M16 in SHO - Feedback request - Forums

Topic starter

Jul 11, 2021

Hi Folks,
So I finally started narrow band imaging as I started to become obsessed with the SHO palette. Of course my first target to try was Pillars of creation. I only acquired about 3 hours of data
1.5 hours of HA
45 minutes of OIII
45 minuted of SII

All exposures were 120 seconds worth as I’m having guiding performance issues with my celestron edghd8 and the Orion 60mm scope on the AVX mount. My mount is 7 years old and probably I think the gears are wearing out. If I try to do 5 minute subs stars don’t look good at all.

In this picture I took, I’m looking for feedback on

a) How to make the stars look better- I’m pretty un satisfied with how bloated they are. I tried using Starnet++ in pixinsight but I’m still not able to reduce or even completely remove them. Is that still related to poor guiding ? Is 120 secs too low for Narrow band ?

b) I tried my best to sharpen the pillars without pixelating the blue background, but still not satisfied with them. How can I get them to be smooth and sharp 😀 I used many mask combinations but still no luck. Is it the data ? I will be more than happy to share the data if some one wants to take a stab.

c) any overall comments - would be appreciated. I’m happy with the colors but if you have feedback around that tell me as well

https://www.astrobin.com/vhtww4/B/

andrea tasselli

Frequent contributor

Jul 11, 2021

For your own benefit: gears do not wear out, not on mounts and not on mounts just 7 years old. Bearings do get old and loose performance and more importantly grease gets stiffer or gluey. That's all.

Going to the image. Are you seriously considering shooting at that image scale and complain about about the mount performance? First thing I would do is use a more realistic image scale, probably around 1"/px or less. It will do a world of good to the overall SNR and improve how the stars look like. And guiding would be a lot easier. I also suspect that you are out of focus (and no surprise here at that image scale), which also goes to explain how the stars look like.

Overall the image is rather poor and I still don't get why the Ha looks grey but maybe that's just me. BTW, given that the preponderance of the signal is in the Ha band you should really capture a lot more OIII and SII to compensate for their relative lack of signal.

Helpful

John

Jul 11, 2021

its not bad..and its possible to get that small image scale,i did it myself. but you need just more subs,so the colors etc will be great.

you have nice round stars..so good job.

mine last month. https://www.astrobin.com/gan7ly/

greetings john

John Hayes

Frequent contributor

Jul 11, 2021

I have a couple of comments:

1) If you want to look at such a small FOV, why are you using a reducer? That is counter productive in a couple of ways. First, you are adding another optical element that may be introducing additional optical aberration. Second, as Andrea pointed out, you are shooting at too small of an image scale. At F/10 under 1.5" - 2.0" seeing, the pixels size should not be much smaller than about 7 microns. Your configuration is throwing away SNR for no good reason. You'll do better if you ditch the reducer and bin that sensor 2x2.

2) Your stars are round but they are also pretty big. How are you focusing and how are you maintaining focus? My first reaction is that your guiding looks ok but that your focus is off. In general, it's good to remember the size of the smallest stars in your image is directly related to the size of smallest feature that you'll see in the object. If your stars are big, you won't see a very detailed image.

John

Well Written Helpful Insightful Engaging

mousta

Topic starter

Jul 11, 2021

John Hayes:
I have a couple of comments:

1) If you want to look at such a small FOV, why are you using a reducer? That is counter productive in a couple of ways. First, you are adding another optical element that may be introducing additional optical aberration. Second, as Andrea pointed out, you are shooting at too small of an image scale. At F/10 under 1.5" - 2.0" seeing, the pixels size should not be much smaller than about 7 microns. Your configuration is throwing away SNR for no good reason. You'll do better if you ditch the reducer and bin that sensor 2x2.

2) Your stars are round but they are also pretty big. How are you focusing and how are you maintaining focus? My first reaction is that your guiding looks ok but that your focus is off. In general, it's good to remember the size of the smallest stars in your image is directly related to the size of smallest feature that you'll see in the object. If your stars are big, you won't see a very detailed image.

John

thanks John. I thought that adding the reducer would put less strain on the guiding, shooting at f/7. I haven’t thought about trying without the reducer. I will give it a shot and use bin2x2

i have seen images at this image scale. Are you saying that my equipment aren’t capable or that purely shooting at this image scale is wrong ? Sorry to be dense but I’m not getting it.

Re: Focus I’m using a bhatinov mask, and it looks pretty good in it. I refocus every hour or so.

mousta

Topic starter

Jul 11, 2021

John:
its not bad..and its possible to get that small image scale,i did it myself. but you need just more subs,so the colors etc will be great.

you have nice round stars..so good job.

mine last month. https://www.astrobin.com/gan7ly/

greetings john

Thank you!! Your image looks amazing. I will keep trying..

