RASA 11 flare on steroids

[symmetal]

Imaging the Flying Bat/Squid nebula recently over several rare clear nights of only nautical darkness in Ha and OIII I managed over 7 hours of OIII and 3 hours of Ha. I also tried SII but all it showed was a very faint version of Ha so didn't use it. Ha was good with just a hint of a flare while the OIII went overboard and gave this

 

SXT gave this 😡

Note that there is a cone shaped area where the flare isn't present and extending this cone outward and overlaying on Stellarium gives Alderamin as the culprit.

The red circle is the area of useable field on the RASA 11,  so the visible field area will be outside this. Bright stars inside or outside this distance don't seem to cause issues. I thought it may be hitting the mirror edge on the opposite side of the scope and reflecting and bouncing off the tube sides into the lens assembly. There is no acceptance cone before the lens assembly to mask off areas outside the mirror.

A much longer dew shield should mask off stars at this distance from entering the scope, or just a long masking section you can attach to the side of the dew shield at the right position. 🤔 Any other thoughts?

On the OIII image I'm assuming the background 'clouds' aren't traces of OIII but dust reflecting in the OIII passband, so apart from the area to the left of the squid I can basically just blank out all the flare area and get a useable image.

Alan

 

Edited July 18 by symmetal

[symmetal]

Just worked out what length dew shield would be needed to block light from Alderamin from reaching the opposite side of the mirror and it's well over 20m. 😁

I think I'll just have to ensure that no bright stars are in the critical zone when I frame the image. Next time out I'll do some test frames positioning Alderamin at various distances from the image centre to find the actual width of the critical zone. A single 5 min OIII exposure will show the brighter flare rings if you run noiseXterminator on it.

Reducing the aperture of exit side of the lens assembly will likely help with flaring but will cause increased vignetting. 

Alan

[ollypenrice]

I find coming closer to the source is usually the solution but I think you'll be lucky with this.

Selective use of Content Aware Fill in Photoshop ought to rescue this great image.

Olly

[gorann]

There are plenty of Squid images taken with RASAs (including 11") and none appear to show this peculiar problem, so I assume it must be something specific to your optical train and should be possible to sort out. Do you only see this with the Oiii filter?

Edited July 18 by gorann

[Fegato]

This looks like you've hit the design flaw in the RASA 11 that has affected many of us RASA 11 users. This is reflections from stars outside the field of view, which we believe is caused from within the lens group itself. It manifests itself using wide image trains (e.g. Baader UFC or the bespoke Octopi / Photon cage trains) and largely with full frame cameras. Are you using your ASI2600MC Pro? I don't think I've seen this with an APS-C sensor, only full frame, but Alderamin is very bright...

We "proved" the problem was in the lens group by taking out the filter draw and flocking the whole image train right up to the camera. No change. As you've suggested - the only solution is to put a small ring (about 4mm) around the edge of the lens group to block the edges of the light cone. This causes a fair amount of vignetting on full frame cameras, but is usable. It's a pain though - I did send a note to Celestron, but no response, and I suspect there's not much they can do without re-designing the lens group.

Full frame users get these arc shaped reflections on many targets - hopefully if you're using APS-C, it will only be a few with very bright nearby stars.

And yes, you're right about the dew shield  - mine will produce these using the large and solid Celestron aluminium shield. 

See - https://www.cloudynights.com/topic/804923-rasa-11-problems-with-reflectionsrainbows-caused-by-bright-stars/

 

[Elp]

I know this might be counter productive but baffling the front objective (thus stopping it down), would that help at all? (I did just brief over the CN thread). Looking at my internally baffled refractors, designed to stop these sort of internal reflections within the optics. Just a thought.

And what manufacturer filter are you using? Have you got a different one to try?

[Fegato]

I guess that's an option - would have a similar effect to the ring at the front I suppose. But you'll have to take the corrector off, which isn't everyone's cup of tea, and working out the size of baffle might be tricky?  With the ring at the front I was able to play around with different sizes to get it as small as I could get away with.

wrt filters - we sort of proved they weren't the problem, but no harm trying a different one if you've got one I suppose.

[Elp]

I was thinking just cutting a ring out of cardboard and sticking it on the front of the corrector like you do when you stop down refractors when doing solar. Similar to what you did on the camera side but you're potentially stopping any light getting into the OTA to begin with by putting it on the corrector. 

The thing with the filters I think this will make a difference, especially with Goran's comment stating other pics don't show this issue. From my experience using Optolong OSC filters for example, I know stray light hitting the objective (experience with refractors and camera lenses) can cause havoc even if the scope isn't pointed directly at said source, even with long dew shields in place. Haven't had an issue like yours with my HS yet, but bright stars in frame do halo like crazy and you can also see the camera cable routing as a shadow around bright stars with my Baaders.

[gorann]

4 hours ago, Fegato said:

This looks like you've hit the design flaw in the RASA 11 that has affected many of us RASA 11 users. This is reflections from stars outside the field of view, which we believe is caused from within the lens group itself. It manifests itself using wide image trains (e.g. Baader UFC or the bespoke Octopi / Photon cage trains) and largely with full frame cameras. Are you using your ASI2600MC Pro? I don't think I've seen this with an APS-C sensor, only full frame, but Alderamin is very bright...

We "proved" the problem was in the lens group by taking out the filter draw and flocking the whole image train right up to the camera. No change. As you've suggested - the only solution is to put a small ring (about 4mm) around the edge of the lens group to block the edges of the light cone. This causes a fair amount of vignetting on full frame cameras, but is usable. It's a pain though - I did send a note to Celestron, but no response, and I suspect there's not much they can do without re-designing the lens group.

