Bloated stars in stacked image

[oyabuns]

Hey All,

I recently got a new camera, the QHY178M.

After the imaging session of NGC 891. I stacked 1 hour of data and then did a stretch and curves adjustment.
However, now I am seeing big bloated stars. What could be the cause of this and how do I get rid of it?
I attached the picture below.

My equipment:

QHY178-M
Skywatcher 150/750
Coma corrector MPPC
Filter-Wheel ZWO
Luminance filter

 Exposure:

1 min exposure @ gain 15

 

[alacant]

6 hours ago, oyabuns said:

What could be the cause

Hi

Nice shot.

A few ideas...

The stars are trailing which give the impression of inflated stars; guiding/tracking/polar alignment. Out of focus (?); use a focusing mask. CC; the Baader makes stars fatter; lose the cc. I don't think you'll need one with your camera.

HTH

[oyabuns]

1 hour ago, alacant said:

Hi

Nice shot.

A few ideas...

The stars are trailing which give the impression of inflated stars; guiding/tracking/polar alignment. Out of focus (?); use a focusing mask. CC; the Baader makes stars fatter; lose the cc. I don't think you'll need one with your camera.

HTH

Hi Alacant,

Since I am using a reflector, won't I get coma when I ditch the coma corrector?

[alacant]

1 minute ago, oyabuns said:

won't I get coma

Hi

My guess is no. Your camera has a very small sensor so you probably won't notice. Worth a try?

[vlaiv]

I agree about CC - at such a small field, maybe only far corners will be affected, and it is questionable if it will show (or be masked by seeing / guiding errors).

Another important fact to realize is that 178 sensor + SW 150 F/5 newtonian is going to give you 0.66"/px sampling rate. That is very high sampling rate and you are oversampling. It is also small sensor so FOV is small. I don't know what you used for imaging previously, but it is likely that your stars appeared tighter not because they were indeed tight in absolute size, but because things were at different scale.

With this setup you are really "zooming" in, and due to laws of physics stars are no longer pin points but start to have rather substantial size (simply because there is lack of resolution to render them as tight dots). Same thing would happen if you use image with tight stars and then zoom in (more than 100%) - stars will look bloated.

[vlaiv]

Btw, here is a random screen shot from a google image search on this galaxy:

If you compare the size of galaxy and size of "star bloat" in this and your image - you will see that you are not far off.

[oyabuns]

3 minutes ago, vlaiv said:

I agree about CC - at such a small field, maybe only far corners will be affected, and it is questionable if it will show (or be masked by seeing / guiding errors).

Another important fact to realize is that 178 sensor + SW 150 F/5 newtonian is going to give you 0.66"/px sampling rate. That is very high sampling rate and you are oversampling. It is also small sensor so FOV is small. I don't know what you used for imaging previously, but it is likely that your stars appeared tighter not because they were indeed tight in absolute size, but because things were at different scale.

With this setup you are really "zooming" in, and due to laws of physics stars are no longer pin points but start to have rather substantial size (simply because there is lack of resolution to render them as tight dots). Same thing would happen if you use image with tight stars and then zoom in (more than 100%) - stars will look bloated.

Hi Vlaiv,

That seems to make sense, before I was using a Canon 700D. So to downsample should I lower the gain or use shorter sub exposures?

[oyabuns]

I will ditch the CC and then post my new results here when I can image again.

[RolandKol]

you can also try a trick with Ha filter data, which will have the smallest stars from all others.

You will most likely take some Ha subs for galaxy anyway, - but once you post-process, after you combined all to HaLRGB (or HaRGB), - you can try to apply H as luminescence again, it should reduce stars also, but you may end up with other problems.

[vlaiv]

14 minutes ago, oyabuns said:

Hi Vlaiv,

That seems to make sense, before I was using a Canon 700D. So to downsample should I lower the gain or use shorter sub exposures?

I'm running a risk of over explaining things here (too much info), or already telling you things that you know, but here we go:

At 0.66"/px you are over sampling - this leads to poorer SNR and less sharp stars when image is viewed at 1:1 (one image pixel to one screen pixel, or 100% zoom). Depending on how good your guiding is, given your aperture and usual seeing conditions, you want to sample somewhere between 1.3"/px and 2"/px (depends on particular night).

With cmos sensors there is a simple way to do this - software binning. You can either bin your subs x2 or x3 prior to stacking (or even bin your stack while still linear, but I think it is slightly better to do it on subs). You can also downsample your image.

Both binning and downsampling will not have impact on image posted on forum (it won't change the FOV) - browser already downsamples them because image is large, unless you bin it to extent that it becomes smaller than browser shows it on forum. There is difference between binning and downsampling however. Both do the same thing but to a different extent. Both produce coarser sampling rate, and both increase SNR. SNR increase is a bit different, and depends on downsampling method used. Binning always provides predictable increase in SNR - by factor of how many pixels you bin. x2 binning will increase SNR by 2, x3 by 3, etc ... SNR increase with downsampling depends on downsampling method used and is always less than this maximum that binning provides. Downsampling also introduces cross pixel correlation that binning does not (not sure if you should care about this, but I pointed it out as a difference). There are more differences between the two, like impact of pixel level blur, etc ...

It is worth doing however - when image is viewed 1:1 it will be sharper looking, although smaller in scale (not so zoomed in), but main benefit will be SNR increase. SNR increase will be visible on screen size image (like here on forum) as well, because you will be able to stretch more and show "deeper".

I would recommend that you bin your image x2 regularly, and x3 on nights of poor seeing. You can try this with existing data (not sure what software are you using for stacking and processing), but do software bin x2 on your calibrated subs and then restack. Process result to see if you can go deeper. Also examine how it looks when viewed at screen size (like here posted on forum), but also what it looks at 1:1.

Btw, there is really simple way to observe how downsampling increases SNR - just look at your image scaled down to be displayed on forum, and also at 1:1, here are screen shots of both:

See how background looks more noisy on bottom image? They are the exact same image, but top one has been downsampled and background is much smoother because of this.