A little advice needed

[CHR15]

Hi, 

I'm just getting started with eaa and I'm over the moon with the results I'm getting so far. 

I'm just wondering if I can enhance the images further without going down the post imaging route? 

I'm not sure what flats and darks are or how they work. Would they help? Should I do more exposures or less gain?

Any advice from experienced to a beginner would be greatly appreciated.

Here are a few photos I have taken. They seem a bit noisey/grainy? 

EDIT

Equipment used

skywatcher ao70500 and st80

Skywatcher azgti mount

Skywatcher wedge 

Zwo asi585mc 

Edited May 16, 2023 by CHR15

[Steve in Boulder]

Flats should be a high priority.  There seems to be moderate vignetting, which flats will help with immensely.  Darks depend more on your camera's (you didn't specify which one) amp glow (aka dark current or thermal noise).  

The second to last one has elongated stars.  This could be the result of imaging near the zenith with an alt-az mount, if that's what you use. 

I'm not sure about your focus in images five through seven.  Do you use any focusing aids?

Noise is directly related to total integration time (number of subs x sub exposure time).  You haven't listed those, so it's hard to say more.  

[Martin Meredith]

Are you using an achromat refractor for these? If so, you can cut out some of the star bloat (eg in the last shot -- M103?) by using a filter. I started with an achromat and eventually switched to a Newt to get tighter stars.

Darks will help if you have either amp glow or hot pixels. I don't see much of either in your images. You can also do hot pixel removal without taking darks. 

Flat will help if you have dust shadows or vignetting. I see some vignetting so I'm guessing flats will help, though many find them a bit of a pain. If you do take flats, remember to take bias frames too. 

Having said that, I think the results you're getting are pretty good. Possibly you could cut back on the black point (ie darken the background) a little.

Could you say more about your equipment and the current exposure times (number of subs and total exposure) you're using?

Martin

 

[CHR15]

48 minutes ago, Steve in Boulder said:

Flats should be a high priority.  There seems to be moderate vignetting, which flats will help with immensely.  Darks depend more on your camera's (you didn't specify which one) amp glow (aka dark current or thermal noise).  

The second to last one has elongated stars.  This could be the result of imaging near the zenith with an alt-az mount, if that's what you use. 

I'm not sure about your focus in images five through seven.  Do you use any focusing aids?

Noise is directly related to total integration time (number of subs x sub exposure time).  You haven't listed those, so it's hard to say more.  

Thanks.

I've edited the equipment above.

I will try doing some flats next time to lose the vignetting.

I've read that my camera has zero amp glow. Does that mean I don't need darks?

The second to last photo is Dubhe. It was pretty much at the zenith, it was in eq mode rather than alt az mode.

I used a bahintov mask to try and focus, maybe too long exposure time on a bright star for photo number 9.

The galaxy photos were around 50 x 10 second exposures.

[CHR15]

1 hour ago, Martin Meredith said:

Are you using an achromat refractor for these? If so, you can cut out some of the star bloat (eg in the last shot -- M103?) by using a filter. I started with an achromat and eventually switched to a Newt to get tighter stars.

Darks will help if you have either amp glow or hot pixels. I don't see much of either in your images. You can also do hot pixel removal without taking darks. 

Flat will help if you have dust shadows or vignetting. I see some vignetting so I'm guessing flats will help, though many find them a bit of a pain. If you do take flats, remember to take bias frames too. 

Having said that, I think the results you're getting are pretty good. Possibly you could cut back on the black point (ie darken the background) a little.

Could you say more about your equipment and the current exposure times (number of subs and total exposure) you're using?

Martin

 

Thanks.

Yes I'm using an achromatic refractor. What filter should I use for the bloating? The last photo is the ET cluster (ngc457).

I shall try using flats. Is bias the same as taking darks but with the quickest exposure possible? 

These were all using the auto histogram on the asiair plus app. I'll have to have a play with the histogram.

I have added the equipment above and the exposure times we 50 x 10 seconds for the galaxies. The stars were only single short exposures. 

What does the gain do?

[PeterC65]

I use an IMX585 based camera and you won't get any amp glow with it, but you will get hot pixels which will be reduced by using dark frames, so they are worth capturing.

