Star Notching

[Somerled7]

I've been setting up my new scope - an Altair Astro 72EDF, and I'm seeing a distinct notch in saturated stars, as if something is protruding into the light path - example image attached.  I've tried changing around the back end of my kit by removing the field flattener and light pollution filter, and rotating the camera, but this has no effect (when I rotate the camera, the notch also rotates, so in same position relative to the scope).  Also tried retracting the dew shield, but again no change.  I've had a good look down the scope, but can't see anything protruding, so I'm at a bot of a loss. I've e-mailed Altair Astro, but wondered if anyone on here could offer any advice or suggestions for more investigating.

Thanks

Gordon

[ollypenrice]

Those 'inverse lighthouse beams' are not unusual and appear in my Tak FSQ106 on bright stars. What's more troubling is that you only have one clear one - on the left, but there's evidence that the other side is trying to produce one as well. You also have another asymmetry: the red channel's stellar image is offset to the upper left on most stars. I'd ention this to the supplier.

Olly

[Martthebass]

I’ve got this on bright stars on my WO GT71, but in my case it’s at 120 degree spacing and even. I assumed it might be slight protrusion of foil spacers (though I can’t see them so it could be something else?) into the light path. I considered sending it back but I’ve seen a number of Skywatcher and WO small apo doublets and triplets with the same issue so there’s no guarantee of getting anything different....and to be honest it doesn’t look too bad.  You can see it on the bright orange star on this image of M76 I took the other night.

[Merlin66]

I had similar issues with an older ED80.

Three distinct notches and flaring. There were no obvious spacers visible.

I marked the orientation of the notches on the outer surface of the cell and found they matched the position of the collimating screws. This scope had three peripheral screws which moved the front element relative to the rear element.

These has obviously been over tightened and nipped the glass. Just an easing of each screw and allow a settling down period seemed to work.

The notches/ flares have never returned - Oh, and the collimation is still perfect!

[blinky]

I had the same on my WO ZS73 - It got send back to WO and is now much better.  They did say initially to try a dew band but when that didnt help they took it back and sorted it, so might be worth trying a dew band to see if that makes a difference.

[Somerled7]

Thanks for the replies.  I've done more investigation, and by sticking a finger into the image found the notch lines up with one of the sets of screws on the lens cell (there are 4 sets of two screws around the cell).  I've very slightly slackened the screws and tried again - only managed a very quick untracked image of Capella between clouds by ramping up ISO, so there's lots of noise and it's not a very good test, but it looks more promising.  

[ollypenrice]

16 hours ago, Merlin66 said:

I had similar issues with an older ED80.

Three distinct notches and flaring. There were no obvious spacers visible.

I marked the orientation of the notches on the outer surface of the cell and found they matched the position of the collimating screws. This scope had three peripheral screws which moved the front element relative to the rear element.

These has obviously been over tightened and nipped the glass. Just an easing of each screw and allow a settling down period seemed to work.

The notches/ flares have never returned - Oh, and the collimation is still perfect!

This confirms Greg Parker's analysis of the same problem.

 

21 hours ago, Martthebass said:

I’ve got this on bright stars on my WO GT71, but in my case it’s at 120 degree spacing and even. I assumed it might be slight protrusion of foil spacers (though I can’t see them so it could be something else?) into the light path. I considered sending it back but I’ve seen a number of Skywatcher and WO small apo doublets and triplets with the same issue so there’s no guarantee of getting anything different....and to be honest it doesn’t look too bad.  You can see it on the bright orange star on this image of M76 I took the other night.

There's an easy Photoshop fix if ever you do want to get rid of them.

The routine is best saved as an action after the initial selection. Select with magic wand then expand by (say) 7 pixels, feather by (say) 2 pixels, then Filter-Blur-Radial Blur set to spin and best quality. Deselect.

Whether it improves the look of the image or not is up to personal preference I guess, but that's how the fix works.

Olly

 

[Martthebass]

Thanks for this Olly, I promised myself I wouldn’t get too precious about star shapes when I started out on this hobby. After years of purely visual endeavours I’m just happy to see colours and some detail. The only concern I had is if it affected the value of the scope, if that was the case I’d take it in to RVO to be adjusted as it’s still under warranty.

[Adam J]

On 27/10/2020 at 13:05, ollypenrice said:

Those 'inverse lighthouse beams' are not unusual and appear in my Tak FSQ106 on bright stars. What's more troubling is that you only have one clear one - on the left, but there's evidence that the other side is trying to produce one as well. You also have another asymmetry: the red channel's stellar image is offset to the upper left on most stars. I'd ention this to the supplier.

Olly

Yes as Olly said colour on one side of a star can imply an off center element in an ED design although it's not common to see that in a doublet. The notch is likely from a over tight adjustment screw or lens spacer in the light path. 

[Adam J]

On 28/10/2020 at 14:27, Martthebass said:

Thanks for this Olly, I promised myself I wouldn’t get too precious about star shapes when I started out on this hobby. After years of purely visual endeavours I’m just happy to see colours and some detail. The only concern I had is if it affected the value of the scope, if that was the case I’d take it in to RVO to be adjusted as it’s still under warranty.

Lots of people are bothered by such things myself included so I would send it back to be adjusted. Don't do it yourself. 

[Martthebass]

Thanks Adam....no way would I touch it myself, certainly not on a triplet. Images have been sent (via RVO) to WO for comment. Hope it doesn’t have to go back with the current waiting list.