Very poor stacking results...

[descott12]

Hello,
I am pretty new to SharpCap and I have seen alot of posts with some amazing EAA images with very short exposures. Unfortunately, I am not having much luck. I am using a Evo 8 with a new Hyperstar 4 and and ASI 294 MC cooled camera. So I would expect much better results. I have posted three screenshots and I am sure hoping somebody can assist.

M31 - All I get is a blurry core. No improvement in stacking at all. The stacked view is not much better than the individual frames and it never gets better than this image. 3.5 minutes of exposure.

M42 Just a horrible muddy mess with only the very core. Usually that is pretty blown out and over-exposed in good photos but it is also usually surrounded by amazing nebulosity. I can't seem to pull out much nebulosity at all. Same with M45 below and in the outer parts of M31 above. There seems to be a common theme in my problems...

M45 - Never improves beyond just the basic stars. Mostly monochrome. No hint of blue. No nebulosity at all. I made a previous post about the Live Stack being all monochrome. It was suggested to up the right-most slider in the histogram. While that worked in some cases, it mostly just put a sickly green hue over most images. And it didn't help to reveal any blue in the stars or the surrounding nebulosity.

I just got done playing with every setting I could think of. 3 hours on these 3 DSOs and these images are the best I could get. In all cases, the stacking was working well, alignment was happy and the FWHM focus tool seemed to think things were ok. No dew problems either. The visibility was not great but it could have been alot worse. I noticed that the histogram peak was very far to the left but I had a hard time affecting that in any way. But even in cases where it was better positioned, things didn't seem to improve much.

Thanks in advance for any help.

Dave in Charlotte, NC , USA

[admin]

Hi Dave,

so the things that immediately jump to mind when looking at your images are the following:

1) there are focus and/or collimation issues visible in all of the images – the stars are showing up much too large especially considering how much the images zoomed out to show it the full image like that within the SharpCap application. If you look at your stars in your Orion nebula image you can see that each start is a central blob of light surrounded by a larger off centre fuzzy blob. You should really be aiming for point stars if at all possible – certainly the fact that the outer blob is off centre hints at poor collimation as well as poor focus. Unfortunately I don't know anything specifically about collimating an SCT with a hyper star attachment – hopefully you can find some advice on that elsewhere.

2) you don't seem to be using a very strong stretch in the histogram tab which means that the faint features aren't being enhanced up to the visible levels. In each of your images you want to move the mid-level slider in the histogram further to the left so that the yellow transfer line starts off close to vertical and then flattens out to the right hand side. You will find that this will help pull out more detail of faint nebulosity. Initially, this fine detail will be noisy since the strong stretch enhances both the image you want to see and the noise. As more frames are stacked the noise level will gradually drop away finally showing you a better and better image.

Hope this helps, Robin

[descott12]

Hi Robin,
Thank you very much for the very helpful reply. I see what you mean about the collimation and the comment about the shape of the transfer line makes alot of sense. Definitely alot to know about the software and image processing in general. I will definitely give that a try tonight hopefully!

Regarding the out of focus stars: I clearly see the collimation issues and lack of focus in the M42 image. In the M45 and M31 shot, the stars appear ok to me and the few FWHM focus tests I ran in Sharpcap seemed to think they were ok? You don't agree? It is my understanding that bright areas, including the main stars in M45, can become over-exposed easily but that does not necessarily indicate a focus problem. Does this make sense or am I way off.

I currently have very little control over focus as the scope is not easily accessible from where my computer is. So I focused very crudely by hand once at the beginning and that was it. I am sure the focus drifted horribly over the 3 hours I was playing with it. I have ordered a motofocus with remote control so that should vastly improve my focus.

Thanks again,
Dave

[turfpit]

Dave

I have a Celestron C8 and JMI Motofocus. The 'hands-off' focusing was helpful but the item that really sorted out my focusing was a Bahtinov Mask (cost around £20 for an 8"). When you have an image like the one below, then you know you are properly focused. I note your comment

I currently have very little control over focus as the scope is not easily accessible from where my computer is.