Overall I think the background sky in your posted images is quite bright and you should be able to do much better. It's hard to say why this is but it could be too much gain. I use a gain of 400 and an exposure time of 4s, usually taking about 50 frames. The sky brightness could also be caused by the level of histogram stretch you are using. With 50 frames you should be able to see M51, say, quite clearly against an almost black sky, even with an 80mm refractor. The sky brightness could also be caused by glow from wispy clouds or a nearby light source, but I'd say these are less likely since all your images are similar.

I'd recommend doing some experiments with different gain and exposure settings using a specific target and comparing the results side by side. So try gains of 250, 300, 350, 400, etc and exposure times of 2s, 4s, 8s, 15s, etc take, say, 30 frame stacks for each, take a snapshot of each stack with the histogram auto stretch, then compare the results side by side. It was by doing this (on M33) that I chose the camera settings I use.

The focus may also be out a bit on some of the images. Bear in mind that the focus will need readjusting unless the kit is properly cooled. I can only get away with focussing once when I've left the kit outside cooling for 45 minutes.

 

[Steve in Boulder]

1 hour ago, CHR15 said:

Thanks.

I've edited the equipment above.

I will try doing some flats next time to lose the vignetting.

I've read that my camera has zero amp glow. Does that mean I don't need darks?

The second to last photo is Dubhe. It was pretty much at the zenith, it was in eq mode rather than alt az mode.

I used a bahintov mask to try and focus, maybe too long exposure time on a bright star for photo number 9.

The galaxy photos were around 50 x 10 second exposures.

 

As Martin mentioned, darks are also helpful for hot pixels.  You can tell if they're a problem by looking for little streaks of color.  As also mentioned by Martin, your live stacking software may have a hot pixel correction feature.

It's helpful to know the gain as well.  Here's a useful explanation about gain.  50 x 10 seconds is a bit more than 8 minutes, which is a typical amount for EAA but you do have to expect some noise, especially if you stretch aggressively to show fainter features.  And by the way, aggressive stretches will emphasize vignetting, all the more reason for flats.  

For the Dubhe photo, perhaps your polar alignment was off?  

And I forgot to say, excellent images for your first effort!  

[Steve in Boulder]

1 hour ago, CHR15 said:

Thanks.

Yes I'm using an achromatic refractor. What filter should I use for the bloating? The last photo is the ET cluster (ngc457).

I shall try using flats. Is bias the same as taking darks but with the quickest exposure possible? 

These were all using the auto histogram on the asiair plus app. I'll have to have a play with the histogram.

I have added the equipment above and the exposure times we 50 x 10 seconds for the galaxies. The stars were only single short exposures. 

What does the gain do?

Yes on bias.  You want the camera to be in total dark.  

The auto histogram on the ASIAir app is a good starting point, but you can do a lot better manually.  Expand the histogram using the button on the right.  Typically you want the black point (left slider) to the left of the histogram peak, and the right slider (mid point I think, not sure because ASIAir docs don't say) pushed to the left enough to show the features you're looking for.  

Unfortunately the ASAir app doesn't have hot pixel correction, so you'll probably want darks.  The dark sub exposure time should match your lights, i.e. 10 seconds.

[Martin Meredith]

If you search for fringe killer filters there are a few threads here and elsewhere that cover the specifics. I must admit I moved on to a reflector and gave the achromat away so I never explored these filters myself.

Gain is a simple multiplier that can be thought of as spreading the count of photons captured at each photosite (~pixel) across the bit range of the camera. Setting the gain is analogous to setting the recording level on a microphone amplifier. Set it too low and your recording occupies just the lower bit range, so you are not making effective use of the bits; while you can boost the resulting digital signal after you capture it, you are boosting a highly-quantised signal, which ends up sounding louder but also noisy. Set the gain too high and you are in danger of saturating. Setting the gain is a bit of an art.

[Aside: forcing a user to set the gain is a hack due to the low bit depth of most CMOS sensors.... personally, I prefer 16-bit CCD cameras with fixed gain -- one fewer decision ;-)]

[CHR15]

1 hour ago, PeterC65 said:

I use an IMX585 based camera and you won't get any amp glow with it, but you will get hot pixels which will be reduced by using dark frames, so they are worth capturing.