John Hayes

Frequent contributor

Jul 11, 2021

thanks John. I thought that adding the reducer would put less strain on the guiding, shooting at f/7. I haven’t thought about trying without the reducer. I will give it a shot and use bin2x2

i have seen images at this image scale. Are you saying that my equipment aren’t capable or that purely shooting at this image scale is wrong ? Sorry to be dense but I’m not getting it.

Re: Focus I’m using a bhatinov mask, and it looks pretty good in it. I refocus every hour or so.

When an image is properly sampled, reducers don't reduce guiding requirements. If your system is properly configured, the one and only thing that a reducer does is to provide a wider field of view. (There are some limits on this statement but for now, that's a good rule to remember.) The subject of matching a sensor to a telescope is a big one but in simple terms, once you pass a certain size, increasing sampling with smaller pixels does nothing to increase image detail. At that point, you are just reducing signal and gaining nothing in "sharpness." When you make pixels bigger, you gain signal but past a certain point, you lose image "sharpness." A rule of thumb is that the sweet spot lies somewhere 2-3 pixels across the blur diameter in the image plane. The blur diameter is the size of the seeing blurred point spread function for your optics. For an 8", F/10 scope under say 2" conditions, the seeing blurred disk diameter will be about 20 microns in the focal plane. Under perfect conditions the Airy disk will be about 13 microns. So in my last message, I simply "ball-parked" the best sampling for your scope at about 7 microns, which works out to be 2x2 binning with your sensor. Sampling with smaller pixels does nothing to produce a sharper image; it only reduces signal and SNR.

A B-mask is a good way to focus but you have to be really picky when you focus. The amount of time between refocusing will determined by how rapidly the temperature is changing. In my C14, I used to have to refocus every 15-20 minutes to get acceptable results. Your 8" won't be that tight, but from what I see in your image, you've got a focus problem. To focus well with a B-mask, you've got to zoom way in and be very picky about getting everything perfectly centered. The Celestron scopes also exhibit a bit of longitudinal color so you have to be careful about focusing at a different bandpass than your filter. This is where it pays to have a fully automatic focusing system.

John

Helpful

mousta

Topic starter

Jul 11, 2021

John Hayes:
thanks John. I thought that adding the reducer would put less strain on the guiding, shooting at f/7. I haven’t thought about trying without the reducer. I will give it a shot and use bin2x2

i have seen images at this image scale. Are you saying that my equipment aren’t capable or that purely shooting at this image scale is wrong ? Sorry to be dense but I’m not getting it.

Re: Focus I’m using a bhatinov mask, and it looks pretty good in it. I refocus every hour or so.

When an image is properly sampled, reducers don't reduce guiding requirements. If your system is properly configured, the one and only thing that a reducer does is to provide a wider field of view. (There are some limits on this statement but for now, that's a good rule to remember.) The subject of matching a sensor to a telescope is a big one but in simple terms, once you pass a certain size, increasing sampling with smaller pixels does nothing to increase image detail. At that point, you are just reducing signal and gaining nothing in "sharpness." When you make pixels bigger, you gain signal but past a certain point, you lose image "sharpness." A rule of thumb is that the sweet spot lies somewhere 2-3 pixels across the blur diameter in the image plane. The blur diameter is the size of the seeing blurred point spread function for your optics. For an 8", F/10 scope under say 2" conditions, the seeing blurred disk diameter will be about 20 microns in the focal plane. Under perfect conditions the Airy disk will be about 13 microns. So in my last message, I simply "ball-parked" the best sampling for your scope at about 7 microns, which works out to be 2x2 binning with your sensor. Sampling with smaller pixels does nothing to produce a sharper image; it only reduces signal and SNR.

A B-mask is a good way to focus but you have to be really picky when you focus. The amount of time between refocusing will determined by how rapidly the temperature is changing. In my C14, I used to have to refocus every 15-20 minutes to get acceptable results. Your 8" won't be that tight, but from what I see in your image, you've got a focus problem. To focus well with a B-mask, you've got to zoom way in and be very picky about getting everything perfectly centered. The Celestron scopes also exhibit a bit of longitudinal color so you have to be careful about focusing at a different bandpass than your filter. This is where it pays to have a fully automatic focusing system.

John

Thank you so much for taking the time to explain to me. That’s really helpful. I’m definitely bookmarking this response and will go research more on this topic. I guess I haven’t been paying enough attention to all these parameters to gain a deep understanding. I will try again and share my results with you. Appreciate the help again

Well Written Respectful

John

Jul 12, 2021

p.s. you can test your config yourself here. http://astronomy.tools/calculators/ccd_suitability

mousta

Topic starter

Jul 13, 2021

John:
p.s. you can test your config yourself here. http://astronomy.tools/calculators/ccd_suitability

yeah Thanks. I tried that yesterday after the discussion here, and it was crystal clear

Bin 2x2 with the reducer or without it. With my one shot color ASI294MC I can do Bin 1 with the reducer 0.67”/pixel. The big mistake I did is not taking inconsideration the new pixel size of the Mono camera after switching from the OSC. Learnt a lot. Thanks