Giant Floating Brain - NGC6888 Crescent Nebula

[badgers]

It's been a while since I finished a subject as I've been wresting with the weather and getting an Off-axis guider installed.

Everything is working now, except for some issues with field tilt. If I can't fix my focal plane tilt problem soon, I'll be in the refractor market : (

Anyway, this is my first crack at combining Ha and RGB data, also my first test of cheeky 2x2 RGB shooting combined with 1x1 data.

• 2 hours of H-alpha acquired in 20 minutes subs at 1x1
  
• 1 hour each of R,G and B in 5 minute subs at 2x2
  
• Darks and Flats for each filter at both 1x1 and 2x2
  

I'm really happy with the H-alpha data everything worked quite well.

The moon caused havoc with my RGB subs and made my flat-frames difficult to apply. I really should have just spent last night capturing more Ha, but wanted to see how this looked with some colour.

The Ha was combined with the RGB by combing a fraction of the Ha to the red channel of the RGB image using PixelMath and then using the Ha channel as a Luminance layer via Pixinsight LRGB combine. I'm really not sure the best way to combine Ha and RGB, I see so many methods around, this was trial and error.

Hope the balance is natural enough and the stars look reasonable.

Should I go and get Luminance data for this too ?

This is the raw H-alpha image (2 hours data in 6 subs).

The raw RGB image was pretty ropey due I think the moon causing gradients through each frame and washing out detail throughout the two nights of imaging. Flats didn't really help, but boosting the red with the Ha and Colour Correction helped.

I can confirm that the Atik 460ex gets away without dark frames for 5 or 10 minute subs, but for 20 minute subs I certainly needed dark frames to reduce hot-pixels marching along the frame.

This was a fun target and I'm looking forward to coming back to it with some other narrowband filters.

Thanks for looking!

Anton

[Delboy_Hog]

Absolutely incredible!

[Biggz]

Incredible indeed.

[swag72]

I like that Anton, I like it alot. Lots going on, nice and sharp and well processed.

[badgers]

Thanks guys and Sara.

I'm really happy with this one and still learning every time I go out there.

I'm finally getting to grips with PixInsight but it's a learning curve. I'm doing data and stats analysis all day at work, but somehow PixInsight makes me feel dumb

The OAG is delivering very well, just got to work out what is tilting/flexing and throwing out collimation and causing the strange star shapes.

Really hope I crack it before Kelling.... fingers crossed

Thanks,

Anton

[Black Knight]

That's a great image Anton, well done!

[RikM]

Very nicely done indeed

[Gina]

Yes, very good indeed Conditions are certainly very difficult!

[davew]

For six subs that Ha data is good. Focus and collimation look just fine to me too.

If that's a full frame, I suspect your star shapes are caused by the coma corrector distance being slightly off. Top right and centre are quite good but bottom left isn't. That could indicate a slight tilt on the focuser as well, just to make it more interesting

Dave.

[steppenwolf]

Really excellent - the long Ha subs have added some great detail and there is a nice sprinkling of star colour.

[badgers]

Thanks!

Would a a coma corrector at the wrong distance cause those kind of effects ? I would have thought it would have symmetrical aberrations outside of the centre.

The Ha shots also had reflections around the bright stars coming out as little mini spikes. A small tub of blackboard paint around parts of the secondary and the focuser reinforcement plate/screws eliminated this for the RGB shots.

The OAG really did the business for long subs. My guidescope and finderguider could manage 10 minutes with just a tiny bit of flex but at 20 minutes stars were badly trailed.

I'm not seeing any difference in star shapes between 5, 10 and 20 minutes with the current guiding setup. My star shapes issue is observed already in short subs (10-60secs).

Maybe a Newtonian scope genius will be at Kelling and can help me diagnose it.

Thanks again,

Anton

[Shibby]

Very nice! I think you've got your 150P working very well actually.

[ollypenrice]

Excellent, as sharp as you like and with super colour. I have imaged this three times and struggled every time. I always avoid binned colour when adding NB but this has worked a treat so I need to think again.

I wouldn't bother with luminance. The Ha is obviously pin sharp and is dealing with that side of things. However, if you have an O111 filter all sorts of new features will appear. This is not always the case but the Crescent is an O111 classic. I add O111 to green and blue, more to blue for aesthetic reasons.