Overall I think the background sky in your posted images is quite bright and you should be able to do much better. It's hard to say why this is but it could be too much gain. I use a gain of 400 and an exposure time of 4s, usually taking about 50 frames. The sky brightness could also be caused by the level of histogram stretch you are using. With 50 frames you should be able to see M51, say, quite clearly against an almost black sky, even with an 80mm refractor. The sky brightness could also be caused by glow from wispy clouds or a nearby light source, but I'd say these are less likely since all your images are similar.

I'd recommend doing some experiments with different gain and exposure settings using a specific target and comparing the results side by side. So try gains of 250, 300, 350, 400, etc and exposure times of 2s, 4s, 8s, 15s, etc take, say, 30 frame stacks for each, take a snapshot of each stack with the histogram auto stretch, then compare the results side by side. It was by doing this (on M33) that I chose the camera settings I use.

The focus may also be out a bit on some of the images. Bear in mind that the focus will need readjusting unless the kit is properly cooled. I can only get away with focussing once when I've left the kit outside cooling for 45 minutes.

 

Thanks.

I'll do some darks too.

I think the gain was between 200-250.

I do have a streetlight across the street unfortunately.

I'll have to try experimenting now I've done all the messier targets in ursa major, although leo is positioned nicely too 😆 

I'll have to try refocusing between targets and see if it helps. Should be plenty of time for the scope to cool down.

[CHR15]

1 hour ago, Steve in Boulder said:

 

As Martin mentioned, darks are also helpful for hot pixels.  You can tell if they're a problem by looking for little streaks of color.  As also mentioned by Martin, your live stacking software may have a hot pixel correction feature.

It's helpful to know the gain as well.  Here's a useful explanation about gain.  50 x 10 seconds is a bit more than 8 minutes, which is a typical amount for EAA but you do have to expect some noise, especially if you stretch aggressively to show fainter features.  And by the way, aggressive stretches will emphasize vignetting, all the more reason for flats.  

For the Dubhe photo, perhaps your polar alignment was off?  

And I forgot to say, excellent images for your first effort!  

Thanks for the link regarding gain I shall have a read through it now.

I'll remember now that streaks of colour are hot pixels and that darks will help to eliminate them. I don't seem to see them with this camera but I do have them in my asi178mc.

Thanks for the compliment on my first try. I'm over the moon with how they've turned out. The more I do the better they'll get through experience and an understanding learned from research and of course you guys.

I thought I must of over exposed Dubhe making it too bright and in turn causing the spike?

[CHR15]

1 hour ago, Steve in Boulder said:

Yes on bias.  You want the camera to be in total dark.  

The auto histogram on the ASIAir app is a good starting point, but you can do a lot better manually.  Expand the histogram using the button on the right.  Typically you want the black point (left slider) to the left of the histogram peak, and the right slider (mid point I think, not sure because ASIAir docs don't say) pushed to the left enough to show the features you're looking for.  

Unfortunately the ASAir app doesn't have hot pixel correction, so you'll probably want darks.  The dark sub exposure time should match your lights, i.e. 10 seconds.

Thanks for clearing up the bias question.

I'll have to have a play with the histogram too. Left pointer, left of the peak and right pointer, either middle or right of the peak.

At the moment I don't get hot pixels but it'll be good practice to take darks just to rule them out in future.

[CHR15]

8 hours ago, Martin Meredith said:

If you search for fringe killer filters there are a few threads here and elsewhere that cover the specifics. I must admit I moved on to a reflector and gave the achromat away so I never explored these filters myself.

Gain is a simple multiplier that can be thought of as spreading the count of photons captured at each photosite (~pixel) across the bit range of the camera. Setting the gain is analogous to setting the recording level on a microphone amplifier. Set it too low and your recording occupies just the lower bit range, so you are not making effective use of the bits; while you can boost the resulting digital signal after you capture it, you are boosting a highly-quantised signal, which ends up sounding louder but also noisy. Set the gain too high and you are in danger of saturating. Setting the gain is a bit of an art.

[Aside: forcing a user to set the gain is a hack due to the low bit depth of most CMOS sensors.... personally, I prefer 16-bit CCD cameras with fixed gain -- one fewer decision ;-)]

Thanks for explaining things through thouroughly