That will make for many wasted nights then - getting focus sorted was one of my big breakthroughs with imaging.

It is my understanding that bright areas, including the main stars in M45, can become over-exposed easily but that does not necessarily indicate a focus problem

There are some M45 images on my astrobin https://www.astrobin.com/users/turfpit/ taken with various exposures - the longest being 180s.

Check out my M42 Chronicles post at viewtopic.php?t=596. The result of 2 years of effort and thinking - started off with stars like golf balls

Dave

bahtinov-mask.JPG (30.48 KiB) Viewed 4088 times

[descott12]

Dave,
Thanks alot for you post. Your first M42 image looks remarkably like mine so that was good to see that there is hope.
Yes, I will get a bahtinov mask right away. I guess I was relying on the Focus Assessment tool in SharpCap a bit much. Is that worth using at all?

Question: do you have to re-apply the mask and re-focus during a longer session or is it good enough just at the start with small focus adjustments later based on what you are seeing on your screen? I ask because my mask will have to fit around a Hyperstar and camera ( I know they make them with a central hole) but that would require me to unplug the USB cable and cooler power cable.

Question 2: In my lame M31 image, the stars looked pretty pinpoint to me. Do you agree or do they look too big there as well?

Thanks again.
Dave

[turfpit]

Dave

With all the answers below, they are things that work for me. YMMV.

I will get a bahtinov mask right away

A picture is worth a thousand words https://www.youtube.com/watch?v=xJXkANpQElU

Best 20 quid I ever spent. With the C8 and the motorised focuser combined with the mask I had a massive improvement in my images. With my refractor I have a 10:1 focuser and that combined with the mask is good enough.

Things to watch out for using the mask:

• Use a medium brightness star. I wouldn't try to focus using a bright star such as Capella, Vega , Sirius etc.

• Focus on a star near the object you are going to image. This will reduce any impact from mirror shifts on the SCT.

• Focus before starting a Live Stack session.

• Exposure in the 0.5s to 1s range seems to work ok for me.

• If you have to endure a meridian flip then re-focus. Think mirror flop with an SCT.

• To help with the diffraction pattern I zoom in to around 125%. Also applying the mini histogram stretch aids clarity. viewtopic.php?f=18&t=1151

do you have to re-apply the mask and re-focus during a longer session or is it good enough just at the start with small focus adjustments later based on what you are seeing on your screen?

I cant really reply to that question how I would like to in a public internet forum
We have already established that achieving focus is hard and needs tools to assist, it doesn't get easier as the night goes on.

When you get the mask and find out how much the slightest movement of the focuser affects the diffraction pattern then you will understand. What does happen over a long session is:

• Airmass and temperature change. These affect focus.

• Altitude of the object affects thickness of the atmosphere and clarity. Again focus is affected.

Enjoy reading about Rayleigh scattering https://www.skyandtelescope.com/astrono ... xtinction/

If I was imaging M31, then M45, then M42, I would (using the mask for each focus):

• Focus on a star near M31

• Carry out the M31 capture

• Focus on a star near M45

• Carry out the M45 capture

• Focus on a star near M42

• Carry out the M42 capture

I carry out traditional imaging i.e. lights, darks, bias, flats with processing a clearly defined separate task. For the 3 objects listed, the lights would typically be 1 hour, darks 30 minutes, bias and flats seconds. So a set of frames would take 1h 45m. So with focusing and test frame I reckon 2 hours per object. The lights might be 60x60s for M42 and 20x180s for M31 and M45 (to capture the blue nebulosity). If I cannot get something decent in an hour then I reckon I need to analyse what I am doing (wrong).

mask will have to fit around a Hyperstar and camera

I suppose your camera is wider than the Fastar? Have a look at
https://www.cloudynights.com/topic/4559 ... hyperstar/
The easy way is hang the mask off the dew shield - I work like this (you might need to cut a notch in the shield for the cables. The other way is cut the mask in half. If you read the post, these can be manufactured to accomodate fastar/hyperstar. Looking on the Farpoint website, there are 3 masks - take care to get the correct one for your scope.

