NGC 6946 Fireworks Galaxy (first time)

[vineyard]

Hello,

This is 6h27 of OSC data on this over 2 nights.  TV102iis f/8.6 reduced by 0.79x.  UHC & CLS-CCD filters double-stacked.  ASI294MCPro -10C, gain 125.  HEQ5Pro.  Guided.  APP then PI.

I used the Photometric Colour Calibration in PI but also ran pretty much identical processes to an image where PCC had not been applied.  I prefer the PCC.

Unfortunately the first night my spacing was a bit off (as the uncropped image shows in the corners) so I tweaked that the second night (meant two sets of flats) - its still not quite right.

It's not going to set the world on fire, but it'll do as a start.  I'm not quite sure why there's like a halo in the centre of the image and a darker ring around the galaxy (see uncropped image).  All critiques v welcome!

It was just nice to get data on a clear night.  Also managed to start the Tadpole Nebula on the 2nd night (5h+ of data in one night )

I'm v curious - had I been using a mono setup, would a lot more data have been captured in the same 6h27 and therefore would it make for a better quality image?

Cheers,

Vin

(PS - this is probably a v silly Q. The 294MCP is an 11.7Mp camera, the bad pixel map that APP builds is 11.7Mb, but each light is 23.4Mb big - why is that?)

Edited November 15, 2021 by vineyard

[woldsman]

Excellent especially soon after fireworks night! Others will doubtless diagnose the halo and ring issues and explain how to fix it. Possibly flats are the issue, assuming you took them. 

[vineyard]

Thanks @woldsman - ha hadn't thought about Nov 5th - yes v coincidental - craxzy how good some of these firecrackers are becoming.

I did take flats (re-took them even).  I reckon it might be a PI thing - I did ABE but while playing around just now with another w-i-p image I've done both ABE & DBE on that and the ABE has a similar dark halo which DBE doesn't.  May re-try this galaxy w DBE in the next few days to see.

Cheers

[vineyard]

So I went back & used DBE instead of ABE, as well as put some negative bias (-0.001) on the R layer in the MMT & MLTs.  Here are a couple of 2x downsampled images (as per the ones above).

I think I prefer this version.

Cheers

[wimvb]

6 hours ago, vineyard said:

PS - this is probably a v silly Q. The 294MCP is an 11.7Mp camera, the bad pixel map that APP builds is 11.7Mb, but each light is 23.4Mb big - why is that?

Each pixel contains 16 bit data = 2 byte

2 x 11.7 = 23.4 MegaByte

The bad pixel map is only 1 byte per pixel, because it only needs to be 0 or 1, and the smallest datanumber is 1 byte (not 1 bit) in size.

Edited November 15, 2021 by wimvb

[vineyard]

Ah that makes sense, thank you @wimvb - much appreciated!

[Richard_]

Nice images! If I had to choose, I do like the one in your follow up post. 

With ABE you may need to play with the function degree to get the correct extraction. Open the tool and go to "Interpolation and Output" and you'll see "Function degree". Have a play around with this value, generate some background models, and compare them against the original image. 

Sometimes this tool works very well, other times you may need to perform DBE where you can manually choose positions which are used to remove background gradients. I find this tool produces the best end effect. 

[vineyard]

Thanks @Richard_ will try that w a w-i-p image I'm playing around with.

Btw your Veil mosaic is v nice - thanks for putting in such a detailed workflow there too!

[Richard_]

You're welcome pal. It took me a while to get to grips with DBE but I think it's up there as one of the most useful tools to use. 

Thanks for your kind words! I like to put the work flow in as it may help newcomers, but also help the more advanced people provide me with constructive criticism (eg "you shouldn't do X until you've done Y, otherwise it causes Z" kind of thing). 

[kirkster501]

The halos in the middle could be caused by reflections or stray light into the tube.  It can have weird effects.

[ollypenrice]

Firstly I'll risk asking you why you used a focal reducer on this target? To speed up capture? It doesn't do that because it doesn't increase the aperture. Have a look at 'The F ratio myth' on Google. (On the other hand, maybe it's also a flattener which you do need?) The reducer is introducing extra glass into the train with an attendant risk of reflections which may or may not have a bearing on your bright centre. The reducer puts the available light onto fewer pixels but you can do that by software binning.

The bright centre is odd because ABE, used on galaxies, usually has the opposite effect. It tends to sample the sky too close to the galaxy and pick up its outer glow which it then over-corrects into a dark region round the galaxy. I prefer DBE on galaxies and use as few markers as possible, certainly keeping well away  from the galaxy.

What's rather lacking in this image is colour. This is a very colourful galaxy and we also see little star colour. I'm sure it will be there in the data. I extract it primarily by using two Photoshop techniques, both of which avoid the saturation tool like the plague. The first is go into LAB colour mode and greatly increase the contrast in the a and b channels. The second is to create two copy layers, set the blend mode to Soft Light, flatten the top layer onto the middle one, convert the blend mode to Colour and flatten that.

I'm sure the mono version of your camera would be a little faster because the luminance time is capturing all colours at once per pixel, but CMOS OSC cameras seem to be very efficient, far better than CCD OSC.

Olly

[vineyard]

Thanks @ollypenrice - yes you're spot on the 0.79x is also a flattener & that's the main reason I was using it.  And you're right about the colour too - when I compare to other images, there's more blue for example.  I wasn't sure whether that's because 6h is still too short (but it's still quite a bit) or whether it's because of the double-stacking of the filters that some parts of the colour spectrum are being attenuated?  I don't have PS - but will see if I can come up w similar workflows either within PI or GIMP.

And thanks re the mono - yes I keep humming & hawing about mono but am quite happy w the OSC & frankly given how few these clear nights are, I'm not sure I can justify the mono expense.  Its mostly the moon-ful nights that become a slight pain b/c I can't image properly then - maybe an L-enhance is called for, or just bite the bullet for narrowband.

@kirkster501yes I did wonder whether it might be internal reflections - the Tadpole nebula I took on the second night didn't seem to have them, so maybe it was the tweak in spacing on the 2nd night that somehow put the reflections out.  Who knows!  Will see if they come back in future images whenever the next imaging night arrives!

Cheers both