RASA 11 strange edge diffraction artifacts

[symmetal]

Long story short I have a new RASA 11 since last week and unlike the others, the collimation is correct and also no tilt issues, with very good stars over the whole frame so could start imaging right away. 😊 I've also fitted the Baader UFC system on this one mainly to avoid the severe vignetting on full frame. The UFC system is 70mm aperture from the scope unlike the Celestron scope adapter which is 48mm.

I started with the ASI2600MC and sure enough, no vignetting. Single subs all looked very good. However after stacking, the highly stretched previews in APP shows noticeable diffraction effects around the edges which vary a lot between targets.

The first was the Horsehead and I assumed the diffraction was a bright star out of frame so wasn't too concerned. However M51 and M101 show quite dramatic effects. M51 was imaged across the Meridian so the flip likely causes the repeated effect in the two corners.

I'm wondering if the UFC wide aperture is allowing reflections from the edge of the lens assembly in the corrector plate through to the sensor. I can try a black card aperture mask after the lens assembly tonight to see if that helps. Maybe the Celestron adapter masks to 48mm at this point for a reason. 🤔

Does anybody have any other suggestions as to the cause?

On normally stretched images the effect is not so obvious, but it upsets the background neutralization.  The Horsehead had the Moon quite close, while M101 had no Moon so the Moon isn't a factor. There are less obvious numerous curved diffraction effects on the right of M101 too, which may contribute to it being brighter there

Here are screen grabs of the APP initial previews. All used the same calibration frames.

Alan

[pipnina]

To be totally clear: You have a dew shield of some sort extending from the scope?
I could see light from the moon, street lamps, house windows or even far off-axis stars/planets refracting or scattering on the front corrector plate if it's not shielded.

Other than that, I don't know what to suggest as I've no experience with this scope, but I wish you a quick resolution!

[symmetal]

46 minutes ago, pipnina said:

To be totally clear: You have a dew shield of some sort extending from the scope?
I could see light from the moon, street lamps, house windows or even far off-axis stars/planets refracting or scattering on the front corrector plate if it's not shielded.

Other than that, I don't know what to suggest as I've no experience with this scope, but I wish you a quick resolution!

Thanks @pipnina. Yes I should have mentioned there is a dew shield in place lined with Protostar flocking. Everything is the same as on my previous RASA 11, which didn't show this behaviour, except for now using the UFC filter system. It could still be a problem with the rest of the scope itself but I would then expect the whole image to show this and not just the edges.

I've now fitted a mask in the place of the clear filter which is removed from the scope if external filters are used. It fits very neatly and now recuces the lens exit aperture from 72mm aperture to 60mm. Should be clear later tonight to see if that cures it. Fingers crossed. 🙂

Alan

[symmetal]

Fixed. 😁 

Tried the 60mm black card mask on M51 again and the coloured diffraction effects and brightening around the edges have gone though the vignetting is quite bad. I used the same flat calibration frame from yesterday. Then tried it without a card mask but just put the Celestron filter holder back in place but without the filter glass present. The holder itself reduces the aperture from 74mm to 68mm and this cured the problems also, but with no real effect on the vignetting.

Here are grabs of the APP stretched stack previews of 12 frames per stack. The Moon was out, but near setting. 

60mm card mask in filter holder

 

68 mm mask by using empty filter holder.

So the moral of the story is, if using the Baader AFC system with a RASA 11, remove the internal filter glass but leave the filter holder in place. 🙂

Hopefully, after 6 RASA scopes this is the end of my RASA woes. Although one was my fault when I dropped it. 😖 Imaging here I come. 😁

Alan