RASA 11 and Baader UFC system flaring

[symmetal]

Over the past good week I've imaged several objects and a couple are spoiled by flares from bright stars around the edge of the RASA 11's FOV circle. I did 3 hours or so on the Leo Triplet to resolve the tidal tail from NGC3628 which it has done but at the cost of a horrible flare from Chertan, a 3.3 mag star near the edge of the RASA 11 FOV. A similar image taken a year ago using the Celestron M48 adapter fares much better in this respect. The M48 adapter is OK with an APS-C sensor but creates massive vignetting on a full frame sensor, which is why I bought the UFC system. Here's a super stretched view of the unprocessed images. The black lines show the hard edges of the flares pointing to the offending star.

My 'super black' conical end tube I made, going from the RASA 11 rear lens to the UFC filter adapter cured the multitude of flares from any bright stars nearby which affected almost all images, but it can't fix them at a that problem distance. I did try an aperture mask on the front of the dew shield to reduce the scope aperture from 279mm to 250mm but it had no effect on the flaring at all, which surprised me as I thought that would block light rays from the edges of the mirror and lenses, so would have some benefit at least.

I can reduce the diameter of the front opening on my conical tube which may possibly reduce the flaring, but at the expense of increased vignetting, but not enough to be worth doing. I suspect the M48 adapter makes it better as there is virtually no light from the problem distance reaching the sensor due to the almost 100% vignetting at the limit of the RASA 11 FOV.  

I tried M78 expecting problems from Alnitak so checked it after a few frames, and tried repositioning M78 to the bottom right to move Alnitak further away which helped to some degree but not enough.

I now use the above Telrad circles in Stellarium to check any targets before imaging. Bright stars inside the RASA 11 FOV but outside the camera FOV don't really cause an issue. A series of heavily overlapping mosaics should help with most problem targets I suppose.

Alan

[AKB]

Thanks for that.  I have recent acquired a RASA 11 and have fitted the UFC system I had previously on my Hyperstar.  Since the weather has been so bad, I still have not fully commissioned the kit, so haven't tried any filters yet.  I will look out for this when the time comes.

Tony

[ollypenrice]

I've had flares from out-of-shot stars with various scopes. I can suggest two solutions, one more 'ethical' than the other.

1) You can make a patch panel by shooting a fairly short run of subs with the source star closer to the image centre but with enough overlap to cover the flare. You just use this patch to replace the flared bit on the original in Photoshop Layers or whatever.

2) If you use a star removal product, once you have a starless image it is dead easy to lasso the flare and use Content Aware Fill in Photoshop - provided there is nothing much going on under the flare. If it goes right through the middle of the Horsehead, this ain't gonna work! 

Olly

[symmetal]

Thanks Olly @ollypenrice,

On my Horsehead I posted recentlyI did use PS layers to change the hue/brightness of the two small blue flares to match the background which worked very well. Looking at the RASA 11 FOV I'm surprised that the flares weren't a lot worse. I assumed they came from Alnilam, outside the camera FOV but well within the RASA FOV so the effects were minor. It seems Mintaka was just far enough away not to cause the hard edge full width flares, along with the rainbows, as the postings above show.

I can use PS layers to reduce the brightness/colour on the triangular flares and 'paint' out the rainbows either with a different image or content aware fill as necessary.

My attempt at reducing the aperture from 279mm to 250mm to reduce the flaring only reduced the f-ratio from 2.2 to 2.48 so maybe it wasn't enough to really affect the result. A more restrictive aperture mask, f2.8, f3.3 or f4 even,and subsequent longer exposures may be an easier solution on problem targets if it works. Do you think it's worth a try or is my reasoning wrong? 🙂

The trials and tribulations of fast scopes I suppose. 

Alan

[ollypenrice]

32 minutes ago, symmetal said:

Thanks Olly @ollypenrice,

On my Horsehead I posted recentlyI did use PS layers to change the hue/brightness of the two small blue flares to match the background which worked very well. Looking at the RASA 11 FOV I'm surprised that the flares weren't a lot worse. I assumed they came from Alnilam, outside the camera FOV but well within the RASA FOV so the effects were minor. It seems Mintaka was just far enough away not to cause the hard edge full width flares, along with the rainbows, as the postings above show.

I can use PS layers to reduce the brightness/colour on the triangular flares and 'paint' out the rainbows either with a different image or content aware fill as necessary.

My attempt at reducing the aperture from 279mm to 250mm to reduce the flaring only reduced the f-ratio from 2.2 to 2.48 so maybe it wasn't enough to really affect the result. A more restrictive aperture mask, f2.8, f3.3 or f4 even,and subsequent longer exposures may be an easier solution on problem targets if it works. Do you think it's worth a try or is my reasoning wrong? 🙂

The trials and tribulations of fast scopes I suppose. 

Alan

I remember having shaft-like out-of-shot flares affecting Tak FSQ images (F5) and ODK14 images (F6.8). I wouldn't be too sure that it had anything much to do with F ratio but I really don't know. With the RASA we do get arcs, on occasion, rather than 'shafts.' I just go for a cosmetic fix on the starless layer. So far so good... I don't regard stopping down as an option on either of our F2 systems. F2 is just too good to mess with! 

Olly

Edited January 15 by ollypenrice
Clarification