In my lame M31 image, the stars looked pretty pinpoint to me. Do you agree or do they look too big there as well

A person who is an experienced imager and capable of writing a capture program suggested that the focus area in your images need attention. What we think is irrelevant, the Bahtinov Mask has the final say. What might look a pinpoint star to you won't necessarily be pinpoint to a stacking program. The classic is with Deep Sky Stacker - if the stars are not round enough the dreaded 'only one frame to stack' appears.

These stars are pinpoint in this M31 work in progress. Why do I say that? Because the diffraction pattern from the Bahtinov Mask on a mini histogram stretch was bang on centre and the 3 perpendicular lines were clearly visible - hard scientific repeatable fact.

M31-focused-stars.JPG (32.79 KiB) Viewed 4068 times

Astro imaging is a humbling experience. It doesn't matter how much or little is spent on equipment, we are all subject to the same issues. ££££££s spent doesn't guarantee success. Time and effort put in will help. We are restricted by the weather and other things. The few clear nights we have can be affected by the moon & wind. If the clear night is a weekday, that is not convenient for someone who has work the next day. This was a single night back in early October viewtopic.php?f=16&t=1199 but there hasn't been skies like that for me since. This one might encourage you viewtopic.php?f=16&t=1088 with M31.

What I did learn (the hard way) is there are some things that cannot be 'fixed' the next day. Examples:

• Polar alignment of mount

• Mount alignment

• Focus

It is easy to get carried away with add-ons which can actually be detrimental to results as they just add complexity. One day I might actually sit down and make a list of what features I actually use for a capture session - I know the list is short.

Dave

[BlackWikkett]

Hi Dave,

A few thoughts about using the Bahtinov Mask with SharpCap. SC has six different focus aids available one of which is the Bahtinov Mask focus assistant. You'll find this and other focus aids on the last icon on the top bar of SharpCap (icon looks like a magnifying glass) There are other tools here that will not require using the mask and may be easier to use with your setup. You will need to be able to see your computer screen running SharpCap to use these effectively. Every image I've posted on these forums was first focused using the Bahtinov Mask focus assistant. Have a look in the gallery for results.

Good luck, Wikkett

[descott12]

Thanks Wikkett,
I was not aware of the Bahtinov mask tool. I will definitely check it out once my mask arrives (gotta love Amazon Prime!)

[oopfan]

Hi Dave,

Sorry I've arrived late to this party. So if I may add...

Poor focus has essentially downgraded your 8-inch telescope to a 2-inch scope. Good focus concentrates the starlight into a 3x3 pixel matrix but poor focus spreads the light over a 10x10 area or worse. Spreading the light over a larger area means that less light reaches each pixel.

I was fortunate to have been involved in AP over 40 years ago when CCD cameras were out of the reach of most amateurs, and CMOS cameras did not exist. We used photographic emulsion back then. Typically we would take one long exposure on a single sheet of film and then use darkroom techniques to bring out faint details. Yes, there were people who stacked emulsion but it was rare.

I see that your stack sizes are only 3.5 minutes in length. That is not enough time. You really should give it at least 30 minutes. Also experiment with longer exposures. Using a 5-second exposure will require much larger stack sizes to achieve a good signal-to-noise ratio. Experiment with 10 second exposures and longer. You will reach a point where star trails will form indicating that your mount's tracking is suffering. Increase the exposure no further, then experiment with various stack sizes. You will find that you get much better results. Work that way for several weeks or months until you achieve proficiency, and only then look into ways to improve your mount's tracking.

The money you've invested in your kit is enough to create stunning results. Use what you've got and squeeze every last drop of performance out of it. Don't be fooled into thinking that throwing more money at the problem can solve your problems. This hobby is all about experimentation.

Brian

[descott12]

Hi Brian,
Thanks for the input. Yes, that makes sense. My new motofocus and Bahtinov mask just arrived so I am looking forward to seeing what a good focus can do! I will also use longer exposures and more stacking as you suggested. Hopefully, I will have a nice sharp image to post soon.