I believe Altair Astro are offering (or going to offer?) a tilt adjuster. This is a much needed accessory since many cameras/focusers suffer from tilt or sag.

Olly

[davew]

Hi Anton,

I downloaded your image and created the ( poor ) illustration below. The top right is not bad at all. Top left and bottom right are just going off and the bottom left is the worst. This indicates to me that the camera / corrector / focus tube / focuser is out of line somewhere. The centre point of the aberrations is going towards the upper right.

One of the problems that causes this is the push fit correctors.

Due to the fact the top left and bottom right are showing slight aberrations it may be the coma corrector distance is a tiny tad out.

This is how it looks to me. If everyone else says it's great then leave it well alone.

Dave.

I too think you've got the 150P working well !

[Shibby]

Could that be explained by field rotation? If you were using a guide star from the upper-right of the image, that could explain it.

[davew]

Excellent point Lewis and you could be correct. I've dismissed it in my mind because it shows the same in 5 to 20 minutes in the original image and Anton says it's the same in 10 / 60 seconds.

It's good to have eyes on these things where people actually want to help. Unfortunately I may be a hindrance. I know at least one person who probably thinks that

Dave.

[Shibby]

Ah sorry, I totally missed that bit. Unlikely to be field rotation, then!

[Moonshane]

This is a great image. I like it when imagers take shots of things I have been observing visually and confirm what I was sure I was seeing. In quite poor light pollution, with my 16" dob I could see the rhomboid asterism and the brighter top section of the nebula quite readily. From a dark site I have seen this a lot better than at home. Used the Oiii filter and 26mm Nagler at 71x.

[RikM]

I also tried to see this one visually last night using my 10", A 20mm Pentax XW and Oiii. I could see the asterism and felt there was a brightening of the background in the outer portion, but it was so faint that it could have been averted imagination. Will have to try again from dark skies.

[Black Knight]

[...] it could have been averted imagination.

I get that quite a lot!

[Austin]

Awesome results Anton! I would be happy if my images were half as good as yours when I eventually get imaging

[alpal]

That's a spectacular picture.

You've done really well.

[badgers]

Thanks guys,

I'll be on the lookout for an OIII filter, I'd love to add to this.

I will double check my 5 and 10 minute shots from this session but if memory serves, there was also precious little rotation if I compare between the 20 minute subs using DSS.

I think I'll try and find a nice dense star field tonight and try some more diagnosis.

Things I can think of:

1) Secondary or Primary mirror movement - This seems possible, especially considering I've been fiddling with my secondary lately.

2) Focuser tilt/slop - Seems less likely, I've reinforced where the focuser attaches and can't get any noticeable tilt hanging weights from the focuser.

3) Tube flex - doesn't seem to be a major issue, I've also tried the tube rings in lots of different locations including flush with the focuser.

4) Corrector misplacement - Definite possibility, and maybe interacting with some focuser tilt.

5) Polar Alignment error - Not looking like an issue, guiding seems smooth and drift between frames seems to be small.

6) Gross Collimation errors - I'm overall very happy with my collimation which is Laser and Cheshire.

For the corrector it is set at 56mm to compensate for 3mm of glass in the light path. I think the Baader UHC is 3mm and the LRGB are 2mm, so maybe I should increase this a little more and see if it helps.

Anyway, none if this will stop me imaging!

Anton

[Jessun]

how this looked with some colour.

It looks good! It's all coming together and very inspirational for many, myself included!

A real treat to view!

Hope you'll be able to iron out the orthogonality or what it now may be!

[badgers]

Well that's the first astro picture I've had printed and framed.

Thank you again all for your kind feedback and advice.

I've had my scope in pieces for two days methodically squaring the focuser and rebuilding everything slowly.

I did a full on collimation with a colli-camera over about 4 hours today and numerous iterations, The view I'm seeing down the cheshire is the best I've ever seen.

I've an idea my secondary was badly placed and possibly not properly circular. Fingers crossed for clear skies tonight for some star-testing.

I'm hoping I've moved the 'sweet spot' back to the centre and can tweak the coma corrector to get round stars into the corners.

Thanks again,

Anton