Repetitive star streak pattern

[chicken george]

Hi, can anyone please advise me how to go about reducing/removing this repetitive streaking in bright stars? It's the same pattern every time.

Skywatcher Evostar 100ED Pro with 0.85x Focal Reducer.

ZWO ASI294MC Pro.

I believe that I have the correct amount of back-focus (55mm). Could it be the silver screws on the face of the camera reflecting somehow off the reducer? The ZWO website shows black screws.

Thanks for any advice. Sorry for the blurry photo.

 

[vlaiv]

Hi and welcome to SGL.

That pattern is most likely produced much further up the optical path - probably in lens cell.

Lens cell has collimation screws and screws holding things together. Those are probably main suspects for this pattern.

People with newtonian telescopes get similar issues from clips holding the primary mirror or when focuser tube protrudes into optical path and blocks some of the light. This is often solved by shortening focuser tube or using aperture mask over primary mirror that hides mirror clips.

Since you have refractor - neither of the two suggestions will help you - but you could try using aperture mask in front of objective lens to stop light hitting sides of lens cell. You don't need to mask off too much - just a few mm on each side - for example, make 90mm aperture mask for testing purposes.

Next time you are under stars and imaging - just pop that mask on and perform single (or few) exposure on a bright star to see if there is difference. If there is - then cause is most likely lens cell. Not sure how to proceed from that point in fixing it - but it will probably involve shortening some screws (for example if they are grub screws - using slightly shorter ones) if they protrude too much into the light path.

Alternative is to just look at front of the lens and see if you can spot any strange looking screws sticking out.

I just thought about another thing that might be the cause (again - mask will show this) - there are objective lens spacers. Small pieces of material that keep two elements of objective lens at correct distance:

Those could have ragged edges or be too much inside optical path to create diffraction effects.

Ok, I finally found suitable image of ED100 lens to show you what to look for:

Spacer bits and any screws that stick too much in the side of the lens cell (ones at the front are not important - as long as they don't sit in optical path - they can't cause issues

[chicken george]

Thanks for the reply. It's very clear all the way down. Just these tiny tabs at the front.

 

[vlaiv]

Next place to check would be focuser tube on far end.

If it is focuser tube, then this pattern will only show in certain parts of the image. For example - bright star in center of the frame won't have it, but same / similar star closer to the edge will have it.

In general - if there is issue that is equally affecting the whole image - it is likely originating in part of the setup that has light rays parallel (before or at lens in front). If it varies with position in the field of view - then cause is in part of the optical path where light is converging.

Focuser sits in converging beam, and as such will produce artifacts (again - look for some sort of protrusion in optical path - either screw or jagged edge or similar) only in some parts of the image, and not in other.

[vlaiv]

Forgot to say - best way to check focuser is to simply turn scope at a white wall or towards the sky (just be careful not to point near the sun) and look at the focuser side with your eye - just empty focuser, don't use eyepieces or anything - you want to see the silhouette of focuser tube edge against the light coming from objective.

Be sure to move focuser through its focusing range as that can change how much it clips incoming light beam.

If you don't find anything suspicious - then just try couple things on your next imaging night:

1. aperture mask

2. placing bright star in corners vs center of the field

to see if any of those change diffraction pattern.

[vlaiv]

Here is an interesting discussion:

https://www.astrobin.com/forum/c/astrophotography/deep-sky-processing-techniques/bright-stars-seem-to-have-some-kind-of-flare-happening/

Conclusion is that "some scopes just simply show this phenomenon".

I'm not entirely sure I'd buy into that conclusion.

[Albir phil]

Have you thought of condensation in the camera . Sometimes I have seen images like this which turned out to be that.Maybe not🤔

[vlaiv]

3 minutes ago, Albir phil said:

Have you thought of condensation in the camera . Sometimes I have seen images like this which turned out to be that.Maybe not🤔

Camera window is too close to focal plane and anything on it will be highly localized (think dust shadows). Sensor usually does not ice or fog up evenly and this leads to distinct pattern on the image - either on one side (icing up) or over whole field for fogging up:

or ice would look like this:

or often in the corner of the sub.

Closest to above pattern is dew forming on objective lens on the front of the scope - but that tends to "soften" up the stars and creates halo around them rather than distinct pattern.

It looks a bit like this:

[chicken george]

This is the view looking back down the telescope. I'll see if I can 3d print a mask at work.

 

If it doesn't work then I'll get rid and buy something different, it's just very obvious in something like this, which is why I didn't bother continuing with taking subs.

 

Thanks all.

[chicken george]

It's been a long time coming. I 3d printed a mask ring to cover the objective lens tabs and the pattern has gone. Just need some good weather 🫤