Tarantula and Surrounds in Narrow Band

[avandonk]

With a near full Moon collected this data over the last two nights as they were the first clear nights for quite some time.

Camera Canon 5DH, 150mm achro at f/4.25, Astronomik 13nm HA and O3 filters. Fridge at -10C.

Exposures 20x( 2m, 4m and 8m ) at 1600 ISO for both HA and O3.
Mapped HA to red, O3 green to green, O3 blue to blue.

Gradients were a problem as the full Moon does get through 13nm NB filters.

Large image 7MB

http://d1355990.i49.quadrahosting.com.au/2009_11/2070nb_.jpg

Bert

[multiweb]

Very nice shot Bert. :thumbsup:

[Paul Haese]

Bert the guiding is superb. In the full sized version the stars are very tight round dots.

The smaller sample looks better though as it does not show up what looks like tendrils of noise every where. Can you explain what is going on there. Or maybe is it the Ha influencing all the stars in the image. It looks like there is no data rejection going on in the image.

I think your handling of the gradients is quite good. There is only the top right that appears to be exhibiting some gradient, but overall it is done nicely.

I like the image and was interested to learn that moon light gets through 13nm filters. I was going to start my new object this weekend with a Ha run. Perhaps I will not now.

[rat156]

Hi Paul/Bert,

I'm also confused about how the moonlight gets through the NB filters.

I'd be looking at another source of LP.

I have both the 4.5nm and the 13nm filters. I was out taking pics of the same object over the past two nights (fullmoon and next to full moon) and none of my Ha images have gradients in them. Also the moon is a long way from the object. I could understand a little background glow, but the area also has lots of low intensity Ha background as well.

Unfortunately the with faster scopes/lenses the filters are less effective, so maybe this is the underlying cause, not the full moon as such.

Looks like you'll have to go to a narrower FOV Bert!

Cheers
Stuart

[multiweb]

I have had gradients on Ha shots (7nm) in the past on 10min exposures on the HH. Although I'm convinced it was caused by moon glow I can't be sure if it was from light coming through the Ha filter or through the side of the camera (externally)? :shrug: The moon was very close to the region I was imaging at the time though and I recall it was a full moon too.

[avandonk]

It is very simple if you have a 13nm filter then you will block all light pollution apart from what is passed by your filter. It is weak but significant. To even get data with a full Moon is amazing.

If you think about it it is 13nm/500nm of white Moonlight.

Bert

[Prickly]

Hi Bert,

That a very nice shot. You have brought out the detail very nicely. The tarantula is a great object for this type of image. Would like to see lagoon too when it is back up.

Can you clarify - for O3 blue and O3 green have you used the same O3 image for both the blue and green channels or did you do something slightly different?

Also did you refocus between the O3 and the Ha imaging? Im presuming so because of the achromat.

Results are extremely sharp and the fridge seems to have worked a treat.

Well done,
David

[rat156]

Not quite so simple though...

If the source of the LP is the full(ish) moon, then why do you have different colour gradients. Surely as the moon and the object pretty much move in unison throughout your exposures the gradient produced from this type of LP should be similar for all filters. This would give you a gradient for sure, but it should not change colour as the contribution from all filters should be similar.

Your picture shows multicoloured gradients, typical of an Earthbound source of LP.

Cheers
Stuart

[avandonk]

David O3 is detected by the green and blue pixels almost equally. That is why O3 is a cyan colour. Cyan is the absence of red light in a colour additive system (see first image of additive colour diagram). Below is an image of the spectral sensitivity of a Canon and Nikon due to their Bayer filters and UV/IR filter or that is unmodded.
From here
http://www.astrosurf.com/buil/d70v10d/eval.htm

You can see how O3 affects both the blue and green pixels equally.

Ha only really affects the red pixels.

By assigning HA to red and O3 green to green, O3 blue to blue you get a far more 'natural' colour of nebulae. Stars also end up white. It is then easy to add full colour data to get the star colours.

Bert

[avandonk]

Stuart you are quite correct. There is a light pollution gradient from scatter due to particles and gases in the atmosphere. There are two contributions to the scattered light the Moon and streetlights etc. This LP gradient gets worse as you move awayfrom the zenith. If you sample this gradient with RGB and for a full night you will get lovely colours. The FOV is 3x2 degrees so this problem is much worse than with longr focal lengths.

I managed to get a new data set of O3 last night. Here is a quick repro using the new data 3MB

http://d1355990.i49.quadrahosting.com.au/2009_11/2070nbnew.jpg


I will recollect the HA even though it is not as badly compromised by LP as the O3. See the third image of 8min O3 data where the gradient is obvious

This 150mm f/4.25 achro is very fast so as well as getting the faint stuff it is also a very good LP detector even in narrow band. The image below shows how big this achro is. That is a 90mm mak on top used as the guidescope.

Bert

[Prickly]

Hi Bert,

Many thanks for your detailed response. Very interesting and helpful.

Its a great image with heaps of details (2nd image even more so!).

Cheers,
David