My first live stacks.

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AndyBooth
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Joined: Thu Mar 01, 2018 6:55 pm

Re: My first live stacks.

Post by AndyBooth » Fri Mar 22, 2019 3:05 pm

I registered years ago Dave,
Haven’t used it much though.

I like the QHY10, but with 6.01 pixels, its not good on high res for galaxies.
I am going to try the shorter exposure but many more of em approach on the next outing,
See how it copes.

MrAstroBen
Posts: 52
Joined: Fri Mar 16, 2018 12:19 pm

Re: My first live stacks.

Post by MrAstroBen » Fri Mar 22, 2019 4:28 pm

Andy,

What is the focal length of your scope (without the 0.75 reducer). ?

Unless short the pixels resolution should be fine at 6.

You will more likely be limited by seeing conditions than pixel resolution.



Rob

AndyBooth
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Joined: Thu Mar 01, 2018 6:55 pm

Re: My first live stacks.

Post by AndyBooth » Fri Mar 22, 2019 5:46 pm

1600mm Rob, but thats at F8 , that gives 0.77” pixel which is OK, but slow.
With the reducer gives F6 ish around 1200mm or just over 1” pixel, great from nebulae etc
The CMOS gives 0.8” pixel at F4 , which is why id like to get it working good,
no wasted FOV on galaxies and planetary nebs, ideal resolution, and fast at F4.
Seeing is about 1.5-2.0”.

To be honest, i have been playing at the imaging game for years on and off,
But have recently retired, so can finally devote some decent time to it,
My work meant only weekends, and I travelled alot to China.

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oopfan
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Re: My first live stacks.

Post by oopfan » Fri Mar 22, 2019 6:19 pm

Andy,

Is your objective to make a galaxy fill your sensor like you did with M81 and your monochrome camera but using the QHY cam instead? The only way to achieve that is to upgrade your scope to an RC20 f/4. The cheaper route is to purchase a 183C and use it with your existing RC8 f/4. That will give you 0.62 arcsec per pixel.

Brian

AndyBooth
Posts: 47
Joined: Thu Mar 01, 2018 6:55 pm

Re: My first live stacks.

Post by AndyBooth » Fri Mar 22, 2019 6:31 pm

Brian,
My view is to use the QHY10 for wider field stuff, at F6 , 1” pixel.

But For galaxies and planetary nebs, globulars,I am trying to get either my CMOS mono/CMOS colour working well for LRGB,
or if its up to it, just the colour CMOS on its own. ive learnt alot about CMOS use from this forum.

Just need some clear nights now to experiment !

MrAstroBen
Posts: 52
Joined: Fri Mar 16, 2018 12:19 pm

Re: My first live stacks.

Post by MrAstroBen » Fri Mar 22, 2019 7:10 pm

AndyBooth wrote:
Fri Mar 22, 2019 5:46 pm
1600mm Rob, but thats at F8 , that gives 0.77” pixel which is OK, but slow.
With the reducer gives F6 ish around 1200mm or just over 1” pixel, great from nebulae etc
The CMOS gives 0.8” pixel at F4 , which is why id like to get it working good,
no wasted FOV on galaxies and planetary nebs, ideal resolution, and fast at F4.
Seeing is about 1.5-2.0”.

To be honest, i have been playing at the imaging game for years on and off,
But have recently retired, so can finally devote some decent time to it,
My work meant only weekends, and I travelled alot to China.
Andy, 1600 x 0.75 = 1200 so a pixel resolution of 1.03 with the CCD. There really is no need to go lower with the seeing in the Uk. That is a good figure.

You can just crop or use RIO to reduce the frame sizes.

Moving away from a cooled 16bit CCD to image with a 12bit cmos is a backwards step.

How do you get to f4 ? Using a different camera doesn't change your f ratio.

Example with QHY10 and a very low resolution.

https://www.astrobin.com/301323/B/?page=4&nc=user

Rob


AndyBooth
Posts: 47
Joined: Thu Mar 01, 2018 6:55 pm

Re: My first live stacks.

Post by AndyBooth » Fri Mar 22, 2019 7:58 pm

Well Rob,
I cant give much of an argument against all those shots!
To get F4 onthe CMOS, im using a 0.5x screw in reducer for the altair, cant use it on the QHY,
so best is F6 for that at 1200mm.
thanks for all the input by the way, I am flip flopping around I know.

having gone through Robins equations etc for the QHY, I Intend to try 120s exposures and alot of em, see if shorter more to stack is better than longer and less.
Unfortunately the readout time is 15 secs per frame, and I usually dither or I get fixed pattern noise on longer subs.
Read out noise is also high at 7.1 e sec.
Take your point on cropping, ROI.

I dont know, just seems so easy to get good results quickly with the CMOS.

🙄

MrAstroBen
Posts: 52
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Re: My first live stacks.

Post by MrAstroBen » Fri Mar 22, 2019 11:43 pm

Your CCD has a deep well, that is an big advantage that permits longer exposures. I dont think very short ones will work well but give it a try. Your Histogram should dictate your exposure length. As long as you can without clipping.

All I will add is search Astrobins for cup winners or images that grab you, then look at the equipment and how they were obtained.

The step up from what you have is a quality mono CCD.

Anyone show me the stunning images on there that were taken with short exposures and Cmos cameras?

Are all those highly experienced top imagers wrong?

People looking for short cuts and magic methods will be looking for a long long time.

Short exposures suit Cmos because longer exposures unveil the issues. Cmos is a cheaper but as yet inferior product.

The wanted signal is random in that not all photons make it to your sensor. The position though is fixed.

Noise is random and its position is not fixed. The longer you expose the more you benefit from both.

Even QHY do not pretend otherwise.

https://www.qhyccd.com/index.php?m=cont ... &catid=102

Also watch this.

https://youtu.be/EO4QFb3ydNM

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admin
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Re: My first live stacks.

Post by admin » Sat Mar 23, 2019 8:25 am

Hi,

CCD cameras are not an automatic upgrade from CMOS cameras these days and don't be suckered in by suggestions to take longer sub-exposures as the only way to improve image quality. The use of long sub exposures by CCD images is not an advantageous thing in and of itself – it is a response to the fundamental flaw of CCD sensors which is that they typically have relatively high read noise values. Unfortunately the idea that long sub exposures are required for deep sky imaging has become a rather ingrained myth in the imaging community because people have spent so long imaging with the CCDs that do require them.

I gave a talk at the practical astronomy show a couple of weeks ago aimed at exposing the incorrect thinking behind this particular myth – you can watch my talk on YouTube if you want –https://youtu.be/3RH93UvP358.

Cheers, Robin

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