CentaurusA with a few problems Help Please!

[Hagar (Doug)]

This image is Centaurus A taken with Tak FSQ106ED with the 1.6X Extender fitted. Taken with QHY9 and QHY filters.
50 minutes of each RGB.

The problem I seem to be struggling with is the red channel doesn't seem to align well and I end up with some form of field rotation between the colours. For the life of me I cannot seem to get the 3 channels to line up correctly. Any ideas or suggestions would be appreciated.
The other thing which has me guessing is the small diffraction spikes on the brighter stars. Nothing used to generate these and I wonder if it is something to do with shutter operation.

Anyway here is the image.

[TheDecepticon]

Hi Hagar, I had some trouble with this until I started using the Astrometric alignment in the Combine dialouge of MaxIm. Now everything seems to line up well.

[gregbradley]

Hi Doug,

I used to have that happen to me sometimes. If your polar alignment is a bit off then even though you are autoguiding and the centre stars in the image stay aligned the outer ones will rotate.

So first off you should check your polar alignment and chances are you will find it was off to some degree.

Secondly CCDstack has an excellent plugin for high accuracy alignment. Once I started using this I found that sort of alignment issue a thing of the past. It costs US$99 but it is worth it. Just one click and it does it all automatically.

What is happening is your polar alignment is a bit off so there is rotation between images as well as any slight drift. You can do it manually in CCDstack and you'll have to move them on top of each other and then rotate them until they line up. The plugin does this automatically.

The 1.6X extender is a nice accessory isn't it? I've been using one lately as well.

Greg.

[Hagar (Doug)]

Originally Posted by TheDecepticon

Hi Hagar, I had some trouble with this until I started using the Astrometric alignment in the Combine dialouge of MaxIm. Now everything seems to line up well.

Thanks Gray I will give that a try.

Originally Posted by gregbradley

Hi Doug,

I used to have that happen to me sometimes. If your polar alignment is a bit off then even though you are autoguiding and the centre stars in the image stay aligned the outer ones will rotate.

So first off you should check your polar alignment and chances are you will find it was off to some degree.

Secondly CCDstack has an excellent plugin for high accuracy alignment. Once I started using this I found that sort of alignment issue a thing of the past. It costs US$99 but it is worth it. Just one click and it does it all automatically.

What is happening is your polar alignment is a bit off so there is rotation between images as well as any slight drift. You can do it manually in CCDstack and you'll have to move them on top of each other and then rotate them until they line up. The plugin does this automatically.

The 1.6X extender is a nice accessory isn't it? I've been using one lately as well.

Greg.

Thanks Greg I will look into this plugin. My alignment is pretty good atthe moment. I get A&E figures on the G11 of -2 and +3 which is probably as good as I will get here. I was wondering what different guide stars might make and perhaps the guide scope alignment to the main scope. It is so frustrating using the RGB with a mono camera when the results are nothing like those of the OSC.
The extender is a lovely piece of custom glass for sure. I bought both the extender and reducer when I purchased the scope but haven't spent much time using either yet.

Thanks again Guy's a few things to look at.

I have re-aligned using Maxim overlay and heavily cropped the image. It does seem quite a bit better. Now I just need to repro this time using darks as well.

[multiweb (Marc)]

Cool pic. There's a PS plug-in here that's great to get rid of green color casts that I use sometimes. There are a couple of dark spots on some of the stars. I find this can be taken care by doing a hot/cold pixel removal prior to stacking. Also stacking separate channels and recombining color in the end helps a lot with that - opposed to debayer then stack the color directly. You won't get as much color ringing due to mis-registration if you do it by channel.

[Hagar (Doug)]

Originally Posted by multiweb

Cool pic. There's a PS plug-in here that's great to get rid of green color casts that I use sometimes. There are a couple of dark spots on some of the stars. I find this can be taken care by doing a hot/cold pixel removal prior to stacking. Also stacking separate channels and recombining color in the end helps a lot with that - opposed to debayer then stack the color directly. You won't get as much color ringing due to mis-registration if you do it by channel.

Thanks Marc, Interesting plug in and for once there is a 64 bit version for windows7 64 bit.

The remainder is pretty much how I am doing it as it's a QHY9 with filters.

[multiweb (Marc)]

Originally Posted by Hagar

The remainder is pretty much how I am doing it as it's a QHY9 with filters.

Of course... still thinking you're imaging with your QHY8. Something you can try as well is to apply a slight deconv to all the channels so you try to match the star sizes in all three prior to recombining. Then if still needed do a star selection with the color range tool in PS, expand/feather then apply a slight gaussian blur then blend back in as luminosity with a mask. Bit fiddly but it works and there aren't that many stars to deal with at this FL. IMO the shot is well worth spending a bit more time on processing as the data is very good. As Greg mentioned CCDIS is great to match star patterns, more in busy fields like wide milkyway shots with heaps of stars. Registar is also very good.

[Hagar (Doug)]

Originally Posted by multiweb

Of course... still thinking you're imaging with your QHY8. Something you can try as well is to apply a slight deconv to all the channels so you try to match the star sizes in all three prior to recombining. Then if still needed do a star selection with the color range tool in PS, expand/feather then apply a slight gaussian blur then blend back in as luminosity with a mask. Bit fiddly but it works and there aren't that many stars to deal with at this FL. IMO the shot is well worth spending a bit more time on processing as the data is very good. As Greg mentioned CCDIS is great to match star patterns, more in busy fields like wide milkyway shots with heaps of stars. Registar is also very good.

Thanks Marc I may well have to try this as even registar cannot get it right. The best method I have found so far is aligning with the overlay method in Maxim. Unusual but I will get to the bottom of it. Work will again slow things down with 12 x 12 hr shifts in the next 14 days then the moon will be up solid again.

[Moon (James)]

+1 vote for CCDStack and CCDIS from me too.
James

[Hagar (Doug)]

Originally Posted by Moon

+1 vote for CCDStack and CCDIS from me too.
James

Thanks James I have downloaded the plugin and will see how it goes. I have a definite rotation problem but think I will get it sorted out OK.

Here is my latest attempt and the link to a higher resolution image just for Martin to look at.

Link to high res version