Full frame users get these arc shaped reflections on many targets - hopefully if you're using APS-C, it will only be a few with very bright nearby stars.

And yes, you're right about the dew shield  - mine will produce these using the large and solid Celestron aluminium shield. 

See - https://www.cloudynights.com/topic/804923-rasa-11-problems-with-reflectionsrainbows-caused-by-bright-stars/

 

Interesting thread Robin! I may have been lucky when I decided to go for a dual RASA8 rig with APS-C cameras rather than a RASA11 with a full frame camera (the total price was about the same).

In your final post you show that you took the corrector plate off since the lens group got lose. I am curious how much collimation issues this caused. I assume you needed to center the lens group carefully.

[Fegato]

11 minutes ago, Elp said:

I was thinking just cutting a ring out of cardboard and sticking it on the front of the corrector like you do when you stop down refractors when doing solar. Similar to what you did on the camera side but you're potentially stopping any light getting into the OTA to begin with by putting it on the corrector. 

The thing with the filters I think this will make a difference, especially with Goran's comment stating other pics don't show this issue. From my experience using Optolong OSC filters for example, I know stray light hitting the objective (experience with refractors and camera lenses) can cause havoc even if the scope isn't pointed directly at said source, even with long dew shields in place. Haven't had an issue like yours with my HS yet, but bright stars in frame do halo like crazy and you can also see the camera cable routing as a shadow around bright stars with my Baaders.

Ah OK, thanks, I see. Well, I guess that might do the trick too, but I'm "happy" with what I've got for now - not enough clear sky time here to try out anything that I don't have to!

Some filters certainly do cause problems, but this issue is almost exclusively about stars outside the field of view (the only target where I had "inside" problems was M45! - but again, that went away with the blocking ring). I've only used L-eNhance and NBZ filters with the RASA, and no specific problems relating to them.

[Fegato]

7 minutes ago, gorann said:

Interesting thread Robin! I may have been lucky when I decided to go for a dual RASA8 rig with APS-C cameras rather than a RASA11 with a full frame camera (the total price was about the same).

In your final post you show that you took the corrector plate off since the lens group got lose. I am curious how much collimation issues this caused. I assume you needed to center the lens group carefully.

Yes possibly so!  I didn't go for the 8 when I bought mine as there wasn't a suitable solution available for backspace and tilt management, but that soon arrived. Having said that, I do like the 11, such a beast! And as I say above, the vignetting is normally well corrected by flats, especially broadband (which is what I mostly focus on).

As for the corrector, I was obviously really careful with rotational position.

For centring, I just unscrewed 2 of the side-on set screws ½ turn (adjacent ones) to get the corrector out. And then when I put it back I tightened these up ½ turn. So in theory it was in exactly the same position. I've barely had any clear sky time since this, and must be honest, I spent time getting my backspace and tilt back where it should be (using my Octopi), but am guilty of not even checking the collimation. Visually, images look OK....    must give it a check though at some point! Mind you - interminable cloud here - I normally image through June and July despite lack of true darkness, but nothing...

 

[symmetal]

Thanks everyone for your replies, especially Robin @Fegato for the info in the extensive CN link.

I am using the Baader UFC system with a full frame 6200MM. The stock Celestron 48mm adapter is OK with an APS-C sensor and would give no flares, but the vignetting on full frame is huge. Using the Baader UFC If I cropped the full frame image to APS-C some circular flaring would still be visible.

I believe I saw the CN link when researching the multiple rainbow flares visible when I started using the Baader UFC system and 3D printed an adapter to go from the RASA 11 rear lens metal ring (clear Celestron filter removed) all the way to the front of the filter holder. I sprayed the front and inside of this adapter with one of the 'Super' black paints. The flange near the top butts against the front of the Baader UFC RASA 11 adapter and tapers out to a 65mm opening at the top which is effectively a 5mm thick aperture reduction from the 75mm wide threaded recess, although the lens front ring is about 1.5mm wide so I'm reducing the lens exit aperture from 72mm to 65mm which is a 3.5mm wide ring.

This cured the rainbow flares a treat but to remove the out of view bright star flares I need to reduce this 65mm to say 62mm. I made the adapter too well and there is no space to insert a flocking ring in front of the adapter so I'll likely print another adapter.

@ollypenrice, yes I'll have a go at removing the flares in PS and see how it looks. 

@gorann & @Elp Oiii shows the effects the worst though the flares are visible on other filters. With the squid, the Oiii is stretched much more than the Ha. On other Oiii images I could fairly easily remove what flares there were in PS. Here's the Flying Bat Ha for comparison. 

I'm using the Astronomik 6nm MaxFR 2" filters. The only other 2" NB filters I have are the not fast FR Astronomik ones. Robin has said that flaring occurs no matter what filters are used though and is almost certainly caused by the lens assembly.

Alan

Edited July 18 by symmetal
Removed excess images

[geeklee]

On 17/07/2024 at 21:49, symmetal said:

only nautical darkness in Ha and OIII I managed over 7 hours of OIII and 3 hours of Ha

Hope you can rescue something Alan.  The squid OIII looks fantastic for only 7 hours in nautical darkness 🤞  The flying bat is looking good too.

[gorann]

This thread makes me think that my decision to buy a Hyperstar for my EdgeHD 11 was a rather good idea. The US made lens group of the Hyperstar (now in version 4) may be better put together than that of the RASA 11. Some even use it with a full-frame camera (https://www.astrobin.com/users/equinoxx/) although I will use mine with an APS-C.

[Elp]

The HS is a nicely made chunk of glass and metal for sure. How people have got nice correction with FF though is amazing, but could also be attributed to the actual edge hd.