reduce it? Single stretched sub below from RAW.

Many thanks.

Bob

 

[ChrisEll]

It'll be that big cross at the front of your scope

I actually love diffraction spikes and usually use them with my refractor.

[happy-kat]

Do you mean the light large line below the star running left right?

Any light reflecting into the telescope tube? 

Any shiny bolt heads?

[bobro]

2 hours ago, happy-kat said:

Do you mean the light large line below the star running left right?

Any light reflecting into the telescope tube? 

Any shiny bolt heads?

Yes - the large line below the nebula running left-right.

Not aware of any unusual light going into the tube - same basic setup as with my 650/130 reflector and no problems with that OTA.

There are shiny bolt heads holding the focus assembly and finderscope, plus shiny screws holding the spider vanes to the outer tube. The focus tube is shiny and projects about 20mm into the tube. Everything else, including the plate under the finderscope, is matt black.

[happy-kat]

I would try black insulating tape over the outside over the top of those screw holes. Light I think is getting in to reflect.

And add some form of light shield extension to the telescope

[ollypenrice]

Par for the course with Alnitak, I'd say. When we did the HH with Yves' 14 inch reflector we had to crop off most of the blessèd image, fortunately saving the HH itself! We had spikes from Alnitak and, worse, an immense flare from Sigma Orionis. The only scope I've used on the Alnitak region which shrugged off bloat, spikes and every other pestilence was the TEC140.

Olly

[almcl]

I had a thread about a very similar problem a little while ago, lines on the images

some of the thoughts offered then may be relevant here?

[bobro]

3 hours ago, almcl said:

I had a thread about a very similar problem a little while ago, lines on the images

some of the thoughts offered then may be relevant here?

Thanks very much for the link. There does appear to be a slight twist to one of the vanes in my scope, as can be seen in the extra diffraction spike from Capella below. 

Aa appropriate part of your previous thread discusses diffraction from more than one star, though there doesn't appear to be anything as bright as Alnitak to create the 'line' coming from both sides of my image and joining in the middle. Perhaps it only comes from Alnitak and there is a reflection?

When clouds allow I will experiment a bit more with rotation and black masking to see what happens.

How did you resolve the problem with lines on your image?

Thanks

Bob

[almcl]

 Well, I blacked all the shiny bolts and adjusted the vanes as suggested, but in the end I used one of Startools functions to reduce the spike.  Haven't really had the opportunity to go back though and re-image, but now, with field flattener, light pollution filter and  upgraded processing software, perhaps I should?