Eta in the eyes of a RedCat

[h0ughy]

taken tonight before the clouds moved in its only a short 2 hours worth of this most imaged region of stars and gas, just waiting for the day it goes boom. Taken with a puddy cat, i thought i saw Eta through the eyes of a 51mm objective with a canon eos Ra, iso 800 with a multi spectrum filter. its sampling what could be a great field of view.

stacked in DSS and processed in pixinsight then PS.

[RB]

Nice and sharp.
Good colours.
Can't wait for you to get more on this one.

:thumbsup:

[xelasnave]

Excellent David I would love to see a high res but I get the idea..I have been trying to talk myself out of buying a Red cat all week ( and goto Startracker) and your photos are not helping me avoiding a purchase.

It seems to be a very flat field.

They seem like a very good unit.

Alex

[JA]

It looks great in detail and colour. :thumbsup:
I'm always curious: Can I ask what sort of subexposure duration you used?

Best
JA

[h0ughy]

I start with 1 second, 5 second, 10 second 30 seconds, 60 seconds 120 seconds and 180 seconds with 20 images of each then stack it all in deep sky stacker and the process in pixinsight then maybe Photoshop for a tweak if needed but that's mainly for resizing after pixinsight.

I use a multispectrum filter on the Canon Eos Ra . My light pollution is hideous and only use iso 800 or 400 if the moon is up

[h0ughy]

That Redcat would be ferral at your place

[h0ughy]

Thanks RB

[JA]

Thanks for the info H. It's useful as a reference for DSLR based imaging of how much H-Alpha is picked up given that your Ra is Canon factory manufactured/"modified" for extended H-Alpha.

Best
JA

[h0ughy]

Well my theory is that the shorter shots get the star colours and the longer ones the nebulae and dust if you go long enough

[multiweb]

Really nice fov. Looks like you'll have a lot of fun with that rig. :thumbsup:

[h0ughy]

Well I hope so

[peter_4059]

Dave, how do you combine the different exposures? Are you layering in ps?

[h0ughy]

Being a lazy astronomer a just shove it all in deepsky stacker and let it stack everything . The save the TIFF file somewhere and run pixinsight. I clip rotate etc with screen transfer, then perform a sin by working on the histogram to align the channels. Then after that boost the colour saturation maybe noise reduction then a few other things and save

[peter_4059]

Which DSS rejection algorithm are you using?

[h0ughy]

Good question, will have to find out.

[RB]

You’re using Kappa-Sigma Clipping Dave.

:)

[peter_4059]

Just wondering if that rejection algo ends up clipping the longest and shortest exposures such that they do not get included in the stack? Shame to spend time capturing subs just to have the software dump them.

[h0ughy]

I thought it was on image quality?

[RB]

Pete, I’m no expert by any stretch of the imagination, but I think David is right.
I’ve used Kappa-Sigma at times and it’s given me the better result.
Other times I’ve used Median Kappa Sigma and it’s given me the better result.
I’ve even gone Average or Median.
I find it depends on the data you’re working with.

It’s been such a long time since I’ve done AP though, so that’s the best I can advise you on.

Sometimes you get a better result, rejecting some of the data.
Well, letting the software reject it for you, actually.

RB
:)

[peter_4059]

Worth reading the manual....
http://deepskystacker.free.fr/english/technical.htm

[RB]

Yes, it is worth reading.

;)

[h0ughy]

Well I'm just not really understanding it, so each group it stacks has the best 80%:shrug:

[peter_4059]

Here's my understanding - others feel free to chime in if you see it differently...

The rejection algorithm is trying to reject outliers. It does this on a pixel by pixel basis. To determine what is real signal and what is an outlier for a given pixel the only information it has is the value of that pixel in each of the subs.

Sigma refers to the standard deviation of the pixel values. If you take ten subs of increasing exposure, for a given pixel (lets say part of a star) you might get values of 1,2,3,4,5,6,7,8,9,10. The mean value is 5.5 and the standard deviation is 3.03.

If the algorithm is set to reject anything that is outside the range of mean +/- one standard deviation (5.5+3.03=8.53) (5.5-3.03=2.47) then data with values less than 2.47 and greater than 8.53 are not included in the stack. So the subs with pixel values 1, 2, 9 and 10 are not included.

The significance of this for you is if you take a whole lot of different exposures and then stack them all together, the rejection algorithm might be rejecting the shortest and longest exposures as outliers. If you have taken 20 x 180 sec subs (ie one hour worth) in an attempt to capture the faint detail, it would be a shame if these were rejected from your final image. The same goes for the short exposures however you obviously spent a lot less time capturing these.

"Kappa-Sigma Clipping
This method is used to reject deviant pixels iteratively.
Two parameters are used: the number of iterations and the standard deviation multiplier used (Kappa).
For each iteration, the mean and standard deviation (Sigma) of the pixels in the stack are computed.
Each pixel which value is farthest from the mean than more than Kappa * Sigma is rejected.
The mean of the remaining pixels in the stack is computed for each pixel."

There are other rejection routines that are designed to combine different exposures for HDR purposes:

Entropy Weighted Average (High Dynamic Range)
This method is based on the work of German, Jenkin and Lesperance (see Entropy-Based image merging - 2005) and is used to stack the picture while keeping for each pixel the best dynamic.
It is particularly useful when stacking pictures taken with different exposure times and ISO speeds, and it creates an averaged picture with the best possible dynamic. To put it simply it avoids burning galaxies and nebula centers.

[h0ughy]

I thought it did it for each set of subs

[peter_4059]

Is that what the manual says?

[h0ughy]

will only stack images that contain at least eight stars that are common between all light framhes. ,.........


Kappa-Sigma Clipping
This method is used to reject deviant pixels iteratively.
Two parameters are used: the number of iterations and the standard deviation multiplier used (Kappa).
For each iteration, the mean and standard deviation (Sigma) of the pixels in the stack are computed.
Each pixel which value is farthest from the mean than more than Kappa * Sigma is rejected.
The mean of the remaining pixels in the stack is computed for each pixel.

But it doesn't say it's not all the images or just the individual timed lots . So I have no idea

[AstroViking]

The image looks great - lots of colour and pin-point stars.

I have to ask, though - why so many different exposure lengths?

[h0ughy]

well my theory is that the shorter shots help give colour to the stars without blowing or saturating them out:question:

[AstroViking]

That makes sense. Thanks, h0ughy.

[Bassnut]

And why is the background green?. I know people at IIS you know (I think), could be all kinds of trouble for you.

[h0ughy]

Does that look green on your screen?

[Bassnut]

well yeah, very. I calibrated my screen many months ago with a spider thing. I might be wrong.

[h0ughy]

Fair enough, though I did run hastalavista green