NGC6723 in Corona Australis - ATiK16IC RGB

[[1ponders]]

NGC6723 with TY Coronae Australis. You can just make out the some of the darker part of the Nebula to the lower left of the TY CorAus

Corona Australis is getting a bit of a hammering from me, seeing as it is in just the right spot at the moment.

A few day ago I imaged the same area using my 135mm lens on the 300D, this time I thought I'd try an LRGB image using the ATiK 16IC, 6.3 Meade FR and the Orion ED80. My first successful effort in RGB for a DSO (though not as planned :P ). I'm just starting to learn how easy it aint. If anyone can offer any suggestions for keeping the stars the same size for each colour channel I'd love to hear it. The biggest problem is bloating in the Red Channel.

10X3min each of L=R, G, B (ie I used the Luminance channel for the red channel as the stars were too bloated). Dark subtracted, aligned and colour combined in ImagesPlus. The filters used are Baader RGB filters (not very happy with the colour outputs.) Final processing in PS. The image in the corner is a crop from the 300D image from the other day and isn't at 100%.

The monochrome image is the stacked luminance, with light stretching before final combining.

[avandonk]

Welcome to my nightmare!

Bert

[h0ughy]

looks like your experimenting is working Paul

[[1ponders]]

I'm glad I'm not the only one having trouble sleeping then Bert. Arrrgh!! Frustrating but fun :lol: I reacon if I keep imaging the same object night after night, sooner or later I'll hit upon a receipe that will work :)


Thanks h0ughy. Experimenting is the word that's for sure. More like trial and error :P

[gbeal]

Now you can see why I have a one shot colour camera, LOL.
Good fun though. Bloating in my case seems to be helped (well that is what the official word is) by an I/R filter, or is that UV?? I normally use a Baader one on the front of the camera all the time, certainly with my refractors and or camera lenses. Your ATIK being mono though should be OK?

[[1ponders]]

From memory the baader filters each block IR and I have an IR for my Luminances. It may also be that I was a bit shoddy on the focusing. Tonight I'm going to start with the red filter and get that as well focused as I can and for the rest of the filters I will try to match the red focus point. I'm using a focus assist that came with the IC Capture program. It measures the intensity and the width of the star in pixels so hopefully that will help.

[[1ponders]]

I'm shooting the Ara Glob atm and already I can see that stars are looking a bit thick. Might have to try to get an extra filter in the line somehow. It will need to be a two inch one though as I'm using the meade FR with an Orion 2" Prime Focus adapter. :confuse3: I'll have to think about this, might be that that is my first purchase when I get to astrofest

[gbeal]

Ordinarily you shouldn't need to refocus between shots, but if there is a significant difference in star size/quality, then maybe you do need to.
What capture program are you using, as the likes of Maxim has a focus routine that really helps, CCDSoft as well.
If your RGB's already have an I/R block, then that cancels that idea.

[[1ponders]]

I'm using the provided program IC Capture. I haven't looked at Maxim yet to see if I can use it with that. I know CCDSoft doesn't work with it.

[jase]

The most common cause of star bloat is out of focus IR light. This typically occurs with refractors and other lens designs as wavelengths reach focus at different points. Mirror based telescopes focus all wavelengths equally, thus don’t exhibit this characteristic. Another thing to consider is that not all IR blocking filters and coatings are created equal. There are differences between manufacturers – some start at 750nm others may be lower or higher. All filters can leak IR, but more so the red filter considering its transmission wavelengths are very close to the start of IR. In some cases, poor quality filters will actually have an overlapping transmission window where major IR leaks can occur. IR contamination with CCDs should be avoided at all costs – especially when colour imaging.

You could try using a UVBRI filter set mapped as R=R; G=V; B=B. I’ve seen some incredible images taken with this combination. There is typically some cross over between R and I, but with the right coatings this is minimised. I've also assumed that your filters are parfocal, so no need to refocus between filter changes.

[[1ponders]]

Thanks Jase. By and large they are parfocal, except for that damn red filter. I have to have a look at getting a 2" to go in front of the filter wheel. I have the B and V filters, maybe it would be a better option to go for the red one instead.

Better yet maybe I should get a 10" newt to put ontop rather than the refractor ;)

[jase]

A commonly used function in processing stellar aesthetics is using the PS minimum filter to control bloating. There are a few pages on the web explaining its use, here is one of them; http://www.astronomy-images.com/Articles/reducing-star-bloat.htm

I've had mixed results with this technique. Depends on your data.

[[1ponders]]

Yes I've used that, in particular an action that enhances DSOs and reduces stars. I works "ok" but I find it can tend to make the star centers a bit hard and artificial looking if the image is a bit scarce in signal.